Enter The Evolution Underground

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

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

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

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

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

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

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

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

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

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

The Evolution Underground: Chapter Titles and Synopses

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

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

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

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

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

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

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

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

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

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

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

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


Traces of the Red Queen

The seagull looked peaceful on that beach, lying on its left side with its eyes closed. Yet it was a permanent quietude, as only its head was there.

This disembodied head stuck out as a white spot with a red edge, perched on top of a pile of dull-brown, dead cordgrass. The torso so recently connected to this head was nowhere to be seen, and I could find no tracks belonging to the gull or any other animal nearby. It looked as if it had been placed there as an object of art, ready for erudite admirers – wine glasses in hand – to comment on its broader themes and nuanced metaphors. To a ichnologist, though, it also spoke of a sudden death, and one likely dealt by a aerial predator.

Seagull-Head-Decapitated-WassawThe place where I saw this gruesome sign was on Wassaw Island, Georgia. Wassaw is the only island on the Georgia coast that was never logged or otherwise developed by European or Americans, hence it retains a more primitive feel compared to most other Georgia islands. You can only get there by boat, and in this instance our boat captain and guide – John Crawford – had taken our field-trip group there to learn about its unique natural history. Because of its intact environments and general lack of human influence on the landscape, I was not surprised to see something new on Wassaw. However, I haven’t seen anything like this since.

Within minutes of arriving on the island, this beheaded seagull presented a little mystery for us. As mentioned before, tracks and the rest of the body were not visible, nor were any droplets of blood around its head, either. Moreover, its dry feathers and the freshness of its fatal wound – a clean severing of its neck vertebrate – also meant it had not washed up on shore. Where did it die, and how did it get there?

After ruling out the land and sea, we looked above the beach, and realized that the attack must have been delivered up there, in the air. We then imagined what could have possessed the bulk, ferocity, and other means to chop through a seagull’s neck while in flight. The list of suspects was a short one, and we quickly narrowed it down to one: a bald eagle.

Our hypothesis was not so far-fetched, as bald eagles don’t just eat fish, but also kill and eat other birds, including gulls. This meant the seagull head we saw that morning was very likely a result of bird-on-bird predation. Extending this a bit further into the evolutionary pasts of these birds, it reflected a time when when their non-avian dinosaur ancestors killed and were killed by similar behaviors, but on the ground.

How did birds evolve flight from non-flighted theropod ancestors? No doubt one of many selection pressures exerted on non-avian dinosaurs was predation. Any means for increasing the likelihood of escape from predators also bestowed a greater probability for passing on genes coding for that “escaping trait” to the next generation of not-quite-flighted dinosaurs.

Of course, flight has evolved for many uses in birds. Nevertheless, making a quick getaway from mortal peril is still one of them. Yet flight has also been used as a means for enhancing predation in the birds that kill other birds, exerting new and different selection pressures on prey. This example of an evolutionary back-and-forth “arms race” between predators and prey is often nicknamed the Red Queen hypothesis, named after Lewis Carroll’s character in Alice in Wonderland. Only now I will change her line (said to a fleeing Alice) about running in place:

Now, here, you see, it takes all the running you can do to keep in the same place.

to a more avian-appropriate one:

Now, here, you see, it takes all the flying you can do to keep in the same place.

Still, In this Georgia-coast example, a more appropriate literary allusion would have been to the Queen of Hearts from Alice in Wonderland, a decapitating character famous for uttering the line, “Off with their heads!” In this sense, the Red Queen and Queen of Hearts meet in the arms race between predators and prey.

Will this “Red Queen of Hearts” scenario happen again during eagle and seagull conflicts? Yes: that is, unless the seagulls’ descendants adapt, which may be followed by the eagles’ descendants adapting to these changes. And on it goes, this evolution of the now blending with the then, a reminder that these days of the dead affect those of the living, as well as those not yet alive.

The Paleozoic Diet Plan

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

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

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

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

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

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

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

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

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

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

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

My reply to this:

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

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

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

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

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

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

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

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

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

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

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

How Did Freshwater Crayfish Get on a Barrier Island?

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.”

Tracking Wild Turkeys on the Georgia Coast

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.

Deconstructing an Ichnology Abstract, with Alligators

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.


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.



Descent with Modification

At this time last year, Fernbank Museum of Natural History was hosting the Darwin exhibit. On loan from the American Museum of Natural History, this exhibit was a major coup for the museum and the Atlanta area, which has enjoyed a growing culture of celebrating science during the past few years. Along with this exhibit, the museum also planned and concurrently displayed an evolution-themed art show, appropriately titled Selections, which I wrote about then here.*

Descent with Modification (2011), mixed media (colored pencils and ink) on paper, 24″ X 36.” Although this artwork might at first look like a tentacled creature infested with crustaceans and living on a sea bottom, its main form actually mimics a typical burrow system made by ten-legged crustaceans (decapods). Yet it’s also an evolutionary hypothesis. Intrigued? If so, please read on. If not, there are plenty of funny cat-themed Web sites that otherwise require your attention. (Artwork and photograph of the artwork by Anthony Martin.)

