Speaking of Life Traces…

Posted on January 18, 2013 by TMartin

With the much-awaited publication of my new book (Life Traces of the Georgia Coast), it’s now time to talk about it. Fortunately, I’ve had plenty of time to prepare for this part of the launching of the book, which is one advantage gained from its publication taking longer than originally anticipated. (I’m not complaining, just saying.)

A brief preview of my book, which I gave to my peers in August as a 20-minute talk at the International Congress on Ichnology meeting (Ichnia 2012) in St. Johns, Newfoundland, Canada. Please note that all subsequent talks about the book will not involve audience members to be screeched in, although folks attending my talk to the Atlanta Science Tavern event on January 26 might be tempted. (Photograph by Ruth Schowalter.)

But what’s been most exciting about this process is the overwhelmingly positive reception to my inquiries about giving talks. Amazingly, no one (so far) has said “no” when I asked if I could speak. This is a lesson for other authors who might be organizing public presentations on your own, without the financial or logistical support of a trade-book publisher: pick what you think are the right venues for speaking about your book, then ask. Until then, you never know who will agree that having you speak about your book would be a fine idea.

I am also blessed with a very good infrastructure for giving talks here in Georgia, particularly in the metro Atlanta area. Despite all of the tired jokes about banjo music – along with urging participants to accompany this music with porcine sounds – Atlanta has a thriving scene of science and natural history enthusiasts. This intellectual richness is exemplified the Atlanta Science Tavern, which was even noticed by some out-of-town newspaper for its “Mars Landing Party” last July.

Lastly, the subject of the book is of great interest to many people in Georgia, especially those who have been to its barrier islands. More than a million visitors are estimated to visit the Georgia coast each year, with many of those driving the 4+ hours from Atlanta to get there. Of these million people, at least a few walk along a beach or marsh, or hike through a maritime forest, and see traces made by the animals that live there on the islands, prompting them to ask, “I wonder what made that?” For those folks and more, these talks are for you.

Here’s my current schedule of appearances for the next few months, but be sure to check in once in a while on this Web site for updates. Hope to see you at one or more of these events!

Wednesday, January 23, 4:00 p.m., Emory University, Atlanta, Georgia. Talk title: Big Burrows through Ecospace and Time. This talk is part of the Department of Environmental Studies Seminar Series for the spring semester, 2013; all seminars are in Math & Sciences Building, Room N304. Free and open to the public.

Saturday, January 26, 7:00 p.m. – Atlanta Science Tavern, at Manuel’s Tavern, Atlanta, Georgia. Talk title: Exploring Tracks and Prints, Marks and Holes on Georgia’s Barrier Islands. Preregistration required, $3 suggested donation. This event is currently FULL, but you can put your name on the waiting list through the preceding link.

Tuesday, February 5, 7:00 p.m. – Georgia Center for the Book, DeKalb Public Library, Decatur, Georgia. Talk title: Life Traces of the Georgia Coast. Free and open to the public

Saturday, February 16, 5:30 p.m. – Jekyll Island Green Screen Event, Jekyll Island Convention Center, Jekyll Island, Georgia. Poster presentation (along with other presenters) summarizing some of my latest research on the Georgia barrier islands (exact title of poster to be updated later). Free and open to the public.

Sunday, February 24, 3:00 p.m. – Andalusia, home of author Flannery O’Connor, in Milledgeville, Georgia. Tentative talk title: Tracks and Traces of Flannery O’Connor’s Favorite Birds. Free and open to the public.

Sunday, March 24, 2:30 p.m. – Fernbank Museum of Natural History, Atlanta, Georgia. Tentative talk title: Tracking Exotic Mammals on the Georgia Coast. Admission fee applies if you’re not a member of the museum, but the lecture is free with admission.

P.S. Bookstores, just remember, if you invite me to speak in your store, I will bring your employees this. Consider yourselves bribed.

Trace Evidence for New Book

Posted on January 7, 2013 by TMartin

This past Friday, I very happily received the first complimentary copy of my new book, Life Traces of the Georgia Coast from Indiana University Press. After years of field observations, photographing, writing, editing, drawing, teaching, and speaking about the plant and animal traces described in this book, it was immensely satisfying to hold a physical copy in my hands, feeling its heft and admiring its textures and smells in a way that e-books will never replace. So for any doubters out there (and I don’t blame you for that), here is a photograph of the book:

A photograph, purportedly documenting the publication of at least one copy of my new book Life Traces of the Georgia Coast. Photo scale (bottom) in centimeters.

Still, given that a photograph of the book only constitutes one line of evidence supporting its existence, I knew that more data were needed. So of course, I turned to ichnology for help. After all, a 692-page hard-cover book should also make an easily definable resting trace. Here is that trace, formed by the book in the same spot shown previously.

Ichnological evidence supporting the existence of my new book, Life Traces of the Georgia Coast. Using the “holy trinity” of ichnology – substrate, anatomy, and behavior – as guides for understanding it better: the substrate is a bedspread; the “anatomy” is the 6 X 9″ outline of the book, with depth of the trace reflecting its thickness (and mass); and the behavior was mine, consisting of placing the book on the bedspread and removing it. E-book versions of the book should make similarly shaped rectangular traces, although these will vary in dimensions according to the reading device hosting the book.

However, I also admit that hard-core skeptics may claim that such photos could have been faked, whether through the manipulative use of image-processing software, or slipping the cover jacket onto a copy of Danielle Steel’s latest oeuvre. As a result, the best and perhaps only way to test such a hypothesis is for you and everyone you know to buy the book (which you can do here, here, or here). Or, better yet, ask your your local bookstore to carry copies of it, which will also help to ensure the continuing existence of those bookstores for future book-purchasing and ichnological experiments, including books of other science-book authors.

Lastly, just to make this experiment statistically significant, I suggest a sample size of at least n = 10,000, which should account for inadvertent mishaps that may prevent deliveries of the book, such as lightning strikes, volcanic eruptions, or meteorite impacts. Only then will you be able to assess, with any degree of certainty, whether the book is real or not.

Thank you in advance for your “citizen science,” and I look forward to discussing these research results with you soon.

Suggested Further Reading

Martin, A.J. 2012. Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press, Bloomington, Indiana: 692 p.

A Sneak Peek at a Book Jacket (with Traces)

Posted on January 4, 2013 by TMartin

After returning from a two-week vacation in California with my wife Ruth, we noticed a cardboard tube awaiting us at home. Intriguingly, the mystery package, which was only about 60 cm (24 in) long and 8 cm (3 in) wide, had been sent by Indiana University Press, the publisher of my new book, Life Traces of the Georgia Coast. We were a little puzzled by it, considering that it couldn’t possibly contain complimentary copies of the book. (As of this writing, I still have not held a corporeal representation of the book, hence my continuing skepticism that it is really published.) What was in this mystery tube?

