‘Dinosaurs Without Bones’ Leaves Its First Marks

Life Traces of the Georgia Coast was published just a little more than a year ago, which as far as authoring goes, seems like yesterday. (Well, unless you’re James Patterson.) Yet as of now, it’s now my second-most recent book.

Dinosaurs-Without-Bones-BookHey, look: it’s a book. How about that? (Photograph by the person whose name is on the cover.)

So I’m proud to announce today is the official launch date of my latest book, Dinosaurs Without Bones: Dinosaur Lives Revealed by Their Trace Fossils (Pegasus Books). What’s it about? Yeah, I know, the main title implies the existence of invertebrate or incorporeal dinosaurs. But the subtitle makes clear that it’s all about the fossil record of dinosaurs apart from just their bones: tracks, nests, burrows, toothmarks, gastroliths, feces, and much more. It’s not only the first comprehensive book written about dinosaur trace fossils, it’s my first overt attempt at popular-science writing in book form. How was it for me? Great fun, and I hope readers feel the same about it.

In a sure sign that authoring might be addictive, I started writing Dinosaurs Without Bones before the publication of Life Traces of the Georgia Coast. The latter book took nearly four years to complete, from proposal to holding that rather hefty volume in my hands. In contrast, I wrote and illustrated Dinosaurs Without Bones in just a little over a year, starting in the summer of 2012 and finishing in December 2013.

This marsupial-like gestation for Dinosaurs Without Bones can be attributed to several fortunate factors coming together, such as my having written two editions of a college textbook on dinosaurs (Introduction to the Study of Dinosaurs, 2001, 2006), writing about dinosaur trace fossils in a 2010-2011 blog (The Great Cretaceous Walk, now defunct), having the fresh experience of writing Life Traces of the Georgia Coast, and the freedom to write with a popular audience in mind. Write? Right.

Although today seems like a firm starting point for its availability to readers, it’s actually been in an incremental “soft launch” during the past few weeks. For example, my publisher made it available for sale by Charis Books in Atlanta, Georgia when I gave a talk to the Atlanta Science Tavern at their annual Darwin Day Dinner on February 9. Other people have told me via Facebook, Twitter, and in person that their pre-ordered copies had already arrived last week. Then just last week, I had a bit of a coming-out party for the book at the annual Science Online 2014 meeting, where it was among the featured new science books, which were all given away in a raffle to lucky meeting participants.

Dinosaurs-Without-Bones-Book-Paleontologist-BarbieMy colleague Paleontologist Barbie, happily posing next to Dinosaurs Without Bones during its first big public viewing at the Science Online 2014 meeting last week in Raleigh, North Carolina. (Photograph by the author again. Unfortunately, Paleontologist Barbie’s arms, much like those of a tyrannosaur, are too short for her to do a selfie.)

I know what you’re thinking: Where can I buy this book? (Your second most likely question is: Does it mention cats? The answer is yes, several times.) If you do get the book and read it, please let me know what you think of it, either via Twitter (@Ichnologist), its Facebook site, e-mail, or most retro of all, in person. Here’s a list of suggested means for acquisition:

  • Your local independent bookstore. Tell the owner I sent you.
  • Order it directly from Pegasus Books here.
  • Order it from Powell’s Books here.
  • Order it from Barnes and Noble here.
  • Order it from that online business that’s trying really hard to make all of those other just-mentioned businesses go extinct. (And I ain’t naming it, because that gives it more power.)

Thanks, hope you like it, and happy tracks, trails, nests, and burrows to you.

 Pertinent Bibliography

Martin, Anthony J. 2014. Dinosaurs Without Bones: Dinosaur Lives Revealed by Their Trace Fossils. Pegasus Books, New York: 460 p.

On the 2nd Day of Ichnology, My Island Gave to Me: 2 Otters Running

On this Christmas of 2013, I thought that the second-to-last post of my “On the __th Day of Ichnology” series would be a gift, one speaking of the beautiful harmony we sometimes are so fortunate to see recorded in the sands of the Georgia barrier islands. The traces composing this gift are the tracks of a male-female pair of river otters (Lutra canadensis).

Otter-Tracks-St-CatherinesSynchronicity expressed in traces: a pair of river otters, running and turning together along a Georgia beach. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale is about 10 cm (4 in) long.)

A normal gait for river otters is a lope, which registers as a 1-2-1 pattern, in which one rear foot is in front, a rear and front foot are next to one another, and a front foot is behind. However, in this instance, I think both otters were galloping, as it looks like both rear feet exceeded their front feet, and a well-defined space is in between each set of four tracks.

What really struck me about these tracks, and made me gasp with joy when I saw them, was their near-perfect symmetry and how they hint of one otter reacting to the other otter’s movement. I can’t say for sure right now what evidence lends to my discerning the following interpretation (sorry, fellow scientists). But my hunch is that the otter on the left was running just in front of the other, maybe separated by a body length at this point, and then turned just slightly to her/his left. The otter on the right was galloping to catch up, saw its partner turn to the left, and decided to turn her/his body in response to this change in direction. Notice how the gap between their trackways is narrowed just a bit, and how the tail of the second otter left an arc-like impression on the sand that points directly to the next set of tracks.

Such a gorgeous set of traces, left by a species we humans often revere (or envy) for its love of play! But I also found these tracks even more gratifying for how they told of two otters linked to one another, perhaps through play, but certainly through their mirrored behaviors, and how this in turn held up a mirror to ourselves. What interactive traces do we similarly leave in our lives? In which instances are we the otter on the left, leading the way and making decisions to change course? In which instances do we follow just behind and to the side of others, and run to catch up? Why do we sometimes lead, why do we sometimes follow, and what makes us come together? Thoughts for Christmas, thoughts for the end of this year, and thoughts for the start of a new year, bestowed by the symbolism of these traces.

Links to Previous Posts in This Theme

On the 12th Day of Ichnology, My Island Gave to Me: 12 Snails Grazing

On the 11th Day of Ichnology, My Island Gave to Me: 11 Plovers Probing

On the 10th Day of Ichnology, My Island Gave to Me: 10 Beetles Boring

On the 9th Day of Ichnology, My Island Gave to Me: 9 Molluscans Hiding

On the 8th Day of Ichnology, My Island Gave to Me: 8 Crab Legs Walking

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

On the 4th Day of Ichnology, My Island Gave to Me: 4 ‘Gators Denning

 On the 3rd Day of Ichnology, My Island Gave to Me: 3 Ghost Shrimp Pooping

On the 5th Day of Ichnology, My Island Gave to Me: 5 Bivalves Drilling

Today’s photo of Georgia-coast traces – similar to yesterday’s about sea turtles connecting to the land through their traces – shows how other marine animals depend on landward environments of the Georgia barrier islands for their livelihoods. In this instance, marine clams require trees to give them homes, and these clams leave marks of their dependency for us to see on formerly forest flotsam that made its way back to land.

