“Worm Burrows” as a Geological Cliché

Posted on November 7, 2011 by TMartin

This past week, I was privileged to have participated in a marvelous three-day field trip to the Triassic and Jurassic sedimentary rocks in and around St. George, Utah. The field trip, organized by paleontologist Andrew Milner and many others in association with the Society of Vertebrate Paleontology meeting in Las Vegas, Nevada, provided our enthusiastic group of nearly forty professional and amateur paleontologists with a grand geological tour of southern Utah and northern Arizona, along with the fantastic dinosaur tracksites in that area.

Foremost among these places where dinosaurs left their marks was one of the most incredible tracksites have seen anywhere, which, like Lark Quarry in Queensland, Australia, is enclosed within a building to protect it. This place, called the St. George Dinosaur Discovery Site at Johnson Farm, has one of the few sitting-dinosaur trace fossils known from the fossil record, along with the world’s best collection of dinosaur swimming tracks, rare examples of dinosaur tail-drag marks, hundreds of other dinosaur tracks, and thousands of invertebrate trace fossils. All were enthralling as detailed records of daily life in the Early Jurassic Period, from about 195 million years ago.

You would think on a field trip like this that Georgia – countering Ray Charles’ memorialized sentiment – would not be on my mind. Yet the modern traces made by living animals of the Georgia barrier islands habitually creep into my thoughts whenever I travel into the geological past. In this instance, the trigger for my thoughts of Georgia traces was through hearing other field-trip participants utter the most recurring of geological clichés connected to invertebrate trace fossils: “worm burrows.”

Invertebrate trace fossils (left) directly associated with theropod dinosaur footprints (right) from the Moenave Formation (Lower Jurassic), southern Utah. These trace fossils are probably the burrows of larval insects made in moist muddy sand, rather than burrows made by earthworms in soils. So don’t be calling them “worm burrows,” or else a baby kitten will get mildly scolded. (Photograph by Anthony Martin.)

Several people spontaneously spoke this ichnological banality as soon as they saw small burrows preserved in the rock, many of which were directly associated with the exquisitely preserved dinosaur tracks. This happened often enough (which is to say, twice) that I just had to call attention to this geological faux pas. “Stop saying ‘worm burrows’!” I said with mock outrage. I quickly followed my joking admonishment with a brief explanation of how most of the burrows were much more likely to be from insects, rather than worms. Traits of the burrows – such as scratchmarks and short, branching, angled tunnels – implied insect tracemakers, such as the larvae of beetles or flies.

Insect traces associated with dinosaur tracks should not be all that surprising to anyone. After all, insects originated in terrestrial environments about 400 million years ago, meaning they were more than halfway through their evolutionary history by the time these Jurassic trace fossils were made. I had seen many similar burrows made by insects on the Georgia barrier islands and elsewhere in Georgia, which gave me enough confidence to propose their more probable identity.

Insect burrows – probably made by “mud-loving” beetles – along the shore of a freshwater pond on Sapelo Island, Georgia. Notice the burrows are relatively younger than (cross-cut) two tail dragmarks made by resident alligators (Alligator mississippiensis). Sandal as scale, which is size 8 1/2 (men’s). (Photograph by Anthony Martin.)

Of course, once you draw attention to a word or phrase among friends that is guaranteed to provoke annoyance, you should expect them to bring it up more frequently later as fodder for their amusement. Indeed, this happened for the remainder of the field trip, and I did not disappoint my audience as I responded with histrionic cringing, flinching, and groaning each time we encountered more of these “worm burrows” in Triassic or Jurassic rocks and they were identified as such.

Look, worm burrows! Ha-ha! The beautiful invertebrate trace fossils, former burrows filled with white sand that contrasts from the surrounding hematite-stained sand, are also in the Moenave Formation (Lower Jurassic) of southern Utah. (Photograph by Anthony Martin.)

All frivolity aside, the point I was trying to make to my field-trip tormenters was this: whenever we look at sedimentary rocks formed in continental environments, and we happen to notice invertebrate trace fossils in those same rocks, we should think before speaking. In other words, we do better as paleontologists, geologists, or naturalists in general when we reexamine our neat, preconceived labels before applying them loudly and confidently to observed phenomena, and particularly with invertebrate trace fossils.

For example, even the word “burrow” can be too glib for interpreting certain invertebrate trace fossils. Many invertebrates do not move underneath a sedimentary surface but along it; traces of such movements are either trackways, which are made with legs and leave impressions of these, or trails, which are made by whole-body movement without legs, such as those formed by worms or snails.

In my experience, trackways and trails are often lumped in with burrows, despite possessing impressions made by legs, furrows, and levees. For example, some of the trace fossils we saw on the field trip were certainly trails, yet I heard these called “burrows” by a few people. Granted, this sort of confusion is actually more understandable than the “worm-burrow” mistake, because trails can segue into shallow horizontal burrows and vice-versa, or some “trails” actually can have tiny leg impressions, meaning they actually are trackways. Thus the distinction between these end members can become blurred quite easily if you don’t pay attention to the details of a given invertebrate trace.

Modern land snail (pulmonate gastropod) making a trail on surface of a coastal dune, Cumberland Island, Georgia; scale in centimeters. (Photo by Anthony Martin.)

Fossil trail, possibly made by a snail, on a former sand dune in the Navajo Formation (Lower Jurassic) of southern Utah. Research funding for scale. (Photograph by Anthony Martin.)

Insect burrow, probably made by a beetle larva, in which it changes from a shallow burrow to a trackway on the surface of a coastal dune, Little St. Simons Island, Georgia. Scale in millimeters. (Photograph by Anthony Martin.)

In the sands and muds of the Georgia barrier islands, insect burrows in particular have often caused me to keep quiet about what I think made them, versus what really made them. Many times I have seen a little lump at the end of a horizontal burrow, scooped up the tracemaker hiding underneath, and been surprised by what was there. Most of these tracemakers have turned out to be small adult beetles or beetle larvae of various species, but I can’t ever predict which life stage or species will be there based just on their traces. (At least, not yet.)

Shallow burrow with short branches in a coastal dune, Cumberland Island, Georgia. Gee, I wonder what worm made it?