Front cover and spine of my new book, Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals (Indiana University Press). The book, newly released this month, is not yet in stores, but supposedly on its way to those places and to people who were kind enough to pre-order it. But if you didn’t pre-order it, that’s OK: you can get it right here, right now.

Upon opening it, we were delighted to find that it held ten life-sized prints of the book jacket: front cover, spine, back cover, and front-back inside flaps. The cover art, done by Georgia artist Alan Campbell, looked gorgeous, and had reduced well to the 16 X 25 cm (6 X 9″) format, retaining details of traces and tracemakers, but also conveying a nice aesthetic sense. I was also amused to see the spine had the title (of course) but also said “Martin” and “Indiana.” Although I’ve lived in Georgia for more than 27 years, I was born and raised in Indiana, so it somehow seemed fitting in a circle-of-life sort of way to see this put so simply on the book.

Back cover of Life Traces of the Georgia Coast, highlighting a few of the tracemakers mentioned in the book – sea oats, sandhill crane, sand fiddler crab, and sea star – while also providing a pretty sunset view of primary dunes, beach, and subtidal environments on Sapelo Island. (P.S. I love that it says “Science” and “Nature” at the top, too.)

I had no idea what the back cover might be like until seeing these prints, but I thought it was well designed, bearing a fair representative sample of tracemakers of the Georgia barrier islands: sea oats (Uniola paniculata), a sandhill crane (Grus canadensis), sand fiddler crab (Uca pugilator), and lined sea star (Luidia clathrata), as well as a scenic view of some coastal environments. I had taken all of these photos, so it was exciting to see these arranged in such a pleasing way. My only scientifically based objection is that I would have like to see it include photos of insects, worms, amphibians, reptiles, or mammals (these and much more are covered in the book), as well as a few more tracks, trails, or burrows. Granted, I suppose they only had so much room for that 6 X 9″ space, and thus I understood how they couldn’t use this space to better represent the biodiversity of Georgia-coast tracemakers and their traces. (Oh well: guess you’ll have to read the book to learn about all that.)

Inside front and back flap material for Life Traces of the Georgia Coast, which also includes a summary of the book (written by me) and a rare photo of me (taken by Ruth Schowalter) in my natural habitat, which in this instance was on St. Catherines Island, Georgia.

I had written the summary of the book on the inside flap nearly a year ago, so it was fun to look at it with fresh eyes, almost as if someone else had written it for me. Fortunately, I banished my inner critic while reading it, and just enjoyed the sense that it likely achieved its goal, which was to tell people about the book and provoke their interest in it.

In short, this cover jacket symbolizes a next-to-last step toward the book being real in my mind. Now, like any good scientist, all I need is some independently verifiable evidence in the form of tactile data, such as a physical book in my hands. Stay tuned for that update, which I’ll be sure to share once it happens. In the meantime, many thanks to all of the staff at Indiana University Press – who I’ll mention by name next time – for their essential role in making the book happen and promoting it in this new year.

Information about the Book, from Indiana University Press

Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals, Anthony J. Martin

Have you ever wondered what left behind those prints and tracks on the seashore, or what made those marks or dug those holes in the dunes? Life Traces of the Georgia Coast is an up-close look at these traces of life and the animals and plants that made them. It tells about the how the tracemakers lived and how they interacted with their environments. This is a book about ichnology (the study of such traces), a wonderful way to learn about the behavior of organisms, living and long extinct. Life Traces presents an overview of the traces left by modern animals and plants in this biologically rich region; shows how life traces relate to the environments, natural history, and behaviors of their tracemakers; and applies that knowledge toward a better understanding of the fossilized traces that ancient life left in the geologic record. Augmented by numerous illustrations of traces made by both ancient and modern organisms, the book shows how ancient trace fossils directly relate to modern traces and tracemakers, among them, insects, grasses, crabs, shorebirds, alligators, and sea turtles. The result is an aesthetically appealing and scientifically accurate book that will serve as both a source book for scientists and for anyone interested in the natural history of the Georgia coast.

Life of the Past – Science/Paleontology

692 pp., 34 color illus., 137 b&w illus.
cloth 978-0-253-00602-8 $60.00
ebook 978-0-253-00609-7 $51.99

More information at:

http://www.iupress.indiana.edu/catalog/806767 ]http://www.iupress.indiana.edu/catalog/806767

Most Intriguing Traces of the Georgia Coast, 2012

Posted on December 18, 2012 by TMartin

The end of another revolution of the earth around the sun brings with it many “best,” “most,” “worst,” “sexiest,” or other such lists associated with that 365-day cycle. Tragically, though, none of these lists have involved traces or trace fossils. So seeing that the end of 2012 also coincides with the release of my book (Life Traces of the Georgia Coast), I thought that now might be a good time to start the first of what I hope will be an annual series highlighting the most interesting traces I encountered on the Georgia barrier islands during the year.

In 2012, I visited three islands at three separate times: Cumberland Island in February, St. Catherines Island in March, and Jekyll Island in November. As usual, despite having done field work on these islands multiple times, each of these most recent visits in 2012 taught me something new and inspired posts that I shared through this blog.

For the Cumberland Island visit, it was seeing many coquina clams (Donax variabilis) in the beach sands there at low tide, and marveling at their remarkable ability to “listen” to and move with the waves. With St. Catherines Island, it was to start describing and mapping the alligator dens there, using these as models for similar large reptile burrows in the fossil record. Later in the year, I presented the preliminary results of this research at the Society of Vertebrate Paleontology meeting in Raleigh, North Carolina. For the Jekyll trip, which was primarily for a Thanksgiving-break vacation with my wife Ruth, two types of traces grabbed my attention, deer tracks on a beach and freshwater crayfish burrows in a forested wetland. So despite all of the field work I had done previously on the Georgia coast, these three trips in 2012 were still instrumental in teaching me just a little more I didn’t know about these islands, which deserve to be scrutinized with fresh eyes each time I step foot on them and leave my own marks.

For this review, I picked out three photos of traces from each island that I thought were provocatively educational, imparting what I hope are new lessons to everyone, from casual observers of nature to experienced ichnologists.

Cumberland Island

Coyote tracks – Coyotes (Canis latrans) used to be rare tracemakers on the Georgia barrier islands, but apparently have made it onto nearly all of the islands in just the past ten years or so. On Cumberland, despite its high numbers of visitors, people almost never see these wild canines. This means we have to rely on their tracks, scat, and other sign to discern their presence, where they’re going, and what they’re doing. I saw these coyote tracks while walking with my students along a trail between the coastal dunes, and they made for good in-the-field lessons on “What was this animal?” and “What was it doing?” Because Cumberland is designated as a National Seashore and thus is under the jurisdiction of the U.S. National Park Service, I’m interested in watching how they’ll handle the apparent self-introduction of this “new” predator to island ecosystems, which may begin competing with the bobcats (Lynx rufus) there for the same food resources.

