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.

How to Track a Vampire (Bat)

The arrival of Halloween reminds us to celebrate mythical creatures that frighten yet also intrigue us, although recent popular crazes have made this less of an annual event and more year-round. Along those lines, probably the top three of such imaginary beings are zombies, werewolves, and vampires. All of these can be classified as changelings of a sort, with two of them dead, but not really. Here in Georgia, public fascination with zombies has even provided employment opportunities, as many people compete for coveted slots as shuffling extras on the TV series The Walking Dead.

Among these inspirations for Halloween costumes, short stories, novels, musicals, TV shows, and movies, which would be the toughest for an aspiring Van Hesling to track down using ichnological methods? Zombies would be far too easy, considering their slow-moving, foot dragging, bipedal locomotion; their trackways would also commonly intersect as they bump into one another in their search for cranial sustenance. In other words, zombie trackway patterns would closely match those of people texting.

As a result, we have many modern analogs for zombie traces, which would also make their recognition in the fossil record that much easier. Traces made by the zombie-like characters portrayed in 28 Days Later, however, would be far different, showing greater distances between tracks and reflecting more rapid movement. (And all kidding aside, we actually do have trace fossil evidence of zombie ants from about 50 million years ago, an example of reality trumping fiction.)

Similarly, tracking werewolves would be straightforward, in that trackway patterns should show normal human bipedal locomotion followed by abrupt changes to quadrupedal patterns that would range from a trot to full gallop, gaits that are comparatively rare in humans. Anatomical details of tracks would also include a transition from five-toed plantigrade tracks to four-toed digitigrade ones, and metatarsal impressions would be replaced by heel-pad impressions. Additional traces to expect from a werewolf would be the direct effects of successful predation, such as blood spatters, scattering of prey body parts, toothmarks, and so on. (Don’t ask me about werewolf scat, though. I don’t even want to think about some of the things that would show up in that, especially if they started consuming suburbanites.)

Mixed assemblage of wolf and human tracks, which no doubt proves the existence of werewolves. Or not. Your choice. (Photograph by Anthony Martin, taken in Yellowstone National Park, Wyoming: scale = 10 cm (4 in).)

A closer look at those supposed “wolf” tracks. Yes, I know, they’re in the same area of Yellowstone National Park where a successful wolf-release program took place. But my doubt means you have to consider the impossible as equally valid.

A gorgeous “wolf” track with evidence of skidding to a halt and turning to the right. Could this have been made immediately after a human transformed into a wolf? My Magic 8-ball says, “Ask again later.”

Scene from some movie I’ll never see, in which one of the characters undergoes a mid-air transformation from a human to a werewolf (Canis lupus hormonensis), abruptly changing his tracks from a more plantigrade bipedal running to digitigrade quadrupedal movement. Sorry, I don’t know if any evidence of teen angst would preserve in such a trackway, nor do I care.

In contrast to zombies and werewolves, vampires would be the most challenging to track, considering their occasional aerial phases of movement, as depicted in Bram Stoker’s novel Dracula (1897) and various popular adaptations. Traces made during a pre-transformation phase – while still in human form – would be indistinguishable from those of a non-undead human, texting or not, and once in the air, no evidence of its movement would be recorded.

A large bat (megachiropteran) in flight, leaving no traces of its passing when traveling in a substrate of air.

So just to leave vampires for a moment, let’s talk about bats, which are real and do leave traces of their activities. Knowing that bats are among the most diverse and abundant of mammals (more than 1,200 species), I made sure to discuss their traces in my upcoming book, Life Traces of the Georgia Coast. Although I personally have not yet seen any of their traces on the Georgia barrier islands, these are predictable and identifiable, so I hold out hope that I or someone else will find them some day.

Probably the most likely traces made by bats that one could encounter on the Georgia barrier islands are their feces, which in other places, through the right geology (think caves) and collective action, can form economic resources (more on that later). About 75% of bat species are insectivores, and because they catch their meals on the fly, their scat will mostly contain winged insect parts. However, the geology of the Georgia barrier islands lacks limestone, and thus precludes the formation of caves or other environments serving as roosting spots for bat colonies. Thus bat feces, such as those dropped by the common brown bat (Myotis lucifugus), will be hard to find unless you look in the right place, such as below a favorite roosting spot. If you are lucky enough to notice these, though, these traces are dark 2-3 mm (0.1 in) wide and 5-15 mm (0.2-0.6 in) long cylinders and filled with parts of flying insects.

