The Ichnology of Jurassic Park

Posted on April 9, 2013 by TMartin

All paleontologists remember their first time. Mine was in 1993, during a hot, steamy summer in Atlanta, Georgia. I had just spent the previous month camping in wilderness areas of Wyoming, so coming back to a big city with all of its urban temptations meant I was weak and susceptible to seeking out unusual sources of pleasure. Although I was not quite ready to be taken for such an exhilarating ride, it was an experience I’d never forget. Afterwards, once I had recovered enough to be able think about it, I wanted to do it again.

I am, of course, talking about seeing the film Jurassic Park on a movie screen. Sure, this movie has been around long enough (20 years) that nearly every paleontologist has also watched it on a TV, computer, or mobile device. But there is something about seeing dinosaurs – which, let’s face it, are most famous for their size – shown big. The initial glimpse of a Brachiosaurus munching on the tops of tall trees, a herd of Paralophosaurus ringing a glistening lake, an ill Triceratops dwarfing its human caretakers, the grand entrance of the Tyrannosaurus: all of these “actors” were meant to be seen large, and fill us with awe. It worked. Plot, acting, and science aside, Jurassic Park was, and probably still is, the best movie made for conveying what it would feel like for us humans, separated by a minimum of 65 million years, to see real, living dinosaurs.

“It’s, it’s a dinosaur.” That pretty much said it all in 1993, and still does. But what traces were being made by this Brachiosaurus as it strolled through its all-you-can-eat salad bar, known to you and me as a “landscape”? Please read on.

In 1993, though, I did not have an appreciation for some of the smaller details included in this film, and how my research specialty of ichnology – the study of traces, like tracks, burrows, and nests – was reflected throughout it. What dinosaur traces were included in the movie, and how were these traces used to serve or advance the plot? I also wondered how 20 years of field experience and scholarly research in ichnology might have changed my perceptions of it since that first viewing.

So with the re-release of Jurassic Park in 3-D last week, I decided it was time to view it from an ichnological perspective and share these thoughts with others. After all, my next book, Dinosaurs Without Bones (Pegasus Press), is about dinosaur trace fossils, and written for a popular audience. Also, in between the movie’s first release and now, I wrote two editions of a college textbook on dinosaurs (Introduction to the Study of Dinosaurs). Thus I went to the theater well justified in watching Jurassic Park once more, to see for myself how dinosaur traces were portrayed in the most well-loved of all dinosaur movies. And oh yes, for the fun.

For the sake of simplicity, I’ve divided these traces into two categories: those viewers could directly observe in the film, and others that could be inferred from the dinosaurs’ behaviors. Wherever possible, I also connect traces shown in the movie to research done on dinosaur trace fossils during the last 20 years, giving a broad sense of how far dinosaur ichnology has progressed since 1993.

(Ichnologist’s note: Even though all of the live dinosaurs in the movie were set in the 1990s, the study of their modern traces still qualifies as neoichnology. In contrast, any reference I make to actual dinosaur trace fossils is paleoichnology. Just so you know.)

Dinosaur Traces in Jurassic Park

• Velociraptor hatchling traces. Jurassic Park shows two different but complementary examples of hatchling traces for “Velociraptor.” (I will call this dinosaur Velociraptor throughout this post, but as most dino-philes know, the director, Steven Spielberg, scaled up the Late Cretaceous Velociraptor to maximize its frightfulness. Hence it is actually more like the Early Cretaceous Deinonychus or Utahraptor.)

One is an egg-emergence trace, which is the hole left in an eggshell where a dinosaur broke out of its egg. In this scene, a cooing Velociraptor hatchling pokes its cute little nose out of its egg. (This nose, if everything worked out for it, would some day would be warmed by fresh human viscera.) We first witness this tracemaking in the Jurassic Park laboratory toward the start of the film, the same day most of the protagonists arrive on the island (Isla Nublar). As far as I know, such trace fossils have not been interpreted from the fossil record, or if they have, they have not been referred to as trace fossils: which they should be.

The next day, after dinosaur paleontologist Alan Grant and his two companions – Lex and Tim Murphy – were sufficiently terrified (and enthralled) by various dinosaur encounters out in the park, they come across a Velociraptor nest. The nest has about 15-20 broken eggs, and the fracture patterns of the eggshells provide clear evidence of hatching. But these traces also had tiny, two-toed tracks leading away from them. The tracks, with two toes studded by digital pads, were typical for deinonychosaurs. However, unlike nearly every theropod track I’ve seen, these tracks lacked claw marks at their ends. (Tsk, tsk, says this nitpicking ichnologist.)

Aw, look at the cute little Velociraptor tracks and hatched eggs. Don’t these traces just make you want to say, “Who’s the cutest little predator in the world?” Still from Jurassic Park (1993), taken from www.anyclip.com.

Even though these tracks were flashed on the screen for only a few seconds, what’s really cool is how they convey three important pieces of information. One is that the Velociraptor chicks hatched after the torrential rainstorm of the previous night, and thus only mere hours before our wandering heroes saw their traces. Second, the tracks demonstrate that the hatchlings were not altricial, but ready to move and leave the nest immediately, presumably without parental care. Third, Dr. Grant realizes that these successfully fertilized and hatched eggs mean the “female-only” genetic fail-safe plan for the dinosaurs just got disproved. In other words, these traces mattered.

One point about that nest: as far as I could tell from, this Velociraptor mother did not make a rimmed structure to protect the eggs, such as those made by another Late Cretaceous theropod, Troodon, or Late Cretaceous sauropods in Argentina. Instead, the eggs were laid out in the open, like some ground-nesting shorebirds might do. In contrast, the Jurassic Park sequels featured Velociraptor nests that were much more overt as traces, such as a rimmed nest seen in Jurassic Park III.

Partially preserved rimmed nest structure of Troodon, a Late Cretaceous theropod that lived in what we now call Montana. The rim has eroded quite a bit since its discovery in the mid-1990s; the Troodon egg clutch was in the area of the foreground before its extraction. (Photograph by Anthony Martin; scale in centimeters.)

• Triceratops feces. “That is one big pile of sh*t,” observes Dr. Ian Malcolm, the “chaotician” (mathematician) who supplies both pessimism and comic relief throughout the movie. In this scene, where the main protagonists attend to an under-the-weather Triceratops, two impressive piles of fecal material inspire Dr. Ellie Satler, a paleobotanist, to figure out whether or not the ceratopsian had eaten any toxic plants.

Somehow I suspect this scene was meant as a metaphor for what most paleontologists have to do in their day jobs in order to do any paleontology at all.

Still, when added together, this amount of still-moist waste was far too voluminous to have been from one or two depositional events: I mean, this dinosaur was sick, but not that much. As a result, park personnel must have been responsible for making these dung heaps from several days worthy of Triceratops contributions. (Strictly speaking, then, these heaps were composite traces.) If so, this would have been a rather unenviable job, but maybe they were better paid than Dennis Nedry, the disgruntled computer programmer who later provided ample fodder for Dilophosaurus.

Unfortunately, fossil Triceratops feces (coprolites) are thus far unknown from the geologic record. What is exciting, though, is that several excellent studies have been done by Dr. Karen Chin on Late Cretaceous hadrosaur coprolites. These coprolites, like the fictionalized Triceratops feces, contain lots of plant material, telling us much about what these hadrosaurs ate 75 million years ago. They also tell us what ate the feces or grazed on them, which were dung beetles and snails, respectively. (Indeed, I now wonder if Isla Nubar had a severe shortage of dung beetles, which might explain how those Triceratops feces got piled higher and deeper.)

Dinosaur coprolite – probably from a large hadrosaur, such as Maisaura – and filled with wood fragments, accompanied by special bonus trace fossils: dung beetle burrows! Specimen from the Two Medicine Formation (Late Cretaceous, Montana) as part of a Museum of the Rockies traveling exhibit at Fernbank Museum of Natural History. (Photograph by Anthony Martin, taken in 2001 and scanned from a 35-mm slide; scale in centimeters.)

• Tyrannosaurus tracks. Probably the most memorable scene in Jurassic Park is the grand entrance of the Tyrannosaurus, whose approach is first detected by “impact tremors” transmitted in cups of water on the dashboard of a jeep. Following this first bout of terror and the arrival of Ellie Sattler and big-game hunter Robert Muldoon, Malcolm, convalescing in the back of a jeep, looks down at a three-toed Tyrannosaurus track. The track, filled with water from the rain, communicates a warning as it vibrates from the footfalls of the approaching giant theropod. This repeats the previous image of the trembling water in the cup, but is made doubly dreadful by happening in a freshly made footprint of the same animal causing the tremors.

So what was by far the most exciting moment in the movie for me, ichnologically speaking? The Tyrannosaurus making a track, in which mud pushes up and out to the sides of its right foot, observed at 2:38 in the following video clip. Just watch:

This was already a great scene for all of its action, suspense, and lawyer eating. But check out that track-making!

Only a few fossil tracks have been attributed to Tyrannosaurus or other tyrannosaurids, mostly for being the right size (really big) and geologic age (Late Cretaceous). One was discovered in New Mexico in 1983, but wasn’t reported in a scientific journal until the year after Jurassic Park came out. More than a decade passed before another was found in Montana in 2007 and reported in 2008. Tragically, both were isolated tracks, and a Tyrannosaurus trackway, with two or more consecutive steps, has not yet been found. If so, it would make for a pretty darned nice find. So please do let the world know if you find one.

A large and well-preserved three-toed theropod track from the Early Cretaceous Glen Rose Formation of Texas, made about 95 million years ago. Although this track was more likely made by Acrocanthosaurus, rather than Tyrannosaurus rex, you can be assured that this theropod, like all living things, was a distant relative of T. rex. (Photograph taken by Anthony Martin; scale in centimeters.)

• Velociraptor tracks (adults). These tracks, shown only for a few seconds, are outside of the Velociraptor enclosure after the power was shut down. Muldoon, accompanied by Sattler, spots the footprints, and he wordlessly identifies them. (His expression also tells the audience that Sattler and he are going to be in deeper doo-doo than the Triceratops piles.) The twisted and broken bars of the enclosure provide additional traces affirming the conclusion that these ‘raptors are on the loose. All of these traces are shown only minutes before Muldoon utters his meme-inspiring last words, “Clever girl.”

Uh oh: Velociraptor tracks, and these don’t look like they’re from hatchlings. Good thing Muldoon is a big-game hunter, who’s skilled at tracking and predicting Velociraptor behavior based on their tracks. But too bad his hypothesis was falsified in such an unpleasant way, but I suppose he could have picked kinder reviewers. Still from Jurassic Park (1993), taken from www.anyclip.com.

Here’s what a real deinonychosaur track looks like. This one, from the Early Cretaceous of Utah, is a right foot impression, and is just slightly smaller than the adult tracks depicted in Jurassic Park. Notice the digits are thinner and end with sharp clawmarks, too. (Photograph by Anthony Martin; scale in centimeters.)

• Bioerosion of fossil dinosaur bones by modern dinosaurs. Toward the end of the film, the main human heroes – Grant, Sattler, Murphy, and Murphy (which sounds like a law firm, but we know how T. rex feels about those) – are confronted by a Velociraptor pack in the Jurassic Park visitor center. During their attempts to flee the ‘raptors, both species end up disarticulating and breaking some of the mounted dinosaur skeletons in the atrium of the visitor center. Their actions were thus a form of bioerosion, in which the fractured dinosaur bones are traces of their activities. Alternatively, the bones may have been artificial casts, in which case their breakage would have constituted bioerosion of modern substrates.

This bioerosion is made more complicated when the Tyrannosaurus rex (who everyone agrees is the ultimate protangonist of the movie) enters the atrium and, among other antics, smashes a skeleton of itself with a recently crunched Velociraptor. As a result, the jumbled assemblage of bones at the end is attributable to three separate, interacting tracemakers: four humans (two adult, two juvenile), two Velociraptors (both adults), and one Tyrannosaurus (adult). What should be noted, though, is that if the Velociraptor was already dead when flung by the Tyrannosaurus, then the broken skeleton is a trace of the Tyrannosaurus, not the Velociraptor. In other words, the Velociraptor body was just being used as a tool.

Bioerosion in action, as a result of Velociraptor and human interactions. Also, at 2:45: T. rex smash!

Dinosaur Trace-Making Behaviors in Jurassic Park

• Brachiosaurus tracks, browsing traces. When Drs. Grant and Sattler experience their first jaw-dropping glimpse of a Brachiosaurus, they watch it rear up on its hind feet, and come down hard on front feet. Considering that a Brachiosaurus this size might have weighed at least 30 tonnes, it surely would have left deep tracks in both the front and rear from the increased stresses imparted by these actions. Also, its cropping the tops of trees would have caused some easily visible gaps in the canopies of forests on Isla Nubar.

• Theropod toothmarks. Part of the fun of Jurassic Park was indulging in our worst nightmares about these non-avian theropods frequently sampling human flesh. Assuming that the theropod teeth in each instance penetrated skin and muscle and contacted bone, toothmarks would have included the following: (1) Tyrannosaurus toothmarks on goat, human, and Velociraptor bones; and (2) Velociraptor and Dilophosaurus toothmarks on human bones.

• Triceratops resting trace. When the paleontologists and others visit the ailing Triceratops, it was lying on its right side. I couldn’t help but think that if Triceratops or any other large ceratopsian dinosaur ever reclined like that, and in the right type of sediment, it would have left a gorgeous body impression. This scene also reminded me of bison traces I’ve seen in Yellowstone National Park, in which bisons roll onto their sides for a dust bath, leaving outlines of their bodies. Did dinosaurs – especially the feathered ones – ever take dust baths, and leave similar body impressions? We don’t know yet, but such trace fossils are something to look for.

• Dinosaur stampede. One of the most astonishing computer-generated effects of Jurassic Park, and one that was especially effective in 3-D, was of a dinosaur stampede. In this scene, a flock of Gallimus run toward and around Grant, Murphy, and Murphy, just before the ambush-hunting Tyrannosaurus rex slaughters one of them (the Gallimus, that is). I’ve written about this scene before, connecting it to a dinosaur tracksite in Queensland, Australia that has more than 3,000 tracks preserved. Although the site was originally interpreted as the only evidence of a dinosaur stampede, the tracks were recently reinterpreted as swim tracks. I’ll write about this topic at length in my upcoming book, so for now, I ain’t saying nothing more.

Run away, run away!