Surprise! It was a tiny adult beetle, found at the end of the burrow. Didn’t see that coming, did you? Well, maybe you did after all of the pedantic foreshadowing. (Both photographs by Anthony Martin.)

As a result of these insect-inspired search images, embedded in my consciousness from years of looking at Georgia-coast insect traces, I cannot ever again look at trace fossils made in formerly terrestrial environments and simply say, “worm burrows,” at least with a clear scientific conscience or a straight face. Hence whenever I see similar burrows in sedimentary rocks that were formed in lakes, streams, or soils from the Devonian Period to the recent, my default hypothesis is “insect burrows,” rather than “worm burrows.” Is this always right? No, as some terrestrial trace fossils, such as Edaphichnium and Castrichnus, were almost certainly made by earthworms, and nematode worms may have formed others, like Cochlichnus. (Although Cochlichnus has also been linked with insect tracemakers – but that’s a another story for another day.) Nonetheless, saying “insect burrows” is more likely to be correct than the alternatives, and in science, it’s good practice to learn from your mistakes.

So geologists and paleontologists everywhere, I beseech you not to limit yourselves descriptively when you encounter the millions of lovely and varied invertebrate trace fossils in sedimentary rocks formed in terrestrial environments. The truth will set you free (or at least put you on parole), and these seemingly simple trace fossils will become more intriguing as you realize their full complexity and potential mystery. Call them something other than “worm burrows,” then see what happens.

Invertebrate trace fossils (burrows) in sandstone from the Moeanave Formation (Lower Jurassic) in St. George, Utah. Do they look a little different to you now that you’re ready to give them a different name than mere “worm burrows”? (Photograph by Anthony Martin.)

(Acknowledgements: Many thanks to Andrew Milner, Jim Kirkland, Tyler Birthisel, Martin Lockley, Brent Breithaupt, Neffra Matthews, and many others for their organizing a most excellent three-day field trip to the Triassic-Jurassic rocks of southern Utah and northern Arizona. We all learned heaps from this direct experience, and greatly appreciate the huge amount of time and effort put into preparing for the field trip.)

Further Reading

Milner, A.R.C., Harris, J.D., Lockley, M.G., Kirkland, J.I., and Matthews, N.A. 2009. Bird-like anatomy, posture, and behavior revealed by an Early Jurassic theropod dinosaur resting trace. PLoS One, 4(3): doi:10.1371/journal.pone.0004591.

Rindsberg, A.K., and Kopaska-Merkel, D. 2005. Treptichnus and Arenicolites from the Steven C. Minkin Paleozoic footprint site (Langsettian, Alabama, USA). In Buta, R. J., Rindsberg, A. K., and Kopaska-Merkel, D. C., eds., = Pennsylvanian Footprints in the Black Warrior Basin of Alabama, Alabama Paleontological Society Monograph No. 1: 121-141.

Smith, J.J., Hasiotis, S.T., Kraus, M.J., and Woody, D.T. 2008. Relationship of floodplain ichnocoenoses to paleopedology, paleohydrology, and paleoclimate in the Willwood Formation, Wyoming, during the Paleocene–Eocene thermal maximum. Palaios, 23: 683-699.

Verde, M., Ubilla, M., Jiménez, J.J., and Genise, J.F. 2006. A new earthworm trace fossil from paleosols: aestivation chambers from the Late Pleistocene Sopas Formation of Uruguay. Palaeogeography, Palaeoclimatology, Palaeoecology, 243: 339-347.

Ghost Crabs and Their Ghostly Traces

Posted on October 31, 2011 by TMartin

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading

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

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

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

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

Of Sandhill-Crane Footprints and Dinosaurs Down Under

Posted on October 24, 2011 by TMartin

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

Georgia Life Traces as Art and Science

Posted on October 17, 2011 by TMartin

This past Friday evening (October 14), Fernbank Museum of Natural History in Atlanta, Georgia hosted the official opening of Selections, a visual-art show themed on evolution, especially as it relates to Charles Darwin. Many other art shows or other creative ventures have revolved around evolutionary themes, especially in 2009, which marked the 150^th anniversary of On the Origin of Species and the 200^th of Darwin’s birth. But two aspects of this display make it distinctive: (1) it was planned more than two years in advance to accompany the traveling exhibit Darwin, on loan at Fernbank from the American Museum of Natural History; and (2) five of the eight participating artists, all local to the Atlanta area, are also scientists.

Other than once again disproving the notion that artists and scientists live in divergent intellectual realms, once lamented by C.P. Snow in 1969 (for a few other examples of how this false dichotomy is becoming less and less defensible, look here, here, here, here, and here), I am pleased to share that my wife Ruth Schowalter and I are two of the artists in this show. Seven drawings and paintings of ours are on display, with three of those collaborative works, in which we freely mixed scientific concepts with our respective artistic expressions.

Here I will focus on just one of those works, a collaborative piece titled Abstractions of a Rising Sea (2011). My reason for taking a closer look at this one exclusively is because of its having been visually inspired by plant and animal traces of the Georgia barrier islands. Also, in keeping with a Darwinian theme, it depicts how changing environments – in this case, rising sea level – can likewise impact the survival of species, thus affecting the types of traces that are formed and preserved in a given place.

Abstractions of a Rising Sea (2011), by Ruth Schowalter and Anthony Martin: watercolor on paper, 66 X 101 cm (26” X 40”), on display at Fernbank Museum of Natural History until January 1, 2012. But this isn’t just abstract art: it’s also a scientific hypothesis. How so? Please read on. (Photograph taken by Anthony Martin.)

Although this painting may look abstract to most viewers, given its strange, funky shapes and patterns expressed with a colorful palette, its basic elements actually embody an evidence-based prediction. The artwork design, shown below, originated as a conceptual drawing I made for my upcoming book, Life Traces of the Georgia Coast; in fact, it will be the last illustration in the book. The drawing, which I later scanned and modified slightly with Adobe Photoshop™, portrays a vertical sequence of traces made by plants and animals on a typical Georgia shoreline, but considerably altered as sea level went up along that shoreline. In short, it reflects my prognosis of how a coastal dune will become inundated by the sea over the next few decades, with traces of marine animals succeeding those of terrestrial plants and animals.