Ghost Shrimp Burrows, Pellets and Buried Whelk – Sometimes the traces on the beaches at low tide are subtle in what they tell us, and the traces in this photo qualify as ones that could be easily overlooked. The three little holes in the photo are the tops of ghost shrimp burrows. Scattered about on the beach surface are fecal pellets made by the same animals; ghost shrimp are responsible for most of the mud deposition on the sandy beaches of Georgia. The triangular “trap door” in the middle of the photo is from a knobbed whelk (Busycon carica), which has buried itself directly under the sand surface. The ghost shrimp are perhaps as deep as 1-2 meters (3.3-6.6 ft) below the surface, and are feeding on organics in their subterranean homes. These homes are complex, branching burrow systems, reinforced by pelleted walls. Hence these animals and their traces provide a study in contrasts of adaptations, tiering, and fossilization potential. The whelk trace is ephemeral, and could be wiped out with the next high tide, especially if the waiting whelk emerges and its shallow burrow collapses behind it. On the other hand, only the top parts of the ghost shrimp burrows are susceptible to erosion, meaning their lower parts are much more likely to win in the fossilization sweepstakes.

Feral Horse Grazing and Trampling Traces – Probably the most controversial subject related to any so-called “wild” Georgia barrier islands is the feral horses of Cumberland Island, and what to do about their impacts on island ecosystems there. A year ago, I wrote a post about these tracemakers as invasive species, and discussed this same topic with students before we visited in February. But nothing said “impact” better to these students than this view of a salt marsh, overgrazed and trampled along its edges by horses. This is a example of how the cumulative effects of traces made by a single invasive species can dramatically alter an ecosystem, rendering it a less complete version of its original self.

St. Catherines Island

Suspended Bird Nest – I don’t know what species of bird made this exquisitely woven and suspended little nest, but I imagine it is was a wren, and one related to the long-billed marsh wren (Telmatodytes palustris), which also makes suspended nests in the salt marshes. This nest was next to one of several artificial ponds with islands constructed on St. Catherines with the intent of helping larger birds, such as egrets, herons, and wood storks, so that they can use the islands as rookeries. These ponds are also inhabited by alligators, which had left plenty of tracks, tail dragmarks, and other sign along the banks. With virtually no chance of being preserved in the fossil record, this nest was a humbling reminder of what we still don’t know from ichnology, such as when this specialized type of nest building evolved, or whether this behavior happened first in arboreal non-avian dinosaurs or birds.

Ant Nest in Storm-Washover Deposit – As you can see, the aperture of this ant nest, as well as the small pile of sand outside of it, did not exactly scream out for attention and demand that its picture be taken. But its location was significant, in that it was on a freshly made storm-washover deposit next to the beach. This “starter nest” gives a glimpse of how ants and other terrestrial insects can quickly colonize sediments dumped by marine processes, such as storm waves. These sometimes-thick storm deposits can cause locally elevated areas above what used to be muddy salt marshes. This means insects and other animals that normally would never burrow into or traverse these marshes move into the neighborhood and set up shop, blissfully unaware that the sediments of a recently buried marginal-marine environment are below them. Ant nests also have the potential to reach deep down to those marine sediments, causing a disjunctive mixing of the traces of marine and terrestrial animals that would surely confuse most geologists looking at similar deposits in the geologic record.

Alligator Tracks in a Salt Marsh – These alligator tracks, which are of the left-side front and rear feet, along with a tail dragmark (right) surprised me for several reasons. One was their size: the rear foot (pes) was about 20 cm (8 in) long, one of the largest I’ve seen on any of the islands. (As my Australian friends might say, it was bloody huge, mate.) This trackway also was unusual because it was on a salt pan, a sandy part of a marsh that lacks vegetation because of its high concentration of salt in its sediments. (According to conventional wisdom, alligators prefer fresh-water environments, not salt marshes.) Yet another oddity was the preservation of scale impressions in the footprints, which I normally only see in firm mud. Finally, the trackway was crosscut by trails of grazing snails and burrows of sand-fiddler crabs (Uca pugilator). This helped me to age the tracks – probably less than 24 hours old, and not so fresh that I should have reason to get worried. (Although I did pay closer attention to my surroundings after finding them.) Overall, this also made for a neat assemblage of vertebrate and invertebrate traces, one I would be delighted to find in the fossil record from the Mesozoic Era.

Jekyll Island

Grackle Tracks and Obstacle Avoidance – These tracks from a boat-tailed grackle (Quiscalus major), spotted just after sunrise on a coastal dune of Jekyll Island, are beautifully expressed, but also tell a little story, and one we might not understand unless we put ourselves down on its level. Why did it jog slightly to the right and then meander to the left, before curving off to the right again? I suspect it was because the small strands of bitter panic grass (Panicum amarum), sticking up out of the dune sand, got in its way. Similar to how we might avoid small saplings while walking through an otherwise open area, this grackle chose the path of least resistance, which involved walking around these obstacles, rather than following a straight line. If we didn’t know about this from such modern examples, but we found a fossil bird trackway like this but didn’t look for nearby root traces, how else might we interpret it?

Acorn Worm Burrow, Funnels and Pile – When I came across the top of this acorn-worm burrow, which was probably from the golden acorn worm (Balanoglossus aurantiactus), and on a beach at the north end of Jekyll, I realized I was looking at a two-dimensional expression of a three-dimensional structure. Acorn worms make deep and wide U-shaped vertical burrows, in which they quite sensibly place their mouth at one end and their anus at the other. These burrows usually have a small funnel at the top of one arm of the “U,” which is the “mouth end.” The “anus end” is denoted by a pile of what looks like soft-serve ice cream, which it most assuredly is not, as this is its fecal casting, squirted out of the burrow. What was interesting about this burrow is the nearby presence of a second funnel. This signifies that the worm shifted its mouth end laterally by adding a new burrow shaft to the previous one, superimposing a little “Y” to that part of the U-shaped burrow.