• Tyrannosaurus drag mark. After the Tyrannosaurus rex decides that a measly goat was just an appetizer and begins seeking out the nearest available mammals for nomming purposes, at some point it overturns and begins pushing an SUV, which still has Lex and Tim Murphy trapped underneath it. Its flipping the SUV with its head certainly would have left a substantial mark on the muddy ground, a trace that then would have been connected to a semi-circular dragmark (clockwise oriented), and with tracks directly adjacent to these traces. Her tracks also may have been deeper in their fronts because of her head being down as she pushed, reflecting a shift in her weight distribution. However, I should again remind everyone that just like with the dead Velociraptor used for bioerosion by this same T. rex later in the film, the SUV is not making the trace. It is only a tool being used by the tyrannosaur, which is the real tracemaker.

• Tyrannosaurus running trackway – This pulse-quickening chase scene, in which the T. rex pursues a jeep driven by Muldoon and with Malcolm and Sattler as passengers, very likely would have caused a wonderful sequence of tracks. These tracks would have shown increasing stride lengths from a standing start to full-speed run, and each successive track would have registered external and internal structures consistent with this acceleration. Even better, the tracks would have cross-cut the jeep tire-tracks at some points, demonstrating to a later observer that the tyrannosaur was very likely following the jeep. (The demolition of a low-hanging tree branch by the T. rex during the chase also counts as some bioerosion along the way.) Some convincing studies have been done since showing that an adult Tyrannosaurus could not have moved as fast as the one in Jurassic Park, but it still could have caught most running humans. And just to repeat what I said earlier, it’d be really nice for someone to find a T. rex trackway, which would give us more direct evidence of how these massive theropods moved.

• Velociraptor scratch marks and other traces. This time, while watching the harrowing and claustrophobic scene in which a pair of Velociraptors hunt the Murphy siblings in a kitchen, I started thinking about the traces they might have left. Did their claws leave scratch marks on the door handles and kitchen counters? Did they indent the steel counters when they jumped up on these surfaces? A broken window is also shown as a trace of their smashing through glass once they became frustrated with a locked door.

OK, enough of the fanciful ichnology. What about other dinosaurs and their traces? Well, it turns out that Jurassic Park saved the real, living dinosaurs for the very end of the movie. These were five brown pelicans (Pelecanus occidentalis), flying in formation as Grant, Sattler, and their companions leave Isla Nubar in a helicopter. For Grant, this is a poignant moment, as he is likely reflecting on how dinosaurs were still with us today as birds. With that thought, I will say “amen,” and add that dinosaur traces – tracks, nests, feces, bite marks, and all – are still here with us, too, and don’t require special glasses to see them in three dimensions. Thanks for that reminder, Jurassic Park.

Want to see some modern dinosaur traces? Here are tracks of a brown pelican, made in wet sand while it was loafing on a beach at low tide on Sapelo Island, Georgia. To see more modern dinosaur traces, just go outside, and you’ll find them wherever birds are found. (Photograph by Anthony Martin; scale in centimeters.)

Further Reading

Chiappe, L.M., Schmitt, J.G., Jackson, F., Dingus, L., and Grellet-Tinner, G. 2004. Nest structure for sauropods: sedimentary criteria for recognition of dinosaur nesting traces. Palaios, 19: 89–95.

Chin, K. 2007. The paleobiological implications of herbivorous dinosaur coprolites from the Upper Cretaceous Two Medicine Formation of Montana: why eat wood? Palaios, 22: 554-566.

Chin, K., and Gill, B.D. 1996. Dinosaurs, dung beetles, and conifers: participants in a Cretaceous food web. Palaios, 11: 280-285.

Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign of North America. Stackpole Books, Mechanicsburg, Pennsylvania.

Erickson, G. M., Van Kirk, S. D., Su, J., Levenston, M. E., Caler, W. E., & Carter, D. R. 1996. Bite force estimation for Tyrannosaurus rex from tooth-marked bones. Nature, 382: 706–708.

Gignac, P.M., Makovicky, P.J., Erickson, G.M., and Walsh, R.P. 2010. A description of Deinonychus antirrhopus bite marks and estimates of bite force using tooth indentation simulations. Journal of Vertebrate Paleontology, 30: 1169-1177.

Hutchinson, J.R., and Garcia, M. 2002. Tyrannosaurus was not a fast runner. Nature, 415: 1018-1021.

Jacobsen, A.R. 1998. Feeding behaviour of carnivorous dinosaurs as determined by tooth marks on dinosaur bones. Historical Biology, 13: 17-26.

Lockley, M.G., and Hunt, A.P. 1994. A track of the giant theropod dinosaur Tyrannosaurus from close to the Cretaceous/Tertiary Boundary, northern New Mexico. Ichnos, 3: 213-218.

Manning, P.L., Ott, C., and Falkingham, P.L. 2008. A probable tyrannosaurid track from the Hell Creek Formation (Upper Cretaceous), Montana, United States. Palaios, 23: 645-647.

Martin, A.J. 2013. Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press, Bloomington, Indiana: 692 p.

Romilio, A., and Salisbury, S.W. 2011. A reassessment of large theropod dinosaur tracks from the mid-Cretaceous (late Albian–Cenomanian) Winton Formation of Lark Quarry, central-western Queensland, Australia: a case for mistaken identity. Cretaceous Research, 32: 135-142.

Romilio, A., Tucker, R., Salisbury, S. 2013. Reevaluation of the Lake Quarry dinosaur tracksite (late Albian-Cenomanian Winton Formation, central-western Queensland, Australia): no longer a stampede? Journal of Vertebrate Paleontology. 33: 1, 102-120.

Sellers, W.I., and Manning, P.L. (July 2007). Estimating dinosaur maximum running speeds using evolutionary robotics. Proceedings of the Royal Society of London, B, 274: 2711-2716.

Thulborn, R.A., and Wade, M., 1979. Dinosaur stampede in the Cretaceous of Queensland. Lethaia, 12: 275-279.

Varricchio, D.J., Jackson, F. and Trueman, C.N. 1999. A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus. Journal of Vertebrate Paleontology, 19: 91-100.

Different Coastlines, Same Traces, and Time

Posted on October 25, 2012 by TMartin

This past week, I visited North Carolina for varied reasons, but all related to paleontology and geology. First, I gave a well-attended evening lecture about polar dinosaurs, graciously invited and hosted by the Department of Geography and Geology at the University of North Carolina-Wilmington (UNCW). Later in the week, I presented a poster at the Society of Vertebrate Paleontology (SVP) meeting in Raleigh (covered last week here), while also taking in a couple of days of talks, posters, and enjoyably catching up with paleo-friends while meeting neo-friends. Regrettably, I had to leave the meeting early, but with good reason, which was for a field trip to look at fossils in a Pleistocene outcrop near Wilmington with faculty and students from UNCW. Overall, it was a fulfilling week, teeming with paleontological and social variety.

This pithy summary, though, omits lots of details (and if it didn’t, then it wouldn’t be pithy). But one item worth explaining a bit more here was a brief trip to Wrightsville Beach, which is a barrier island was just east of Wilmington. Dr. Doug Gamble, a geography professor in the UNCW Department of Geography and Geology, offered to take me there just before my talk, which I eagerly accepted. Considering all of the field work I had done on the Georgia barrier islands to the south of there, and that I would be teaching a course on barrier islands next semester, going to this beach was an opportunity to learn more about the similarities and differences between Georgia and North Carolina beaches.

Panorama of Wrightsville Beach on the coast of North Carolina, replete with human locomotion traces and dwelling structures. These features make it very different from most beaches in Georgia. But what about other traces? Don’t you just love rhetorical questions? Including this one? (Photograph by Anthony Martin.)

Many North Carolina beaches are famous (or infamous) as examples of what can go wrong with unrestrained development of barrier islands. Many such case studies have been explored through the research, writings, and activism of geologist Dr. Orrin Pilkey of Duke University, as well as other coastal geologists who have looked at the effects of human alterations of these habitats. Wrightsville Beach is such a barrier-island beach, having been heavily modified by human activities during the past 150 years or so. When comparing it in my mind to the Georgia barrier islands, it most resembled Tybee Island, which is also next to a relatively large city (Savannah), easily accessible by a bridge, and developed as a sort of “vacation destination” for people who like beaches, but also want them to have all of the amenities of the places they left behind. Otherwise, it held little resemblance to the mostly uninhabited and undeveloped beaches I prefer to peruse on the Georgia barrier islands.

After driving over the bridge to the island, we walked onto the beach in several places, and I began looking for traces. At first there was little to see, which was a direct result of there being too much to see. Because it was a pleasant day and we were visiting in the afternoon, much of the beach had been heavily trampled by humans, with more than a few of these people aided in their bioturbation by canine companions. Obvious restructuring of the beach included a jetty at the north end that combined a concrete wall and boulders, and pilings of concrete blocks at the south end. Dunes were modest, low-profile, and capped by sparse stands of sea oats (Uniola paniculata), and behind these were hotels, condominiums, and houses, all chock-a-block. It would be too strong to say this beach was alien to me, let alone post-apocalyptic, but it did seem like an altered reality compared to my experiences in Georgia.

A jetty at Wrightsville Beach (North Carolina) composed of concrete and rocks, intended to preserve sand on the beach, which it is doing here, but also results in an imbalanced distribution of sand along it. Note the abundant human and canine tracks on the right, shouting out any other animal traces that might have been in the sand. (Photograph by Anthony Martin.)

Another view of the jetty at Wrightsville Beach, sharply contrasting sand deposition and erosion on either side of it. (Photograph by Anthony Martin.)

A pile of broken concrete being used as rip-rap at the south end of Wrightsville Beach in an attempt to slow erosion there. Or something. (Photograph by Anthony Martin.)

Only with more walking toward the south end of the beach did we see less of an overwhelming human-dog ichnoassemblage and start noticing signs of the native fauna. With this, I became comforted by the familiar. These traces included some I had seen many times on Georgia beaches, including: the soda-straw-like burrows of parchment worms (Onuphis microcephala); the volcano-like sand mounds and chocolate-sprinkle-like feces of callianassid shrimp (either Biffarius biformis and Callichirus major); the soft-serve-ice-cream-like fecal mound of acorn worms (Balanoglossus aurantiactus); and the hole-in-the-ground-like burrows of ghost crabs (Ocypode quadrata). (OK, so I ran out of metaphors.) Seagull tracks abounded as well, lending more of a dinosaurian flavor to the trace assemblage.

Two burrows of parchment worms (Onuphis microcephala) on Wrightsville Beach, exposed by a little bit of erosion, with tiny fecal pellets at their bases. Scale in millimeters. (Photograph by Anthony Martin.)

Burrow aperture and fecal pellets of a ghost shrimp (either Biffarius biformis or Callichirus major) on Wrightsville Beach. Scale in millimeters again. (Photograph by Anthony Martin.)

Fecal casting of an acorn worm, and probably that of a golden acorn worm (Balanoglossus aurantiactus) on Wriightsville Beach. One end of its burrow is underneath this pile, and that would be its anal end, which is sensibly located in a different place from its oral end. And I think you know the scale by now. (Photograph by Anthony Martin.)

Ghost crab (Ocypode quadrata) burrow and tracks, out of the intertidal zone and more into the dunes on Wrightsville Beach. (Photograph by Anthony Martin.)

These traces thus showed us that this North Carolina beach, one majorly changed by humankind during the past 150 years, actually was more biodiverse than one might think at first glance. In my mind, then, it became just a bit more wild through these signs of life hinting at what laid beneath our feet.

At this point, I could depress everyone by listing what traces and biota were not there, but that’s not the point, so I won’t. In a more progressive sense, what traces we saw represented traces of hope, of life hanging on despite environmental change, living almost invisibly beneath our feet. So as human development continues on beaches like these, and sea level rises through the rest of this century, I felt assured of their being survivors of this change, and of their traces outlasting our humanity. The trace fossils of the future are now, and recording our effects on the life that makes these traces. How many will wink out with our species, and how many of their marks will outlast us?

An intergenerational stroll – a grandmother and grandson? - alongside the pier on Wrightsville Beach in North Carolina. Did she have memories of this beach in her childhood? How do these compare to what she sees there now? What memories will this child have of it in the future, especially as the sea continues to rise? If these memories are not recorded, what will be left behind? (Photograph by Anthony Martin.)

Further Reading

Pilkey, O., and Fraser, M.E. 2005. A Celebration of the World’s Barrier Islands. Columbia University Press, New York: 309 p.

Thieler, E.R., Pilkey, O., Cleary, W.J., and Schwab, W.C. 2001. Modern sedimentation on the shoreface and inner continental shelf at Wrightsville Beach, North Carolina, U.S.A. Journal of Sedimentary Research, 71: 958-970.

Deconstructing an Ichnology Abstract, with Alligators

Posted on October 19, 2012 by TMartin

Many people from outside of the realm of academia (or is it a fiefdom?) prefer to get the latest scoops on new paleontological or geological research directly from the source, rather than just reading a press release or news article about it. As someone looking from the inside out, I’m pleased to see so many non-scientists try to probe one layer deeper with their understanding of a beloved scientific topic that interests them, and I try to encourage it through my own blogging, speaking, teaching, and other forms of outreach.

An alligator den on St. Catherines Island, (Georgia), with baby alligator and “big momma” alligator for scale. This week, I presented a poster with about these big burrows and their makers at the Society of Vertebrate Paleontology meeting in Raleigh, North Carolina. The original field work we did for this research was reported back in March here, and now we’re ready to share more of what we found out. (Photograph by Anthony Martin.)

Unfortunately, many of the original research articles that become subjects of media attention are behind paywalls, requiring a reader to pay for access to read those articles, even if the research was publicly funded. This practice is especially common if the research is published in one of those glamorous journals that seemingly make or break academic careers in science, regardless of the lasting quality of the research. (I won’t name them directly, but let’s just say that’s the nature of science nowadays.)

So one option for these curious folks is to read abstracts from proceedings volumes of professional meetings. Abstracts, which ideally are succinct summaries highlighting the most significant findings of a given study, can thus serve as a way for the public to at least get a few insights on the latest scientific research happening in their favorite disciplines.

Want to get below the surface with this research? Oh, sorry, I was just being metaphorical. You really don’t want to go below the surface of an alligator den, which is why we mostly studied abandoned ones, mapped them, and otherwise tried to use methods that didn’t bother the alligators or otherwise have uncomfortable encounters with them.

Along those lines, the annual meeting of the Society of Vertebrate Paleontology (SVP) has been taking place this week in Raleigh, North Carolina, and it has an abstract volume associated with the meeting. Regrettably, though, the general public does not have access to these abstracts, only SVP members and people who have registered for the meeting. The Society of Vertebrate Paleontology also has a policy regarding researchers who publicly share their research results based on these abstracts, muddied by the word “embargo.” In short, this policy holds that people working for the media, which include reporters and bloggers (the latter of whom are also sometimes reporters), cannot write about and otherwise publicize research results presented at the meeting. That is, unless the researchers have given their permission to do so, or the results have been freely distributed by the researchers through a press release, blog, or other forms of outreach.