The original illustration that inspired the artwork, which I drew to portray the sequence of traces that would be made in a given place on the Georgia coast as sea level goes up in the next few hundred years. (Illustration by Anthony Martin.)

So if you’ll bear with me for a few minutes, here’s a more detailed explanation. The traces at the bottom of the illustration represent those of a coastal dune, with plant-root traces, insect burrows, and sea-turtle nests. Just above, those traces are replaced by the burrows of ghost crabs, which are semi-terrestrial animals, but dependent on the sea. A typical Y-shaped burrow of a ghost crab (Ocypode quadrata), viewed in longitudinal section in the eroded face of a coastal dune on Sapelo Island, Georgia. This formerly open burrow was filled from above by sand of a slightly different composition, making it easier to spot. But also note that it cuts across the layering (bedding) of the dune, showing that the crab burrow is relatively younger than the dune deposit. (Photograph by Anthony Martin.)

Next are burrows made by marine invertebrates that live in the intertidal and shallow subtidal areas of a beach, such as polychaete worms, sea cucumbers, and acorn worms.

A variety of abandoned polychaete worm burrows, all washed out of their original places by a vigorous waves and tides and found along a beach on Sapelo Island, Georgia. Although each burrow is distinctive, what they share are behavioral adaptations to living in sandy environments dominated by the surf, shown by their reinforced walls. All four species of worms also orient their burrows vertically, which helps prevent too-frequent exhumation. (Photograph by Anthony Martin.)

Accompanying these is a snail shell (lower third, center) with a drillhole, a cannibalism trace made when a moon snail preyed on its own kind.

Drillhole in the shell of a common moon snail (Neverita duplicata) caused by another moon snail, a trace of both predation and cannibalism: Sapelo Island, Georgia (Photograph by Anthony Martin.)

A broken clam shell to the right of the snail is a likewise a predation trace, but attributable to a seagull. (The bird flew up with the clam in its beak, dropped it onto a hard-packed beach sand at low tide, and dined on its freshly killed contents.)

Broken shell of the giant Atlantic cockle (Dinocardium robustum), caused by a sea gull that picked it up, flew with it, and dropped it onto a sandflat at low tide on Sapelo Island, Georgia. Scale in centimeters. (Photograph by Anthony Martin.)

The upper half of the figure is then dominated by traces of marine invertebrates that live fully submerged offshore, such as ghost shrimp and other crustaceans, other polychaete worms, sea urchins, and brittle stars.

Labeled version of the illustration, depicting an overall progression from onshore traces (bottom) to offshore traces (above). If this sequence of sand and mud were to fossilize, this is how paleontologists and geologists would interpret it. (Illustration by Anthony Martin.)

The preceding artistic-scientific deconstruction should also help a viewer to better understand how geologists think when they look at a vertical sequence of sedimentary rock. For example, geologists follow several basic principles when trying to figure out the relative timing of different events in the geologic past.

One of these is called superposition, in which the effects of the oldest (first occurring) event in a given sequence of sedimentary rock are at the bottom, and the effects of subsequent events are recorded in progressively younger rocks toward the top.

The second principle is cross-cutting relationships, in that whatever is cutting across a previously existing structure must be younger than it. Think about how an animal burrow may cut across burrows made by previous generations of animals, and how you could unravel the sequence of “burrowing events” by simply observing which intersects which burrow.

A third principle is Walther’s Law, named after German geologist Johannes Walther (1960-1937) which states (more-or-less) that laterally adjacent environments succeed one another vertically. In other words, where a maritime forest and coastal dune are next to one another today on the Georgia coast, a drop in sea level means that coastal dunes might by succeeded vertically by the forest. Conversely, sea level going up implies that sediments of offshore environments, which are currently next to the beach and dunes, will some day overlie those of the dune.

Hence the illustration shows all three principles at play with a rising sea. For example, ghost-crab burrows cut across a sea-turtle nest from above, vertical burrows of a polychaete worm in turn dissect ghost-crab burrows below them, and a ghost-shrimp burrow from above interrupts one limb of a U-shaped acorn-worm burrow. Even better, a trained ichnologist can look at this sequence of traces and discern the environmental change that happened over the time represented by the sediments.

You can test this supposition by showing the illustration to other ichnologists, and I predict they will say, “Looks like sea level went up.” As a result, seemingly abstract patterns can become meaningful as we apply these images within the context of time passing, a concept we think Darwin – as a geologist and biologist – would have appreciated.

When I first showed this illustration to Ruth, she was quite taken by its forms and compositions, and she imagined what it would look like made much larger and in color. So we got to work on it, purposefully choosing a large piece of watercolor paper, onto which I drew the ichnological design. She then composed the color scheme, using a combination of water-color pencils and brushes, and I painted in a few details here and there, but most of the hard work was hers.

Ruth and my artistic styles are quite different – she’s a visionary artist, whereas I’m a more of a surrealist – but we both agree that meaningful art should provoke thought. So we very much like how this artwork also addresses and combines two contentious issues in American society: evolutionary theory and global-climate change. In Georgia, as in many other places in the U.S., scientists and science-educators still encounter resistance to the teaching of evolution, despite its extensive testing during the past 150 years and its consequent acceptance by virtually all scientists worldwide. Likewise, in recent years, so-called “global-warming deniers” have put much effort into rebuffing, ignoring, or otherwise downplaying the effects of human-caused climate change – despite near-universal scientific consensus – resulting in the twisting of scientists’ words or outright censorship.

For the plants, animals, and people who live on the Georgia coast, politically charged arguments become pointless as the shoreline moves up and over the land. As global climate continues to change and sea level goes up along the Georgia coast, how will life respond to these changes, especially if the sea rises faster than most organisms can adapt? This is a question we could have put to Charles Darwin, and one we attempt to pose through this synthesis of art and science.

(Acknowledgements to my wife and art-science collaborator, Ruth Schowalter, for her invaluable input on this post: thank you! Selections, featuring the artwork discussed here as well as others by us and six other artists, will be showing at Fernbank Museum of Natural History in Atlanta, Georgia until January 1, 2012. Admission to the museum includes viewing of the artwork, permanent exhibits, and the Darwin exhibit.)