Ghost Crab Dragging Its Claw – As ubiquitous and prolific tracemakers in coastal dunes of the Georgia barrier islands, and as many times as I have studied their traces, I can always depend on ghost crabs (Ocypode quadrata) to leave me signs telling of some nuanced variations in their behavior. In this instance, I saw the finely sculpted, parallel, wavy grooves toward the upper middle of its trackway, made while the crab walked sideways from left to right. A count of the leg impressions in the trackway yielded “eight,” which is the number of its walking legs. This means the fine grooves could only come from some appendage other than its walking legs: namely, one of its claws. Why was it dragging its claw? I like to think that it might have been doing something really cool, like acoustical signaling, but it also might have just been a little tired, having spent too much time outside of its burrow.

So now you know a little more about who left their marks on the Georgia barrier islands in 2012. What will 2013 bring? Let’s find out, with open eyes and minds.

How Did Freshwater Crayfish Get on a Barrier Island?

Posted on December 6, 2012 by TMartin

Two weeks ago, during an all-too-brief visit to Jekyll Island (Georgia) over the Thanksgiving holiday weekend, I decided to check in on some old friends. When I was first introduced to them about four years ago (2008), their presence on Jekyll was a big surprise for me. But thanks to their distinctive traces and a little bit of detective work, I now know they’re on other Georgia barrier islands, too.

Why look, miniature volcanoes in the middle of a maritime forest on Jekyll Island! Or, could they be something else? (In science, that’s what we like to call an “alternative hypothesis.”) Photo scale (left) in centimeters. (Photograph by Anthony Martin.)

These “friends” were conical towers, which look like small lumpy volcanoes (stratovolcanoes, that is, not shield volcanoes), were the traces of freshwater crayfish. A few of the structures, composed of piled balls of sandy mud, also had circular holes in their centers, and they had all seemingly popped out of the forest floor along the edge of a pool of fresh water. All I needed to do to find them was look in the same place where I was first introduced to them, which was by a Jekyll Island resident who knew about their whereabouts.

The towers were 10-25 cm (4-6 in) wide at their bases, 7-10 cm (3-4 in) tall, and each of the rounded, oval balls of sediment was about 1-1.5 cm (0.4-0.6 in) wide. The overall appearance of the towers said “still fresh,” having not been appreciably weathered, and all that I saw in the area looked about the same age. Knowing a little bit about crayfish behavior, I figure they were made just after the last rainfall on Jekyll, maybe a week or so before I spotted them.

Close-up of a crayfish tower, with a circular hole in the center (that’s the burrow). Scale in centimeters. (Photograph by Anthony Martin, taken on Jekyll Island, Georgia.)

Crayfish that dig burrows adjust their depth according to the water table, which they must do to stay alive because they have gills. If the water table drops, they burrow deeper, but if the water table rises, they move their burrows up. For example, where I live here in the metro Atlanta area, crayfish towers often pop up in people’s backyards the day after a hard rain. (This also means that these people need to get flood insurance, because their backyards are on a floodplain. Thus also demonstrating yet another practical reason to know a little basic ichnology.)

Burrowing was (and still is) accomplished by crayfish using their prominent claws (chelipeds) as spades, rolling up the balls of sediment and placing them outside of the burrow entrance, and thus building up towers. But they also smooth out burrow interiors with their bodies through up-and-down and back-and-forth movement, resulting in their burrows having near-perfect circular cross sections. Crayfish burrow systems can be complicated, with vertical shafts connecting the surface with the below-ground parts, which can consist of branching horizontal tunnels and chambers; the last of these can even be occupied by multiple crayfish.

When I first saw these these towers and burrow cross-sections on Jekyll Island in 2008, I immediately knew they were from crayfish. My certainty was because such traces had been described in loving detail by crayfish researchers and ichnologists, linking these directly to their crustacean makers. In fact, just a few months ago, I saw an example of this connection between traces and tracemakers in my home of Decatur, Georgia, where the drying of a human-made pond there caused the crayfish to burrow into the former pond bottom and move about on its sediments in a desperate attempt to stay wet.

A high density of crayfish burrows in a recently drained human-made pond in Decatur, Georgia. Note the similarity of the towers, circular burrow cross-sections, and rounded balls of sediment to those of the Jekyll Island crayfish burrows. Scale with centimeters. (Photograph by Anthony Martin.)

“Are you looking at me?” Crayfish, about 5 cm (2 in) across, and probably a species of Procambarus, copping an attitude while guarding its burrow entrance. (Photograph by Anthony Martin, taken in Decatur, Georgia.)

With about 70 species documented in the state, Georgia is quite rich in crayfish diversity, qualifying it and bordering states in the southeastern U.S as a “biodiversity hotspot” for these animals. Freshwater crayfish are also geographically widespread – occurring in North and South America, Europe, Madagascar, Australia, New Zealand, New Guinea – a direct result of plate tectonics, which spread and isolated populations from one another during their evolutionary history.

In terms of that history, these crustaceans (decapods, more specifically) split from a common ancestor with marine lobsters about 240 million years ago, an age based on molecular clocks, which have been integrated with fossil evidence. I’ve also seen trace fossils that look very much like crayfish burrows in Late Triassic rocks, from about 210 million years ago, which suggests that burrowing began in this lineage early in the Mesozoic Era.

In a 2008 article I co-authored and published with six other scientists – three paleontologists and three zoologists – we described fossil burrows in rocks from the Early Cretaceous Period (about 115-105 million years ago) of Australia, and named what is still the oldest fossil crayfish in the Southern Hemisphere, Palaeoechinastacus australanus. In this article, we pointed out how burrowing was an adaptation that likely helped these crayfish survive polar winters in Australia during the Cretaceous, but also how burrowing abilities in general have given crayfish an upper claw, er, hand in making it past environmental crises in the geologic past.

Here’s the oldest known fossil freshwater crayfish in Australia and the rest of the Southern Hemisphere, Palaeoechinastacus australanus (= “ancient spiny crayfish of Australia”), found in 105-million-year-old rocks (Early Cretaceous) of southern Victoria. Not everything is there, but you can see most of its tail to the left and the right-side legs. Specimen is Museum Victoria, Melbourne, Australia. (Photograph by Anthony Martin.)

And here’s a bedding plane (horizontal) view of trace fossils attributed to freshwater crayfish burrows, preserved in 115-million-year-old rocks (also Early Cretaceous) near Inverloch, Victoria (Australia). The burrows were filled with sand originally, which cemented differently from the surrounding sediment, making them stand out in positive relief as they weather on the outcrop. Scale = 10 cm (4 in). (Photograph by Anthony Martin.)

So how did these crayfish get onto the Georgia barrier islands? Before answering that, I can tell you how they did not get there, which was from people. Because these are burrowing (infaunal) crayfish, and not ones that hang out on lake or stream bottoms (also known as epibenthic), it’s not very likely that humans purposefully introduced them on the islands for aquaculture. Let’s just say that digging up each crayfish burrow, which may or may not contain a crayfish, would require too much work to make crayfish etoufee worth the effort, no matter how good your recipe might be.