So in the spirit of the public having easier access to this primary scientific information, the following is our SVP abstract, which I presented as a poster at the meeting yesterday. The abstract is co-authored with Michael Page (Emory University), Sheldon Skaggs (Georgia Southern University), and R. Kelly Vance (also Georgia Southern University), and we worked together on the research, writing, and editing of the abstract. Because this abstract also includes a lot of scientific shorthand (charitably referred to as “jargon”), I also included a sentence-by-sentence explanation of it, in which the abstract text is in italics and my explanation is in formal typeface. So I hope you, the gentle reader, get something from this exercise in explanation, and we look forward to sharing more of this research with you as it continues to evolve and we publish it sometime next year as a peer-reviewed paper.

DENS OF THE AMERICAN ALLIGATOR (ALLIGATOR MISSISSIPPIENSIS) AS TRACES AND THEIR PREDICTIVE VALUE FOR FINDING LARGE ARCHOSAUR BURROWS IN THE GEOLOGIC RECORD

MARTIN, Anthony J., Emory University, Atlanta, GA, United States; PAGE, Michael, Emory University, Atlanta, GA, United States; SKAGGS, Sheldon, Georgia Southern University, Statesboro, GA, United States; VANCE, Robert K., Georgia Southern University, Statesboro, GA, United States

Large archosaur burrows are rarely interpreted from the geologic record, a circumstance that may be attributable to a lack of search images based on modern examples, rather than actual rarity.

Archosaurs make up an evolutionarily related group of vertebrates that include crocodilians (alligators and crocodiles), dinosaurs (the non-bird ones, that is), birds, and their extinct relatives. A few of the larger extinct archosaurs may have dug burrows, but paleontologists have reported very few of these, with one exception being the small Cretaceous ornithopod dinosaur Oryctodromeus cubicularis, found in its burrow with two juveniles of the same species. The authors are proposing here that this “rarity” of archosaur burrows in the fossil record might be more attributable to paleontologists not knowing what modern archosaur burrows look like. So they don’t recognize the fossil ones, leading to a perceived rarity rather than an actual one.

To test this idea, we measured, imaged, and mapped den structures of the American alligator (Alligator mississippiensis) on St. Catherines Island (Georgia, USA).

By “measured,” I mean that my colleagues and I used a low-tech instrument known as a “tape measure” to assess the width and height of an alligator den entrance. By “imaged,” we used a much more technologically complex instruments and method, called ground-penetrating radar (GPR) in combination with computers to figure out what these dens looked like below the surface. By “mapped,” I mean that we looked for alligator dens on St. Catherines Island (Georgia) and recorded their locations using a handheld GPS (global positioning system) unit, then plotted the distribution of these points to see if any patterns emerged.

St. Catherines is an undeveloped barrier island on the Georgia coast, consisting of Pleistocene and Holocene sediments.

St. Catherines Island is undeveloped in the sense that very few buildings or people live on the island year-round. It is privately owned and reserved for researchers’ uses under the direction of the St. Catherines Island Foundation. Like most of the Georgia barrier islands on the southern part of its coast, St. Catherines also has a geologically complex history. Its northwestern end is made of sediments deposited about 40,000 years ago – during the Pleistocene Epoch – whereas its southeastern end is made of much more recent sediments from the Holocene Epoch.

Alligators dug most dens along the edges of freshwater ponds in loosely consolidated Holocene or Pleistocene sand.

This sentence doesn’t need much more explanation other than to reemphasize that alligators gravitate to freshwater ecosystems to dig their dens (pictured below), not saltwater ecosystems, like salt marshes or coastal dunes.

Adult female alligators use dens to protect offspring, but burrows also aid in thermoregulation or serve as refugia for alligators during droughts and fires.

This is probably the neatest insight we gained from doing the research, is that the dens aren’t just used by big momma ‘gators for raising baby ‘gators, but also to make sure alligators of all ages are cozy during winters, stay wet during droughts, and are safe from fires. For instance, because southern Georgia has been going through a drought the past few years, some of the occupied dens we saw were in places that were high-and-dry, but the dens themselves intersected the local water table (seen in one photo above).

Some dens are evidently reused and modified by different alligators after initial construction.

This is an important point for paleontologists to know, and probably shouldn’t have been buried so far into the abstract, but we couldn’t very well put it at the beginning, either. Dens, like other homes, get used again, and probably by generations of alligators. This means that once a den is dug, stays open, and has a wetland nearby, alligators may just move into an abandoned den and modify it if needed, an alligator form of “home improvement.”

Drought conditions along the Georgia coast have exposed many abandoned dens, thus better allowing for their study while increasing researcher safety.

The drought is bad for alligators but was good for us when we did our field work, because so many dens were abandoned and exposed on dry land. This also eased any concerns we had about bothering the alligators, but especially alleviated worries we might have had about close encounters with protective parents near occupied dens. To be sure, we ran into a few of those, but not as many as we would have if conditions had been wetter.

Den entrances have half-moon cross-sections, and based on one sample (n = 20), these range from 22-115 cm wide (mean = 63 + 23 cm) and 14-55 cm high (23 + 9 cm).

I like throwing numbers into ichnology, just to remind people that this is a part of it as a science. Although our sample size is small compared to other studies of traces and trace fossils, it gives people an idea of the range of sizes of these dens, or at least their entrances. As an exercise in the imagination, think about whether you could squeeze into one of these. You know, if you were crazy enough to do such a thing.

In addition to field descriptions, we applied geographic information systems (GIS) and ground-penetrating radar (GPR) to help define the ecological context and subsurface geometry of these structures, respectively.

Computer-aided mapping methods like GIS helped us to test how alligators decided to make dens as a function of the landscape. For instance, we found most of their dens were in lower-elevation areas, which made sense when you think about water accumulating in those places. And the GPR served the dual purpose of not bothering the alligators if they were in their dens, while also keeping us away from their, um, denizens. (Sorry.)

GIS gave spatial data relatable to alligator territoriality, substrate conditions, and proximity to potential nest sites. GPR produced subsurface images of active dens, which were compared to abandoned dens for a sense of taphonomic history.

Big alligators tend to stay away from other big alligators. They also tend to burrow in sediments that don’t take too much effort for them. Female alligators also make their nests close to water bodies and dens, so their little tykes don’t have to travel so far to the water. Newer, active dens were also compared to those no longer being used to see what happens to them over time with neglect, kind of like how an old, abandoned house tends to fall apart and collapse on itself over time.

Most den entrances are southerly facing, with tunnels dipping to the northwest or northeast.

This is pretty self-explanatory, but I’ll just ask readers to think about why these dens are oriented like this.

From entrances, tunnels slope at about 10-15°, turn right or left within a meter, and lead to enlarged turn-around chambers.

Pure description here too, but by “turn-around chamber,” that means the den has enough room inside the den for a big adult alligator to go in head-first and turn around so that it’s head is right at the entrance. (See the photo of “big momma” at the top for an example of that.)

Collapsed dens in formerly ponded areas (secondary-succession maritime forests) provided further insights into subsurface forms of these structures.

Dens left high-and-dry from years ago and taken over by forests collapsed in a way that we could see the full outline of the den and measure these.

These features are: 3.1-4.6 m long; 30-40 cm deep, relatively narrow at either end (35-60 cm), and 1.2-1.6 m wide in their middles.

Dude. Those are big burrows. Dude.

Expansive areas were probable turn-around chambers, and total volumes of collapsed dens accordingly reflect maximum body sizes of their former occupants.

The bigger the den, the easier it was for a large occupant to turn around in it. And although smaller, younger alligators could have lived in these dens, some of the dens were too small to allow the really big alligators from moving into them.

One sampled area (8,100 m²), an almost dry former pond, had 30 abandoned dens, showing how multiple generations of alligators and fluctuating water levels can result in dense concentrations of alligator burrows over time.

Think of an area about the size of an American football field, and put 30 alligator dens in that area. (Now that would make for an interesting game, wouldn’t it?) These dens weren’t all made at the same time, though, and were constructed or abandoned as the pond filled or dried out, respectively.

In summary, the sheer abundance, distinctive traits, and sizes of these structures on St. Catherines and elsewhere in the Georgia barrier islands give paleontologists excellent search images for seeking similar trace fossils made by large semi-aquatic archosaurs.

That’s the big take-home message here for vertebrate paleontologists. All of the information we gathered about these alligator dens from the Georgia barrier islands, especially what they look like, can be applied to test the fossil record of archosaurs. In other words, did archosaurs actually leave lots of dens for us to find, but we just didn’t know what to look for? Hopefully we’ll find out because of this research.

Later, denning ‘gator. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

(Special thanks to Ruth Schowalter for assisting with the field work, and to the St. Catherines Island Foundation for funding some of the research.)

Source of Abstract (Reference):

Martin, A.J., Page, M., Vance, R.K., and Skaggs, S. 2012. Dens of the American alligator (Alligator mississippiensis) as traces and their predictive value for finding large archosaur burrows in the geologic record. Journal of Vertebrate Paleontology, 32 [Suppl. to No. 3]: 136.

The Traces We Leave Behind: A Tribute to Jordi Maria de Gibert

Posted on September 28, 2012 by TMartin

Paleontologists have an odd relationship with death. We often joke about how our livelihoods depend on what has died before us, or we experience great delight when we find an exquisite fossil, which probably was buried alive for it to be so well preserved. We also blithely talk about “death assemblages” and happily explain this gruesome term to non-paleontologically inclined students, friends, spouses, and partners without much thought about how it sounds to people outside of our field.

For ichnologists, who mostly study the tracks, burrows, and other vestiges of these lives that preceded us, our perspectives become even more skewed. Once-live animals, through their behavior, made trace fossils. Yet we almost never see what made them. Hence we also spend much of our time among the living, watching them make traces that we can use as analogs for those trace fossils left by their ancestors. Sometimes we find ourselves identifying with modern animals, developing empathy for what they experience as they form traces, a sensitivity that can extend to their trace-fossil equivalents. Hence for ichnologists, these parts of the fossil record become just a bit less removed from death, and we end up feeling for our tracemakers, both long gone and extant.

Jordi Maria de Gibert, contemplating and lamenting the loss of dinosaurian tracemakers from mass extinctions. The window display was in Basel, Switzerland, one of many places visited by Jordi in his quest to learn all things ichnological. (Photograph by Anthony Martin, who is also pictured in the reflection, along with ichnologists Luis Buatois and Gabriela Mángano, taken in 2003.)

In this sense, our small and close-knit international community of ichnologists was shocked to learn about the sudden loss of one of our own “tracemakers” this past weekend, Jordi Maria de Gibert. His death was unexpected and its impact accentuated because he and the rest of us had just gathered together only last month for the International Congress of Ichnology (Ichnia) in St. Johns, Newfoundland. We also anticipated seeing him again in his home city of Barcelona in 2016, where he died on Sunday. None of us had prepared ourselves to reflect on his legacy, let alone contemplate the possibility that his cognitive traces would cease any time soon.

The aftermath of the first Ichnia football match (sometimes known as “soccer” to you Yanks) between ichnologists of Team Gondwana and Team Laurasia, which took place on a pitch near Trelew, Argentina. Jordi, in the middle of the back row, is either signaling “Peace,” “Victory,” or, most likely, ordering two beers: one for him, and one for you so he can sit down to argue about trace fossils with you. (Photograph by Anthony Martin, taken in 2004.)

Most of our dismay about Jordi’s departure is because we loved him as a person, but it is also surely connected to our witnessing an ascendancy cut short. For instance, at the end of the meeting in St. Johns, Jordi addressed all of us as the newly elected president of the International Ichnological Association, and he had volunteered to serve as the main organizer for the next Ichnia meeting four years from now. His larger-than-life personality was on full display during his informal and impromptu speech: enthusiastic, cheerful, witty, earnest. In the days before then, he delivered multiple presentations on ongoing research projects, most of which revolved around his continuing interests in crustaceans and their traces, as well as those of marine bioeroders, animals that make a living by boring into rocks. Jordi was a prolific publisher of peer-reviewed papers on these topics, and was well known for his cooperative spirit, coauthoring with many ichnologists and other types of paleontologists on these papers.

Jordi (right, seated), in his preferred life habit, talking about fossils with colleagues (and friends) at an outcrop. And this wasn’t just any outcrop, but was at Mistaken Point, Newfoundland, which has one of the most spectacular Ediacaran fossil assemblages in the world. This had to have been a dream come true for him, as it was for many of us.

Jordi showing off his “Bama booties,” required footwear for the sacred ground of Mistaken Point, as some other ichnologist vainly attempts to “photobomb” him with his own blue-footed bootie. (Photograph by Ruth Schowalter.)

I had known Jordi since 1995, having first met in Bornholm, Denmark at a small ichnological meeting there. He and I were still new to our discipline (we were about the same age) and quite green, but eager to learn from our elders. As is typical with many academic friendships, over the next 17 years we would see each other at various meetings, and by my count we saw trace fossils and toasted one another in six countries (Denmark, U.S., U.K., Switzerland, Poland, Canada). Each time together, I grew more impressed with his intense and tenacious will to seek out more knowledge, digest it, and pass it on to others. He was a fierce intellectual who relished the debating of ideas, and was never satisfied with a conversation if he did not leave it wiser. This, of course, benefited all who were brave enough (and lucky enough) to enter into such discussions with him.

A happy time at the Ichnia 2004 banquet in Trelew, Argentina, with (from left to right) Renata Guimarães Netto, Jordi, and Ludvig Loewemark, where the exchange of ideas and good cheer flowed nearly as fast as the wine.

Jordi was young as far as ichnologists go, and as I argued in my previous post, the best ichnologists are the most experienced ones. So he knew as well as any of us that gaining more experience in the field was essential, and traveled to many places and studied traces of all ages – from the Ediacaran to the present – and traces of all kinds, from plant roots to dinosaur tracks. Accordingly, because of his dedication and broad interests, he had already become one of our best. In this vein, one of the metaphorical jokes ichnologists tell is how our academic success can be measured by how deeply we can burrow: shallow tiers are the least successful, whereas the deepst tiers are the most successful. Jordi was assuredly well on his way to the deepest tier, but we are all saddened about his unexpectedly reaching the historical layer before so many of us.

Los quatros amigos, posing happily toward the end of an ichnology field trip in Switzerland in 2003: from left to right, ichnologists Noelia Carmona, Gabriela Mángano, Luis Buatois, and Jordi, sporting some distinctive headgear and proudly flouting conformity. (Photograph by Anthony Martin.)

I learned about Jordi’s death on Sunday through our mutual ichnologist friend, Renata Guimarães Netto, who had likewise known Jordi for more nearly 20 years. Quickly the word spread through social media, e-mails, and phone calls, our sadness multiplying and magnifying worldwide. Only last month, we had celebrated with him, and now we mourned him, and expressed our sorrow to his family members, and close friends.