Further Reading

Pilkey, O.H., and Fraser, M.E., 2005. A Celebration of the World’s Barrier Islands. Columbia University Press, New York: 400 p.

Purcell, W.S., and Gould, S.J., 2000. Crossing Over: Where Art and Science Meet. Three Rivers Press, New York: 159 p.

Trusler, P., Vickers-Rich, P., and Rich, T.H., 2010. The Artist and the Scientists: Bringing Prehistory to Life. Cambridge University Press, Cambridge, U.K.: 320 p.

Gopher Tortoises, Making Deep and Meaningful Burrows

Posted on October 10, 2011 by TMartin

As I wrote this post, I was flying from Atlanta, Georgia to Minneapolis, Minnesota to attend the annual meeting of the Geological Society of America (GSA), where I’ll be with about 7-8,000 geoscientists from across and outside of the U.S. Why am I not doing something else, such as field work on the Georgia coast? Well, other than to learn the latest of what’s happening in the world of geology, seeing old friends, and meeting new ones, I’m here to share new scientific knowledge coming out of the Georgia coast with my fellow geologists and paleontologists. The subject of the presentation I will give tomorrow – Tuesday, October 11 – is about the wondrous burrows of a humble-looking, slow-moving, and seemingly lethargic reptile that actually is an ichnological force of nature: the gopher tortoise (Gopherus polyphemus).

A gopher tortoise in captivity, but living a safe and happy life at the 4-H Tidelands Nature Center on Jekyll Island, Georgia. Although it may not look like a big deal, it is a very impressive tracemaker, deserving the rapt attention of geologists and paleontologists. (Photograph taken by Anthony Martin.)

So you’re probably wondering why geologists and paleontologists should hear about gopher tortoises from me. It’s a good question, because I’m not a biologist, and these animals are famous for their very important role in ecosystems. Specifically, they are well known as keystone species in the sandy soils of longleaf pine-wiregrass communities of the southeastern U.S. Just like the keystone to a building, once you remove gopher tortoises from their ecosystems, a lot of other species disappear with it. Surprisingly, their ecological worth all revolves around their burrows.

And oh, what marvelous and grandiose burrows they make! The lengthiest of their measured burrows approach 14 meters (45 feet) long and as much as 6 meters (20 feet) vertically below the ground surface. These burrows commonly twist to the right or left on their way down, which probably helps protect its tortoise occupant against predators, while maintaining a constant temperature and humidity in the burrow. With so much digging, of course, a lot of sand has to be excavated, so the locations of their burrows are easily spotted by looking for piles of sand in the middle of a grassy field or in a longleaf-pine forest. For female tortoises, these sand piles also serve as nesting sites, where they bury their eggs to incubate.

Satellite view of gopher-tortoise burrows on St. Catherines Island, Georgia. Nearly all of the white spots you see in the photo – indicated by the yellow arrows – are the sand piles (aprons) outside of their burrows. Look closely, and you can see some of the trails worn down by tortoises traveling between burrows. Yes, these are animal traces you can see from space! (Original image from the U.S. Geological Survey and Google Earth, taken in May 2008.)

Close-up view of a sand apron outside of a gopher-tortoise burrow entrance. The large amount of sand tells you that this must be a very deep burrow. Field notebook is about 15 cm (6 in) long. (Photograph taken by Anthony Martin on St. Catherines Island, Georgia.)

In cross-section, their burrows have flat bottoms and rounded tops, similar to a tortoise body. Burrow widths varies with the length of the tortoise, as it needs to be wide enough for the tortoise to turn around in the burrow. So this means a 30-cm (12 in) wide burrow can accommodate a tortoise of that length or less. The powerful front limbs of tortoises are specially adapted for digging, ending in flat, spade-like feet with stout claws. Burrow walls are compacted by the hard shell of the tortoise as it moves up and down the burrow. These burrows descend steeply, at angles of 20-40°, which means they have to be good climbers to get out of their deep burrows.

Down-tunnel view of a gopher-tortoise burrow, with the light at the end of that tunnel not from an oncoming train, but reflected morning sunlight on the tunnel wall at one of its turns. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Now think about a tunnel that’s about 10 m (33 ft) long and 30 cm (12 in) wide, and how much space that represents underneath the ground, and you’ll see what I mean about the vital role of these burrows ecologically, geologically, and (most importantly) ichnologically. In terms of ecology, about 200-300 species of invertebrate and vertebrate animals cohabit these burrows (whether a gopher tortoise is in it or not), including the longest snake in North America, the eastern indigo snake (Drymarchon couperi), the secretive gopher frog (Rana capito), the Florida mouse (Podomys floridanus), and a bunch of different insects. At least a few of the insects and the Florida mice make their own burrows, thus adding their little homes to the main burrow, like small anterooms to a big mansion.

Idealized conceptual sketch showing a cut-away view through a gopher-tortoise burrow with many additional burrows made by other animal species. Note especially the short horizontal tunnels near the burrow top, which would have been made by hatchling tortoises, and the vertical shafts that connect to these, which would have been made by Florida mice. (Illustration by Anthony Martin.)

So now you can see why this ichnologist (that would be me) became rather enamored with these burrows. For one thing, they have great preservation potential in the fossil record. A general rule in ichnology for the preservation of burrows is “deeper is better,” in that burrows that go to great depths are less likely to be eroded by surface weathering and erosion, and more likely to be fossilized. Secondly, we know that vertebrate animals in the geologic past also made big burrows, such as synapsids and even small dinosaurs. I’ve done research on the few dinosaur burrows interpreted from the geologic record, and am especially interested in how such large burrows might compare with similar burrows made by modern animals, such as gopher tortoises.

But how to study these burrows without digging them out and leaving the tortoises undisturbed? Fortunately, two colleagues of mine at Georgia Southern University – Sheldon Skaggs and Robert (Kelly) Vance – came up with an elegant solution, which was to use ground-penetrating radar, also known by its acronym of GPR. This method uses a portable unit to transmit microwaves underground (don’t worry, not these aren’t intense enough to cook the tortoises), which reflect off surfaces with different qualities, especially the curved, compacted surfaces of burrow walls. Computers then process and render these reflections into three-dimensional images that more-or-less represent the forms and geometries of the burrows.