Marine-Bivalve-Bored-DriftwoodA piece of driftwood rendered holey by abundant and active wood-drilling marine bivalves, some of which also left their bodies in their former homes. These borings are probably all the work of wedge piddocks (Martesia cuneiformis), which settled onto the wood as wee little clams (larvae, actually), then started drilling.(Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

After the larvae of these clams latched onto these woody substrates – whether these were floating on ocean currents or sunken on sea bottoms – they then lived out their lives drilling into the wood. They drill into wood by rotating or otherwise moving their ridged shells against the hard substrate, like a self-propelled screw.

A few species of wood-drilling clams – sometimes nicknamed “shipworms,” despite their molluscan heritage – actually eat the wood for food. But others, including wedge piddocks, are just making tight, secure homes, similar to how some animals make snug burrows for themselves. Once in a while we get to see the handiwork of these clams in pieces of wood that wash up on Georgia shorelines, a special delivery brought to us by tides and waves.

Wood-boring clams probably evolved about 150-200 million years ago during the Mesozoic Era, and their trace fossils are common in fossil driftwood from the Jurassic Period to just recently. For marine clams to start drilling into wood – whether for food, homes, or both – is pretty remarkable as a behavior, when you think about it evolving in response to the growth of forests on land. After all, bivalves lived in the world’s oceanscapes long before forests spread across landscapes, with the former starting in the Cambrian Period (more than 500 million years ago) and the latter starting in the Devonian Period (about 350 million years ago).

But it’s also interesting to think about how marine clams apparently did not take advantage of these terrestrial tissues for several hundred million years after wood first started floated out to sea. In contrast, mites, insects, and other land-dwelling invertebrates began chewing wood right away, and consequently left their own distinctive traces (mentioned last week with beetle borings). But thanks to trace fossils, we can better tell when terrestrial animals commenced wood-eating behaviors, and when certain marine clams began mixing their traces with those of their land-lubbing compatriots.

Further Reading

Martesia cuneiformis (Say, 1822) Wedge Piddock. Jaxshells.org, by Bill Frank, images by Joel Wooster.

The Second World That Forms on Sunken Trees. Ed Yong, Not Exactly Rocket Science, National Geographic Phenomena.

Wood: It’s What’s For Dinner. Craig McClain, Deep Sea News.

Links to Previous Posts in This Theme

On the 12th Day of Ichnology, My Island Gave to Me: 12 Snails Grazing

On the 11th Day of Ichnology, My Island Gave to Me: 11 Plovers Probing

On the 10th Day of Ichnology, My Island Gave to Me: 10 Beetles Boring

On the 9th Day of Ichnology, My Island Gave to Me: 9 Molluscans Hiding

On the 8th Day of Ichnology, My Island Gave to Me: 8 Crab Legs Walking

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

 

 

On the 6th Day of Ichnology, My Island Gave to Me: 6 Hatchlings Crawling

For today’s photo of Georgia-coast traces and explanation of their meaning, we’ll look at some that are made only very briefly in the first moments of active life for their tracemakers, and in an environment very few of them will ever revisit. Moreover, those exceptional individuals who do make it back to the same environment – sometimes the same place where they took their baby steps – may take as long as three decades to do so. The traces are the trackways made by hatchling sea turtles, in this instance loggerhead turtles (Caretta caretta).

Hatchling-Turtle-Trackways-St-CatherinesYeah, I know, there are a lot more than just six sea-turtle hatchling trackways here, but it’s at least six. Regardless, they’re still in the spirit of the holiday season. These tracks were made only moments after the hatchlings emerged from a buried hole nest, which was behind coastal dunes on St. Catherines Island, Georgia. They hatched early in the morning of July 31, 2011 and promptly began making tracks. (Photograph by Anthony Martin; scale in centimeters.)

The trackways are relatively simple, consisting of alternating front-back flipper impressions and central body drag marks that are vaguely defined in dry sand (like in the photo) but become gorgeously expressed in wet sand. Overall trackway patterns tend to loop and intersect one another close to the nest as they tried to get oriented toward the sea, then become more linear and cross one another less often once they find the beach and waddle down slope. Trackways may be tens of meters long, depending on how far hatchlings must travel from their nests to the surf zone.

As an ichnologist, what I find remarkable conceptually about hatchling traces is knowing that their makers only leave traces on land for a few minutes after they’re born. All successfully hatched sea-turtle eggs are located in nests above the high-tide mark, and on the Georgia coast these are normally behind the first line of coastal dunes along a sandy shoreline. Then, assuming the hatchlings don’t die during their brief and vulnerable time on land (raccoons and herons and ghost crabs – oh my!) and that they do make it into the sea, almost all of their remaining tracemaking behaviors will be done in the Atlantic Ocean.

In other words, you’ll have to be very patient and relatively young to see traces made by those same individual turtles on Georgia beaches and dunes again, and these will only be from adult females. About 15-30 years pass before sea turtles reach sexual maturity, meaning it will take at least that long for pregnant mother turtles to come ashore. Even then, they do this seasonally, from about May through August, so you will only see adult female and hatchling tracks in the middle of any given year. Thus loggerheads and other sea turtles collectively are important tracemakers on the Georgia coast, but individually are rare.

For paleontologists, the huge trackways, covering pits, hole nests, and other marks of sea turtles comprise a trace assemblage with a great potential for preservation in the fossil record. Sure enough, sea turtle trace fossils have been reported from Cretaceous Period rocks in the western U.S. More are likely out there, and I have little doubt that these will be found by applying the right search images. Who will recognize the first trace fossils of sea-turtle hatchlings? My bet is that it will be someone who saw a lot of modern ones, perhaps even some from the Georgia coast. Good luck!

Further Information

St. Catherines Island Sea Turtle Conservation Program. (Mostly done by Gale Bishop.)

Georgia Sea Turtle Center. Jekyll Island, Georgia.

Links to Previous Posts in This Theme

On the 12th Day of Ichnology, My Island Gave to Me: 12 Snails Grazing

On the 11th Day of Ichnology, My Island Gave to Me: 11 Plovers Probing

On the 10th Day of Ichnology, My Island Gave to Me: 10 Beetles Boring

On the 9th Day of Ichnology, My Island Gave to Me: 9 Molluscans Hiding

On the 8th Day of Ichnology, My Island Gave to Me: 8 Crab Legs Walking

On the 7th Day of Ichnology, My Island Gave to Me: 7 Lizards Looping

On the 9th Day of Ichnology, My Island Gave to Me: 9 Molluscans Hiding

For today’s entry in the holiday-and-ichnology inspired countdown of Georgia-coast traces, we will move from the maritime forest to the shoreline, where a trace made through the behavior of one species of molluscan – the knobbed whelk (Busycon carica) – influenced the behavior (and hence traces) of another molluscan – the dwarf surf clam (Mulinia lateralis).