Mmmmm, flavorful freshwater decapod concoction [drooling sounds]. But first imagine having to dig up every single crayfish for this dish. Just to prevent this from happening, your recipe should have some qualifying statement, such as, “Make sure to use epibenthic crayfish, not infaunal ones!” (Original image, modified slightly by me, from Wikipedia Commons here.)

Another point to remember about crayfish is that they are freshwater-only animals, incapable of tolerating salt-water immersion, let alone swimming kilometers through marine-flavored waters to reach offshore islands. Yet I’ve seen their traces on Jekyll and two other Georgia barrier islands, and crayfish species have been reported from two additional islands. (For now I won’t say which other islands or identify the probable species of these crayfish until they’ve been properly studied. Sorry.)

What might seem strange to most people, though, is that I still haven’t seen a single living crayfish on any of the Georgia barrier islands. Nonetheless, seeing and documenting their traces is good enough for me to know where they’re living and how they’re behaving. This again demonstrates one of the many advantages of ichnology: you don’t actually have to see an animal to know it’s there, just as long as it leaves lots of identifiable traces.

Oh yeah: almost forgot about the title of this post. What’s my explanation for how the crayfish got to the islands, including Jekyll? I think they lived on the islands before they were islands. In other words, present-day crayfish on the islands descended from ones that originally lived on the mainland part of Georgia, but these were cut off from their original homeland by the last major sea-level rise (well before the current one, that is). This rise started as long as 11,000 years ago, when the last great ice age of the Pleistocene ended, shedding water from continental glaciers and expanding the seas.

So think of a salty moat filling in the low areas between what are now the Georgia barrier islands and the rest of Georgia, with crayfish on either side of it, metaphorically waving goodbye to one another with their claws. In this scenario, the crayfish of the Georgia barrier islands may represent relics left behind and isolated from their ancestral populations. They may have even undergone genetic drift and became new species, or are well on their way to reproductive isolation from their mainland relatives. But that’s just speculation on my part right now. Like I said, these critters need to be studied before anything can be said about them.

All of this neatly illustrates how our knowledge of the geological past ties in with the present, as well as how ichnology can be applied to conservation biology. With regard to the latter, these little muddy crayfish towers exemplify one of the dangers associated with any rapid, careless development of the Georgia barrier islands. What if most people aren’t aware of the unique plants and animals on the islands because at least some of this biodiversity lies below their feet? Without such knowledge, unheeded development may very well wipe out rare or previously unknown species that have been part of the ecological legacy of the Georgia coast for the past 10,000 years.

This is one of many reasons why environmental protection of the islands is still needed, even on semi-developed one like Jekyll. Fortunately, motivated people are working toward such protection on Jekyll, and most other Georgia barrier islands are under some sort of state or federal protection, or privately owned as preserves.

Nice maritime forest you got there. It’d be a shame if something happened to it. (Photograph by Anthony Martin, taken on Jekyll Island.)

What’s also happened on Jekyll Island is increased ecotourism, highlighted by the success of the Georgia Sea Turtle Center. The center, which opened in 2007, has a rehabilitation center for injured turtles, educates the public about sea turtles nesting on the Georgia coast, and helps to monitor turtle nests on Jekyll during the nesting season. And just how is this monitoring done? By looking for tracks of the nesting mothers on the beaches of Jekyll during nesting season, of course. (Say, didn’t I say something previously about using ichnology in conservation biology?)

So can a Jekyll Island Crayfish Center be far behind? Um, no. Still, it’s time to start thinking of species on the Georgia barrier islands and their traces as assets, bragging points that can be used to bolster ecotourism on the coast. Barrier-island biodiversity is an economic resource that will continue to pay off as long as the species survive and their habitats are protected, while simultaneously feeding our sense of wonder at how these species, including burrowing freshwater crayfish, got to the islands in the first place.

Further Reading

Breinholt, J., Ada, M. P.-L., and Crandall, K.A. 2009. The timing of the diversification of the freshwater crayfish. In Martin, J.W., Crandall, K.A., and Felder, D.L. (editors), Decapod Crustacean Phylogenetics, CRC Press, Boca Raton, Florida: 343-355.

Hobbs, H.H., Jr. 1981. The Crayfishes of Georgia. Smithsonian Institute Press, Washington, D.C.: 549 p.

Hobbs, H.H., Jr. 1988. Crayfish distribution, adaptive radiation and evolution. In: Holdich, D.M., Lowery, R.S. (editors), Freshwater Crayfish: Biology, Management and Exploitation. Croom Helm, London: 52-82.

Martin, A.J. 2011. Ichnology in a time of climate change: predicted effects of rising sea level and temperatures on organismal traces of the Georgia coast. Geological Society of America, Abstracts with Programs, 43(2): 86. Link here.

Martin, A.J., Rich, T.H., Poore, G.C.B., Schultz, M.B., Austin, C.M., Kool, L., and Vickers-Rich, P. 2008. Fossil evidence from Australia for oldest known freshwater crayfish in Gondwana. Gondwana Research, 14: 287-296.

P.S. So you’d like to hear more details on the crayfish of the Georgia barrier islands? Well, then you’re going to have to read my book, which starts out Chapter 5 (on terrestrial invertebrate traces) with a section titled The Crayfish of Jekyll Island. Yes, that’s a sales pitch, but you can also request your public library to get it, or borrow a copy from a friend. Which makes this more of a “knowledge pitch.”

Deer on a Beach

Posted on November 29, 2012 by TMartin

In the southeastern U.S., the most common large herbivorous mammal native to this region is the white-tailed deer (Odocoileus virginianus). Accordingly, deer traces, such as their tracks, trails, scat, and chew sign are abundant, easy to identify, and interpret. Some of these traces I discuss in my upcoming book, which has, like, you know, the same title as this blog. (Oh, all right, here’s the link.) But since writing the book, I’ve encountered many more examples of deer traces that surprise me, with implications for better understanding the behavioral flexibility of these mammals.

Yours Truly taking a break from biking to look at some deer tracks on a beach. Yes, that’s right: deer on a beach. Which I’ll take any day over, say, snakes on a plane. (Photograph by Ruth Schowalter, taken on Jekyll Island, Georgia.)

The ecology and ichnology of deer is a big subject, and I began writing a much longer post addressing just that, explored in exquisite detail, with stunningly brilliant insights and witty bon mots sprinkled throughout. Fortunately for all of us, I realized I was being a typical perfectionist (and pedantic) academic, instead of just getting to the point of this post. Thus the gentle reader will be spared such a tome for now, and instead I’ll talk about the cool deer traces my wife Ruth and I encountered while on Jekyll Island (Georgia) last week.