Sure enough, we tried out this technique on gopher-tortoise burrows on St. Catherines Island of the Georgia coast in January and July this year. Although we can’t share all of our results just yet, we did successfully make three-dimensional images of the burrows, all without us having to burrow ourselves, or bother the tortoises by becoming homewreckers. Veronica Greco, a wildlife biologist on St. Catherines Island who has studied the behavior and breeding of the tortoises, also helped us to better understand the biology of these reptiles.

Although it looks like Sheldon (center) is mowing the lawn and I’m (right) just supervising, he’s actually pushing a portable ground-penetrating radar (GPR) unit over a field that has some gopher-tortoise burrows in it, while I walk alongside to look at the reflection profiles. Kelly (background) is no doubt monitoring our every move, but is also recording our location. (Photograph by Ruth Schowalter, taken on St. Catherines Island, Georgia.)

My talk at the GSA meeting will be about how we used GPR to study the burrows in a non-invasive way, and how our results might be applied to studying similar burrows in the fossil record. After the meeting is over, we plan to summarize our results in a research article, which we’ll submit to a journal later this year for peer review.

Unfortunately, gopher tortoises are endangered because of huge losses in acreage of longleaf-pine forests in the southeastern U.S. during the past 200 years or so. Knowing this makes our study of their burrows even more meaningful, for if these wonderful tracemakers go extinct in the near future, we will not have the chance to study them and their burrows. In this sense then, only geologists and paleontologists who know about their ichnology through studies like ours will be able to study their burrows, which would be a sad thing indeed. Let’s hope they survive and thrive, and we can continue to learn more about these superb burrowing animals and their traces.

(P.S. Many thanks to the St. Catherines Island Foundation for their support of our research!)

Further Reading

Aresco, M.J., 1999. Habitat structures associated with juvenile gopher tortoise burrows on pine plantations in Alabama. Chelonian Conservation and Biology, 3: 507-509.

Doonan, T.J., and Stout, I.J., 1994. Effects of gopher tortoise (Gopherus polyphemus) body size on burrow structure. American Midland Naturalist, 131: 273-280.

Epperson, D.M., and Heise, C.D., 2003. Nesting and hatchling ecology of gopher tortoises (Gopherus polyphemus) in southern Mississippi. Journal of Herpetology, 37: 315-324.

Guyer, C., and Hermann, S.M. 1997. Patterns of size and longevity for gopher tortoise burrows: implications for the longleaf pine-wiregrass ecosystem. Bulletin of the Ecological Society of America, 78: 254.

Jackson, D.R. and Milstrey, E.R. 1989. The fauna of gopher tortoise burrows. In Diemer, J.E. (editor), Proceedings of the Gopher Tortoise Relocation Symposium, State of Florida, Game and Freshwater Fish Commission, Tallahassee, Florida: 86-98.

Jones, C.A., and Franz, R. 1990. Use of gopher tortoise burrows by Florida mice (Podomys floridanus) in Putnam County, Florida. Florida Field Naturalist, 18: 45-68.

Lips, K.R. 1991. Vertebrates associated with tortoise (Gopherus polyphemus) burrows in four habitats in south central Florida. Journal of Herpetology, 25: 477-481.

Martin, A.J., Skaggs, S.A., Vance, R.K., and Greco, V. 2011. Ground-penetrating radar investigation of gopher-tortoise burrows: refining the characterization of modern vertebrate burrows and associated commensal traces. Geological Society of America Abstracts with Programs, 43(5): 381.

Varricchio, D.J., Martin, A.J., and Katsura, Y. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society of London, B, 274: 1361-1368.

Witz, B.W., and Wilson, D.S., and Palmer, M.D. 1991. Distribution of Gopherus polyphemus and its vertebrate symbionts in three burrow categories. American Midland Naturalist, 126: 152-158.

The Lost Barrier Islands of Georgia

Posted on October 3, 2011 by TMartin

The Georgia coast is well known for its historic role in the development of modern ecology, starting in the 1950s and ongoing today. But what about geologists? Fortunately, they were not long behind the ecologists, starting their research projects on Sapelo Island and other Georgia barrier islands in the early 1960s. Indeed, through that seminal work and investigations afterwards, these islands are now renown for the insights they bestowed on our understanding of sedimentary geology.

Why would geologists be attracted to these islands made of shifting sand and mud that were nearly devoid of anything resembling a rock? Well, before sedimentary rocks can be made, sediments are needed, and those sediments must get deposited before solidifying into rock. So these geologists were interested in learning how the modern sands and muds of the barrier islands were deposited, eroded, or otherwise moved in coastal environments, a dynamism that can be watched and studied every day along any Georgia shoreline. The products of this sediment movement were sedimentary structures, which were either from physical processes – such as wind, waves, or tides – or biological processes, such as burrowing. Hence sedimentary structures can be classified as either physical or biogenic, respectively.

Cabretta Beach on Sapelo Island at low tide, its sandflat adorned with beautiful ripples and many traces of animal life. Sand is abundant here because of a nearby tidal channel and strong ebb-tide currents that tend to deposit more sand than in other places around the island. This sand, in turn, provides lots of places for animals that live on or in the sand, making trails and burrows, demonstrating how ecology and geology intersect through ichnology, the study of traces. Speaking of traces, what are all of those dark “pipes” sticking out of that sandy surface? Hmmm… (Photograph by Anthony Martin.)

These geologists in the 1960s were among the first people in North America to apply what they observed in modern environments to ancient sedimentary deposits, and just like the ecologists, they did this right here in Georgia. For example, in 1964, a few of these geologists – John H. Hoyt, Robert J. Weimer, and V.J. (“Jim”) Henry – used a combination of: geology, which involved looking at physical sedimentary structures and the sediments themselves; modern traces made by coastal Georgia animals; and trace fossils. Through this integrated approach, they successfully showed that the long, linear sand ridges in southeastern Georgia were actually former dunes and beaches of ancient barrier islands.