To ease some of this pain and enjoy an otherwise beautiful Sunday in Decatur, Georgia, my wife Ruth and I went for a walk. Without thinking, I suggested that we meander in one of the largest, quietest green spaces in Decatur, which turned out to be its cemetery. (Yes, I know. All I can say is that the subconscious is more powerful than we know.) While we strolled, I thought about times spent with Jordi on field trips and in conferences, while also recalling papers he had written and discoveries he had made. As mentioned earlier, Jordi’s interests were varied, but perhaps his favorite research topic was crustacean burrows, especially the burrows of crabs, shrimp, lobsters, and other 10-legged crustaceans. Too bad we were nowhere near the Georgia coast, I thought, as it would have been a fitting and comforting homage for Ruth and I to take in the many decapod burrows of the Georgia beaches and salt marshes, which Jordi had never seen in person.

That’s when an eerie coincidence happened. During our walk, we spotted a former pond on the cemetery grounds, now drained for dredging. There’s something about a big pit of mud that appeals to every ichnologist, so I excitedly suggested that we go take a look to see what traces were there. We expected to find lots of tracks, such as those of birds, raccoons, squirrels, and coyotes, and maybe a few other urban fauna. Instead, though, the muddy surface was perforated by decapod tracks and burrows.

Need to see some crustacean traces, but you live in the landlocked part of Georgia? Just go to a dried pond and look for tracks like these.

These were the traces of crayfish, decapods that diverged from a common ancestor to modern lobsters more than 250 million years ago to live in freshwater environments as their brethren dispersed throughout the seas. A few years back, I studied Cretaceous crayfish and their burrows in Australia, but had never seen a live crayfish in its burrow here in Georgia, let alone seen so many of their tracks in one place. We even saw some crayfish (probably a species of Procambarus) poking their heads and claws out of their burrows, or walking around on the mudflat. So it turned out we did not need to go to the Georgia coast after all to see traces reminding us of Jordi: they had been right here with us the whole time.

A crayfish at its burrow entrance in the mudpit now in Decatur Cemetery, either defending its territory, or waving goodbye to people who study its kind and their traces. Your choice, but I know which one I’m picking.

In April, Jordi began writing about ichnology and invertebrate zoology for a more public audience through his cleverly titled blog, Infaunal Epiphany. His first entry was titled First Post, Hope Not Last!, in which he expressed a growing aspiration to connect with more than just his academic colleagues:

We scientists produce science. We scientists consume science. Most of us do that. We do our research, we publish it and other scientists read it. We are keeping all the fun for ourselves!!! It is true that there are scientists, journalists and writers who devote an effort to popularize science results. They are the ones building a bridge to society and I think it is fair to do that as many of us are investigating on public money.

Jordi wanted to share the fun of science, and in that respect, field trips with him were always a delight. These are probably what I will miss most about him, a pang that becomes particularly acute when I realize that one of our last conversations was about his some day visiting the Georgia coast to see its modern traces with me and our like-minded friends.

Lastly, in the light of his most recent life departing us, perhaps Jordi’s most poignant post on his nascent blog was Seven Reasons to Reincarnate as a Cephalopod. I won’t spoil it for anyone who hasn’t read this wonderful piece, but will just say that this post alone showcases how Jordi’s fine sense of humor blended readily with his science.

We will never know whether Jordi’s wish came true, let alone which cephalopod he might have become, or whether some element of his considerable spirit is now somehow connected to one of his beloved crustacean tracemakers or bioeroders in the past or present. But we can be assured that he will continue to live with us through his works and our memories of him. When our ichnological community meets again in his home town of Barcelona four years from now, his traces will all around us, continuing to inspire us to learn and live more.

Salud y un abrazo grande, mi amigo Jordi. Vaya con las trazas.

Correction: Someone pointed out to me that the newly elected International Ichnological Association (IIA) president is actually Alfred Uchman, not Jord. Jordi only seemed presidential to me because of his inspiring report given at Ichnia 2012 as outgoing secretary of the IIA and his agreement to host Ichnia 2016. (I am pleased to report that Alfred likewise gave an excellent speech to those gathered.) Apologies for the mistake, and thanks (as always) to anyone who points them out to me.

A Mirror Less Distant in Time

Posted on September 17, 2012 by TMartin

(This post is the third in a series about my recent field experiences in Newfoundland, Canada in association with the International Congress on Ichnology meeting (Ichnia 2012) in August, 2012. The first dealt with the unusualness of the Ediacaran Period and the second was about the transition from the Ediacaran to the Cambrian Period for burrowing animals.)

The Ordovician Period, a time represented by rocks from 488-443 million years ago, is an old (and I mean, really old) friend of mine. In my master’s thesis, I studied Ordovician fossils from southwestern Ohio, and for my Ph.D. dissertation, I described and interpreted Ordovician trace fossils and strata in Georgia and Tennessee. Thus for the formative years of my academic career, the Ordovician had a strong presence in my life, overshadowing most other geologically inspired opportunities in my adopted home state of Georgia.

Nice outcrop, eh? It’s composed of Lower Ordovician sedimentary rocks, formed more than 450 million years ago, and is on Bell Island, just offshore from St. Johns, Newfoundland (Canada). It’s a place I had never visited before last month, but its trace fossils took me back to Georgia. How? Guess you’ll have to read some more to find out. (Photograph by Anthony Martin.)

This Ordovician-dominated worldview contrasted with a much later focus on the present-day Georgia barrier islands. Between when I first arrived in Georgia, in 1985 through 1998, my only foray to its coast was a three-day field trip as a graduate student to Sapelo Island in 1988. Fortunately, I’ve been a more regular visitor to Sapelo and other Georgia barrier islands throughout the past 14 years or so, and my geologic perspective has accordingly traveled more than 400 million years forward to study modern plant and animal traces.

However, as I’ve embraced the present and the lessons it offers, what also happened over those years was a personal distancing from the Ordovician. This separation was unfortunate for several reasons. One is that Ordovician body and trace fossils are a mere 1.5-2 hour drive from where I live in the metropolitan Atlanta area, just south of Chattanooga, Tennessee. In contrast, the Georgia coast takes a minimum of four hours to reach by car.

Granted, northwest Georgia was part of my dissertation field area, so my leaving behind a place already prospected, poked, prodded, and otherwise inspected thoroughly more than 20 years ago is understandable and forgivable. Yet a day trip there with a colleague last spring (March 2011), along with a recent field trip to view Ordovician rocks in Newfoundland, Canada last month, reminded me of what was in my geological backyard, while also provoking new thoughts about the intersections between the Ordovician and the Georgia coast.

So what happened during those 20+ years of not studying the Ordovician rocks close to me in Georgia? Well, I gained lots more experience, went to many places with rocks and trace fossils of varying ages, and thus – I like to think – became a better ichnologist. So that leads to an imperiously pronounced statement, so please read it, take it in, and revel in its truth: Ichnology is a skill-based science.

People who study the earth sciences have an old saying, often stated during field trips to students: “The best geologist is the one who’s seen the most rocks.” The same sentiment might be applied to ichnologists. To excel as an ichnologist, it’s not your publication record (let alone impact factors of journals publishing your work), the number or size of your grants, accolades of your peers, “big-idea” review papers, erudite tomes, or any number of trappings imposed by academia that matter. What really matters in becoming a better ichnologist is how many traces you’ve seen, measured, sketched, journaled, photographed, pondered, argued over, and folded into your consciousness.

Hey, look – it’s ichnologists, trying to learn more by studying trace fossils in the field! (Photograph by Ruth Schowalter, taken on Bell Island, Newfoundland, Canada.)

Sure, peer review from your colleagues is still an important part of this learning process. Otherwise, as a tracking instructor once told me and other nascent trackers, “When you always track by yourself, you’re always right.” You don’t want to be that ichnologist who gets things wrong, then insists every other ichnologist is wrong, while also imagining that they’re teeming with jealousy over your brilliance. You know, the “they laughed at Galileo, too” fallacy.

Behold my genius! Only I can clearly see these are the tracks of an eight-legged river otter. Oh, so you think they’re from two four-legged otters, with one following the other? Dolt! Don’t you know who I am?

So am I the best ichnologist? Not just no, but hell no. The acknowledged master of ichnology is Dolf Seilacher. And the main reason I enthusiastically bestow Dr. Seilacher with a crown of back-filled and spreiten-laden burrows is because of the extraordinary amount of experience he has as an ichnologist. Granted, he’s also done all of that academic-type stuff that persuades far less-accomplished members of tenure-review committees to nod their heads with utmost seriousness and say, “Well, I suppose we can make an exception in this case.” But he also has seen, measured, sketched, journaled, photographed, pondered, argued over many, many trace fossils during his 87 years on this planet. Dolf knows traces.

Dolf Seilacher, the widely hailed master of ichnology in the world. Even when he’s wrong, he’s really good at it. (Photograph by Anthony Martin, taken in Krakow, Poland.)

So let’s go back to the Ordovician, and how it relates to Dolf and my claim about the importance of experience in ichnology. In 1997, I invited Dolf to visit Emory University as a distinguished speaker in an evolutionary biology lecture series we had then (since gone defunct, like many things at Emory). Because he had never before visited Georgia, he insisted that we also arrange a field trip for him to see some trace fossils here. So with my friend and colleague, Andy Rindsberg, we organized a day trip to an outcrop near Ringgold, Georgia to look at the Ordovician trace fossils there. Andy had done his master’s thesis on the Ordovician and Silurian trace fossils in that area, and as mentioned earlier, I had done my Ph.D. dissertation about the Ordovician rocks, in which I interpreted them mostly through an ichnological lens.

Dolf Seilacher in Georgia (USA) for the first time in November 1997, coffee in one hand and a trilobite burrow in the other. See all of those Ordovician rocks in the background? Even though he’d never been there before, he noticed trace fossils in them in less time than most of us take to read a Huffington Post headline. Gee, you think it had anything to do with his experience? (Photograph by Anthony Martin, taken near Ringgold, Georgia. And just so you know, no paleontologists were “Dolfed” in this photo.)

Andy and I knew the rocks and their trace fossils at this outcrop better than anyone in the world. Yet within five minutes of arriving at the outcrop, Dolf laid his hand on a large slab of Ordovician rock and began talking matter-of-factly about the trilobite burrows in it. Andy and I looked at each other, and said (almost simultaneously), “Trilobite burrows?”

Dolf was right. This rock and many others there were filled with circular, back-filled burrows, which were made by small trilobites that burrowed into mudflats more than 400 million years ago. During a futile attempt to disprove him the following year, Andy and I found these burrows connected to trackways, and one even ended in a resting trace, perfectly outlining the body of a small trilobite. (Did I mention Dolf was right?)

Burrow (upper right, circular structure) connected to tracks made by little legs from a little trilobite. Trace fossils are on the bottom of a sandstone from the Upper Ordovician Sequatchie Formation of northwest Georgia. Scale in centimeters. (Photograph by Anthony Martin.)

Later on that same day, we looked more carefully at some other fossil burrows at the outcrop. These broad, banana-shaped trace fossils were ones that Andy and I had noted in our respective studies, called Trichophycus. Dolf continued his trilobite–tracemaker theme, insisting that these were also trilobite burrows. This idea was supported by scratchmarks on the burrow walls, which linked the burrows to the small legs of whichever arthropod lived in the burrows. Again, trilobites made sense as the tracemakers, and we haven’t yet found a reason why this would be wrong.

Trusted field assistant Paleontologist Barbie, pointing to a cluster of Trichophycus (interpreted as trilobite burrows) in the Sequatchie Formation of northwest Georgia. She is pointing to some scratchmarks on the burrow walls, which are preserved in natural casts of the burrows. (Photograph by Anthony Martin.)

Almost 13 years later, in March 2011, Andy and I went back to this same Ringgold outcrop to re-study the trace fossils there, done in preparation for a presentation he gave the next month at a regional Geological Society of America meeting (abstract here). He and I were surprised at how much the outcrop had changed since we last visited. Vegetation, particularly of the thorny variety, covered the ground and impeded our progress. Nonetheless, we found many excellent examples of trilobite burrows (Trichophycus), a beautiful trilobite resting trace (Rusophycus), and, for the first time for either of us, a sea-star resting trace.

Resting trace of a trilobite (Rusophycus), with a small part of its trackway leading to the trace, in the Upper Ordovician Sequatchie Formation of northwest Georgia. These trace fossils are preserved as natural casts on the bottom of a sandstone, so you’re seeing the underside of where the trilobite hunkered down more than 400 million years ago. (Photograph by Anthony Martin.)

Resting trace of a sea star (Asteriacites) in the Upper Ordovician Sequatchie Formation of northwest Georgia. This trace fossil, like that of the trilobite resting trace, is also preserved as natural casts on the bottom of a sandstone, so you’re looking underneath where the sea star moved into the mud. (Photograph by Anthony Martin.)

Our discovery of the latter two trace fossils – the trilobite and sea-star resting traces – took me from the Ordovician to the Georgia coast and back again. Throughout the late 1980s, I recall my Ph.D. advisor, Robert (“Bob”) Frey placing many of his articles in my graduate-student mailbox, all of which dealt with the traces of the modern Georgia coast. That’s odd, I thought. What did the traces of the modern Georgia coast have to do with these 440-million-year-old rocks?

In my limited worldview at the time, I did not see that the Georgia barrier islands and their traces composed a mirror, however removed by time, for looking into that Ordovician past. But eventually, given enough articles read, field work done, and trace fossils examined at these Ordovician outcrops, I slowly realized these 440-million-year-old rocks had been formed in estuaries, similar to those along the Georgia coast. When I first published an article about these rocks and their trace fossils in 1993 (link here), these strata represented the oldest known estuary deposits in the world, and some of the trace fossils could be readily compared to those on the Georgia coast. The beauty of this realization was that Frey, a master ichnologist in his own right and a contemporary of Seilacher, had allowed me to discover it for myself: he just provided the clues.

Remember that small, circular trilobite burrow with tracks connecting to it? Now compare it to the same sort of traces made by a modern beach mole crab (Albunea paretii), which left its burrow on the right, walked to the left, and is here rapidly burying itself in the sand. Scale in centimeters. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Resting trace and attached trackway of a juvenile horseshoe crab (or limulid, specifically Limulus polyphemus). So think about a similarly sized trilobite making this, and what the bottom of the trace would like like, then compare it to the Ordovician trilobite resting trace fossil shown earlier. Scale in centimeters. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Resting trace of a lined sea star (Luidia clathrata), with the original tracemaker just below its trace. This sea star was stuck above the high tide mark, burrowed into the underlying moist sand, but then tried to move to a better place once its spot started to dry out. Now compare this resting trace to the Ordovician trace fossil shown before. No scale, but sea star is about 8-10 cm wide. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

The following year and only a month ago (August 2012), Andy and I had another Ordovician learning opportunity presented to us, but this time in Newfoundland, Canada. A day trip to see Ordovician rocks and trace fossils on Bell Island, just a 30-minute ferry ride from St. Johns, Newfoundland, was a welcome break from the butt-numbing sessions of the previous two days of the Ichnia 2012 conference at Memorial University.

