Tag Archives: burrow

Enter The Evolution Underground

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It seemed all too fitting that author copies of my new book, The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet (Pegasus Books, 2017) arrived on February 2. In the U.S., this is Groundhog Day, which is named after a burrowing animal and one in which its burrow plays a key role in its mythology. Did it cast a shadow or otherwise predict the weather for the next six weeks? No, but it may enlighten as you travel through geologic time, learning all about how animals and their burrows altered the world, and how animals used burrows to survive the worst the earth (or solar system) could toss at them.

It’s here! After about two years from start to end, The Evolution Underground is out of its literary bunker and into your hands. (Photo by Anthony Martin.)

Is a book about burrows and burrowing animals too far beneath you to read? Well, as the immortal Kenny Loggins might say: Do what you like, and do it naturally.

The Evolution Underground is my seventh book and the second written overtly for a popular-science audience, following Dinosaurs Without Bones: Revealing Dinosaur Lives through Their Trace Fossils (2014, also by Pegasus Books). Dinosaurs Without Bones was a successful debut for me as a popular writer, with not-bad sales and mostly positive reviews (such as this, this, and this). That book was also my first attempt to make the word “ichnology” (the study of traces) more mainstream, and by using those always-charismatic dinosaurs as a hook. It worked, and I now think the percentage of people confusing ichnology with ichthyology has gone down ever so slightly since that book came out.

It’s ichnology, not ichthyology. Make sure you get it right, because you do not want to be slapped by Batman.

For fans of Dinosaurs Without Bones, I’m happy to report my new book – which is officially published today, February 7, 2017 – includes dinosaurs and it’s about ichnology. But it also includes plenty of paleontology, geology, ecology, and good, old-fashioned natural history throughout. Moreover, this book gave me a chance to introduce readers to a panoply of animals representing the past 550 million years of earth history, while also exploring the big idea that burrowing impacted the evolution of many animals and their ecosystems.

What’s it like to be a gopher tortoise? Kind of like being a subterranean landlord, considering that you might be sharing your burrow with 300-400 other species of animals.(Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Along those lines, main themes of the book are expressed in subtitles I considered for it: How Burrows Changed the World and Better Surviving through Burrows. For the former, the mere collective action of burrowing animals – from the deep seafloor to mountaintops – is an essential part of how most ecosystems function. For the latter, burrows were all-natural bunkers enabling animals to escape the worst the Earth (or solar system) could throw at them and allowing their evolution to continue underground. Want to survive a mass extinction? Start digging.

Lungfishes since the Devonian Period (more than 350 million years ago) have burrowed to avoid droughts, and their lineage has survived four mass extinctions. Coincidence? Probably not. (Original illustration by Anthony Martin, in The Evolution Underground (2017).)

Must you buy this book, or at least persuade your local public library to get it? Well, yes, if you insist. Still, just in case you first need to know a bit more about the burrowing animals and geologic times represented in between its front and back covers, here’s a chapter list with brief descriptions of their contents. Thanks in advance, and I hope you and other readers enjoy reading it.

The Evolution Underground: Chapter Titles and Synopses

Chapter 1: The Wondrous World of Burrows – Did you know that alligators make burrows? They do indeed, and they’re awesome burrows. Learn how these body-armored saurians straight out of central casting from the Mesozoic Era provide superb living examples of how many animals use (or used) burrows to survive and thrive, thus symbolizing many of the main themes of the book.

Chapter 2: Beyond “Cavemen”: A Brief History of Humans Underground – Since the time of living in caves, humans have gone beneath the Earth’s surface during times of environmental or societal stress, and we still do. In this chapter, travel to Turkey, China, Russia, Australia, Canada, and the far-off exotic land of Pennsylvania (home of weather-predicting groundhogs) to marvel at how humans, time and time again, have looked below when seeking safety.

One of these is a map of a naked mole rat burrow system, and the other is of an underground city made by humans in central Turkey. Which is which? That might be one of many questions answered by reading my new book. (Original illustration by Anthony Martin, in The Evolution Underground.)

Chapter 3: Kaleidoscopes of Dug-Out Diversity – Gopher tortoises of the southeastern U.S. dig burrows that are both deep and meaningful, as these burrows host underground menageries of many other species, boosting the biodiversity of their ecosystems. How did tortoises and other turtles evolve and survive mass extinctions of the past? If you answered “burrowing,” you’re catching on to what this book is about.