For the past four years, Ruth and I have traveled to Jekyll during our Thanksgiving break for a much–needed escape from teaching, grading, and urban environments of Atlanta, trading these in for wide beaches, beautiful salt marshes, fresh air, and exercise. Like previous years, we took our bicycles with us and spent several days there riding on its plentiful bike trails, or on the beaches at low tide.

Jekyll, unlike most other Georgia barrier islands, is partially developed, with about a thousand residents, and is amenable to tourists staying on the island. This made it convenient for us to pull up on Thursday, check into a hotel, saddle up, and start riding. Of course, we don’t just ride our bikes, but we also look for traces and other interesting tidbits of natural history while speeding along Jekyll’s beaches. For example, last year while riding there, we discovered interesting interactions happening between small burrowing clams, whelks, and shorebirds (links to those here and here), a phenomenon we had never noticed before on other Georgia barrier islands.

This year, on a gorgeous Friday morning on the south beach of Jekyll, we breezed past thousands of human and dog tracks, but grew bored with the ichnological homogeneity wrought by these two tracemakers. But then, something different popped out in the midst of these ordinary, domestically produced ones, prompting us to stop and look more closely. These were deer tracks, and from two deer walking together in the intertidal zone of the beach, where a dropping tide had cleaned the beach surface.

A broad expanse of sandy beach on the south end of Jekyll Island, exposed at low tide, and with two sets of deer tracks pointing downslope and then parallel to the shoreline. Note how these trackways are more-or-less equally spaced from one another, implying that the deer were next to one another and maintained their respective “personal spaces” at this point. (Photograph by Anthony Martin.)

We had seen deer tracks on Georgia barrier-island beaches before, but these are typically in the upper parts of Georgia beaches, closer to the dunes and above the high tide mark. Hence these trackways were unusual for us, showing an unexpected foray into a habitat that was not life-sustaining at all for these deer: no food, no cover, no bedding material, or other creature comforts provided by the forests and back-dune meadows. Just open beach.

Still, there they were, so we enjoyed this opportunity to figure out what they were doing while there. For one, we wondered exactly when they were on the beach. Fortunately, this was relatively easy to answer, as one of the nicer aspects of tracking animals in intertidal zones of beaches (other than being on a beach, of course) is that their tracks can be aged accurately in accordance with the tides. In this instance, high tide was in the early morning, at 3:43 a.m., and the low tide was at 10:18 a.m. We spotted the tracks at about 11:30 a.m., so it was still low tide then, but rising. The furthest down-beach extent of the deer tracks was in the middle of the intertidal zone. This implied that about three hours had elapsed after the high tide receded sufficiently to allow the deer to travel this far down the beach slope: so at 6:45-7:00 a.m. Dawn that morning was at 7:00 a.m., so their presence in this area just before dawn also synched well with the well-known crepuscular movements of deer.

Two sets of deer tracks, showing them moving downslope from above the high-tide mark (look at the rackline in the bottom third of the photo), and heading toward a runnel before turning to the left and paralleling the surf zone. You may have also noticed where their trackways cross over further down the beach. Say, looks like there’s some differences in their trackway patterns. I wonder why? (Photograph by Anthony Martin, taken on Jekyll Island.)

Further evidence of the freshness of these tracks was the moistness of the fine-grained sand, still holding their shape. The morning sunlight had dried them slightly along the edges, and especially the plates or ridges (pressure-release structures) outside of the tracks. The ocean breeze coming out of the east, though, was too gentle to have eroded the tracks, so they looked as if they had been made only a few hours before. Which they had.

Tracking deer doesn’t get much easier than this, folks. Fine-grained and well-packed sand, still moist enough to hold the shape of the tracks and pressure-release structures, gentle wind, and fresh tracks, only about four hours old. (Photograph by Anthony Martin, taken on Jekyll Island.)

We backtracked the deer to their entry point on the beach, which was from the eroded scarp of the primary dunes. One deer must have been following the other, as their tracks came together at this point. The lead deer made the decision to step down onto the beach, a drop of a little more than a meter (3.3 ft), and then the second one followed it down.

The decision point, where one of two deer took the lead and stepped down from the primary dunes to the beach (indicated by tracks at top and bottom of the photo). Note the ghost-crab burrow in the middle-right part of the photo, just above the photo scale. (Photograph by Anthony Martin, taken on Jekyll Island.)

What was really interesting for me, as an ichnologist and just a plain ol’ tracker, was to see the differences in how they stepped down and moved once both deer were on the beach. Based on the trackway patterns, the lead deer simply took a big step down, landed with little drama, and began moving in a normal (baseline) gait for a deer, which is a diagonal pattern with indirect and direct register (rear-foot track on top of front-foot track on the same side). In contrast, the second deer leaped nearly two meters from the dune scarp to the beach, landed heavily, and broke into a gallop, denoted by a set of four tracks – both rear footprints ahead of both front footprints – followed by a space, then another set of four tracks.

Me taking a closer look at the tracks of the “jumper,” whose first tracks show up just behind me, whereas the other deer preceding it simply took a big step down. (Photograph by Ruth Schowalter, taken on Jekyll Island.)

A contrast in trackway patterns by deer on a beach: one that made a normal, diagonal-walking pattern with direct or indirect register (rear foot registering totally or partially on the front-foot impression), and the other galloping, in which front feet landed, then were exceeded by both rear feet, followed by a suspension phase. (Photograph by Anthony Martin, taken on Jekyll Island.)

A close-up of those tracks, in which Deer #1 (right) was strolling relaxedly, not kicking up so much sand, whereas Deer #2 (left) was taking sand with it as it forcefully punched through and extracted its feet from the sand while galloping. (Photograph by Anthony Martin, taken on Jekyll Island.)

This stark difference in their gait patterns led me to ask a simple question: why? This is where a bit of intuition came into play, in which I imagined the following scenario:

The first deer arrived at the dune scarp first, surveyed the scene, saw no threats in the immediate area, stepped down onto the beach, and walked normally.
The second deer, following behind the first, must have temporarily lost sight of the first deer once it stepped off the dune scarp. Not wanting to be left behind, it quickened its pace up to the scarp edge, spied its companion walking nonchalantly down the beach, and jumped.
The best way to catch up with its companion from there was to gallop, which it did.

With this hypothesis in mind – that maybe one deer was trying to catch up with the first one to join it – I had to be a good scientist and test it further. Looking down the beach, we saw how the tracks of the walking and the galloping deer eventually crossed one another, with the walking one crossing left, and the galloping one crossing right. Aha! I could use the old tried-and-true method used by generations of geologists, cross-cutting relations! This principle states that whatever cross-cuts another medium (say, a fault cross-cutting bedrock) is the younger of the two events. In this instance, I tracked the galloping deer to where it crossed and stepped on the tracks of the walking deer. Hence it came afterwards, but perhaps only a few minutes later, as the preservational quality of its tracks were identical to the first deer’s tracks. So it was very likely following and trying to catch up with its companion.