In our first few minutes at the outcrop and its numerous boulders – spoil piles from an iron-ore mine – we realized that one of the dislodged slabs in front of me was loaded with specimens of Trichophycus. It was a pleasant surprise to get reacquainted with this trace fossil, and in a place far away both geographically and experientially from Georgia.

Multiple specimens of Trichophycus in Lower Ordovician rocks of Newfoundland, Canada, preserved as natural casts of the burrows. See all of those scratchmarks on the burrow walls? These were also made by trilobites, but probably different ones from those in Georgia. Scale in centimeters (and that ain’t no real maple leaf.) (Photograph by Anthony Martin.)

Multiple specimens of Trichophycus in the Upper Ordovician Sequatchie Formation of Georgia, USA, also preserved as natural casts of the burrows and showing some scratchmarks on the walls. Do they look familiar to you, too? If so, welcome to the Ordovician. (Photograph by Anthony Martin.)

Here’s that trilobite resting trace (Rusophycus) from Georgia that I showed earlier. Now take a gander at the one below…

Why, that seems to be a trilobite resting trace (Rusophycus), too, but in Lower Ordovician rocks of Newfoundland. Surprise, surprise, surprise! Scale in centimeters. (Photograph by Anthony Martin.)

Suddenly, much of Andy’s and my previous experience with the Ordovician rocks of Georgia came back to us. We were, paradoxically, home, only in this instance, “home” was a time, not a place. Ichnological colleagues who had no idea Andy and I had worked with Ordovician trace fossils stared at us quizzically (and skeptically) as we excitedly discussed the burrows. But once we informed them that we had seen these trace fossils before, our experience was recognized, egos were set aside, and learning was enhanced. Funny how that works sometimes.

So with our trip to Newfoundland, we went from the alien world of the Ediacaran Period, with its trace fossils unlike anything I had seen before, to the more familiar and accommodating Ordovician Period rocks and their trace fossils. What I learned from this trip, combined with many others to Ordovician rocks elsewhere, as well as the modern sediments of the Georgia coast, was that the mirror was not so foggy after all, and that more field experiences can only further clarify these connections between life traces from the present and the not-so-distant past.

Further Reading

Buatois, L.A., Gingras, M.K., MacEachern, J., Mángano, M.G., Zonneveld, J.-P, Pemberton, S.G., Netto, R.G., and Martin, A.J. 2005. Colonization of brackish-water systems through time: Evidence from the trace-fossil record. Palaios, 20: 321-347.

Eldredge, N., 1970. Observations on burrowing behavior in Limulus polyphemus (Chelicerata, Merostomata), with implications on the functional anatomy of trilobites. American Museum Novitates, 2436: 17 p.

Fillion, D. and Pickerill, R.K. 1990. Ichnology of the Lower Ordovician Bell Island and Wabana Groups of eastern Newfoundland. Palaeontographica Canadiana, 7: 1-119.

Martin, A.J. 1993. Semiquantitative and statistical analysis of bioturbate textures, sequatchie formation (upper ordovician), Georgia and Tennessee, USA. Ichnos, 2: 117-136.

Seilacher, A. 2007. Trace Fossil Analysis. Springer, Berlin: 240 p.

Mistaken Point and the Limits of Actualism

Posted on August 23, 2012 by TMartin

Sometimes we paleontologists, especially those who also study modern organisms and their behaviors, get a little too sure of ourselves, thinking we have a clear vision of life during the pre-human past. So it’s good to have that confidence shaken a little, made uneasy by a glimpse at a much deeper past, one that preceded the bulk of fossils that shape our accepted norms and basic expectations in paleontology.

Welcome to the Ediacaran Period, the span of earth history from 635-542 million years ago, and a time when actualism – the precept that the present is the key to the past – becomes a naïve, idealistic dream, a glib summary of a world that has only existed for a mere 12% of earth history.

What are these? They’re fossils, but otherwise I’m not sure what else to tell you: guess I’ve been spending too much time in the present. But for for those people who have studied them and know better than me, they’re called Charniodiscus, and they’re frond-like fossils with holdfasts (those circular parts connected to their stems) that kept them attached to the seafloor about 565 million years ago. All you have to do to see these fossils is go to Newfoundland, Mistaken Point Ecological Reserve in Newfoundland, Canada, get permission from the Reserve to visit them, have a guide accompany you, and walk 40-45 minutes to the site from a car park. Incidentally, there will be absolutely no cafes or toilets on the way there. You know, just like how it was in the Precambrian. (Photograph by Anthony Martin; scale in centimeters.)

These discomforting realizations started a little less than two weeks ago, inspired by a field trip to the Ediacaran-Cambrian rocks of eastern Newfoundland, Canada. Why was I in cool, temperate Newfoundland, instead of sweating it out on the summertime Georgia coast? The occasion was a pre-meeting trip associated with the International Congress on Ichnology, simply known among ichnologists as Ichnia. This was the third such meeting, a once-every-four-years event (coinciding with years of the summer Olympics). The previous two were in Krakow, Poland (2008) and Trelew, Argentina (2004), and thus far these meetings also include fabulous field trips.

For Ichnia 2012, upon seeing an announcement of a field trip to Mistaken Point and other localities associated with the Precambrian-Cambrian boundary, I eagerly signed up for it. You see, Mistaken Point is world famous for its extraordinary preservation of more than 1,000 body fossils of those weird and wonderful fossils known as the Ediacaran fauna, Ediacaran biota, Vendian fauna, or Vendobionts (take your pick). This was the main reason why my fellow ichnologists on the field trip – 16 of us from 9 countries – were along for the ride, despite the trip’s clear emphasis on body fossils.

A rare photo of ichnologists getting really excited about seeing body fossils, which is totally understandable when we’re talking about the Ediacaran fossils at Mistaken Point, Newfoundland. Eventually, though, they later became unruly and started demanding, “Show me your trace fossils!” Fortunately for the sake of international ichnological relations, the field-trip leaders happily obliged that same day. (Photograph by Ruth Schowalter.)

These rare fossils, which are strange enough to even cause paleontologists to question whether or not they are animals (hence the cautious use of the more inclusive term “biota” instead of “fauna”), are abundantly exposed on broad bedding planes in Mistaken Point Ecological Reserve on the southeastern coast of Newfoundland. Discovered in 1967, these fossils have since proved to be one of the best examples of easily visible body fossils from more than 542 million years ago, and the Newfoundland fossils comprise the only such assemblage that originally lived in deep-marine environments. They evidently died in place when suffocated by a layer of volcanic ash that settled onto the seafloor, hence the fossils reflect a probable sample of their original ecosystem. This ash layer neatly preserved the fossils, and its minerals provided a means to calculate absolute age dates for the assemblage, which is from 565 +/- 3 mya (million years ago).

Bedding-plane exposure at Mistaken Point with many frond-like fossils, broadly referred to as rangeomorphs. (Photograph by Anthony Martin, Canadian-themed scale is in centimeters.)

A close-up of one of the more exquisitely preserved rangeomorphs, which I think is Fractofusus misrai. But you really shouldn’t trust this ichnologist with that identification, so it’d be wise to double-check that with a real expert. (Photograph by Anthony Martin.)

Just a few years ago, though, Mistaken Point became paleontologically famous again, and this time for its trace fossils. Researchers from Memorial University in Newfoundland and Oxford University looked at bedding planes near those holding the the body fossils, and were surprised to find a few trails there. At that time, it was the oldest evidence of animal movement from the fossil record, and although these finds have been disputed and others have tried to stake this claim for trace fossils elsewhere, it is still holding up fairly well.

A surface trail, probably made by a < 1 cm wide animal moving along the seafloor about 565 mya. The animal moved from left to right, which is indicated by the crescentic ridges inside the trail, which open in the direction of movement. (Photograph by Anthony Martin, taken at Mistaken Point, Newfoundland.)

Another surface trail, but this one without the internal structure of the other one, and with levees on either side of the central furrow. (Photograph by Anthony Martin, taken at Mistaken Point, Newfoundland.)What’s this? Don’t have a clue. It looks like a series of overlapping trails, some looping, but would have taken me several hours to unravel. Anyway, it generated some good discussion at the outcrop, and they’re probably trace fossils, which made us ichnologists both happy and perplexed. (Photograph by Anthony Martin, taken at Mistaken Point, Newfoundland; scale in centimeters.)

What made these trace fossils? It’s hard to say, and that’s a humbling statement for me to make. In public talks I’ve given about my upcoming book, and in a presentation I gave the following week at Ichnia on the Memorial University campus, I’ve assured how the actualism of the Georgia barrier islands and its traces can reliably serve as models for interpreting many trace fossils formed in different environments, and trace fossils of various geologic ages from around the world. But in this instance, I didn’t have a inkling of what made the Mistaken Point trace fossils. These trace fossils were also made in deep-marine environments, which are lacking from the Georgia coast, and I haven’t learned much about deep-marine trace fossils from elsewhere.

In short, my ignorance was showing, and these trace fossils were completely out of my realm of experience. The only feeble hypothesis I could conjure on the basis of what I’ve seen in modern sediments of the Georgia barrier islands are small marine gastropod trails. Sorry, that’s all I got.

Oooo, look, it’s snail! Making a trail! Isn’t that neat? And if you squint really hard and have a couple of beers, you might agree that it almost resembles one of the fossil trails from Mistaken Point. Don’t see it yet? Here, have another beer. (Photograph by Anthony Martin, taken at Sapelo Island, Georgia; scale in millimeters. )

But if ignorance loves company, I can feel good in knowing that others have grasped at the same straw of actualism and found it far too short. I could tell a few of my ichnological colleagues were likewise a little challenged by what they saw at Mistaken Point, and I knew that for some of them – like me – they normally deal with trace fossils in much younger rocks. But hey, that’s what geology field trips are supposed to do: challenge us with what’s really there in the rock record, right there in front of us, rather than what we wish were there.

Fortunately, a little more information provided during the meeting after the field trip helped my understanding of the trace fossils we saw at Mistaken Point, and actually connected to modern tracemakers. Alexander Liu, the primary author of the paper that first reported the trace fossils, gave a talk that reviewed the evidence for Precambrian trace fossils, including those from Mistaken Point. In experiments he and his coauthors did with living anemones in a laboratory setting, they were able to reproduce trails similar to the Mistaken Point trace fossil with the internal structure. Thus these researchers were able to use actualism to assist in their interpretation, which also meant that neoichnology was not so useless after all when applied to the Ediacaran. That made me feel a little better.

Let’s take a look at that first surface trail again, but this time with the help of my trustworthy colleague Paleontologist Barbie, who was along for the field trip. The crecentic ridges in the interior of the trail may represent marks where the basal disc of a anemone-like animal pushed against the surface as it moved. Even more interesting, the arrow points to an oval impression, which may be a resting trace that shows the approximate basal diameter of the tracemaker. What was the tracemaker? It’s currently identified as a small anemone, which is based on modern traces. Neoichnology rules! (Photograph by Anthony Martin.)

Ediacaran trace fossils still engender debate, though, and especially with people who don’t necessarily accept that animals made trails during the Ediacaran. For instance, about four years ago, some scuba-diving researchers observed a giant protozoan making a trail on a sediment surface in the Bahamas. Accordingly, they proposed that one-celled organisms – not animals – could have made similar trails during the Ediacaran Period. Interestingly, this shows how actualism can produce conflicting results when applied to Ediacaran fossils. After all, it’s still a big world out there, and we humans haven’t really observed everything in it yet.

So I’ll make one last point about Ediacaran fossils here, then will move on to more recent times. If you think that at the very least we paleontologists should be able to tell the difference between trace fossils and body fossils in Ediacaran rocks, you’re also in for some confusion. In the only research article I have ever attempted on Ediacaran fossils, which were much closer to Georgia – coming from the Carolina Slate Belt of North Carolina – my coauthors and I struggled with exactly that question with some fossils found in that area. In the end, we said they were body fossils, not trace fossils. And as everyone knows, I love trace fossils, and I really wanted these to be trace fossils. But they were not. That’s science for you: denying your deepest desires in the face of reality.

So surely the Cambrian would be easier to interpret, right? I meanl, after 542 mya, animals started burrowing merrily, to and fro, hither and tither, with uninhibited and orgiastic abandon, and, well, you get the idea. But, not really. Another part of the field trip involved looking at what happened with the departure of the relatively unbioturbated alien world of the Ediacaran, pre-542 mya, to the more familiar sediment mixing of the Cambrian and Ordovician Periods, post-542 mya. Yet even these rocks and their trace fossils were still not quite like what we see today.

This will be the subject of my next post, which will again explore the theme of how we should approach strict actualism like any scientifically based idea: with a mixture of astonished wonder, but also with a hard-edged look at what is really there.

As we bid adieu to Mistaken Point and began our walk back to the car park, we could swear we saw lifeforms emerging from the mist-covered rocks, resurrected from the deep time and deep water of the Avalonian Precambrian. Then we realized those were just some of our group behind us. Oh well. Maybe next time. (Photograph by Anthony Martin.)

(Acknowledgements: Much appreciation is extended to the field trip leaders – Liam Herringshaw, Jack Matthews, and Duncan McIlroy – for their organization and execution of a fantastic three-day field trip; to Valerie and Richard of the Mistaken Point Ecological Reserve for guiding us to the site; to my ichnological colleagues for their cheery and knowledge-broadening company; and my wife Ruth for being with me and providing an artist’s perspective about her experiences with us crazy ichnologists, shared here and here.)

Further Reading

Fedonkin, M., Vickers-Rich, P. Grey, K., and Narbonne, G. 2007.The Rise of Animals: Evolution and Diversification of the Animalia. Johns Hopkins Press, Washington: 320 p.

Liu, A.G., McIlroy, D., and Brasier, M.D. 2010. First evidence for locomotion in the Ediacaran biota from the 565Ma Mistaken Point Formation, Newfoundland. Geology, 38: 123-126.

Matz, M.V., Frank. T.M., Marshall, N.J., Widder, E.A., and Johnsen, S. 2008. Giant deep-sea protest produces bilaterian-like traces. Current Biology, 18: 1-6

Tacker, R.C., Martin, A.J., Weaver, P.G., and Lawver, D.R. 2010. Trace vs. body fossil: Oldhamia recta revisited. Precambrian Research, 178: 43-50.