Chapter 4: Hadean Dinosaurs and Birds Underfoot – Although burrowing dinosaurs of the Mesozoic past were apparently rare, a few of their living descendants (birds) evolved to put their nests not in trees, but underground. In this chapter, penguins, puffins, shearwaters, owls, kiwis, bee-eaters, and other birds raising underground families are lauded for their digging family values.

Chapter 5: Bomb Shelters of the Phanerozoic – This chapter opens with a piece of fiction about a Lystrosaurus (or two) embarking on a post-apocalyptic journey. This allegory conveys how burrowing helped their kind and a few other animals to survive the worst mass extinction in the history of life at the end of the Permian Period (about 250 million years ago). This chapter also summarizes other mass extinctions and how burrowing provided an advantage for making it through the worst ecological crises of the geologic past.

Chapter 6: Terraforming a Planet, One Hole at a Time – When did animals move from the sea to freshwater and then onto land? Burrowing may have helped animals to make transitions from such environmental extremes, which ultimately resulted in their shaping landscapes as we know them today. Featured animals in this chapter include trilobites, horseshoe crabs, lungfish, amphibians (frogs, toads, salamanders), lizards, and snakes.

Chapter 7: Playing Hide and Seek for Keeps – For a long time, all animal life was superficial, living either on seafloor surfaces or just underneath. Then about 550-540 million years ago, animals starting plumbing deeper. What caused this downward shift, and how did animals’ churning of oceanic sands and muds forever change the oceans, atmosphere, and the evolution of life? Also, the evolution of predators gave animals yet another reason to burrow: That is, before the predators started burrowing, too, starting an underground “arms race” that continues through today.

Chapter 8: Rulers of the Underworld – What animals are the real ecosystem engineers for our planet? Mostly the small and spineless ones, invertebrates. This chapter starts with those marvelous earthworms that so beguiled Charles Darwin, then pays tribute to the amazing feats of burrowing and animal architectures created by ants, crayfish, crabs, lobsters, and more.

Chapter 9: Viva La Evolución: Change Comes from Within – This chapter starts with the second fictional story in the book, following the exploits of an ecological hero – a pocket gopher – following the 1980 volcanic eruption of Mount St. Helens. The rest of the concluding chapter of The Evolution Underground looks at burrowing mammals (especially rodents), but also considers the largest burrowing animals of all time. Also, what can we as mammals learn from our fellow furry underground relatives as we head into an uncertain future posed by rapid climate change?

Appendix: Genera and Species Mentioned in The Evolution Underground – A listing of the animals name-dropped in the book, some of which may surprise you.

What are you waiting for? Leave your underground hidey-hole and get my book! P.S. Thanks for reading it. (Photograph by Anthony Martin, taken in Decatur, Georgia.)

 

Into the Dragon’s Lair: Alligator Burrows as Traces

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American alligators (Alligator mississippiensis) tend to provoke strong feelings in people, but the one I encounter the most often is awe, followed closely by fear. Both emotions are certainly justifiable, considering how alligators are not only the largest reptiles living on the Georgia barrier islands, but also are the top predators in both freshwater and salt-water ecosystems in and around those islands. I’ve even encountered them often enough in maritime forests of the islands to regard them as imposing predators in those ecosystems, too.

Time for a relaxing stroll through the maritime forest to revel in its majestic live oaks, languid Spanish moss, and ever-so-green saw palmettos. Say, does that log over there look a little odd to you? (Photo by Anthony Martin, taken on St. Catherines Island.)

But what many people may not know about these Georgia alligators is that they burrow. I’m still a little murky on exactly how they burrow, but they do, and the tunnels of alligators, large and small, are woven throughout the interiors of many Georgia barrier islands. Earlier this week, I was on one of those islands – St. Catherines – having started a survey of alligator burrow locations, sizes, and ecological settings.

Entrance to an alligator burrow in a former freshwater marsh, now dry, yet the burrow is filled with water. How did water get into the burrow, and how does such traces help alligators to survive and thrive? Please read on. (Photograph by Anthony Martin and taken on St. Catherines Island, Georgia.)

In this project, I’m working cooperatively (as opposed to antagonistically) with a colleague of mine at Emory University, Michael Page, as well as Sheldon Skaggs and Robert (Kelly) Vance of Georgia Southern University. As loyal readers may recall, Sheldon and Kelly worked with me on a study of gopher tortoise burrows, also done on St. Catherines Island, in which we combined field descriptions of the burrows with imaging provided by ground-penetrating radar (also known by its acronym, GPR). Hence this project represents “Phase 2” in our study of large reptile burrows there, which we expect will result in at least two peer-reviewed papers and several presentations at professional meetings later this year.