These traces then attracted shorebirds, which added their tracks and beak probe marks to the molluscan traces. This is a excellent modern example of how the interaction of one species of animal with a sediment can affect the interactions of other species with that same sediment, leading to their creation of composites traces.

Whelks-Dwarf-Surf-Clams-Burrowing-JekyllSee all of the knobbed whelks (Busycon carica) in this photo? I know, you don’t actually see their shells because they buried themselves, but you see their outlines on this sandy beach surface because of the many dwarf surf clams (Mulinia lateralis) that burrowed around them. Also look for all of the bird tracks and probe marks around the whelks. (Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

I’ve already written about these composite traces and the ecological story they tell, so for those details, go to this link and this link. But the summary version goes like this:

  • Low tide stranded the whelks and clams on the beach.
  • Whelks used their muscular feet to pull themselves into the still-wet sand to avoid desiccation and predation.
  • Clams took advantage of disturbed (liquified) sand around the whelks and buried themselves, also to avoid predation and desiccation.
  • Shorebirds saw whelk-shaped concentrations of clams, chowed down on them.

The story gets more complicated in places, especially when seagulls decided they also wanted to eat the whelks, but that’s most of it. So next time you’re on a beach and you see a triangular-shaped concentration of small clams, take a second look to see whether there’s a live whelk underneath it: traces begetting traces.

Further Reading

Busycon carica: Knobbed Whelk. Smithsonian Marine Station at Fort Pierce.

Mulinia lateralis: Dwarf Surf Clam. Smithsonian Marine Station at Fort Pierce.

Links to Previous Posts in This Theme

On the 12th Day of Ichnology, My Island Gave to Me: 12 Snails Grazing

On the 11th Day of Ichnology, My Island Gave to Me: 11 Plovers Probing

On the 10th Day of Ichnology, My Island Gave to Me: 10 Beetles Boring

Erasing the Tracks of a Monster

Life can certainly imitate art, as can life traces. I was reminded of this last week while doing field work on St. Catherines Island (Georgia), and after encountering traces made by two very different animals, alligators and fiddler crabs. What was unexpected about these traces, though, was how they intersected one another in a way that, for me, evoked scenes from the recent blockbuster summer movie, Pacific Rim.

Alligator-Tracks-Fiddler-Crab-Burrows-1

Could these be the tracks of a kaiju, making landfall on the shores of Georgia? Sorry to disappoint you, but they’re just the right-side and very large tracks of an American alligator (Alligator mississippiensis), accompanied by its tail drag-mark, left on a sandy area next to a salt marsh. Note the scale impressions in its rear-foot track, a symbol of the awesome reptilian awesomeness of its tracemaker. But wait: what nefarious nonsense is happening to the tail drag-mark, which is being covered by tiny balls of sand? Who made that hole next to the drag-mark? And what the heck was a raccoon (Procyon lotor) doing in the neighborhood, leaving its track on the tail drag-mark? With such a monster on the loose, shouldn’t that raccoon be hiding in the forest? (Photo by Anthony Martin, taken on St. Catherines Island; scale in centimeters.)

For anyone who has not seen Pacific Rim, you can read what I wrote about its distinctive fictional ichnology here. But what came to my mind while I was doing field work was one of the themes expressed early on in the film: how quickly humanity returned to normalcy following a lull in attacks by gigantic monsters (kaiju) that emerged from the ocean, destroyed major cities, and killed millions of people. It reminded me of how horrific hurricanes can strike a coast, such as the 1893 Sea Islands Hurricane that hit Georgia, but because no hurricane like it has happened there since, coastal developers think it’s hunky-dory to start building on salt marshes.

But enough about malevolent evil as exemplified by kaiju and coastal developers: let’s get back to traces. Last week, I was on St. Catherines Island for a few days with my wife (Ruth) and an undergraduate student (Meredith) to do some field reconnaissance of my student’s proposed study area. The area was covered by storm-washover fans; these are wide, flat, lobe-shaped sandy deposits made by storm waves, which span from the shoreline to more inland on barrier islands. We were trying to find out what traces had been left on these fans – tracks, burrows, scrapings, feces, and so on – which would tell us more about the distribution and behaviors of animals living in and around the washover fans.

Alligator-Trackway-St-Catherines-2Part of a storm washover fan on St. Catherines Island (Georgia), with the sea to the left and salt marsh (with a patch of forest) in the background. Say, I wonder what made those tracks coming out of the tidal creek and toward the viewer? (Photograph by Anthony Martin.)

It didn’t take long for us to get surprised. Within our first half hour of walking on a washover fan and looking at its traces, we found a trackway left by a huge alligator, split in half by a wavy tail-drag mark. I recognized this alligator from its tracks, as I had seen them in almost exactly the same place more than a year before. Besides their size, though, what was remarkable about these tracks was their closeness to a salt marsh behind the washover fan. When we looked closer, we could see long-established trails cutting through the salt-marsh vegetation, which were the width of a large adult alligator.

Alligator-Trackway-St-Catherines-1That ain’t no skink: the distinctive tracks and tail drag-mark of a large alligator, strolling through a storm-washover fan and next to a salt marsh. You think these animals are “freshwater only”? Traces disagree. Scale = 10 cm (4 in). (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Alligator-Trail-Salt-Marsh-SCIAlligator trail cutting through a salt marsh. Trail width was about 45-50 cm (18-20 in), which is about twice as wide as a raccoon trail. And it wasn’t made by deer or feral hogs either, because, you know, alligators. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

So although the conventional wisdom about alligators is that these are “freshwater-only” animals, their traces keep contradicting this assumption. Sure enough, in the next few days, we saw more alligator tracks of varying sizes going into and out of tidal creeks, salt marshes, and beaches.

Based on a few traits of these big tracks, such as their crisp outlines (including scale impressions), the alligator had probably walked through this place just after the tide had dropped, only a couple of hours before we got there. But when we looked closer at some of the tracks along the trackway, we were astonished to see that something other than the tides had started to erase them, causing these big footprints to get fuzzy and almost unrecognizable.

The culprits were sand fiddler crabs (Uca pugilator), which are exceedingly abundant at the edge of the storm-washover fans closest to the salt marshes. These crabs are industrious burrowers, making J-shaped burrows with circular outlines corresponding to their body widths. They also scrape the sandy surfaces outside of their burrows to eat algae in the sand, then roll up that sand into little balls, which they deposit on the surface.

In this instance, after this massive alligator had stomped through their neighborhood, they immediately got back to work: digging burrows, scraping the surface, and making sand balls. Within just a few hours, parts of the alligator trackway was obscured. If these parts had been seen in isolation, not connected to the clear tracks and tail drag mark, I doubt we would have identified these slight depressions as large archosaur tracks.