Two small samples of bat poop for you. You’re welcome. (Image from Internet Center for Wildlife Damage Management.)

Most other bats are fruit-eaters; this means these bats, like many birds, are also important seed dispersers through their excreting indigestible seeds covered in fertilizer. Speaking of fertilizers, massive deposits of bat feces (guano) also accumulate in caves and other places where millions of bats have roosted. These nitrogen- and phosphorous-rich deposits have been mined for fertilizers used in agriculture, an example of feeding traces helping to feed people.

Do bats come to the ground and leave tracks? Yes, once in a while they do, where they might forage and walk on all fours. When they do this, they make diagonal walking patterns, contacting with the thumbs on the tips of their wings – which are skin membranes connected to their other, elongated fingers – and their rear feet.

OK, now back to vampires, or rather, vampire bats. There are only three species of parasitic bats, all of which subsist on the blood of other mammals. For feeding, they slice skin with their sharp teeth, which leaves a small (several centimeters long, millimeters thin) incision. They then lap up whatever blood comes out, and the victim often isn’t aware of its blood loss. These wounds also heal, but leave visible scars.

What about other traces left by vampire bats? Surprisingly, scientists have actually asked themselves, “Hey, I wonder how vampire bats get around on the ground?”, and conducted experiments on terrestrial movement of the common vampire-bat (Desmodus rotundus), as well as the short-tailed bat of New Zealand (Mystacina tuberculata).

Just in case you needed another reason why science is cool, these scientists constructed bat-sized treadmills and placed these bats on them. This experiment confirmed that bats, including the common vampire bat, perform an alternating-walking movement in which the rear foot (pes) registers just behind the thumb, which also bears a claw. (This claw comes in handy as a sort of grappling hook at they climb onto their blood sources.)

Walking on Wings from Science News on Vimeo.

Based on this video, here is what I would hypothesize as the trackway pattern of a walking vampire bat. Note that the rear foot has five digits, nearly equal in length, and that the feet point away from the midline of the trackway.

But then they found out something most people didn’t expect. When they increased treadmill speeds, the bats bound and almost gallop, in which their rear feet nearly move past their wings. While bounding, these bats land on one of the digits on their wings, then push off with their rear feet, causing a suspension phase, reaching maximum speeds of 1.2 m/s. (Which, incidentally, is about the same speed as most people walking while texting, or slow zombies.) The resulting trackway patterns would be in sets of four – rear feet paired behind thumb impressions – separated from one another by about a body length. Based on my viewing of the videos, the trackways would show both half-bound and full-bound patterns, in which the rear feet are either offset or parallel, respectively.

Vampire Running from Science News on Vimeo.

And here is the hypothesized trackway pattern for a running vampire bat, which is almost like a gallop pattern, but more like a half-bound or full-bound. The feet actually should point a little more inward than during walking, and depending on the substrate, deformation structures might be associated with track exteriors.

Just to insert a little paleontology into this consideration of bat traces: has anyone found a trackway, feces, or other traces made by bast in the fossil record? No, unless you count old guano deposits as trace fossils (which I would if they exceed 10,000 years old). The body fossil record for bats extends back to the Eocene Epoch, about 50 million years ago, but such fossils are rare, too. Far more impressive than a bat body fossil, though, would be a fossil bat trackway would be the discovery of a lifetime, almost as noteworthy as finding an actual vampire. And if you found a fossil bat trackway where it was running? Time to start playing the lottery.

More readily available in ancient strata, though, are pterosaur tracks, whose makers likely walked in a manner similar to bats when on land. Hence bats, although not directly related to these flying reptiles, may provide analogues for how some small pterosaurs moved about when on the ground. Despite their long study and many pterosaur fossils, though, a few people are still arguing about how pterosaurs moved on the ground. So hopefully more studies of bat locomotion will help us to better understand the earthbound behaviors of pterosaurs.

The take-home message of the preceding is that even though zombies, werewolves, and vampires still garner plenty of attention from the public, the truth is that real animals of the past and present – like bats and pterosaurs – are actually more fantastic than we sometimes know. Sure, let’s continue to have fun with our mythical creatures, but in the meantime, also keep an eye out for traces left by the marvelous animals of today and yesteryear.