Why is a paleontologist (that would be me) looking at alligator burrows? Well, I’m very interested in how these modern burrows might help us to recognize and properly interpret similar fossil burrows. Considering that alligators and tortoises have lineages that stretch back into the Mesozoic Era, it’s exciting to think that through observations we make of their descendants, we could be witnessing evolutionary echoes of those legacies today.

Indeed, for many people, alligators evoke thoughts of those most famous of Mesozoic denizens – dinosaurs – an allusion that is not so farfetched, and not just because alligators are huge, scaly, and carnivorous. Alligators are also crocodilians, and crocodilians and dinosaurs (including birds) are archosaurs, having shared a common ancestor early in the Mesozoic. However, alligators are an evolutionarily distinct group of crocodilians that likely split from other crocodilians in the Late Jurassic or Early Cretaceous Period, an interpretation based on both fossils and calculated rates of molecular change in their lineages.

Archosaur relatives, reunited on the Georgia coast: great egrets (Ardea alba), which are modern dinosaurs, nesting above American alligators (Alligator mississippiensis), which only remind us of dinosaurs, but shared a common ancestor with them in the Mesozoic Era. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

Along these lines, I was a coauthor on a paper that documented the only known burrowing dinosaur – Oryctodromeus cubicularis – from mid-Cretaceous rocks in Montana. In this discovery, we had bones of an adult and two half-grown juveniles in a burrow-like structure that matched the size of the adult. I also interpreted similar structures in Cretaceous rocks of Victoria, Australia as the oldest known dinosaur burrows. Sadly, these structures contained no bones, which of course make their interpretation as trace fossils more contentious. Nonetheless, I otherwise pointed out why such burrows would have been likely for small dinosaurs, especially in Australia, which was near the South Pole during the Cretaceous. At least a few of these reasons I gave in the published paper about these structures were inspired by what was known about alligator burrows.

Natural sandstone cast of the burrow of the small ornithopod dinosaur, Oryctodromeus cubicularis, found in Late Cretaceous rocks of western Montana; scale = 15 cm (6 in). (Photograph by Anthony Martin, taken in Montana, USA.)

Enigmatic structure in Early Cretaceous rocks of Victoria, Australia, interpreted as a small dinosaur burrow. It was nearly identical in size (about 2 meters long) and form (gently dipping and spiraling tunnel) to the Montana dinosaur burrow. (Photograph by Anthony Martin, taken in Victoria, Australia.)

What are the purposes of modern alligator burrows? Here are four to think about:

• Dens for Raising Young Alligators – Many of these burrows, like the burrow interpreted for the dinosaur Oryctodromeus, serve as dens for raising young. In such instances, these burrows are occupied by big momma ‘gators, who use them for keeping their newly hatched (and potentially vulnerable) offspring safe from other predators.

Two days ago, Michael and I experienced this behavioral trait in a memorable way while we documented burrow locations. As we walked along the edge of an old canal cutting through the forest, baby alligators, alarmed by our presence, began emitting high-pitched grunts. This then provoked a large alligator – their presumed mother – to enter the water. Her reaction effectively discouraged us from approaching the babies; indeed, we promptly increased our distance from them. (Our mommas didn’t raise no dumb kids.) So although we were hampered in finding out the exact location of this mother’s den, it was likely very close to where we first heard the grunting babies. I have also seen mother alligators on St. Catherines Island usher their little ones through a submerged den entrance, quickly followed by the mother turning around in the burrow and standing guard at the front door.

Oh, what an adorable little baby alligator! What’s that? You say your mother is a little over-protective? Oh. I see. I think I’ll be leaving now… (Photograph by Anthony Martin, taken on St. Catherines Island.)

• Temperature Regulation – Sometimes large male alligators live by themselves in these burrows, like some sort of saurian bachelor pad. For male alligators on their own, these structures are important for maintaining equitable temperatures for these animals. Alligators, like other poikilothermic (“cold-blooded”) vertebrates, depend on their surrounding environments for controlling their body temperatures. Even south Georgia undergoes freezing conditions during the winter, and of course summers there can get brutally hot. Burrows neatly solve both problems, as these “indoor” environments, like caves, provide comfortable year-round living in a space that is neither too cold nor too hot, but just right. The burrowing ability of alligators thus makes them better adapted to colder climates than other crocodilians, such as the American crocodile (Crocodylus acutus), which does not make dwelling burrows and is restricted in the U.S. to the southern part of Florida.