Alligator-Tracks-Burrowed-Fiddler-CrabsHey, what’s going on here? Who would dare to erase and fill in giant alligator tracks? Don’t they know who they’re dealing with? (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Alligator-Tracks-Destroyed-Fiddler-Crab-Burrows-1Going, going, gone: alligator tracks nearly obliterated by burrowing, surface scraping, and sand balls caused by feeding of sand fiddler crabs (Uca pugilator). (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)

What was even neater, though, was how some of the fiddler crabs took advantage of homes newly created by this alligator. In at least a few tracks, we could see where fiddler crabs had taken over the holes caused by alligator claw marks. In other words, fiddler crabs saw these, said, “Hey, free hole!”, and moved in, not caring what made them.

Alligator-Tracks-Destroyed-Fiddler-Crab-BurrowsDon’t believe me about fiddler crabs moving into alligator claw marks? OK, then what’s that I see poking out of that alligator claw mark (red square)? (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)

Fiddler-Crab-Burrow-Alligator-Claw-MarkWhy, it’s a small sand fiddler crab! Does it care that its new home is an alligator claw mark? Nope. Does ichnology rule? Yup. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Fiddler-Crab-Burrow-Alligator-Claw-2Need a free burrow? Then why start digging a new one when alligator claw marks (arrow) gives you a nice “starter burrow”? Notice the sculpted, round outline, showing the claw mark was modified by a crab. Also check out the sand balls left outside of the other claw marks, meaning these have probably been occupied and mined for food by fiddler crabs, too. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)

As a paleontologist, the main lesson learned from this modern example that can be applied to fossil tracks, is this: any tracks made in the same places as small, burrowing invertebrates – especially in intertidal areas – might have been destroyed or otherwise modified immediately by the burrowing and feeding activities of those much smaller animals. The secondary lesson is on how large vertebrate tracks can influence the behaviors of smaller invertebrates, resulting in their traces interacting and blending with one another.

More symbolically, though, these alligator tracks and their erasure by fiddler crabs also conjured thoughts of fictional and real analogues: Pacific Rim and coastal development, respectively. With regard to the latter, it felt too much like how, as soon as a hurricane (a meteorological “monster”) passes through a coastal area, we begin to talk about rebuilding in a way that, on the surface, wipes out all evidence that a hurricane ever happened.

Yet unlike fiddler crabs, we have memories, we have records – including the plotted “tracks” of hurricanes – and thanks to science, we can predict the arrival of future “monsters.” So the preceding little ichnological story also felt like a cautionary tale: pay attention to the tracks while they are still fresh, and be wary of those that vanish too quickly.

The Ichnology of Pacific Rim

Last week I surrendered to geekdom peer pressure and went to see the new summer blockbuster Pacific Rim. Living up to my namesake, St. Anthony, I normally don't have a problem resisting such temptations, and just wait to see a movie like this in some other format: DVD, Netflix, or the way movies were originally intended to be seen, on a tiny screen on the back of an airplane seat. But what really pushed me to go was the following image, only glimpsed for a few seconds in one of the trailers:

Kaiju-Track-IntertidalOoo, look, a trace made in an intertidal sandflat! Perhaps it's from a ghost crab, moon snail, or shorebird. Hey, wait a minute, something doesn't quite look right. Are those people next to it? (Image from http://www.comicbook.com.)

Yes, that’s right: it's a gigantic footprint, and in what looks like an intertidal coastal environment, between the low tide mark and coastal dunes. That was all the incentive I needed, as I further wondered what other ichnological wonders would be included in the film. I was also encouraged to see where other scientifically inclined bloggers had fun with Pacific Rim by taking a look at its biology (here, here, and most recently, here) and physics (here here, and most recently, here). So given a $5 afternoon matinee and a spouse (Ruth) willing to indulge my sci-fi inner nerd (OK, so it’s not so “inner”), I had every reason to document the various traces and tracemaking activities in the film. You know, for science and science education.

The verdict? Well, I have to admit some mild disappointment with how the director – Guillermo del Toro - chose to focus on the conflicts between massive amphibious creatures (kaiju) constructed by interdimensional aliens and human-guided fighting machines (jaegers), rather than on their traces. Nonetheless, I managed to find some ichnological gems scattered throughout. For example, the footprint shown in the trailer did indeed look glorious on a big screen, and the human figures associated with it reminded me of Jason Isley’s whimsical underwater photos. But let’s take a closer look at what this footprint tells us about its maker.

Although viewed from an oblique angle, the track seems longer than wide, and has four clearly defined digits, although a probable fifth digit is visible on the side farthest from the viewer. All of the digits are forward-pointing and taper abruptly at their ends. The tracks also has an indentation on the “heel” (proximal) part of the foot, and is more-or less-bilaterally symmetrical. Pits inside of the track may represent additional anatomical traits, such as scales or other bumps on its skin, or could be sediment that underwent liquefaction or other soft-sediment deformation.

Kaiju-TrackInterpreted kaiju track, extrapolated from oblique view. Scale = 10 m (33 ft).

Using the people around the tracks as informal units of measurement, and assuming from the hiragana-katakana in the newscast image that this track - like many items - was made in Japan, we can estimate the dimensions of the track. Average heights for Japanese males and females are 1.71 m and 1.58 m, respectively, and the average of those is 1.64 m. Using one figure (boxed) as a unit that equals 1.64 m (5.4 ft), the footprint had about 18.4 Japanese-Person-Units (JPU) length and 10.1 JPU width, which converts to about 30 m (98 ft) long and 17 m (56 ft) wide. This results in a length:width ratio of about 1.8.

Kaiju-Track-MeasuredLength and width measurements for kaiju track, including figure used as 1.0 JPU = 1.64 m. Width measurement is assumed on basis of probable fifth digit impression on side of track furthest from the viewer.

Unlike in most articles published in high-impact journals, I'll actually admit potential sources of error in these measurements before I'm forced to retract this blog post under a cloud of scandal, followed by my accepting a high-paying position on Wall Street, where such inaccuracies are rewarded without penalty. For example, the width measurement, because it is being taken from an oblique angle (not so accurate) instead of from directly above (much more accurate) probably underestimates the actual width. So the actual width is probably closer to 20 m (67 ft), which reduces the length:width ratio to about 1.5. The length measurement would also benefit from more of an overhead view, and probably would best be studied using aerial high-resolution LiDAR scanning. So there.

To put this in ichnological perspective, when these dimensions are compared to typical sauropod dinosaur tracks from the Early Cretaceous of Texas - where everything is supposed to be bigger - the sauropod comes out looking pretty puny indeed. In this example, the rear track length is 87 cm (34 in) and width is 59 cm (23"), and although its length:width ratio comes out fairly close to my estimation for the kaiju track (1.47), it is only about 2% of its size. Some "thunder lizard."