Further Reading

Elbroch, M. 2003. Mammal Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, Pennsylvania: 778 p.

Mazin, J.-M., Billon-Bruyat, J.-P., and Padian, K. 2009. First record of a pterosaur landing trackway. Proceedings of the Royal Society of London, B, 276: 3881-3886.

Padian, K., and Fallon, B. 2012. Meta-analysis of reported pterosaur trackways: testing the corrspondence between skeletal and footprint records. Journal of Vertebrate Paleontology, 32 [Supplement to 3]: 153.

Riskin, D.K. et al. 2006. Terrestrial locomotion of the New Zealand short-tailed bat Mystacina tuberculata and the common vampire bat Desmodus rotundus. Journal of Experimental Biology, 209: 1725-1736.

Of Sandhill-Crane Footprints and Dinosaurs Down Under

Last week, while in Athens, Georgia, I found myself musing about footprints from the barrier islands of Georgia and the Cretaceous rocks of Australia, despite their separation by half a world and more than 100 million years. These seemingly random thoughts came to me during a visit to the Department of Geology at the University of Georgia to give a lecture in their departmental seminar series.

It was a pleasure speaking at the geology department for many reasons, but perhaps the most gratifying was how it was also a homecoming. I had worked on my Ph.D. there in the late 1980’s, and in 1988-1989 had taught introductory-geology classes in the very same lecture hall where I gave my presentation. Several of my former professors, who were junior faculty then, are still there and now comprise a distinguished senior faculty. So seeing them there now, their smiling faces in the audience along with the latest generation of undergraduate and graduate students, generated all sorts of warm-and-fuzzy feelings.

But enough about the present: let’s go back about 100 million years to the Cretaceous Period, which was the subject of my talk. I had actually asked to speak about the modern Georgia barrier islands and their traces: you know, the main theme of this blog and my upcoming book of the same title (Life Traces of the Georgia Coast, just in case you need reminding). Nonetheless, my host and valued friend, paleontologist Dr. Sally Walker, figured that a summary of my latest research on the Cretaceous trace fossils of Victoria, Australia would bring in a wider audience, especially if I used the magical word “dinosaur” in the title (which I did).

For my talk at the UGA Department of Geology, I could have talked about this place – St. Catherines Island, Georgia – and it’s modern traces. After all, it’s only about a four-hour drive and short boat ride from Athens, Georgia.

But instead I talked about this place – coastal Victoria, Australia – and its trace fossils from more than 100 million years ago. Which wasn’t such a bad thing.

In retrospect, she was right, and I thoroughly enjoyed putting together an informative and (I thought) entertaining presentation that shared highlights of fossil discoveries from that part of Australia during the past five years. For the benefit of the students in the audience, basic geology was woven throughout the talk, as I included facets of sedimentology, stratigraphy, geochemistry, paleobotany, paleoclimatology, plate tectonics, evolution, history of science, field methods, and oh yes, dinosaurs. (If you are interested in hearing more about the science and personal experiences behind these recent findings in Australia, these are related in another blog of mine written previous to this one, The Great Cretaceous Walk.)

So how do the barrier islands of the Georgia coast and their animal traces relate to the Cretaceous of Australia? I mentioned the main reason briefly in my talk, but will elaborate more here: I likely owed one of my most important fossil discoveries in Australia to track-imprinted memories gained from field work on the Georgia coast. The fossil find, which happened in June 2010, was of about two dozen thin-toed theropod dinosaur tracks in Cretaceous rocks along the Victoria coast. These tracks represent the best assemblage of dinosaur tracks found thus far in southern Australia, and the largest collection of polar-dinosaur tracks in the Southern Hemisphere. Moreover, some of these tracks just happened to be about the same size and forms of footprints made by sandhill cranes (Grus canadensis).

Comparison between the footprint of a sandhill crane (Grus canadensis), made in moist sand next to a freshwater pond, St. Catherines Island, Georgia (top), and a footprint made by a theropod dinosaur about 105 million years ago on a river floodplain, Victoria, Australia (bottom). Notice the resemblance?