• Protection against Fires – Burrows protect their occupants against a common environmental hazard in the southeastern U.S., fire. This is an advantage of alligator burrows that I did not appreciate until only a few days ago while in the field on St. Catherines. Yesterday, the island manager (and long-time resident) of St. Catherines, Royce Hayes, took us to a spot where last July a fire raged through a mixed maritime forest-freshwater wetland that also has numerous alligator burrows. The day after the fire ended, he saw two pairs of alligator tracks in the ash, meaning that these animals survived the fire by seeking shelter, and further reported that at least one of these trackways led from a burrow. The idea that these burrows can keep alligators safe from fires makes sense, similar to how gopher tortoises can live long lives in fire-dominated long-leaf pine ecosystems.

An area in the southern part of St. Catherines Island, scorched by a fire last July, that is also a freshwater wetland inhabited by alligators with burrows. The burrow entrances are all under water right now, which would work out fine for their alligator occupants if another fire went through there tomorrow. (Photograph by Anthony Martin, taken on St. Catherines Island.)

• Protection against Droughts – Burrows also probably help alligators keep their skins moist during droughts. Because these burrows often intersect the local water table, alligators might continue to use them as homes even when the accompany surface-water body has dried up. We saw several examples of such burrows during the past few days, some of which were occupied by alligators, even though their adjacent water bodies were nearly dry.

For example, yesterday Michael and I, while scouting a few low-lying areas for either occupied or abandoned dens, saw a small alligator – only about a meter (3.3 ft) long – in a dry ditch cutting through the middle of a pine forest. Curious about where alligator’s burrow might be, we approached it to see where it would go. It ran into a partially buried drainage pipe under a sandy road, a handy temporary refuge from potentially threatening bipeds. Seeing no other opening on that side of the road, we then checked the other side of the road, and were pleasantly surprised to find a burrow entrance with standing water in it. This small alligator had made the best of its perilously dry conditions by digging down to water below the ground surface.

Alligator burrow (right) on the edge of a former water body. Notice how water is pooling in the front of the burrow, showing how it intersects the local water table. The entrance also had fresh alligator tracks and tail dragmarks at this entrance, showing that it was still occupied despite the lack of water outside of it. (Photograph by Anthony Martin, taken on Cumberland Island, Georgia.)

Alligator burrows (left foreground and middle background) in a maritime forest, also not associated with a wetland but marking the former location of one. Although the one to the left was unoccupied when we looked at it, it had standing water just below its entrance. This meant an alligator could have hung out in this burrow for a while after the wetland dried up, and it may have just recently departed. Also, once these burrows are high and dry, bones strewn about in front of them also add a delicious sense of dread. Here, Michael Page points at a deer pelvis, minus the rest of the deer. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)

What is especially interesting about the American alligator is how the only other species of modern alligator, A. sinensis in China, is also a fabulous burrower, digging long tunnels there too, which they use for similar purposes. This behavioral trait in two closely related but now geographically distant species implies a shared evolutionary heritage, in which burrowing provided an adaptive advantage for their ancestors.

Thus like many research problems in science, we won’t really know much more about alligator burrows until we gather information about them, test some of the questions and other ideas that emerge from our study, and otherwise do more in-depth (pun intended) research. Nonetheless, our all-too-short trip to St. Catherines Island this week gave us a good start in our ambitions to apply a comprehensive approach to studying alligator burrows. Through a combination of ground-penetrating radar, geographic information systems, geology, and old-fashioned (but time-tested) field observations, we are confident that by the end of our study, we will have a better understanding of how burrows have helped alligators adapt to their environments since the Mesozoic.

Juvenile alligators just outside two over-sized burrows, made and used by previous generations of older and much larger alligators. How might such burrows get preserved in the fossil record? How might we know whether these burrows were reused by younger members of the same species? Or, would we even recognize these as fossil burrows in the first place? All good questions, and all hopefully answerable by studying modern alligator burrows on the Georgia barrier islands. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Further Reading

Erickson, G.M., et al. 2012. Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation. PLoS One, 7(3): doi:10.1371/journal.pone.0031781.

Martin, A.J. 2009. Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia and their relation to Cretaceous polar environments. Cretaceous Research, 30: 1223-1237.

Martin, A.J., Skaggs, S., 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.

St. John, J.A., et al., 2012. Sequencing three crocodilian genomes to illuminate the evolution of archosaurs and amniotes. Genome Biology, 13: 415.

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.