These sand ridges, barely discernible rises on a mostly flat coastal plain, are southwest-northeast trending and more-or-less parallel to the present-day shoreline. Remarkably, these ridges denote the positions of sea-level highs during the last few million years on the Georgia coastal plain. The geologists applied colorful Native American and colonial names to each of these island systems – Wicomico, Penholoway, Talbot, Pamlico, Princess Anne, and Silver Bluff – with the most inland system reflecting the highest sea level. So how did these geologists figure out that a bunch of sand hills were actually lost barrier islands? And what does this all of this have to do with traces and trace fossils?

Map showing positions of sand ridges that represent ancient barrier islands, with each ridge marking the fomer position of the seashore. The one farthest west (Wicomico) represents the highest sea level reached in the past few million years, whereas the current barrier islands reflect an overlapping of two positions of sea level, one from about 40,000 years ago (Silver Bluff), and the other happening now. (Photograph by Anthony Martin, taken of a display at the Sapelo Island Visitor Center.)

Here’s how they did it. They first observed modern traces on Georgia shorelines that were burrows made by ghost shrimp, also known by biologists as callianassid shrimp. On a sandy beach surface, the tops of these burrows look like small shield volcanoes, and a burrow occupied by a ghost shrimp will complete that allusion by “erupting” water and fecal pellets through a narrow aperture.

Top of a typical callianassid shrimp burrow, looking much like a little volcano and adorned by fecal pellets, which coincidentally resemble “chocolate sprinkles,” but will likely disappoint if you do a taste test. (Photograph by Anthony Martin, taken on St. Catherines Island.)

A couple of ghost shrimp, which are either a male-female pair of Carolina ghost shrimp (Callichirus major) or a Carolina ghost shrimp and a Georgia ghost shrimp (Biffarius biformis). Sorry I can’t be more accurate, but I’m an ichnologist, not a biologist (although I could easily play either role on TV). Regardless, notice they have big claws, which they use as their main “digging tools.” The tracemakers look a little displeased about being outside of their protective burrow environments, but be assured I thanked them for their contribution to science, and promptly threw them back in the water so they could burrow again. Scale = 1 cm (0.4 in) (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Just below the beach surface, these interior shafts widen considerably, making these burrows look more like wine bottles than volcanoes. This widening accommodates the ghost shrimp, which moves up and down the shaft to irrigate its burrow by pumping out its unwanted feces (understandable, that) and circulating oxygenated water into the burrow. Balls of muddy sand reinforce the burrow walls like bricks in a house, stuck together by shrimp spit, and the burrow interior is lined with a smooth wall of packed mud.

A small portion of a ghost-shrimp burrow, showing its wall reinforced by rounded pellets of sand and stuck together with that field-tested and all-natural adhesive, shrimp spit. Photograph by Anthony Martin, taken on Sapelo Island.

Amazingly, these shafts descend vertically far below the beach, as much as 2-3 meters (6.5-10 feet) deep. Here they turn horizontal, oblique, and vertical, and tunnels intersect, branch, and otherwise look like a complex tangle of piping, perhaps reminding baby-boomers of “jungle gyms” that they used to enjoy as children in a pre-litigation world. Who knows what goes on down there in such adjoining ghost-shrimp burrow complexes, away from prying human eyes?

The deeper part of a modern ghost-shrimp burrow, exposed by erosion along a shoreline and revealing the more complex horizontally oriented and branching networks. Gee, do you think these burrows might have good fossilization potential? (Photograph by Anthony Martin, taken on Sapelo Island.)

See all of those burrow entrances on this sandy beach? Now imagine them all connecting in complex networks below your feet the next time you’re walking along a beach. Feels a little different knowing that, doesn’t it? (Photograph by Anthony Martin, taken on Sapelo Island.)

Interestingly, these burrows are definitely restricted to the shallow intertidal and subtidal environments of the Georgia coast, and their openings are visible at low tide on nearly every Georgia beach. Hence if you found similar burrows in the geologic record, you could reasonably infer where you were with respect to the ancient shoreline.

I think you now know where this is going, and how the geologists figured out what geologic processes were responsible for the sand ridges on the Georgia coastal plain. Before doing field work in those area, the geologists may have already suspected that these sandhills were associated with former shorelines. So with such a hypothesis in mind, they must have been thrilled to find fossil burrows preserved in the ancient sand deposits that matched modern ghost-shrimp burrows they had seen on the Georgia coast. They also found these fossil burrows in Pleistocene-age deposits on Sapelo Island, which helped them to know where the shoreline was located about 40,000 years ago with respect to the present-day one. This is when geologists started realizing that the Georgia barrier islands were made of both Pleistocene and modern sediments as amalgams of two shorelines, and hence unlike any other known barrier islands in the world.

Vertical shaft of a modern ghost-shrimp burrow eroding out of a shoreline on Cabretta Beach, Sapelo Island. Scale in centimeters. (Photograph by Anthony Martin.)

Vertical shaft of a fossil ghost-shrimp burrow eroding out of an outcrop in what is now maritime forest on Sapelo Island, but we know used to be a shoreline because of the presence of this trace fossil. Scale in centimeters. (Photograph by Anthony Martin.)

Geology and ecology combined further later in the 1960s, when paleontologists who also were well trained in biology began looking at how organisms, such as ghost shrimp, ghost crabs, marine worms, and many other animals changed coastal sediments through their behavior. So were these scientists considered geologists, biologists, or ecologists? They were actually greater than the sum of their parts: they were ichnologists. And what they found through their studies of modern traces on the Georgia barrier islands made them even more scientifically famous, and these places became recognized worldwide as among the best for comparing modern traces with trace fossils.

Further Reading:

Hoyt, J.H., and Hails, J.R. 1967. Pleistocene shoreline sediments in coastal Georgia: deposition and modification. Science, 155: 1541-1543.

Hoyt, J.H., Weimer, R.J., and Henry, V.J., Jr. 1964. Late Pleistocene and recent sedimentation on the central Georgia coast, U.S.A. In van Straaten, L.M.J.U. (editor), Deltaic and Shallow Marine Deposits, Developments in Sedimentology I. Elsevier, Amsterdam: 170-176.