Sandhill cranes do not normally live on the Georgia barrier islands, and nearly all of them simply fly over or stop briefly during their annual migrations from south of Georgia to the Great Plains, or vice versa. However, at least a few have settled on St. Catherines Island, the same place on the Georgia coast where I recently studied gopher tortoise burrows. According to Jen Hilburn, the island ornithologist, some of these cranes found life so comfortable on the island that they stayed. This turned out to be fortunate for me, as I became familiar with their tracks after repeated visits to St. Catherines. Even though these tall, beautiful, and majestic birds restrict themselves to just one island year-round, St. Catherines is big enough to hold a wide variety of habitats and substrates, so I have seen their tracks in salt marshes, next to fresh-water ponds, and along dusty roads throughout the entire length of the island.

Who are you calling a “dinosaur”? A sandhill crane on St. Catherines Island graciously poses for its portrait, helping this ichnologist get a better idea of what an anatomically similar tracemaker might have looked like more than 100 million years ago.

Sandhill-crane trackway on the sandy substrate of a high salt marsh, St. Catherines Island, Georgia. In this environment, its tracks are accompanied by fiddler-crab burrows and feeding pellets, as well as the tracks and dig marks of raccoons hunting the fiddler crabs. Scale (toward the top of the photo) in centimeters.

So to make a long story short, while walking along the Victoria coast last year, I also carried with me mental picture of these tracks in Georgia. These images, I am sure, contributed to my stopping to look at a rock surface that held faint but nearly identical impressions made by dinosaurian feet on the once-soft sediments of a river floodplain. This is how ichnology is supposed to work, and it did.

A comparison between sandhill-crane tracks on the Georgia barrier islands and those of Cretaceous dinosaurs in Australia is actually not as far-fetched as one might think at first. For one, we now know that birds are dinosaurs, evolutionarily speaking. This formerly vague hypothesis is now a certainty, and is based on an ever-improving fossil record of feathered theropod dinosaurs, as well as studies from modern biology that show genetic and developmental affinities between modern birds and theropods. Even so, this idea is not new, either. For example, evolutionary biologist Thomas Huxley (1825-1895), friend and noted proponent of Charles Darwin, readily connected Archaeopteryx, the Late Jurassic bird (or dinosaur, depending on evolutionary perspective) with theropod dinosaurs.

Preceding Huxley, though, was one of the first scientists to formally apply ichnology to fossilized dinosaur tracks, Edward Hitchcock (1793-1864). Hitchcock interpreted the abundant dinosaur tracks of the Connecticut River Valley – many made by theropods – as those of large, flightless birds that lived before humans. Although he never made the evolutionary connection between dinosaurs and birds, his hypothesis reflected anatomical similarities between their feet.

A close-up look at sandhill crane feet while it takes a step. Notice the left foot has a little toe facing backwards, but off the ground. This is the equivalent of our “big toe,” also known as digit I, and it rarely registers in their tracks unless a crane walks in soft mud or sand. Instead, you will see impressions of the other three toes with clawmarks, and the middle toe normally makes the deepest mark.

Theropod dinosaurs, like many modern birds, mostly made three-toed tracks, a condition also called tridactyl. Although theropod tracks are occasionally confused with similar tracks made by ornithopod dinosaurs, they have the following traits: (1) three prominent, forward-facing digit impressions; (2) a footprint that is longer than wide; (3) angles of less than 90° between the outermost digits; and (4) well-defined clawmarks. One of the many changes that happened to bird feet as they evolved from non-avian theropods was the dropping of and rearward projection of their first digit (equivalent to our big toe). This condition was a great adaptation for grasping branches in trees and otherwise getting around off the ground. Bird tracks from the Cretaceous Period also tend to be wider than long, a function of the angles between the outermost toes becoming greater than 90°, and most of these also show the impression of a backward-pointing toe. Sandhill-crane footprint made in firm sand of a high salt marsh, St. Catherines Island, Georgia. Like many bird tracks, this one is wider than it is long, which is unlike most theropod dinosaur tracks. Still, these are very similar to tracks made by certain types of thin-toed theropod dinosaurs during the Cretaceous Period. Scale in centimeters.

Much later in their evolutionary history, though, some lineages of birds became either flightless or otherwise spent more time on the ground than in the trees, such as wading birds and shorebirds. These circumstances resulted in their first digit becoming reduced or absent, or vestigial. Violá, the tridactyl theropod-dinosaur footprint came back in style, so to speak, and now dinosaur ichnologists regularly study the tracks and behaviors of birds with such feet to better understand how their theropod relatives may have moved during the Mesozoic Era.