Close-up of the where Deer #2 stepped on the tracks of Deer #1 as it tried to catch up. This cross-over point is also where Deer #2 started going to the right of Deer #1, and was on the ocean side of it once they started traveling together, side-by-side. (Photograph by Anthony Martin, taken on Jekyll Island.)

Close-up of where Deer #2 stepped on the tracks of Deer #1 as it crossed its trackway, eventually traveling to the right of Deer #1. Scale in centimeters. (Photograph by Anthony Martin, taken on Jekyll Island.)

The tracks went down-slope for a distance further, and at some point turned to the left (north), showing where they walked next to one another, about 1.5 m (5 ft) apart and paralleling the surf zone. Where did they go from there? We don’t know, but I suspect they soon went back up into the dunes and back-dune meadows, just in time to avoid all of the humans and dogs who would be on the beach in the next few hours following sunrise. Still, the tracks conjured a beautiful image, of two white-tailed deer walking down the beach together, side-by-side, as the sun came up over the ocean to their right.

Not wanting to spend our entire morning tracking these two deer, we said, “OK, that was neat,” and got back on our bikes for more riding. Later, though, while reflecting on this lesson imparted by the deer tracks in a paleontological sense, I extended their range back into prehistory. How might such tracks from terrestrial mammals have been preserved in ancient beach sediments? If they did get preserved, how would we would recognize them for what they were, or would we just assume they must be traces from some marine-dwelling animal (probably an invertebrate)? And even if we did realize these traces came from big terrestrial mammals, would we have the skills to interpret how two or more animals were affecting each others’ behaviors, which we did so easily with modern, fresh tracks directly in front of us, and knowledge of the daily tides and sunrise? This is the power of ichnology, in which these life traces motivate us to move mentally from the present, to the past, and back again.

As it was, we ended up not seeing a deer during the four days we spent on Jekyll. Nevertheless, we came away with a good story of at least two deer, knowing about their almost-secret trip to the beach, just a few hours before our own.

Further Reading

Elbroch, M. 2003. Mammal Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, Pennsylvania: 779 p.

Halls, L.K. 1984. White-tailed Deer: Ecology and Management. Stackpole Books, Mechanicsburg, Pennsylvania: 864 p.

Hewitt, D.G. (editor). 2011. Biology and Management of White-tailed Deer. Taylor & Francis, Oxon, U.K.: 674 p.

Webb, S.L., et al. 2010. Measuring fine-scale white-tailed deer movements and environmental influences using GPS collars. International Journal of Ecology, Article ID 459610, doi:10.1155/2010/459610: 12 p.

Tracking Wild Turkeys on the Georgia Coast

Posted on November 20, 2012 by TMartin

Of the many traditions associated with the celebration of Thanksgiving in the U.S., the most commonly mentioned one is the ritual consumption of an avian theropod, Meleagris gallopavo, simply known by most people as “turkey.” The majority of turkeys that people will eat this Thursday, and for much of the week afterwards, are domestically raised. Yet these birds are all descended from wild turkeys native to North America. This is in contrast to chickens (Gallus gallus), which are descended from an Asian species, and various European mammals, such as cattle, pigs, sheep, and goats (Bos taurus, Sus scrofa, Ovis aries, and Capra aegagrus, respectively).

Trackway of a wild turkey (Meleagris gallopavo) crossing a coastal dune on Cumberland Island, Georgia. Notice how this one, which was likely a big male (“tom”), was meandering between clumps of vegetation and staying in slightly lower areas, its behavior influenced by the landscape. Scale = 20 cm (8 in). (Photograph by Anthony Martin.)

American schoolchildren are also sometimes taught that one of the founding fathers of the United States, Benjamin Franklin, even suggested that the wild turkey should be elevated to the status of the national bird, in favor of the bald eagle (Haliaeetus leucocephalus). With an admiring (although I suspect somewhat facetious) tone, he said:

“He [the turkey] is besides, though a little vain & silly, a Bird of Courage, and would not hesitate to attack a Grenadier of the British Guards who should presume to invade his Farm Yard with a red Coat on.”

There are eight of us, and only one of you. Do you really want to mess with us? (Photograph by Anthony Martin, taken on Cumberland Island, Georgia.)

Unfortunately, because I live in the metropolitan Atlanta area, I never see turkeys other than the dead packaged ones in grocery stores. Nonetheless, one of the ways I experience turkeys as wild, living animals is to visit the Georgia barrier islands, and the best way for me to learn about wild turkey behavior is to track them. This is also great fun for me as a paleontologist, as their tracks remind me of those made by small theropod dinosaurs from the Mesozoic Era. And of course, as most schoolchildren can tell you, birds are dinosaurs, which they will state much more confidently than anything they might know about Benjamin Franklin.

Compared to most birds, turkeys are relatively easy to track. Their footprints are about 9.5-13 cm (3.7-5 in) long and slightly wider than long, with three long but thick, padded toes in front and one shorter one in the back, pointing rearward. In between these digits is a roundish impression, imparted by a metatarsal. This is a trait of an incumbent foot, in which a metatarsal registers behind digit III because the rear part of that toe is raised off the ground. The short toe is digit I, equivalent to our big toe, but not so big in this bird. Despite the reduction of this toe, its presence shows that turkeys probably descended from tree-dwelling species, as this toe was used for grasping branches. Clawmarks normally show on the ends of each toe impression, and when a turkey is walking slowly, it drags the claw on its middle toe (digit III), thus making a nicely defined linear groove.

Wild turkey tracks made while it was walking slowly up a gentle dune slope, dragging the claw on the middle digit of its right foot, making a long groove. Also notice the bounding tracks of a southern toad (traveling lower right –> upper left), cross-cutting the turkey tracks. (Photograph by Anthony Martin, taken on Cumberland Island.)

A normal walking pace (right foot –> left foot, left foot –> right foot) for a turkey is anywhere from 15-40 cm (6-16 in), and its stride (right foot –> right foot, left foot –> left foot) is about twice that, or 30-80 cm (12-32 in), depending on the age and size of the turkey. Their trackways show surprisingly narrow straddles for such wide-bodied birds, only 1.5 times more than track widths. This is because they walk almost as if on a tightrope, with angles between each step approaching 180°; so they still make a diagonal pattern, but nearly define a straight line. However, turkeys meander, stop, or change direction often enough to make things interesting when tracking them. Their flocking behavior also means their tracks commonly overlap with one another or cluster, making it tough to pick out the trackways of individual turkeys. However, in such flocks, the dominant male’s tracks are noticeably larger than those of the females or younger turkeys, so these can be picked out and help with sorting who’s who.