Vickers-Rich, P., and Komarower, P. (editors). 2007 The Rise and Fall of the Ediacaran Biota. Geological Society of London, Special Publication 286: 448 p.

Traces of Toad Toiletry and Naming Trace Fossils

Posted on July 12, 2012 by TMartin

Sometimes I envy those people on the Georgia barrier islands who, through sheer number of hours in the field, come upon animal traces that I’ve never seen there. But this was one of those instances where the find was so extraordinary that I will suppress my jealous urges, celebrate the person who found it, marvel at it, and share its specialness with others.

Gale Bishop, a fellow ichnologist who is currently on St. Catherines Island, found an intriguing sequence of traces during a morning foray on its dunes and beaches there last week. In his second life – his first was as a geology professor at Georgia Southern University – he has transformed into an indefatigable sea-turtle-nesting monitor on St. Catherines and coordinator of a teacher-training program. Part of his daily routine there, among many other duties, includes looking for mother-turtle traces – trackways and nests – during the nesting season, which in Georgia is from May through September.

Along the way, with his eyes well trained for spotting jots and tittles in the sand, Gale often notices oddities that likely would be missed by most people, including me. The following photograph, which he shared on the St. Catherines Island Sea Turtle Program page on Facebook, is from a find he made about 7:15 a.m. on Saturday, July 7. Take a look, and please, if you haven’t already, forget the title of this post as you ponder its clues.

A mystery in the dune sands of St. Catherines Island on the Georgia coast, begging to be interpreted. No, not the shovel: those are never mysterious. Look at the traces to the left and above the shovel. What made these, what was it doing, and who else was in the neighborhood afterwards? Oh, and again, stop staring at the shovel. (Photograph by Gale Bishop.)

Gale called me out specifically when he posted this and several other related photos on Facebook, and asked me to tell a story about it. I gave him my abbreviated take in the comments, kind of like an abstract for the research article:

Looks like southern toad (Bufo terrestris) to me. What’s cool is the changes of behavior: hopping, stopping, pooping, and alternate walking (which people don’t expect toads to do – but they do).

That was my knee-jerk analysis, which took a grand total of about a minute to discern and respond. (After all, this was Facebook, a forum in which prolonged and deep thinking is strongly discouraged.) But I also kept in mind that quick, intuitive interpretations later need introspection and self-skepticism, especially when I’m making them. (See my previous post for an example of how wrong I could be about some Georgia-coast traces.) So rather than fulfill some Malcolm Gladwell-inspired cliché through my intuition, I sat down to study the photo with this series of questions in mind:

Why did I say “Southern toad” as the tracemaker for the sequence of traces that start from the lower left and extend across the photo?
What indicates the behaviors listed and in that order: hopping, stopping, pooping, and alternate walking?
What signified the changes in behavior, and where did these decisions happen?
Why did I assume that most people don’t expect toads to walk (implying that they think they just hop)?

The first leap in logic – how did I know a Southern toad (Bufo (Anaxyrus) terrestris) was the tracemaker – was the easiest to make, as I’ve often seen their tracks in sandy patches of maritime forests and coastal dunes. These hardy amphibians leave a distinctive bounding pattern, with the front-foot impressions together and just preceding the rear-foot ones; the toes of their front feet also point inward. In the best-expressed tracks, you will see four toes on the front feet and five toes on the rear.

Close-up of bounding pattern (from lower left of previous photo), showing front-foot impressions just in front of and more central than the rear feet impressions. Direction of movement is from bottom to top of photo. (Photograph enhanced to bring out details, but originally taken by Gale Bishop.)

The only other possible animal that could make a trackway pattern confusable with a toad in this environment is a southeastern beach mouse (Peromyscus polionotus). Still, mice mostly gallop, in which their rear feet exceed their front feet as they move. Mouse feet are also very different from those of a toad, with toes on both feet all pointing forward (remember, toad toes point inward). So although dune mice live in the same environment as these tracks, these weren’t mouse tracks. The only alternative tracemakers would be spadefoot toads (Scaphiopus holbrookii) or a same-sized species of frog, such as the Southern leopard frog (Rana sphenocephala). But neither of these species is as common in coastal dunes as the Southern toad, so I’ll stick with my identification for now.

Mouse tracks – probably made by the Southeastern beach mouse (Peromyscus polionotus) – on costal dunes of Little St. Simons Island, Georgia. The two trackways on the left are moving away from you, whereas the one on the trackway on the right is heading toward you. All three show a gallop pattern, in which the larger rear feet exceeded the front feet. Scale = 10 cm (4 in). (Photograph by Anthony Martin)

The second conclusion – the types of behaviors and their order – came from first figuring out the direction of travel by the tracemaker, which from the lower left of the photo toward its middle. This shows straight-forward hopping up to the point where it stops.

From there, it gets really interesting. The wide groove extends to the left past the line of travel and had to be made by the posterior-ventral part of the toad’s body (colloquially speaking, its butt). This, along with the disturbed sand on either side of the groove, shows that the toad turned to its right (clockwise) and backed up with shuffling movement. That’s when it deposited its scat, which I’ve also seen in connection with toad tracks (and on St. Catherines, no less). This really helped me to nail down the identity of the tracemaker, almost being able to declare, “Hey, I know that turd!”

Southern toad bounding pattern that abruptly stops, followed by clockwise turning, backing up, and, well, making a deposit. (Photograph by Gale Bishop, taken on St. Catherines Island.)

How about the alternate walking? Turns out that toads don’t just hop, but also walk: right side, left side, right side, and so on. This pattern – also called diagonal walking by trackers – is in the remainder of the photo (with the direction of movement left to right). When toads do this, the details of their front and rear feet are better defined, and you can more clearly see the front foot registers in front of the rear and more toward the center line of the body.

This side-by-side movement is also what imparted a slight sinuosity to the central body dragmark, which was from the lower (ventral) part of its body, or what some people would call “belly.” In my experience, most people are very surprised to find out that toads can walk like this, which I can only attribute to sample bias. In other words, they’ve only seen frogs and toads hop away from them when startled by the approach of large, upright bipeds.

Close-up of alternate walking pattern and body dragmark made by Southern toad. Direction of movement is from upper left to lower right. (Photograph enhanced to bring out its details, but original taken by Gale Bishop on St. Catherines Island.)

But wait, what are those two dark-colored depressions in the center of the alternate-walking trackway? Well, it doesn’t take much imagination to figure those out, especially if you’ve already had a couple of cups of coffee. Yes, these are urination marks, and even more remarkable, there are two of them in the same trackway. So not only did this toad do #2, but also #1 twice.

Southern toad urination mark #1, not too long after doing #2. (Photograph by Gale Bishop.)

Urination mark #2 , which you might say was #2 of #1, but with both #1′s after #2, or, oh, never mind.

Notice that the second mark seems to have had less of a stream to it, which makes sense in a way that I hope doesn’t require any more explanation or demonstration.

So to answer to one of the questions above – what signified the changes in behavior – you have to look for the interruptions in the patterns, much like punctuation marks in a sentence. The commas, semi-colons, colons, dashes are all part of a story too, not just the words.

The summary interpretation of what happened. Let’s just say that this frog (or toad, whatever) didn’t come a courtin’.

Through the series of photos Gale shared in an album on Facebook, he also showed that he was following a protocol all good trackers do: he changed his perspective while observing the traces. Likewise, I teach my students to use this same technique when presented with tracks and other traces, that it’s a good idea to walk around them. While doing this, they see changes in contrast and realize how the direction and angle of light on the traces alters their perceptions of it. At some points, a track or other trace may even “disappear,” then “reappear” with maximum clarity with just a few more steps.

A different perspective of the same traces, viewed from another angle. Do you notice something new you didn’t see in the previous photo and its close-ups? (Photograph by Gale Bishop, taken on St. Catherines Island.)

Now, because I’m also a paleontologist, this interesting series of traces also prompts me to ask: what if you found this sequence of traces in the fossil record? Well, it’d be a fantastic find, worthy of a cover story in Nature. (That is, if the tracks somehow went across the body of a feathered dinosaur.) Right now, I can’t think of any trace fossils like this coming from vertebrates – let alone toads or frogs – so let’s go to invertebrate trace fossils for a few examples of similarly interconnected behaviors preserved in stone.

In 2001, a sequence of trace fossils was reported from Pennsylvanian Period rocks (>300 million years old), in which a clam stopped, fed, and burrowed along a definite path, with all of its behaviors clearly represented and connected. The ichnologists who studied this series of trace fossils – Tony Ekdale and Richard Bromley – reckoned these behaviors all happened in less than 24 hours; hence the title of their paper reflected this conclusion.

Ichnologists have a sometimes-annoying and always-confusing practice of naming distinctive trace fossils, giving them ichnogenus and ichnospecies names. (For a detailed discussion of this naming method, I talked about it in another blog from the dim, dark, distant past of 2011 here.) For instance, Ekdale and Bromley stated in their study that three names could be applied to the distinctive trace fossils made by a single clam, with each a different form made by a different behavior: Protovirgularia (burrowing), Lockeia (stopping), and Lophoctenium (feeding).

Along those lines, another ichnologist (Andy Rindsberg) and I also suggested that an assemblage of trace fossils in Early Silurian rocks (>400 million years old) of Alabama, with many different ichnogenera, were all made by the same species of trilobite. The take-home message of that study, as well as Ekdale and Bromley’s, is that a single species or individual animal can make a large number of traces. This also means that ichnodiversity (variety of traces) almost never equals biodiversity (variety of tracemakers).

So let’s go back to the toad traces, put on our paleontologist hats, and think about a “what if.” What if you found this series of traces disconnected from one another: the hopping trackway pattern, the diagonal walking pattern, the urination marks, the groove, and the turd, all found in disparate pieces of rock? Taken separately, such trace fossils likely would be assigned different names, such as “Bufoichnus parallelis,” “B. alternata,” “Groovyichnus,” “Tinklichnus,” and “Poopichnus.” (Please do not use these names beyond an informal, jovial, and understandably alcohol-fueled setting.)

Color, present-day version of the variety of traces made by a Southern toad (above), and a grayscale imagining of it fossilizing perfectly (below). Key for whimsically named ichnogenera in fossilized version: Bp = “Bufoichnus parallelis,” Ba = “Buofichnus alternata,” G = “Groovyichnus,” P = “Poopichnus,” and T = “Tinklichnus.” Please don’t cite this.

Granted, the environment in which Gale noted these traces – coastal dune sands – are not all that good for preserving what is pictured here, but other environments might be conducive to fossilization. To quote comedian Judy Tenuta, “It could happen!”

So if someone does find a fossil analogue to Gale’s evocative find on St. Catherines Island, I will understand their giving a name to each separate part, even if I won’t like it. The most important matter, though, is not what you call it, but what it is. And in this case, the intriguing story of toiletry habits left in the sand one July morning by a Southern toad is worth much more than any number of names.

Further Reading

Ekdale, A.A., and Bromley, R.G. 2001. A day and a night in the life of a cleft-foot clam: Protovirgularia-Lockeia-Lophoctenium. Lethaia, 34: 119–124.

Halfpenny, J.C., and Bruchac, J. 2002. Scats and Tracks of the Southeast. Globe Pequot Press, Guilford, Connecticut: 149 p.

Jensen, J.B. 2008. Southern toad. In Jensen, J.B., Camp, C.D., Gibbons, W., and Elliott, M.J. (editors), Amphibians and Reptiles of Georgia. University of Georgia Press, Athens, Georgia: 44-46.

Rindsberg, A.K., and Martin, A.J. 2003. Arthrophycus and the problem of compound trace fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 192: 187-219.

Life Traces as Cover Art

Posted on April 30, 2012 by TMartin

I’ve been a long-time admirer of the artistic appeal of tracks, trails, burrows, nests, and other traces of animal behavior. My fondness for the beauty of traces also no doubt contributes to my science: after all, the longer I look at a trace, the more I learn about it. This sentiment accords with a long-time principle of paleontology, botany, and other facets of natural history, which is, “If you draw it, you know it,” with the added benefit of expressing your appreciation of natural objects to others through visual depictions.

Here it is: the cover for my upcoming book, Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals! The book is scheduled to be published by Indiana University Press in the fall of 2012, so be watching out for it then. But in the meantime, look at the beautiful cover art. Who created it, what inspired it, and what science lies behind its aesthetically pleasing composition? Please read on to find out.

My thinking about traces as objects of art is not very original, though, and in fact has been preceded by most of humanity. For example, animal tracks and other traces were common subjects of rock art extending back to the Pleistocene Epoch. Whether made as pictographs or petroglyphs, these traces of traces are in Australia, southern Africa, Australia, and Europe, with some tens of thousands of years old. Based on this tantalizing evidence, one could reasonably propose that the representation of animal traces through art composes an intrinsic part of our heritage as a species. Yes, I know, that’s a tough hypothesis to pursue any further. So I’ll leave it to my paleoanthropologist colleagues to work out (or not).

Petroglyphs that likely represent bird tracks, etched in Triassic sandstone by Native Americans hundreds of years ago (sorry, I’m a paleontologist, not an archaeologist). The pair of marks on the right is similar to the tracks made by a perching bird with three forward pointing toes and one rearward-pointing toe – such as an eagle – whereas those to the right may be like those of a roadrunner, which has an X-shaped foot. Petroglyphs are in northeastern Arizona, near Petrified Forest National Park.

Much more recently, trace fossils similarly inspired renowned ichnologist Dolf Seilacher, who also saw these vestiges of past behavior as lovely objects that fill us with wonder. As a result, in the mid-1990s, he conceived of a traveling exhibit and book showcasing tableaus of trace fossils and other sedimentary structures, titled Fossil Art. For this show – embraced by natural-history venues but mostly rejected by art museums – Seilacher prepared it by: (1) making latex molds of sedimentary rock surfaces; (2) pouring epoxy resin into the molds to make casts mimicking the original bedding planes; and (3) using indirect lighting to enhance details; and (4) assigning creative titles to each piece as if they were works of art.

So these artificial slabs are not human-made art in the traditional sense, but nonetheless invoke marvel, project splendor, and otherwise make us think, engaging the same senses and thought processes that accompany an appreciation of art. Moreover, the slim book Seilacher authored for the exhibit contains explanatory text about each of the objects, illuminated further by his marvelous illustrations and visual interpretations. I remember first seeing a version of this exhibit in Holzmaden, Germany in 1995, near Seilacher’s home in Tubingen, and most lately enjoyed strolling through it with other many ichnologists – and Seilacher himself – in Krakow, Poland in 2008.

World-renowned ichnologist and (oh yeah) Crafoord Prize winner, Dolf Seilacher, lecturing about the planning and execution of Fossil Art as an exhibit while it was showing at the Geological Museum of Jagiellonian University in Krakow, Poland in September 2008. Photograph by Anthony Martin.