Comparison of a track made by a greater rhea (Rhea americana, right), which is a large flightless bird native to Argentina, to that of an equivalent-sized theropod dinosaur track (right). Both tracks have three forward-facing digits ending with sharp clawmarks and are longer than wide. Scale = 15 cm (6 in). The dinosaur track is a replica of an Early Jurassic theropod (from about 200 million years ago) from the western U.S. Photograph of the rhea track is by Anthony Martin, and of the dinosaur-track replica is by Ty Butler of Tylight™. Scale in the photo to the left = 15 cm (6 in).

Thus while writing the research paper on the dinosaur tracks, I kept in mind the comparison between sandhill-crane footprints in Georgia and the Australian dinosaur tracks. I also recalled how paleontologists had previously measured theropod skeletons – feet and rear limbs, specifically – and proposed a relationship between foot length and probable hip height.

Based on these studies, you can take a theropod track, multiply it by 4.0, and you get the approximate hip height of its trackmaker. When I applied this calculation to the Australian tracks, their hip heights ranged from about 25 to 60 centimeters (10-23 inches). The smallest of these dinosaurs I imagined as chicken-sized; perhaps these were juveniles of the larger ones. But what might be living today that would compare to the largest of the trackmakers? Immediately I thought of herons, but then it struck me that sandhill cranes provided a more apt analogy.

So I think you know where this is going. Adult sandhill-crane tracks are about 12 centimeters (4.7 inches) long, so if you apply the same formula for theropod-dinosaur tracks to them, their hip heights should be 48 centimeters (19 inches). Would this relationship also hold up on a modern dinosaur, such as a sandhill crane?

Just to satisfy my curiosity, I wrote to Jen Hilburn (St. Catherines Island) and asked her to do me a little favor: could she measure the hip height of a living, adult sandhill crane for me? Fortunately, Jen carried out my unusual request (she said it was not easy, so I definitely owe her), and she wrote back with an answer: 58 centimeters (22 inches). This wasn’t a perfect fit with regard to the footprint formula, but it certainly worked for the size of the Australian dinosaurs I had in mind as trackmakers. Based on my study of the Australian tracks, they were made by small ornithomimids, which likewise made thin-toed tridactyl tracks.

After thanking Jen, I delighted in explaining how her measurement of a Georgia-island-dwelling sandhill crane related to a dinosaur-track discovery on the other side of the world. Furthermore, in the Emory University press release that accompanied the publication of the dinosaur-track discovery in August 2011, the reporter (Carol Clark) used my analogy of the trackmakers as “…theropods ranging in size from a chicken to a large crane.”

Sandhill crane walking down a sand pile next to a fresh-water pond and maritime forest on St. Catherines Island, Georgia, and leaving lovely tracks for an ichnologist to study and keep in mind while tracking non-avian theropod dinosaurs.

Artist conception of Struthiomimus, a Late Cretaceous non-avian theropod dinosaur from western North America. Although not a perfect fit, the tracks of cranes and other similarly sized birds can be compared to those of ornithomimid dinosaurs to better discern the presence and behaviors of these dinosaurs. Artwork by Nobu Tamura and from Wikipedia Commons.

What other modern traces from the Georgia coast will contribute to our better understanding the fossil record? Time will tell, and I hope some day to again share those thoughts at my former home – the Department of Geology at the University of Georgia – with friends, students, and colleagues, new and old.

Further Reading

Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, PA: 456 p.

Forsberg, M. 2005. On Ancient Wings: The Sandhill Cranes of North America. Michael Foreberg Photography: 168 p.

Henderson, D.M. 2003. Footprints, trackways, and hip heights of bipedal dinosaurs: testing hip height predictions with computer models. Ichnos, 10: 99–114.

Johnsgard, P.A. 2011. Sandhill and Whooping Cranes: Ancient Voices over America’s Wetlands. University of Nebraska Press, Lincoln, NB: 184 p.

Lockley, M.G. 1991. Tracking Dinosaurs: A New Look at an Ancient World. Cambridge University Press, Cambridge, UK: 264 p.

Martin, A.J., Anthony J., Rich, T.H., Hall, M., Vickers-Rich, P., and Gonzalo Vazquez-Prokopec. 2011. A polar dinosaur-track assemblage from the Eumeralla Formation (Albian), Victoria, Australia. Alcheringa: An Australiasian Journal of Palaeontology, article online August 9, 2011. DOI: 10.1080/03115518.2011.597564