Weimer, R.J., and Hoyt, J.H. 1964. Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments. Journal of Paleontology, 38: 761-767.

Georgia Salt Marshes: Places Filled with Traces

Posted on September 26, 2011 by TMartin

The Georgia coast has long captured the attention of scientists interested in its biological and geological systems and how these two realms overlap. For example, starting in the 1950s, ecologists – people who study the connections between living and non-living things in ecosystems – began investigating the exchange of energy and matter between the plants and animals of the Georgia barrier islands. In particular, they were interested in the Georgia salt marshes, most of which are between the mainland and the upland portions of the barrier islands. Why study salt marshes in Georgia, and not somewhere else? And how do the traces of plants and animals in these marshes, such as root disturbances, scrapings, burrows, and feces, actually play a major role in the functioning of these ecosystems?

The muddy bank of a tidal creek in a typical salt marsh on St. Catherines Island, Georgia. See the traces? No? It’s a trick question: it’s made of nothing but traces. Photograph by Anthony Martin.

Georgia salt marshes are flat, extensive coastal “prairies” dominated by a tall, marine-adapted grass, smooth cordgrass (Spartina alterniflora) in their lowermost parts. These ecosystems turned out to be fantastic places for scientists to study basic principles of ecology, and are among the most productive of all ecosystems, besting or equaling tropical rain forests in this respect. Georgia salt marshes also represent about one-third of all salt marshes in the eastern U.S. by area. How did this happen?

Such an unusual concentration of salt marshes along the relatively small Georgia coastline is a result of several factors. One is its semi-tropical climate, only rarely dipping below freezing, which allows marsh plants and animals to thrive and actively participate in their ecosystems nearly year-round. Another is the high tidal range of the Georgia coast of about 2.5-3 meters (8-10 feet), which causes enormous amounts of organic material – living and nonliving – to get cycled in and out of the marshes by this moving water.

A third reason, and perhaps the most important, is what people did not do to the marshes, which was to develop them in ways that would have completely altered their original ecological characters. (Take a look at the barrier islands of New Jersey as examples of what could have happened in Georgia.) Salt marshes that were not drained, filled in, paved over, or otherwise irreparably altered could be studied for what they were, not what we supposed.

Salt marsh and tidal creek adjacent to a maritime forest on Cumberland Island. Fortunately, this is a typical sight on the Georgia barrier islands, which gladdens ecologists and lots of other people who prefer to see their landscapes unpaved.

The scientists interested in the Georgia salt marshes, among them Eugene (“Gene”) Odum, Mildred Teal, and John Teal, were astonished by the amount of organic matter produced in these marshes, especially in their lower parts, which were appropriately called low marsh. Amazingly, much of this flux is controlled by tides and just five species of organisms you can easily see any given day in these marshes:

Smooth cordgrass (Spartina alterniflora)
Ribbed mussels (Geukensia demissa)
Eastern oysters (Crassostrea virginica)
Marsh periwinkles (Littoraria irrorata)
Mud fiddler crabs (Uca pugnax)

Just to oversimplify matters, but to assure that you get the big picture, the flow of matter and energy goes like this. Smooth cordgrass is the primary producer of organic material in the salt marshes, converting sunlight into food for it and, as it turns out, lots of other organisms. This is relatively easy for these plants because they are powered by intense Georgia sunlight much of the year. Smooth cordgrass also has extensive and complicated root systems, which help to hold most of the marsh muds in place when marshes are flushed by the tides. These roots locally change the chemistry of the surrounding mud and otherwise leave visible traces of their deeply penetrating networks, which are noticeable long after the plants had died and decayed.

Cross-section of a relict salt marsh preserved on Sapelo Island, Georgia, buried for about 500 years but just now being exhumed by shoreline erosion. See how deeply those roots of smooth cordgrass (Spartina alterniflora) penetrate the mud and still hold it in place? Modern ones do the same thing. Scale = 15 cm (6 in).

What produces the mud in a marsh? Mostly the ribbed mussels, oysters, and similar suspension-feeding animals, which: suck in water made cloudy by suspended clays; consume any useful organics that might be in that water; and excrete massive amounts of mud-filled feces, packaged with mucous as sand-sized particles. The oysters, along with cordgrass roots, stabilize the banks of tidal creeks, keeping these from washing away with each ebb tide.

If you’ve ever wondered what ribbed mussel (Geukensia demissa) feces look like, you’re in luck. Each one is only about 1 millimeter (0.04 inches) across, which makes them behave more like sand instead of much tinier clay particles. Also think of them as little packets of mud shrink-wrapped by mucous. Illustration by Anthony Martin, based on a figure by Smith and Frey (1985).

A view of what used to be a marsh surface – the relict marsh on Sapelo Island, that is – with stubs of long-dead smooth cordgrass accompanied by equally long-dead clusters of ribbed mussels (Geukensia demissa). Back in the day (about 500 years ago), these mussels were happily pumping out mud-filled feces, and their modern descendants are still doing the same thing. Sandal (left) is size 8½ (mens). Photograph by Anthony Martin.

Prominent clumps of eastern oysters (Crassostrea virginica), exposed at low tide in the middle of a tidal creek on Sapelo Island. These not only help to produce mud, but keep it in place, while also slowing down flow and helping to deposit mud. Photograph by Anthony Martin.

A close-up look at more oysters surrounded by smooth cordgrass, with both working together to bind and accumulate mud on Sapelo Island. Now that’s ecological teamwork! Photograph by Anthony Martin.

Both the cordgrass and oysters also baffle and otherwise slow down the water flow, causing mud – fecal or otherwise – to get deposited. In short, a Georgia salt marsh with its thick deposits of beautifully dark, rich, gooey mud, much of which consists of the traces of mussels and oysters, would cease to exist without these bivalves and smooth cordgrass, and would become more like an open lagoon.

Meanwhile, marsh periwinkles are constantly moving up and down the stalks and leaves of the smooth cordgrass, grazing on algae growing on the cordgrass. This activity causes visible damage to the plants, tearing them into small bits and pieces that fall onto the marsh surface.