Sauropod-Tracks-TexasSauropod tracks from the Early Cretaceous (about 120-million-years-old) Glen Rose Formation of central Texas.The larger track is from the left rear foot, and the smaller one in front of it is the left front foot; this sauropod was walking slowly with an "understep" gait, in which its rear foot stayed behind its front. Please read the preceding text for all of that measurement stuff, which ichnologists sometimes call "data." (Photograph taken by Anthony Martin in Dinosaur Valley State Park, near Glen Rose, Texas.)

Kaiju+Sauropod-Tracks copyTo-scale comparison between sauropod track (arrow, lower left) and kaiju track (right) to same scale. Looks like some cute little saurischian would be feeling a little inadequate. As Cowboy Curtis once said on Pee Wee's Playhouse, "You know what they say: Big feet, big boots!" Scale = 10 m.

Speaking of high impact, how about track depth and other features of this individual track that might tell us about behavior of the kaiju tracemaker? Oddly enough, the kaiju track looks too shallow to me, measuring only about 1.6 JPU, or about 2.5 m (8 ft) deep. It also lacks pressure-release structures, which are sedimentary structures caused by the tracemaker applying and releasing pressure against the wall of the track. Considering that kaiju were supposed to weigh tens of thousands of tons, this track should have a greater depth, along with major ridges and plates outside of the track outline that would have been imparted by any forward or lateral movement of its foot.

Alternatively, this track may represent more of what I would call a “stamp,” which would have been made by placing a foot directly down onto a soft substrate and pulling it straight up, rather than from moving forward or laterally. Based on this evidence, the kaiju might have been attempting to squish pesky humans, rather thank performing its normal, forward-walking, city-destroying gait. Unfortunately, the preceding and next track are not shown in the photo, which would help to test this hypothesis.

Other than size, how does the form of this track compare to those of other known dinosaur tracks? The length: width ratio comes out close to that of a sauropod dinosaur, yet other qualitative traits of the track, such as thin digits that taper and end with sharp clawmarks, are more like that of a theropod. But I do want to point out a little coincidence. Have you ever seen the front-foot track of a typical raccoon? Hmmm...

Raccoon+Kaiju-TracksI give you you raccoon tracks, and I give you kaiju track. That is all. (Photo of raccoon tracks taken by Anthony Martin on Cumberland Island, Georgia.)

What’s really fun, though, is if you compare the kaiju track to known theropod tracks. Theropod tracks bearing four or more forward-pointing toes are quite rare, and the few identified probably belong to a group of theropods called therizinosaurs, which - by a strictly enforced paleo-nerd law - cannot be mentioned in a sentence without also using the descriptor "bizarre." Late Cretaceous dinosaur tracks recently reported from Alaska with four long, forward-pointing digits have been attributed to therizinosaurs. Were the creators of the kaiju track trying to compare it to that of a really strange theropod dinosaur? Maybe, maybe not.

Therizinosaur-Tamara-TrackArtistic rendition of Nothronychus mckinleyi, a therizinosaur from the mid-Cretaceous of North America (left) and a four-toed rear-foot track credited to a therizinosaur from Late Cretaceous rocks of Alaska (right). Therizinosaur artwork by paleoartist Nobu Tamara and available in Wikipedia Commons here; photo of track by David Tomeo and reproduced from Everything Dinosaur.

Although the Pacific Rim kaiju designers used a mix of invertebrate and vertebrate elements for anatomical details appearances of their monsters (detailed splendidly by Darren Naish here), I do wonder how they came up with the track, and which real-life animals - modern or extinct - were supposed to be evoked by this track's brief appearance onscreen. Hopefully the DVD and its Special Features will reveal all once that comes out.

(Incidentally, this attempt to divine the evolutionary relatedness of a science-fictional animal from a single track reminds me of a scene from the classic science-fiction film Forbidden Planet. At some point, an invisible monster comes aboard a spaceship on the aforementioned planet and kills its chief engineer. The ship scientist, Dr. Ostrow, then gave a fine interpretation of the monster based on a plaster cast made from one of its footprints, including how it traits seemed to go against all known evolutionary principles. It's such a fun scene, I've shown it in some of my classes as an example of "extraterrestrial ichnology.")

Other tracemaking in the movie, of course, included wholesale destruction of major population centers by the kaiju, clawmarks left on various city substrates, as well as kaiju scat. Unlike other fans of the movie, I've only seen it once so far, and cannot recall whether the following picture of its droppings was flashed on the movie screen or not.

What-a-load-of-kaiju-crapThe banner for this news clip says it all: kaiju excrement, and you can bet this much did indeed contaminate a portion of Manila, Philippines (or the "Phillipines," which may be a gated community just outside of Philadelphia.) On the flip side, I'll bet a certain sick Triceratops in the movie Jurassic Park is now a little less self-conscious about having its wastes piled higher and deeper on the big screen.

One line about their excrement – uttered by kaiju-organ harvester, Hannibal Chau (played by a hilarious Ron Perlman) - alludes to its commercial value based on its phosphorus content. This would accord with the economic importance given to bat or bird guano, which has been mined and sold as fertilizer, and even inspired wars. (I am not making that up.) Still, it would have been beyond awesome to have just one scene showing a deposit of its scat enveloping a large, recognizable monument to a politician in one of those cities.

Hannibal Chau (Ron Perlman), selling kaiju products for whatever might ail you. Alas, their scat is not mentioned in this ad, but he could easily do another one directed at Whole Foods. After all, it would be 100% organic and free-range fertilizer!

What about the jaegers? Their traces are much tougher to discuss, semantically speaking. Ichnologists classify tools themselves as traces of behavior, but most do not count marks made by tools (or machines) as traces. Nonetheless, because the jaegers are being controlled by humans, the marks they leave on the landscape, seascapes, and upside some kaiju’s head, might count as traces, too.

However, in one scene of the movie, in which a kaiju picked up a jaeger and threw it – inflicting much destruction of private and public property – these traces would be those of the kaiju, not the jaeger. I pointed out a similar situation with Jurassic Park. Toward the end of the movie, the poor, misunderstood protagonist of the film - the Tyrannosaurus rex - in an action tinged with self-loathing, hurled a Velociraptor at a mounted T. rex skeleton, no doubt expressing doubt about her place in a post-Mesozoic world. Existentialist angst aside, the destruction of the skeleton was a trace of the tyrannosaur's behavior, not that of the Velociraptor.

So next time you go to a movie featuring multi-ton monsters emerging from the deep sea and massive fighting machines, look for them to make traces, note the traces they make, how these traces may reflect some sort of evolutionary history for the tracemakers, and ask yourself what constitutes a trace. Then no matter how bad the movie, you'll still be guaranteed to enjoy it. Happy movie viewing and tracking!