Waters, D.G. 2008. Crocodlians. In Jensen, J.B., Camp, C.D., Gibbons, W., and Elliott, M.J. (editors), Amphibians and Reptiles of Georgia. University of Georgia Press, Athens, Georgia: 271-274.

Acknowledgements: Much appreciation is extended to the St. Catherines Island Foundation, which supported our use of their facilities and vehicles on St. Catherines this week, and Royce Hayes, who enthusiastically shared his extensive knowledge of alligator burrows. I also would like to thank my present colleagues and future co-authors – Michael Page, Sheldon Skaggs, and Kelly Vance – for their valued contributions to this ongoing research: we make a great team. Lastly, I’m grateful to my wife Ruth Schowalter for her assistance both in the field and at home. She’s stared down many an alligator burrow with me on multiple islands of the Georgia coast, which says something about her spousal support for this ongoing research.

Coquina Clams, Listening to and Riding the Waves

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A little more than a week ago, I co-led a class field trip to Cumberland Island, Georgia and the nearby Okefenokee Swamp for a course titled Ecosystems of the Southeastern U.S. Although I had been to both places more than a few times, none of the students – and a few of my colleagues – had never visited either, potentially casting these already special places in a more exciting light for them.

Nonetheless, as is typical with any field trip to a Georgia barrier island, I also noticed new phenomena while on Cumberland, once again demonstrating how field trips with students ideally also cause the instructors to be filled with wide-eyed wonder. Even better, a few seemingly lowly small bivalves – coquina clams (Donax variabilis) – provided the intellectual highlight for me while we were on Cumberland Island. This is saying something for an island bearing charismatic livestock as a touristic draw.

Resting traces (or are they escape traces?) of coquina clams (Donax variabilis) in the upper intertidal zone of a beach on Cumberland Island, Georgia. These clams are buried just underneath each bump of sand, but some others are much deeper and safer. How do I know that? You’ll find out. (Photograph by Anthony Martin.)

We first noticed the clams as a death assemblage just above the uppermost part of the surf zone on the beach. Their shells, some evident as single valves and others as pairs still hinged together, had been deposited by waves following a high tide, then moved slightly by the wind. These finely ribbed and polished shells readily showed why the specific name of coquina clams (D. variabilis) is applied to them, as they display a gorgeous variety of colors: yellow, orange, beige, blue, pink, and other schemes that surely would inspire interior decorators seeking paisley themes.

Coquina clams with both valves intact or apart, some partially covered by windblown sand, and variably colored. (Photo by Anthony Martin, taken on Cumberland Island, Georgia.)

Just a little bit lower on the beach and in the freshly scoured intertidal zone, we then noticed many small bumps of sand. Underneath these bumps were living clams that buried themselves, which helps to avoid drying out between tides or predation by ravenous shorebirds. With regard to the latter, these bivalves still would have been easy targets for shorebirds intent on acquiring some fresh clam snacks: think of a person ducking under a blanket to avoid being eaten by a lion and how well that might work as a tactic. (Please, just think about it and don’t actually test this idea.)

However, instead of simply writing off all coquina clams as inept burrowers who deserve to die at the beaks of their avian overlords – a similar fate experienced by dwarf surf clams (Mulinia lateralis) – we should look well below the surface, and I mean the sand surface. Look again at the photo first shown above. See all of those tiny, paired holes in between the bumps? Those are the traces of siphons from more deeply buried coquina clams, which are much more likely to escape from bivalve-munching birds while also keeping moist until the next high tide.

Here’s the same photo as above, but zoomed in so you can see the details. Did you notice all of the little holes in between shallowly buried clams? If so, bravo. If not, oh well. (Despite the cropping, turning, and otherwise shuffling electrons, this photograph is still by Anthony Martin, and was still taken on Cumberland Island.)

Coquina clams are actually accomplished burrowers, a necessary adaptation for nearly any small animal living in the high-energy surf of a Georgia beach. In the event of a wave breaking on a beach and washing away the top layer of sand, thus exposing a coquina clam, it will open its valves only enough to stick out its foot, which it then vibrates rapidly. This movement loosens the wet sand underneath, and the clam’s smooth, streamlined shell does the rest of the job, allowing it to glide into its self-made local pit of quicksand and vanish from the surface.

Once under the sand, this clam remains in a vertical position and projects its siphons upwards, making paired holes visible at the sand surface. The resulting burrow is Y-shaped, with the clam body making the lower part of the “Y” and the two siphons leaving thinner traces above. On the Cumberland Island beach the given day we observed their traces, I suspect the more deeply buried clams had the benefit of wetter sand early on as the tide dropped, then the ones hiding under mere caps of sand did the best they could with less wet sand later.