A close-up of Wrong Sided Hands, one of the pieces displayed in Fossil Art, cast from a latex mold of a sample from Lower Triassic Buntsandstein of Germany. The piece is so-called because the false appearance of a “thumb” on the outside of the tracks originally led to the mistaken idea that the animal awkwardly crossed its own path with each step. This turned out to be wrong. Also, check out the mudcracks! Photograph by Anthony Martin.

Another close-up of a piece from Fossil Art, titled Shrimp Burrow Jungle (helpfully translated into Polish here). This one is based on burrow systems made by crustaceans during the Late Triassic in Italy, which became densely populated over time and hence contributed to overlapping systems. Photograph by Anthony Martin.

Hence during my writing of a book about the modern traces of the Georgia barrier islands, I was well aware of how some of these traces could likewise lend to artistic expression. Some of this mindfulness was applied to a collaborative artwork done with my wife, Ruth Schowalter, in which we took an illustration of mine from the book and used it as the inspiration for a large watercolor painting depicting traces that would form with rising sea level along the Georgia coast (discussed in detail here).

Nonetheless, it was especially important to think about traces as art when considering a potential cover for the book. Book authors know all too well that a well-designed, attractive cover is essential for grabbing the attention of a potential reader, so I had that practical consideration in mind. But I also wanted a cover that pleased me personally, sharing my love of beautiful traces with others, especially those varied and wondrous tracks, burrows, and trails I had seen and studied on the Georgia barrier islands during the past 15 years.

In such an endeavor, I also faced the added pressure of precedence set by my publisher, Indiana University Press. My book is part of a series by IU Press, called Life of the Past, which is widely admired not only for its comprehensive coverage of paleontological topics, but also for its fine cover art, showcasing works done by a veritable “who’s who” of “paleoartists,” So I knew the cover art for my book needed to both conform to this legacy of artistic excellence, but also stand out from other books in the series because of its unique themes. After all, this would be first book in Life of the Past focusing specifically on ichnology. Moreover, the book is more concerned on modern tracemakers and their environments, rather than plants and animals of pre-human worlds. This was done with the intention of demonstrating how our knowledge of modern traces helps us to better understand life from the geologic past, an intrinsic principle of geology called uniformitarianism.

Ideally, as an ichnological purist, I would have had a cover devoid of any animals, and just shown environments of the Georgia of the Georgia coast with their traces. Indeed, I did just that in some of my illustrations in the book, in which I purposefully omitted animals and left only their traces. This “ichno-centric” mindset actually serves a pedagogical purpose, in that it would echo the truism that many sedimentary rocks are devoid of body fossils, yet are teeming with trace fossils.

Figure 1.3 from Life Traces of the Georgia Coast, conveying a sense of the variety and abundance of traces on a typical Georgia barrier island, from maritime forest (left) to shallow intertidal (right). I purposefully drew this illustration using a more cartoonish technique to introduce broad search images of traces for people who may not ordinarily think about these. But also notice what’s missing from the figure: the animal tracemakers. Instead, only immobile plants are depicted. Would this make for good cover art? No and no, especially if you’ve seen the typical covers done for Indiana University Press books. Illustration by Anthony Martin.

Realistically, though, I also knew that modern traces, particularly those made in places as easy to visit as parts of the Georgia coast, would be more eye-catching if accompanied by some of their charismatic tracemakers in a beautiful, natural setting. After all, the Georgia coast has lengthy sandy beaches, dunes, maritime forests, and salt marshes, inhabited by a wide variety of animals, such as sea turtles, shorebirds, alligators, horseshoe crabs, ghost crabs, and many others.

I also knew that a paleoartist would not be as well suited to the task of creating a cover as someone who works more with modern environments. A better pick would be someone who was familiar with the landscapes, plants, and animals of the Georgia barrier islands, but also a fine artist. I briefly toyed with the idea of doing it myself, but already felt like far too much of the book had been “DIY,” and was not confident enough in my skills to put together a compelling cover in enough time before the book came together. An artfully done photograph was another possibility, so I sent several prospective examples to the editors for their appraisal, but these were all shot down for not having enough aesthetic elements for an attention-getting cover (i.e., traces + landscapes + sky + water = very difficult to get into a single photo).

Fortunately, through social connections that still happen despite the Internet and its incentives for becoming increasingly introverted, I met Alan Campbell through mutual friends in December 2008 at a dinner party on the Georgia coast. Fortuitously enough, our meeting was also just before Ruth and I did three weeks of field work on the barrier islands for the book. It was only fitting, then, that our first meeting was spent dining with both of us facing a Georgia salt marsh, filled with fiddler crab burrows and other such traces. Alan is a Georgia artist with much life experience along its coast, he has often portrayed its environments through gorgeous watercolors, and he has worked with scientists in the field.

Consequently, I kept Alan in mind as a potential cover artist for the next few years, and after I had finished the text and all figures for the book, I contacted him last year about my idea, while simultaneously suggesting him to the editors at IU Press. After much back-and-forth negotiations, with me in the middle, both parties finally came to an agreement, and Alan had a contract to do the artwork for the cover by December 2011.

To help Alan in researching his task, I sent him all of my illustrations and photos used in the book so that he would have an extensive library of trace images on hand for reference. He also had this blog as a source, in which I regularly write about Georgia-coast traces, explanations that are always accompanied by photographs and an occasional illustration. We also exchanged many e-mails and talked on the phone whenever needed. I told Alan my preferred cover would feature a coastal scene, but one filled with traces. He voiced a concern that the painting might become too “busy,” and the details might be lost in reduction of the image to the size

Alan’s contract specified that he would have preliminary study sketches would be done by February 1, and the final cover art was to be finished by March 30. He was only a little late with the study sketches (delayed by a minor operation), and I was delighted to see the following sketch in mid-February.

Study sketch by Alan Campbell for the cover of Life Traces of the Georgia Coast. Reprinted with his permission, and anyone else who want to use it, you have to ask him, too. By the way, every time you use original artwork without permission, a little kitten dies.

After a little bit of feedback from both me and graphic designers at IU Press, Alan went back to the drawing board (so to speak), and came up with the following watercolor painting.

Life Traces of the Georgia Coast, 2012, watercolor on paper, 14” X 18” by Alan Campbell. Again, if you want to use it, you have to ask him first and get permission. Remember those kittens? They’re alive now, but there’s no guarantee they’re going to stay that way.

I gave this artwork a big thumbs up, as did the people at IU Press. So once approved and the scan was sent to IU Press, it was up to the graphic designers there to pick out the typeface, color of the type for the main title, subtitle, author name, and placement of type without covering up the main composition of the painting. I had no say in this, and that’s a good thing, because they really knew what they were doing. It is a very nicely designed cover, and the only thing that would please me more is if they had produced a holographic image of it. (Maybe next year.)

The final cover art for Life Traces of the Georgia Coast revisited. Does it look a little different, now that you know more about how it came about?

I won’t spoil the fun for potential readers, scientists, and art appreciators by explaining in detail all of the ichnological, ecological, and geological elements incorporated into the cover. After all, I’d like to sell a few copies of the book, while also letting readers make their own personal discoveries. But hopefully all of you now have a better appreciation for how traces made by animals, our recognition and admiration for these, and artistic expression of them can all combine to contribute to a book that can be accurately judged by its cover.

Further Reading

Leigh, J., Kilgo, J., and Campbell, A. 2004. Ossabaw: Evocations of an Island. University of Georgia Press, Athens, Georgia.

Martin, A.J., in press. Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press, Bloomington, Indiana.

Morwood, M.J. 2002. Visions from the Past: The Archaeology of Australian Aboriginal Art. Allen & Unwin, Sydney, Australia.

Seilacher, A. 2008. Fossil Art: An Exhibition of the Geologisches Institut. Tubingen University, Tubingen, Germany.

Tomaselli, K.G. 2001. Rock art, the art of tracking, and cybertracking: Demystifying the “Bushmen” in the information age. Visual Anthropology, 14: 77-82.

Fossils in Progress

Posted on November 22, 2011 by TMartin

Despite whatever lamentations are made about the “incompleteness” of the fossil record, fossils are actually quite common. This truism is brought home even more so whenever trace fossils – tracks, burrows, and other evidence of organismal behavior – are included in a fossil checklist (as well they should be) when examining any given outcrop of sedimentary rocks formed in the past 550 million years or so.

For example, many a time I have visited an outcrop described previously as “lacking fossils,” and instead found it filled with trace fossils; hence what people meant was “lacking fossils” equals “no body fossils.” Normally these trace fossils are invertebrate burrows, which might be glibly identified as “worm burrows,” but tracks or other trace fossils may also reveal themselves to those who are looking for them. Indeed, this expectation of finding fossils is such that on occasions when geologists find a sedimentary rock layer devoid of either body or trace fossils, this is odd enough to cause geologists to scratch their heads and ask why.

But how do the former bodily remains of plants or animals, or traces of their behaviors, become preserved as fossils in the first place? This question other related ones are answered by the science of taphonomy. Coined by Russian paleontologist Ivan Yefremov, the etymology of this term stems from Greek, in which taphos ( = burial) and nomos (= law). In such a term, he was thus alluding to an expectation that natural processes that result in fossils becoming preserved are orderly and predictable.

An overview of taphonomy as a field of study would be far too lengthy to explore here, so instead I will use one example from the Georgia coast to show how it is supposed to work. This superb case in point is a relict marsh. It is what’s left of a salt marsh from about 500 years ago, and it has been revealing its nature to paleontologists, geologists, and students for the past few decades.

Overall view of relict marsh exposed on Cabretta Beach, Sapelo Island, Georgia. Me for scale, but photo taken 7 years ago, so the scale might now be slightly wider now. (Photograph taken by Ruth Schowalter.)

Just a little more than a week ago, my colleague Steve Henderson and I took a group of students from Emory University to Sapelo Island for a weekend field trip (detailed last week). One of our goals on this trip was to take them to a relict marsh on Cabretta Beach so that they could better appreciate how a sedimentary deposit makes a transition from living ecosystem to inert rock, yet filled with evidence of its formerly teeming life. Similar relict marshes are on St. Catherines Island and other Georgia-coast islands, but when it comes to teaching about taphonomy in the field, I prefer using the one on Sapelo.

Closer view of relict marsh on Sapelo Island, showing 500-year-old remains of smooth cordgrass (Spartina alterniflora), cross section of its muddy sediments, and quartz sand deposited on top by tides, waves, and wind. (Photograph by Anthony Martin.)

As mentioned in a previous entry, modern salt marsh on the Georgia coast have a few key components that make them among the most productive of all ecosystems: smooth cordgrass (Spartina alterniflora), marsh periwinkles (Littoraria irrorata), mud fiddler crabs (Uca pugnax), and ribbed mussels (Geukensia demissa). So if a Georgia salt marsh were to be buried quickly – say, by a storm that dumps a thick layer of sand on it – what would be preserved? The Cabretta relict marsh partially answers that question, showing us incipient trace and body fossils of these biota. They are not quite fossils, but on their way there, giving us a glimpse of the fossilization process well before it is completed.

For example, the tall, green or golden stalks of smooth cordgrass that we see today, adorned my millions of marsh periwinkles (Littoraria irrorata), are absent from the relict marsh. Only the lowermost ochre-colored stubs and extensive root systems remain, and traces made by the roots below what was the marsh surface.

Modern smooth cordgrass (Spartina alterniflora) and its constant companions, marsh periwinkles (Littoraria irrorata) on Sapelo Island, Georgia.

Cross-sectional view of relict marsh, what is left from a formerly magnificent marsh: stubs, roots, root traces, and not many periwinkles. (Both photographs by Anthony Martin.)

Once in a while, I also find old marsh periwinkle shells scattered on the surface of the relict marsh. These are made of calcium carbonate and will dissolve in slightly acidic waters, so these might not last for long once exposed. The real reason for why these tend to disappear quickly, though, is modern hermit crabs. Hermit crabs encounter these periwinkle shells on the relict marsh surface, say “Hey, free shells!”, then happily trot away with these, not caring that their “new” homes are actually 500 years old.

No mud-fiddler crab remains were apparent on the surface, nor have I seen them in 20-30 visits to this relict marsh. This is not surprising, as their exoskeletons are made of chitin and dissolve more quickly than molluscan shells. Nonetheless, their burrows are always abundantly evident on the surface as perfectly round holes, which are sometimes accompanied by new burrows made by modern fiddler crabs, as well as bivalves that will bore into this firmground.

Modern salt marsh surface on Sapelo Island with mud fiddler crabs (Uca pugnax) showing off a few of the behavioral traits they do best: eating, fighting, mating, and burrowing. Note that burrows, surface scrapings, and pellets are a few of the traces they make. Which of these traces get preserved?

Close-up of eroded relict marsh surface, showing cross-sections of old fiddler-crab burrows now being filled with modern beach sand. Think of how this will look in the fossil record. (Scale in centimeters).

Longitudinal view of former fiddler-crab burrows associated with smooth-cordgrass root traces. Fill the deeper parts of these burrows with sand, and they’re more likely to get preserved as trace fossils. Scale to right is 15 cm (6 in) long. (All photographs by Anthony Martin.)

Modern ribbed mussels are harder for us to see in the field because we would have to wade into soft, deep, sulfurous mud to get close to them, and however amusing that might be, we don’t have time to do our laundry before getting back into our rental vans for the ride home. So the students take our word for it that those mussels are indeed in the marsh, then we point to the old ones clumped on the relict-marsh surface that are still in life position.

Cluster of ribbed mussels (Guekensia demissa) directly associated with stubs of smooth cordgrass on relict marsh surface. Now that they’re exposed, how long will these shells last on the surface? (Photograph by Anthony Martin.)

Oysters (Crassostrea virginica) are less common in the relict marsh, but given the right exposure, these can be observed on some visits too. These clumps of oyster shells mark the edges of tidal creeks that wound through the marsh.

(Top) Modern salt marsh with tidal creek cutting through it and oyster bank exposed at low tide, Sapelo Island.

Former oyster bank peeking out of relict marsh, formerly buried for about 500 years, now revealed by erosion of the modern shoreline. (Both photographs by Anthony Martin.)

Because it was all too easy to spot the similarities between this relict marsh and a modern one less than 100 meters (330 feet) from where we stood, I then asked about other differences. For instance, take the fact that we were standing on the relict marsh while discussing its traits: could we do the same in the modern marsh nearby? No, was the universal answer, and I affirmed that they would likely be up to their waists in ribbed-mussel-produced mud. (I asked for volunteers to test this hypothesis, and they very smartly declined.)