2013 International Ichnofabric Workshop: Çanakkale, Turkey

After a week of traveling and “touristing” in Turkey with my wife Ruth, which included a 16+ hour overnight bus ride from Goreme to Çanakkale (thus proving that “bus lag” is a real thing), it is now time to learn some ichnology. Every two years, ichnologists from around the world – normally 30-40 people hailing from 10-15 countries – gather for the 2013 International Ichnofabric Workshop which consists of six days of talks, field trips, and congenially conducted arguments about trace fossils, traces, and ichnology in general. The workshop, which I mentioned in the next-to-last chapter of my book (Life Traces of the Georgia Coast) is typically held in some beautiful place with lots of trace fossils and beer nearby (and not necessarily in that order of priority). This time is taking place in Çannakale – located on the western side of Turkey, across from Gallipoli.IIW-2013-Banner[Darth Vader voice] The ichnologists are here: I can sense them. Ichnologists, such as this one (who, incidentally, owes me money) are emerging from their burrows everywhere – much like periodical cicadas – to discuss ichnology here in Çanakkale, Turkey. (Photograph by Ruth Schowalter.)

Based on the program, most of the talks will be about marine invertebrate trace fossils and their uses in interpreting ancient animal behavior and their environments, although a few will also talk about how ichnofabrics – the sum effect of tracemakers on a given substrate, such as sediment or rock – can be applied to learning more about environmental change and evolution. You know, that weighty stuff we natural scientists crave doing.

Goreme-Ichnofabric-1So what the heck is an “ichnofabric”? Here’s an example provided by a paving stone in Goreme, Turkey, in which the people who placed it there thought, “Gee, this rock has some cool looking fabrics in it,” (although they likely thought that in Turkish.). But they probably did not know these fabrics were the result of trace fossils made on a sea bottom by marine invertebrate animals millions of years ago. (Photograph by Anthony Martin.)

So for those people who are not able to attend – most of the world, in other words – I will do my best to provide a regular report on what we are doing and learning here. For this, you can come back to this blog every few days, or for more instant gratification, you can follow me on Twitter (@Ichnologist), which I will use for quick assessments of the events as they’re happening. For those of you familiar with how the Twitterverse works, I and other ichnologists should be using the hashtag #Ichnofab2013 to denote the workshop. Teşekkür eridim!

The Paleozoic Diet Plan

Given the truth that the Atlantic horseshoe crab (Limulus polyphemus) is more awesome than any mythical animal on the Georgia coast (with the possible exception of Altmaha-ha, or “Altie”), it’s no wonder that other animals try to steal its power by eating it, its eggs, or its offspring. For instance, horseshoe-crab (limulid) eggs and hatchlings provide so much sustenance for some species of shorebirds – such as red knots (Calidris canutus) and ruddy turnstones (Arenaria interpres) – that they have timed their migration routes to coincide with spawning season.

Ravaged-Limulid-SCISomething hunted down, flipped over, and ate this female horseshoe crab while it was still alive. Who did this, what clues did the killer leave, and how would we interpret a similar scenario from the fossil record? Gee, if only we knew some really cool science that involved the study of traces, such as, like, I don’t know, ichnology. (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, on May 4, 2013.)

Do land-dwelling birds mammals eat adult horseshoe crabs? Yes, and I’ve seen lots of evidence for this on Georgia beaches, but from only three species: feral hogs (Sus crofa) and vultures (Coragyps atratus and Cathartes aura: black vultures and turkey vultures, respectively). In all of these interactions, no horseshoe-crab tracks were next to their bodies, implying they were already dead when consumed; their bodies were probably moved by tides and waves after death, and later deposited on the beach. This supposition is backed up by vulture tracks. I’ve often seen their landing patterns near the horseshoe-crab bodies, which means they probably sniffed the stench of death while flying overhead, and came down to have an al fresco lunch on the beach.

Nonetheless, what I just described is ichnological evidence of scavenging, not predation. So I was shocked last month when Gale Bishop, while he was monitoring for sea-turtle nests on St. Catherines Island (Georgia), witnessed and thoroughly documented an incident in which a raccoon (Procyon lotor) successfully preyed on a live horseshoe crab. Yes, that’s right: that cute little bandit of the maritime forest, going down to a beach, and totally buying into some Paleozoic diet plan, a passing fad that requires eating animals with lineages extending into the Paleozoic Era.

Limulid-Death-Spiral-SCISo what’s the big deal here? Horseshoe crab comes up on beach, gets lost, spirals around while looking for the ocean, and dies in vain, a victim of its own ocean-finding ineptitude: the end. Nope, wrong ending. For one thing, those horseshoe crab tracks are really fresh, meaning their maker was still very much alive, then next thing it knows, its on its back. Seeing that horseshoe crabs are not well equipped to do back-flips or break dance, I wonder how that happened? (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, and you can see the date and time for yourself.)

Here is part of the field description Gale recorded, which he graciously shared with me (and now you):

“Female Horseshoe Crab at 31.63324; 81.13244 [latitude-longitude] observed Raccoon feeding on upside-down HSC [horseshoe crab] on south margin of McQueen Inlet NO pig tracks. Relatively fresh HSC track. Did this raccoon flip this HSC?”

Raccoon-Tracks-Pee-Limulid-Eaten-SCIWell, well. Looks like we had a little commotion here. Lots of marks made from this horseshoe crab getting pushed against the beach sand, and by something other than itself. And that “something else” left two calling cards: a urination mark (left, middle) and just above that, two tracks. I can tell you the tracks are from a raccoon, and Gale swears the urination mark is not his. (Photograph by Gale Bishop, taken on St. Catherines Island, Georgia, and on May 4, 2013.)

I first saw these photos posted on a Facebook page maintained by Gale Bishop, the St. Catherines Island Sea Turtle Program (you can join it here). This was one of this comments Gale wrote to go with a photo:

GB: “This HSC must have been flipped by the Raccoon; that was NOT observed but the fresh crawlway indicates the HSC was crawling across the beach and then was flipped – only tracks are Rocky’s!”

[Editor's note: "Rocky" is the nickname Gale gives to all raccoons, usually applied affectionately just before he prevents them from raiding a sea-turtle nest. And by prevent, I mean permanently.]

My reply to this:

AM: “VERY fresh tracks by the HSC, meaning this was predation by the raccoon, not scavenging.”

In our subsequent discussions on Facebook, Gale agreed with this assessment, said this was the first time he had ever seen a raccoon prey on a horseshoe crab, and I told him that it was the same for me. This was a big deal for us. He’s done more “sand time” on St. Catherines Island beaches than anyone I know (every summer for more than 20 years), and in all my wanderings of the Georgia barrier island beaches, I’ve never come across traces showing any such behavior.