Vertical sections of the Y-shaped burrows made by a coquina clam, dwarf surf clam, or similar small, burrowing bivalves on the Georgia coast, in which the clam body is removed and sediment filled in the empty spaces from above. Both views, taken at right angles from one another, assume the clam is not moving up or down in the sand, which actually isn’t very realistic. (Illustration by Anthony Martin.)

How do we apply this knowledge to the fossil record? Paleontologists have found similar small, Y-shaped burrows in fine-grained sedimentary rocks, trace fossils named Polykladichnus. Some of these burrows are interpreted as the works of suspension-feeding bivalves, although polychaete worms or small arthropods are possible tracemakers, too. Where the lower parts of bivalves rested in sediment and left impressions of their lower halves, which were later filled by overlying sediments, are trace fossils called Lockeia. As a result, paleontologists can reliably identify a former presence of bivalves in rocks that might not have any of their shells. (By the way, please remember that trace fossil names are not the names of the bivalves that made the traces, but of the trace fossil itself. Yeah, I know, that’s confusing. But if you need more of an explanation, I have one for you here.)

So what else is significant about coquina clams? Well, for one, their shells were abundant enough to have formed the framework for a loosely cemented limestone common in Florida, coquina limestone. This is a rock encountered by nearly anyone who has enjoyed (or suffered through) an introductory geology lab exercise on sedimentary rock identification. Students of mine have often compared these rock samples to popcorn balls or similar sugar-cemented treats, although I’ve noticed that no one has been tempted to eat them, no matter how long I kept them in lab that day.

Coquina limestone from Florida. Notice that some of the pieces are from other species of bivalves with rougher, corrugated shells, so these rocks are not made entirely of coquina clams. (Photograph by Mark Wilson of Wooster College, and taken from Wikipedia Commons here.)

But I saved the best tidbits of information for last, and something I’ll bet most people don’t know about these clams that makes them, like, totally cool. These little bivalves respond to sound and migrate seasonally. Yes, that’s right: these clams have their own form of “listening” and they can move en masse once the seasons change on the Georgia coast. Here’s how they do both:

Stop, Look, Listen – Of course, clams lack ears (although rumors still persist that they have legs). Thus they do not hear in the sense we do, but instead respond to low-frequency vibrations caused by waves striking the shore. Once these vibrations are detected, they react by: popping out of the sand; jumping up into the water; body surfing on the wave; and quickly reburying themselves in the sand once dumped by that same wave. Like some aficionados of heavy metal or punk rock, louder is better, as higher-decibel waves cause more clams to jump up out of the sand and into the water, an aquatic version of a molluscan mosh pit.

Shelled Migration – Coquina clams, like caribou, wildebeest, and arctic terns, migrate. Unlike the vast distances covered by those animals, though, coquina clams simply move up and down the slope of a beach with the changes of the seasons. Using their wave-surfing and burrowing abilities, they move from the lower intertidal zone – which is where they live during the spring, summer, and fall – to the upper part of the beach, which becomes their winter homes.

So I hope that all of this pondering over a few shells, bumps, and holes on a Cumberland Island beach has helped lend an appreciation for the small wonders on any given Georgia barrier island. Who knows what little discovery the next field trip will bring, or whether some facsimile of what is seen might also be preserved in the fossil record? As the old saying goes, time will tell, whether that time is in the present or the geologic past.

Further Reading

Ellers, O. 1995a. Discrimination among wave-generated sounds by a swash-riding clam. Biological Bulletin, 189: 128-137.

Ellers, O. 1995b. Behavioral control of swash-riding in the clam Donax variabilis. Biological Bulletin, 189: 120-127.

Pemberton, S.G., and Jones, B. 1988. Ichnology of the Pleistocene Ironshore Formation, Grand Cayman Island, British West Indies. Journal of Paleontology, 62: 495-505.

Ruppert, E.E., and Fox, R.S. 1988. Seashore Animals of the Southeast. University of South Carolina Press, Columbia, South Carolina: 429 p.

Turner, H.J., Jr., and Belding, D.L. 1957. The tidal migrations of Donax variabilis Say. Limnology and Oceanography, 2: 120-124.

Gopher Tortoises, Making Deep and Meaningful Burrows

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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.

Georgia Salt Marshes: Places Filled with Traces

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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.]