Turkey trackway in which it walked across the wind-rippled surface of a coastal dune on Cumberland Island, meandering while moseying. Same photo scale as before. (Photograph by Anthony Martin.)

An abrupt right turn recorded by a turkey’s tracks. Check out that beautiful metatarsal impression in the second track from the right, and how the claw dragmark in the thrid track from the right points in the direction of the next track. (Photograph by Anthony Martin.)

One of the more remarkable points about these Georgia barrier-island turkeys, though, is how their tracks belie their stereotyped image as forest-only birds. Although they do spend much of their time in the forest, I’ve tracked turkeys through broad swaths of coastal dunes, and sometimes they will stop just short of primary dunes at the beach. So however difficult it might be to think about these birds as marginal-marine vertebrates, their tracks overlap the same places with ghost-crab burrows and shorebird tracks. Geologists and paleontologists take note: this exemplifies the considerable overlap between terrestrial and marginal-marine tracemakers that can happen in coastal environments. This also happened with dinosaurs that strolled onto tidal flats or otherwise passed through marginal-marine ecosystems.

Turkey tracks heading toward the beach, with the open ocean visible just beyond. Is this close enough to consider turkeys as marginal-marine tracemakers? (Photograph by Anthony Martin.)

Do these turkeys also have an impact on the dunes themselves? Yes, although these effects vary, from trackways disrupting wind ripples to more overt changes to the landscape. For instance, one of the most interesting effects I’ve seen is where they’ve caused small avalanches of sand downslope on dune faces. Interestingly, this same sort of phenomenon was also documented for Early Jurassic dinosaurs that walked across dry sand dunes, which caused grainflows that cascaded downhill with each step onto the sand.

Grainflow structure (arrow), a small avalanche caused by a turkey walking down a dune face. (Photograph by Anthony Martin.)

Close-up of grainflow structure (right) connected to turkey tracks, which become better defined once the turkey reached a more level surface. (Photograph by Anthony Martin, taken on Cumberland Island.)

What other traces do turkeys make? A lot, although I’ve only seen their tracks. Other traces include dust baths, feces, and nests. Dust baths, in which turkeys douse themselves with dry sediment to suffocate skin parasites, must be awesome structures. These are described as 50 cm (20 in) wide, 5-15 (1-3 in) deep, semi-circular depressions, and feather impressions show up in those made in finer-grained sediments. Although such structures would have poor preservation potential in the fossil record, I hold out hope that if paleontologists start looking more at modern examples, they are more likely to find a fossil dust bath, whether in Mesozoic or Cenozoic rocks.

Turkey feces, like most droppings from birds, have white caps on one end, but are unusual in that these can tell you the gender of their depositor. Male turkeys tend to make curled cylinders that are about 1 cm wide and as much as 8 cm long (0.4 X 3 in), whereas females make more globular (not gobbular) droppings that are about 1 cm (0.4 in) wide. These distinctive shapes are a result of their having different digestive systems. Turkeys are herbivores, so their scat normally includes plant material, but don’t be surprised to see insects parts in them, too. Still think about how exciting it would be to find a grouping of same-diameter cylindrical and rounded coprolites in the same Mesozoic deposit, yet filled with the same digested material, hinting at gender differences (sexual dimorphism) in the same species of dinosaur maker.

Turkeys normally make nests on the ground by scratching out slight depressions with their feet, but evidently this is a flexible behavior. On at least one of the Georgia barrier islands (Ossabaw), these birds have been documented as building nests in trees. Although this practice seems very odd for a large, ground-dwelling bird, it is an effective strategy against feral hogs, which tend to eat turkey eggs, as well as eggs of nearly every other species of bird or reptile, for that matter. Just to extend this idea to the geologic past, ground nests are documented for several species of dinosaurs, but tree nests are unknown, let alone whether species of ground-nesting dinosaurs were also capable of nesting in trees.

As everyone should know from their favorite WKRP episode, domestic turkeys can’t fly. But wild turkeys can, and use this ability to get into the branches of live oaks (arrow), high above their predators, or even curious ichnologists. (Photograph by Anthony Martin, taken on Cumberland Island.)

So whether or not you have tryptophan-fueled dreams while dozing later this week, keep in mind not just the evolutionary heritage of your dinosaurian meal, but also what their traces tell us about this history. Moreover, it is an understanding aided by these magnificent and behaviorally complex birds on the Georgia barrier islands. For this alone, we should be thankful.

Paleontologist Barbie, tracking wild turkeys on the Georgia coast to learn more about how these tracemakers can be used as modern analogs for dinosaur behavior and traces, and once again demonstrating why she is the honey badger of paleontologists. (Yes, photograph by me, and taken on Cumberland Island. P.S. Happy Thanksgiving!)

Further Reading

Dickson,J.G. (editor). 1992. Wild Turkeys: Biology and Management. Stackpole Books, Mechanicsburg, Pennsylvania: 463 p.

Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign of North America. Stackpole Books, Mechanicsburg, Pennsylvania: 456 p.

Fletcher, W.O., and Parker, W.A. 1994. Tree nesting by wild turkeys on Ossabaw Island, Georgia. The Wilson Bulletin, 106: 562.

Loope, D.B. 2006. Dry-season tracks in dinosaur-triggered grainflows. Palaios, 21: 132-142.

How to Track a Vampire (Bat)

Posted on October 31, 2012 by TMartin

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Walking on Wings from Science News on Vimeo.

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

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

Vampire Running from Science News on Vimeo.

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

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

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

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

Further Reading

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

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

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

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

Different Coastlines, Same Traces, and Time

Posted on October 25, 2012 by TMartin

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading

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

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

Deconstructing an Ichnology Abstract, with Alligators

Posted on October 19, 2012 by TMartin

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Dude. Those are big burrows. Dude.

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

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

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

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

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

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

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

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

Source of Abstract (Reference):

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

Life Traces of the Georgia Coast

Unseen lives of the Georgia barrier islands

Category Archives: Blog post

Speaking of Life Traces…

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A Sneak Peek at a Book Jacket (with Traces)

Most Intriguing Traces of the Georgia Coast, 2012

How Did Freshwater Crayfish Get on a Barrier Island?

Deer on a Beach

Tracking Wild Turkeys on the Georgia Coast

How to Track a Vampire (Bat)

Different Coastlines, Same Traces, and Time

Deconstructing an Ichnology Abstract, with Alligators

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

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

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