(Yes, that’s right, I know you’re all in shock now, and it’s not that this was our first observance of this phenomenon. Instead, it is that we used Facebook for exchanging scientific information, hypotheses, and testing of those hypotheses. In other words it is not just used for political rants, pictures of cats and food, or political rants about photos of cat food. Which are very likely posted by cats.)

Now, here’s where ichnology is a pretty damned cool science. Gale was on the scene and actually saw the raccoon eating the horseshoe crab. He said it then ran away once it spotted him. (“Uh oh, there’s that upright biped with his boom stick who’s been taking out all of my cousins. Later, dudes!”) And even though I trust him completely as a keen observer, excellent scientist, and a very good ichnologist, I didn’t have to take his word for it. His photos of the traces on that Georgia beach laid out all of the evidence for what he saw, and even what happened before he got there and so rudely interrupted “Rocky” from noshing on horseshoe-crab eggs and innards.

Raccoon-Galloping-Limulid-Death-Spiral-Traces-SCIAnother view of the “death spiral” by the horseshoe crab, which we now know was actually a “life spiral” until a raccoon showed up and updated that status. Where’s the evidence of the raccoon? Look in the middle of the photos for whitish marks, grouped in fours, separated by gaps, and each forming a backwards “C” pattern. Those are raccoon tracks, and it was galloping away from the scene of the crime (toward the viewer).

Raccoon-Galloping-Pattern-SCISo you don’t believe me, and need a close-up of that raccoon gallop pattern? Here you go. Both rear feet are left, both front feet are right, and the direction of movement was to the left; when both rear feet exceed the front, that’s a gallop, folks. Notice the straddle (width of the trackway) is a lot narrower than a typical raccoon trackway, which is what happens when it picks up speed. When it’s waddling more like a little bear, its trackway is a lot wider than this. Conclusion: this raccoon was running for its life.

Although this is the only time Gale has documented a raccoon preying on a horseshoe crab – and it is the first time I’ve ever heard of it – we of course now wonder whether this was an exception, or if it is more common that we previously supposed. The horseshoe crab was a gravid female, and was likely on the beach to lay its eggs. Did the raccoon somehow know this, and sought out this limulid so that – like many shorebirds – it could feast on the eggs, too, along with some of the horseshoe crab itself? Or was it opportunistic, in that it was out looking for sea-turtle eggs, saw the horseshoe crab, and thought it’d try something a little different? In other words, had it learned this from experience, or was it a one-time experiment?

All good questions, but when our data set is actually a datum set (n = 1), there’s not much more we can say about this now. But given this new knowledge, set of search patterns, and altered expectations, we’re more likely to see it again. Oh, and now that you know about this, so can you, gentle reader. Let us know if you see any similar story told on the sands of a Georgia beach.

You want one more reason why this was a very cool discovery? It shows how evolutionary lineages and habitats can collide. Horseshoe crabs are marine arthropods descended from a 450-million-year-old lineage, and likely have been coming up on beaches to spawn all through that time. In contrast, raccoons are relative newcomers, coming from a lineage of land-dwelling mammals (Procyonidae) that, at best, only goes back to Oligocene Epoch, about 25 million years ago. When did a horseshoe crab first go onto land and encounter a land-dwelling raccoon ancestor? Trace fossils might tell us someday, especially now that we know what to look for.

So once again, these life traces provided us with a little more novelty, adding another piece to the natural history of the Georgia coast. Moreover, a raccoon preying on a horseshoe crab was another reminder that even experienced people – like Gale, me, and others who have spent much time on the Georgia barrier islands – still have a lot more to learn. Be humble, keep eyes open, and let the traces teach you something new.

(Acknowledgement: Special thanks to Dr. Gale Bishop for again spotting something ichnologically weird on St. Catherines Island, documenting it, and sharing what he has seen during his many forays there.)

Horseshoe Crabs Are So Much More Awesome Than Mermaids

Given all of the controversy over a recent cable-TV program, in which its broadcasting channel decided mythical marine animals deserved more air-time than real ones, I thought it was important to highlight one extant animal that never fails to surprise me. This animal’s lineage is more ancient than dinosaurs, reptiles, or even amphibians, with its oldest fossils dating from about 450 million years ago. It is also the largest living marine invertebrate animal you are likely to see on beaches of the eastern U.S. and Gulf Coast. And at this time of year, if you see it crawling around on a beach, it’s because of sex. For the past month or so, this animal has been participating in massive orgies. Pictures of this gamete-laden frenzy somehow made it past prudish censors of Facebook and other social-media sites, titillating prurient invertebrate enthusiasts everywhere and filling them with cockle-warming glee.

Juvenile-Limulid-SapeloBehold, a fine juvenile specimen of the Atlantic horseshoe crab (Limulus polyphemus)! Although it lives in the ocean, it can walk on land for hours, like some sort of reverse Aquaman, but totally cooler than him. And some day, if this one lives long enough, it will use those legs to walk on land again, but in pursuit of sex. Sounds to me like this animal deserves its own planet. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

As you already know from reading the title of this post, I’m talking about horseshoe crabs. More properly known as limulids by real marine biologists and paleontologists, these ultra-cool, über-hip, but totally retro critters are more closely related to spiders than they are to true crabs, but their common name is so, well, common, that scientists just sigh and begrudgingly go along with it for the sake of public communication.

Modern limulids are represented by four species, three of which are in Asia, but the grandest of them all is the Atlantic horseshoe crab, Limulus polyphemus. This species is at its largest here in Georgia, which may be a function of the Georgia Bight, an extensive offshore shelf that affords more food and habitat than other areas. How big? I’ve seen some as long as 70 cm (27 in) – tail included – and 40 cm (16 in) wide, big enough to scare both of our cats at home. They grow to these sizes after hatching as little limulids not much bigger than the period on this sentence, an astonishing increase in mass if they make it to adulthood (which most don’t).

Baby-Limulid-TrailThe circuitous trail of a baby limulid, made on a sandflat at low tide. Its body width can be estimated by the width of the interior of the trail, and its body length was slightly more than that, meaning it was smaller than my fingernail. See that central groove? That’s from its tail, but if you want to impress your friends, call it a telson. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Horseshoe crabs are so astounding that I could go on endlessly about all sorts of facts about them. Fortunately for you, gentle reader, other folks have written entire books about them and heaps of popular and scientific articles. (For starters, try going here.) So I don’t want to needlessly duplicate what others have done, and done well. Instead, I’ll focus on my main interest in these animals – their traces – and will regale you with tales of the traces they can make with their tails.

Horseshoe crab tails are spiky projections called telsons. Based on lots of the traces I’ve seen on the Georgia coast and a few direct observations, the main function of a telson is to help a horseshoe crab to get back on its feet after being knocked onto its back. That is, whenever a limulid is upside-down, it immediately start using its telson as a sort of sideways pole vault to lever itself into a less vulnerable position.