This led to a discussion of why the relict marsh could be so firm, which introduced them to the concept of diagenesis: how a sedimentary deposit can change over time, an important consideration in taphonomy. Such alterations are especially apparent in muds, which lose considerable volume as these lose their water content, causing a “softground” to become a “firmground,” then eventually a “hardground.” The students were surprised when I told them that the relict marsh acting as the floor of our “classroom” was likely 2-3 times as thick as what was there now.

Would these students so blithely walk around on a modern salt marsh? I don’t think so, and please don’t experiment with this yourself. Nevertheless, a relict marsh, thanks to dehydration of its muds and compaction, is just fine for exploring on foot. (Photograph by Anthony Martin.)

We spent only about an hour at the relict marsh before regretfully walking back to our field vehicle, followed by a ferry ride to the mainland part of Georgia and a long drive home to Atlanta. Yet I felt assured that the lessons about taphonomy, ancient environments, ichnology, and diagenesis imparted by this relict marsh encompassed enough material to fill 4-5 class sessions in an indoor classroom. Moreover, if we had been all enclosed by four walls and a ceiling, and without a former marsh underfoot, there was no guarantee that these concepts would be understood or retained.

This is why we geoscientist-educators take our students outside, enriching our collective awareness of how environments change through time and how we piece together the clues left behind from ancient environments. It’s memorable, it’s fun, and it works. But don’t take my word for it. Whether you’re an educator or student, try it yourself sometime, whether on the Georgia coast or elsewhere, and see what happens.

Further Reading

Basan, P.B., and Frey, R.W. 1977. Actual-palaeontology and neoichnology of salt marshes near Sapelo Island, Georgia. In Crimes, T.P., and Harper, J.C. (editors), Trace Fossils 2. Liverpool, Seel House Press: 41-70.

Edwards, J.M. and Frey, R.W. 1977. Substrate characteristics within a Holocene salt marsh, Sapelo Island, Georgia. Senckenbergiana Maritima, 9: 215-259.

Frey, R.W. and P.B. Basan. 1981. Taphonomy of relict Holocene salt marsh deposits, Cabretta Island, Georgia. Senckenbergiana Maritima, 13: 111-155.

Frey, R.W., Basan, P.B. and Scott, R.M. 1973. Techniques for sampling salt marsh benthos and burrows. American Midland Naturalist, 89: 228-234.

Letzsch, W.S. and Frey, R.W. 1980. Deposition and erosion in a Holocene salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 50: 529-542.

Morris, R. W. and H. B. Rollins. 1977. Observations on intertidal organism associations on St. Catherines Island, Georgia. I. General description and paleoecological implications. Bulletin of the American Museum of Natural History, 159: 87-128.

Smith, J.M., and Frey, R.W. 1985. Biodeposition by the ribbed mussel Geukensia demissa in a salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 55: 817-825.

Ghost Crabs and Their Ghostly Traces

Posted on October 31, 2011 by TMartin

The ghost crabs of the Georgia barrier islands – all belonging to the species Ocypode quadrata – are among my favorite tracemakers anywhere, any time. My ichnological admiration for them stems from the great variety of behaviors they record in the beach and dune sands of the islands, telling many fascinating tales of what they were doing while no one was watching. Thus I thought it only appropriate that a blog entry posted close to Halloween deserved a story about an animal that not only has the word “ghost” in its common name, but one that also leaves mystifying marks of its unseen behavior.

On the dawn of June 22, 2004 on Sapelo Island (Georgia), my wife Ruth and I were presented with one of the most intriguing of ghost-crab mysteries related to their vestiges. We were scanning the freshly scoured surfaces of Nannygoat Beach on the south end of the island; high tide only a few hours before had cleansed the beach of the previous day’s traces. The low-angle rays of early-morning sunlight were optimal for contrasting any newly made animal signs on the beach, which is why we were there then. We went to the beach with our minds open to anything novel; indeed, my experience with the Georgia barrier islands is that no matter how many times you visit them, they always hold previously unsolved puzzles.

Sure enough, within about 15 minutes of stepping foot on the beach, Ruth paused and asked one of the most simple – yet important – of scientific questions: “What is this?” She pointed to a depression on the sandy surface, and what I saw was astonishing. It was a trace perfectly outlining the lower (ventral) half of a ghost crab, preserving in detail: impressions of all eight walking legs (pereiopods), including their pointed ends (dactyli); its smaller claw (inferior cheliped) and larger claw (superior cheliped); and its main, rectangular body.

A perfect outline of the bottom side of a ghost crab (Ocypode quadrata), found just after dawn and high tide on Nannygoat Beach, Sapelo Island, Georgia. Why would a ghost crab make such a trace? (Scale in centimeters, and photograph taken by Anthony Martin.)

Even more strangely, only one set of tracks connected with this body imprint, leading away from it, and none moved toward it. This was not an impression made by the dead body of a crab. Instead, the tracks showed that the crab was very much alive when it made its resting trace and immediately afterwards. But what happened just before then? It looked as if the crab floated through the air, dropped vertically, made a perfect 10-point landing, sat there for a while, and walked away.

Another exquisitely defined ghost-crab body impression, and with tracks leading away from it, showing this is not a crab “death mask,” but one made by a live crab. (Scale in centimeters, and photograph taken by Anthony Martin.)

The same ghost-crab impression as above, but this time with the crab anatomy labeled and direction of movement after it stopped and sat down on the sand. What happened to the tracks that must have led to its resting spot? And what’s with that word “hydration”? Let’s just say this is what you call “foreshadowing” in the story. (Scale in centimeters, and photograph taken by Anthony Martin.)

Knowing that ghost crabs can do a lot of things, but not aerial acrobatics, we wondered how this could have happened. Well, single observations can be the start of good science, but for this inquiry to progress any further, we had to see if this seemingly unusual observation could be repeated. So we walked further south along the beach to test whether this was an isolated incident, or if we could find any other ghost-crab outlines with single trackways attached. With such a search image in mind, we quickly found about a dozen more such marks made by crabs of various sizes, but showing an identical behavior. Even better, all were located just below the high-tide mark of the previous night.

Yet another beautiful ghost-crab resting trace, surrounded by a scoured beach surface. Lot of these traces and all just below the high-tide mark meant something was happening that could be answered by the awesome power of science. (Scale in centimeters, and photograph taken by Anthony Martin.)

Time to think. These crabs must have walked to their resting places, but why didn’t they leave any tracks? We soon realized that the tracks were certainly made, but not preserved. So like all other surface traces on the beach, they must have been made erased during high tide. Yes, that was it! The crabs walked to the surf zone just after the high tide, sat down, waited long enough for the tide to drop a little bit, and walked away.

Mystery solved? Well, not quite. This was an incomplete explanation, one with a big, unanswered question. Why did the ghost crab walk to – and sit down in – the surf? (With a prompt like that, feel free to create your own intertidal-crab equivalent of “chicken-crossing-road” punch lines.) Ghost crabs normally spend much of their time in deep, J- or Y-shaped burrows close to or in the dunes, and above the high-tide mark. They are most active at night, when they come out of their burrows to scavenge delectable dead things dumped on the beach by waves and tides, or to prey on smaller invertebrates, like dwarf surf clams (Mulinia lateralis). They also leave their burrows to seek mates, which might involve one crab enticing another to check out its burrow.

A seemingly indignant and defiant ghost crab outside of its burrow during the day, either looking for new territory, food, mates, or all three. They’re greedy that way. In this instance, though, it was mostly running away from me and my camera. (Photograph taken by Anthony Martin.)

None of the crabs that made these traces were scavenging, preying, or mating, yet something in the surf was life-sustaining enough for them to risk becoming meals for early-morning predatory shorebirds. I searched my memory for what I had read previously about ghost crabs and their biological needs, and finally realized what could have driven them to the surf in the middle of the night: they were thirsty.

You see, ghost crabs are living examples of so-called “transitional animals” that evolution-deniers insist do not exist, having an interesting mixture of adaptations to different environments. These crabs are descended from fully marine crabs, so they still have gills that allow them to filter oxygen from marine water. Yet they also have little lungs and can breathe air, enabling them to stay out of the water for hours. Having both gills and lungs makes them semi-terrestrial, living in a world between the land and ocean, and dependent on both realms. They live close to the sea for their food, reproduction (females lay their fertilized eggs in sea water), and water, but their main livelihood is gained from the beach and dunes.

In this respect, ghost-crab burrows in the upper parts of beaches and lower parts of dunes provide protection against predators, but also keep the crabs hydrated. One of the functions of a ghost-crab burrow – which can be more than one meter (3.3 feet) deep, is to intersect the water table below. That way, when a crab needs water for proper respiration, it crawls down the burrow to that saturated area and replenishes it bodily fluids. But they can’t stay down there as the tide rises, so they move higher up the burrow to where there’s some air. Unlike blue crabs (Callinectes sapidus), which have completely developed gills and hence fully marine, if you keep a ghost crab in sea water too long, it drowns.

The previous night was a higher tide than normal, which probably flooded many of the ghost-crab burrows and causing these crabs to abandon their homes. This meant the crabs spent most of the night outside of their burrows, resulting in dehydration, but having to wait out the high tide. As soon as the tide turned and began to drop, the crabs ran to the surf zone, settled down into the wet sand, and soaked up water through small openings where the legs connect to the main body. Spiky “hairs” (setae) on their legs help with this water up-take, drawing up moisture from the sand through capillary action.

My legs? Sorry, I meant to shave. Guess you’ll have to deal with it. Hey, wait a minute: does that pose look like it could make anything you’ve already seen, like, oh, I don’t know, a resting trace? Keep reading. (Photograph by Anthony Martin.)

Ghost crabs are amazingly efficient at pulling water out of sand. So their hunkering down onto a saturated sandy surface with waves breaking on top of them must have been like the ghost-crab equivalent of drinking from a funnel, quenching their thirst in a most satisfying way. Meanwhile, waves washed away their tracks leading to these resting spots. They stayed a while, long enough for the tide to drop and expose the sandy beach surface. Only then did they get up and walk away, fully rehydrated, refreshed, and ready to go back to their burrows or dig new ones.

This was a detailed explanation, but one based entirely on traces and what little I knew about ghost crabs from the scientific literature. How else to test it and see whether it was right or not?

If you just said, “By directly observing this interpreted behavior in a ghost crab,” you would be right. A little more than a month later, on July 30, 2004, I actually got to witness this behavior, and on Nannygoat Beach. Back without Ruth this time, and by myself, I was looking for more traces following a high tide, when I saw a small, wraith-like movement out of the corner of my eye. It was a beautiful adult ghost crab, flat-out running in full daylight and heading straight from the dunes to the surf zone. I stood back and watched it reach the surf, where it promptly sat down and became still.

Here’s a ghost crab that doesn’t mind getting a soggy bottom. This one sprinted from the dunes to the surf, stopped abruptly, and sat a spell. (Photograph by Anthony Martin.)

I took photos while walking toward this crab, expecting it to bolt at any moment. Instead, I was instead surprised to see it remain where it sat, even as its eye stalks rotated to look warily at me. Amazed, I grasped that this one must have been thirsty enough to risk being eaten or stomped. The photo you see shows just how close I got to it, and I was thrilled to see it in exactly the same position depicted by the traces Ruth and I had seen the month before.

Although scientists aren’t always right, if you practice good science, you sometimes hit the nail on the head. Or the crab on the sand. Or, well, never mind. Anyway, this ghost crab is making a trace just like the ones documented the month before and in the same place, and it is a direct result of the same behavior interpreted from just the traces and some knowledge of their physiology. It’s almost as if science has predictive power. Who’d have thought? (Photograph by Anthony Martin.)

With the “resting trace = rehydration” hypothesis now supported by both traces and direct observation, I wrote the results into a formal, peer-reviewed paper. Unexpectedly, such traces had never been documented for ghost crabs, and especially from the perspective of a paleontologist. In the paper, published in 2006, I pointed out that this behavior would explain similar-looking trace fossils in the geologic record, or the preservation of crab bodies frozen in the same position by death, perhaps reaching the surf too late and being buried by wave-borne sands. The geological significance of such trace fossils would be their value in pointing exactly to where the surf may have washed across an ancient shore, millions of years ago. Geologists really like this kind of precision, and become grateful to ichnologists who give them such tools they can easily use in the field.

A fossil crab from the Miocene Epoch (about 15 million years old), preserved in a sandstone bed cropping out on a beach near Comodora Rivadavia, Argentina. This crab and others like it in the sandstone were all preserved the same way: nearly entire, implying they were buried quickly, and parallel to the original sandy surface on which they settled. Could these have died after dehydration near the surf, and then been buried? How long ago did some crabs evolve to become semi-terrestrial? I don’t know, but now we have a hypothesis that can be applied to fossils like these and tested. (Coin is about 2.5 cm (1 in) wide; Photograph by Anthony Martin.)

Since then, I have seen these resting traces on the beaches of every Georgia barrier island, in the Bahamas, and other places where ghost crabs dwell. Although trace fossils echoing this behavior in ghost crabs or their ancestors have not yet been found, I predict that with the right images now in mind, geologists and paleontologists will recognize them some day.

So with this ichnological lesson from ghost-crab traces, I hope they have become just a bit less “ghostly” and much more alive in your imaginations.

Further Reading

Duncan, G.A. 1986. Burrows of Ocypode quadrata (Fabricus) as related to slopes of substrate surfaces. Journal of Paleontology, 60: 384-389.

Martin, A.J. 2006. Resting traces of Ocypode quadrata associated with hydration and respiration: Sapelo Island, Georgia, USA. Ichnos, 13: 57-67.

Wolcott, T. G. 1978. Ecological role of ghost crabs, Ocypode quadrata (Fabricius) on an ocean beach: Scavengers or predators? Journal of Experimental Marine Biology and Ecology, 31: 67-82.

Wolcott, T. G. 1984. Uptake of interstitial water from soil: mechanisms and ecological significance in the ghost crab Ocypode quadrata and two gecarcinid land crabs. Physiological Zoology, 57: 161-184.

Life Traces of the Georgia Coast

Unseen lives of the Georgia barrier islands

Tag Archives: trace fossils

The Ichnology of Jurassic Park

Different Coastlines, Same Traces, and Time

Deconstructing an Ichnology Abstract, with Alligators

Martin, A.J., Page, M., Vance, R.K., and Skaggs, S. 2012. Dens of the American alligator (Alligator mississippiensis) as traces and their predictive value for finding large archosaur burrows in the geologic record. Journal of Vertebrate Paleontology, 32 [Suppl. to No. 3]: 136.

The Traces We Leave Behind: A Tribute to Jordi Maria de Gibert

A Mirror Less Distant in Time

Mistaken Point and the Limits of Actualism

Traces of Toad Toiletry and Naming Trace Fossils

Life Traces as Cover Art

Fossils in Progress

Ghost Crabs and Their Ghostly Traces