Marsh periwinkles (Littoraria irrorata) doing what they do best, which is graze on the stalks and leaves of smooth cordgrass, leaving many traces from damaging these plants while also contributing plant debris to the marsh surface. Photograph by Anthony Martin, taken on Sapelo Island.

Fungi and bacteria further break down this “gentle rain from heaven” of cordgrass debris once it reaches the marsh surface. Here, mud fiddler crabs consume this stuff, along with any algae that might be growing on marsh surfaces. Their scrape marks and discarded balls of processed sediment are everywhere to be seen on the marsh surface and add to the sediment load of a marsh. Furthermore, as we may have learned in grade school, all animals poop, so in this way these fiddler crabs and other species of crabs living in the salt marshes donate even more enriched organic material. They also dig millions of burrows, some adorned by prominently pelleted turrets, churning the uppermost part of the marsh mud like earthworms would do to a soil in a forest or field.

Mud fiddler crabs (Uca pugnax) and their many traces on a salt marsh surface, including feeding pellets, scrape marks, and burrows. Photo by Anthony Martin, taken on Sapelo Island.

Mud-fiddler crab burrows exposed at low tide in a marsh, many with pellet-lined turrets. Why do they make these structures? Good question, which I’ll try to answer in the future. Photo by Anthony Martin, taken on Sapelo Island.

Hence you cannot go to a Georgia salt marsh and say, “I can’t see any traces,” unless you are closing your eyes or are otherwise sight deprived. The entire salt marsh is composed of traces, and these traces, which are the products of plant and animal behavior, actually control the ecology of the salt marshes. Thus I often refer to Georgia salt marshes as examples of “ichnological landscapes,” places that are the sum of all traces. This concept then better prepares us for viewing these and other Georgia coastal environments as places where geologists can begin to understand how organisms can leave their marks – both big and small – in the geologic record.

Further Reading:

Craige, B.J. 2001. Eugene Odum: Ecosystem Ecologist and Environmentalist. University of Georgia Press, Athens, Georgia: 226 p.

Odum, E.P. 1968. Energy flow in ecosystems; a historical review. American Zoologist, 8: 11-18.

Odum, E.P., and Smalley, A.E. 1959. Comparison of population energy flow of a herbivorous and a deposit-feeding invertebrate in a salt marsh ecosystem. Proceedings of the National Academy of Sciences, 45: 617-622.

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

Teal, J.M. 1962. Energy flow in the salt marsh ecosystem of Georgia. Ecology, 43: 614-624.

Teal, J.M., and Teal, M. 1983. Life and Death of a Salt Marsh. Random House, New York: 274 p.

Teal, M., and Teal, J.M. 1964. Portrait of an Island. Atheneum, New York: 167 p. [reprinted by University of Georgia Press in 1997, 184 p.]

Why Study Traces in Georgia? A Celebration of the Familiar

Posted on September 19, 2011 by TMartin

For those of us who live in Georgia, we either forget or don’t know about the ecological and geological specialness of this part of the U.S. For example, my undergraduate students here in Atlanta often talk dreamily about their desire to visit the Amazon River basin, Costa Rica, Kenya, Australia, or other places far removed from Georgia, beguiled as they are by the exotic “other” qualities of those places with their biota and landscapes. On the other hand, almost none of these students have been to the Okefenokee Swamp, the Blue Ridge Mountains, the Cumberland Plateau, the long-leaf pine forests of Ichauway, or the Georgia barrier islands, unless my colleagues or I have taken them there on field trips. Yet these places, especially those with freshwater ecosystems, collectively hold a biodiversity nearly matching that of the Amazon River basin, an evolutionary consequence of the long geologic history of the Appalachian Mountains.

To be fair, I have likewise found myself succumbing to such place-based deflection and lack of appreciation for what is more-or-less in my backyard. In 2001, I realized that I had been to Brazil (three times) more often than Fernbank Forest (two times), even though Fernbank was only a five-minute bicycle ride from home in Decatur, Georgia. This imbalance was soon corrected, though, and many visits later, I learned to appreciate how this old-growth southern Appalachian forest in the middle of metropolitan Atlanta is a gem of biodiversity, every native species of plant and animal a facet testifying to their long evolutionary histories. Still, I wonder why we often ignore what is nearby, even if it is extraordinary?

Related to this quandary is one of the most common questions I encountered from friends, family, and colleagues while writing my book – Life Traces of the Georgia Coast – which was, “Why are you, a paleontologist and geologist, writing about the traces of modern plants and animals in Georgia?” This is a legitimate inquiry, but my answer surprises most people. I tell them that my main reason for staying here in Georgia to study the tracks, trails, burrows, nests, and other traces of its barrier islands is because these traces and their islands are world-class and world-famous. This high quality is directly linked to the biodiversity of the Georgia barrier islands, but also their unique geological histories compared to other barrier-island systems. Furthermore, these islands have inspired more than a few major scientific discoveries related to modern ecology and geology, some of which, made nearly 50 years ago, are still applicable to diagnosing the fossil record and the earth’s geologic history. In short, the Georgia barrier islands and their traces also reflect a legacy recognized by scientists far outside the confines of Georgia.

How so? I’ll explain in upcoming posts, and hope to demonstrate how the marvelous ecosystems of the Georgia coast and its geological processes are the proverbial gift that keeps on giving, continually helping us to better understanding the earth’s geologic past. Now that’s special!

Burrows at dawn: a partial view of the thousands of ghost-shrimp burrows dotting a Georgia beach at low tide, their entrances looking like tiny volcanoes. What makes these burrows so important, scientifically speaking, and why are they something that would cause scientists from outside of Georgia to travel and see in person? Photo by Anthony Martin and taken on Sapelo Island, Georgia.

Life Traces of the Georgia Coast

Unseen lives of the Georgia barrier islands

Author Archives: TMartin

“Worm Burrows” as a Geological Cliché

Ghost Crabs and Their Ghostly Traces

Of Sandhill-Crane Footprints and Dinosaurs Down Under

Georgia Life Traces as Art and Science

Gopher Tortoises, Making Deep and Meaningful Burrows

The Lost Barrier Islands of Georgia

Georgia Salt Marshes: Places Filled with Traces

Why Study Traces in Georgia? A Celebration of the Familiar

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