Without a telson, an upside-down horseshoe crab is stuck; its legs run furiously, but to no avail. However, with a telson, it can put the pointy end into the sand or mud underneath its body, and push itself up from a surface. This gives a limulid a fighting chance to get back to where it once belonged and start walking. This strategy works best if it turns to its right or left side, as limulids are longer than wide. They may be wonders of nature, but they’re not doing back flips or somersaults.

Limulid-Telson-Windshield-Wiper-TraceA large adult horseshoe crab that was right-side-up when trying to get back to the sea, got tired, and tried to use its telson to move itself along. In this instance, it didn’t work, but the traces made by the telson show its range of motion, working like a windshield wiper. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

OK, all of the preceding information I already knew. After all, I have: coauthored an edited book chapter about juvenile limulid traces and their close resemblance to trace fossils made by trilobites; coauthored another article on the history of limulid-trace studies (which go back to the 1930s!) that’s now in review; and devoted a lengthy section of a chapter in my book to limulids as tracemakers. So you could say I’ve been feeling pretty cocky about what I knew about these animals as tracemakers. That is, until one horseshoe crab showed me how much I still need to learn about them and what they can make.

The humility-inspiring traces showed up in a photo on a Facebook page I follow (and so should you), the St. Catherines Island Sea Turtle Conservation Program. The program organizers – Gale Bishop and Robert (Kelly) Vance – regularly add photo albums showing sea turtle traces (trackways, body pits, nests), and otherwise report on other facets of natural history they observe on St. Catherines Island beaches. As a result, I live vicariously through these pictures while marooned in the metro-Atlanta area. But they also like to throw me ichnological stunners once in a while, such as the following photo that Kelly posted last week.

Limulid-Telson-Trace-1Who needs made-up animals on TV when traces like these, made by awesome invertebrates like horseshoe crabs, turn up on a Georgia beach? (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia; scale is about 15 cm (6 in) long.)

Kelly found these traces while patrolling the beaches of St. Catherines Island for other traces, namely those of expectant mother sea turtles. Although these distracted briefly from his mission, I was very happy he stopped to document these, as I had never seen anything like them, despite much looking at traces on Georgia beaches.

The holes in the sand, defining a nearly perfect circle, were made by the telson of an adult horseshoe crab that kept on trying to right itself after landing on its back. Each puncture mark shows where it inserted the telson into the sand and then pushed itself up and to its side. Based on the number of holes, direction of sand flung out of each hole, and little “commas” made by extraction of the telson, it tried to flip itself a minimum of 16 times, and all to the right. These separate actions culminated in a 360° clockwise rotation of its body. Also check out the central depression with smaller drag marks; this is where its head shield was in contact with the sand. To imagine the movement represented by these traces, think of a horseshoe crab doing a slow-motion, step-by-step, break-dance backspin.

Seeing the evidence for such persistence was wow-inducing in itself, but in my ichnologically influenced euphoria, I figured the limulid finally succeeded in righting itself. After all, the trackway just to the left of the trace, indicates where it walked away from the scene of its gravitationally challenged situation.

But then I realized there was no “impact mark.” This large horseshoe crab flipping itself onto the sandy surface should have registered an outline of its body before it started walking. Instead, the place where it started walking showed no such impression, meaning it must have made a soft landing, with only its legs and telson digging into the sand. What happened? Did it use mind over matter and levitate itself through telekinesis? Or was it gently picked up and placed on its feet by a merciful mermaid? (Or merman: let’s make sure we’re being inclusive when talking about made-up stuff.)

It turned out that Kelly was the dues ex machina that entered this limulid’s drama, providing divine intervention just when it was needed. When I expressed my puzzlement to Kelly about how this large arthropod finally turned itself over, he confessed to saving it, in which he lifted it and put it back on its feet, where it promptly walked away in a series of tight spirals. The spiraling is something I’ve seen before in their tracks, a method used to find the downslope direction, which normally leads horseshoe crabs to the low-tide mark and the comfort of a watery environment.

Limulid-Telson-Trace-2Another perspective of the “escape” traces made by the limulid’s telson (background), but this time with its tracks, showing how it started spiraling clockwise in an attempt to make its way back to the sea. Check out those telson drag marks in the trackway, doing a little bit of back-and-forth movement as its owner walked. (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia.)

Limulid-Telson-Trace-3OK everyone, start singing “Born Free!” The spiraling helped this limulid (arrow) to find a downslope direction, which took it in the right direction to the sea. But it’s not all sunshine and lollipops for other limulids, some of which are visible in the background, and look like they’re still stuck. Given the tidal range on the Georgia coast – 2.5-3 m (8.2-9.8 ft) – strong wave energy, and wide beaches, lots of big limulids that come in with the flood tide get knocked onto their backs by waves and left behind. It’s almost as if some sort of natural selection is taking place, and something similar might have happened in the geologic past, affecting the evolution of its lineage. (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia.)

In the last photograph, I was glad to see how the story told by these traces promised a happy ending for this limulid that had so stubbornly tried to put itself back on its feet. Yet when you also notice how many of its compatriots did not make it back into the life-nourishing sea, it also serves as a sobering reminder that storybook endings don’t always happen in nature, and what we wish to be true sometimes isn’t.

In this instance, I don’t know whether this horseshoe crab made it back into the sea to live another day or not. Still, the lesson it left for us in the sand lives on, and I am now slightly more confident that if any limulids were stuck on their backs at any point in their 450-million-year history, made similar traces with their tails, and these marks were preserved as trace fossils, we just might recognize them for what they are. For that alone, I am grateful. Thank you, horseshoe crabs, for being real, making traces, and continuing to share this planet with us today.

(Acknowledgement: Special thanks to Drs. Robert Kelly Vance and Gale Bishop for being my ichno-scouts on St. Catherines Island, and feeding my mind with such tasty treats while I am landlocked.)

Further Reading

Brockmann, H.J. 1990. Mating behavior of horseshoe crabs, Limulus polyphemus. Behaviour, 114: 206-220.

Martin, A.J. Life Traces of the Georgia Coast. Indiana University Press, Bloomington, Indiana, 692 p.

Martin, A.J., and Rindsberg, A.K. 2007. Arthropod tracemakers of Nereites? Neoichnological observations of juvenile limulids and their paleoichnological applications. In Miller, W.M., III (editor), Trace Fossils: Concepts, Problems, Prospects, Elsevier, Amsterdam: 478-491.

Shuster, C.N., Jr., Barlow, P.B., and Brockmann, H.J. (editors). 2003. The American Horseshoe Crab. Harvard University Press, Cambridge, Massachusetts: 427 p.