One unusual feature of this art show was that five of the eight artists were also scientists (full confession: I was one of them). Furthemore, one of the other artists was married to a scientist (fuller confession: that would be my wife Ruth). The show stayed up for more than three months, which was also as long as the Darwin exhibit resided at Fernbank. Thus we like to think it successfully exposed thousands of museum visitors to the concept that scientists, like many other humans, have artistic inspirations and abilities, neatly refuting the stereotype that not all of us are joyless, left-brained automatons and misanthropes.

Last week I was reminded of this anniversary and further connections between science and art during a campus visit last week by marine biologist and crustacean expert Joel Martin (no relation). Dr. Martin was invited to Emory University to give a public lecture with the provocative title God or Darwin? A Marine Biologist’s Take on the Compatibility of Faith and Evolution. His lecture was the first of several on campus this year about the intersections between matters of faith and science, the Nature of Knowledge Seminar Series. This series was organized as a direct response to the university inviting a commencement speaker this past May who held decidedly strong and publicly expressed anti-science views.

Dr. Martin, who is also an ordained elder in his Presbyterian church and has taught Sunday school to teenagers in his church for more than 20 years, gave an informative, organized, congenial, and otherwise well-delivered presentation to an audience of more than 200 students, staff, faculty, and other people from the Atlanta community. In his talk, Martin effectively explored the false “either-or” choice often presented to Americans who are challenged by those who unknowingly misunderstand or deliberately misrepresent evolutionary theory in favor of their beliefs. Much of what he mentioned, he said, is summarized in a book he wrote for teenagers and their parents, titled The Prism and the Rainbow: A Christian Explains Why Evolution is Not a Threat.

I purposefully won’t mention any of the labels that have been applied to the people and organizations who promote this divisiveness between evolutionary theory and faith. After all, words have power, especially when backed up by Internet search engines. Moreover, it is an old and tired subject, of which I grow weary discussing when there is so much more to learn from the natural world. Better to just say that Martin persuasively conveyed his personal wonder for the insights provided by evolutionary theory, how science informs and melds with his faith, and otherwise showed how science and faith are completely compatible with one another. You know, kind of like science and art.

Previous to his arrival, his host in the Department of Biology asked Emory science faculty via e-mail if any of us would like to have a one-on-one meeting with Dr. Martin during his time here. I leaped at the chance, and was lucky enough to secure a half-hour slot in his schedule. When he and I met in my office, we had an enjoyable chat on a wide range of topics, but mostly on our shared enthusiasm for the evolution of burrowing crustaceans, and particularly marine crustaceans.

Ophiomorpha nodosa, a burrow network in a Pleistocene limestone of San Salvador, Bahamas. In this instance, the burrows were probably made by callianassid shrimp, otherwise known as “ghost shrimp,” and are preserved in what was a sandy patch next to a once-thriving reef from 125,000 years ago. (Photograph by Anthony Martin.)

Interestingly, during this conversation we also touched on on how art and science work together, and I was pleasantly surprised to find out that Dr. Martin is a talented artist, too. It turns out he has illustrated many of his articles with exquisite line drawings of his beloved subjects, marine crustaceans. Yes, I realize that some artists like to draw a line (get it?) between being an “artist” and an “illustrator,” with the latter being held in some sort of disdain for merely “copying” what is seen in nature. If you’re one of those, sorry, I don’t have the time or inclination to argue about this with you. (Now go back to putting a red dot on a white canvas and leave us alone.)

Cover art of branchiopod Lepidurus packardi from California, drawn by Joel W. Martin, for An Updated Classification of the Recent Crustacea, also co-authored by Joel W. Martin and George E. Davis: No. 39, Science Series, Natural History Museum of Los Angeles County, Los Angeles, California.

During our discussion in my office, I pointed out a enlarged reproduction of a drawing of mine depicting the burrow complex of an Atlantic mud crab (Panopeus herbstii). He immediately recognized it as a crustacean burrow, for which I was glad, because it is an illustration of just that in my upcoming book, Life Traces of the Georgia Coast.

Burrow complex made by Atlantic mud crab (Panopeus herbstii), originally credited to a snapping shrimp (Alpheus heterochaelis). Scale = 5 cm (2 in). (Illustration by Anthony Martin, based on epoxy resin cast figured by Basan and Frey (1977).

After his campus visit, though, I realized that an even more appropriate artistic work to have shown him was the following one made for the Selections art exhibit last fall, titled Descent with Modification. This title in honor of the phrase used by Charles Darwin to describe the evolutionary process, but also is a play on words connecting to the evolution of burrowing crustaceans.

Descent with Modification again, but this time look at it as an evolutionary chart, where the burrow junctions in the burrow system reflect divergence points (nodes) from common ancestors. For example, from left to right, the ghost shrimp is more closely related to a mud shrimp, and both of these are more closely related to the ghost crab (middle) than they are to the lobster and freshwater crayfish (right). The main vertical burrow shaft represents their common ancestry from a “first decapod,” which may have been as far back as the Ordovician Period, about 450 million years ago.

The image shows five burrowing crustaceans, or to be more specific, ten-legged crustaceans called decapods. Along with these is a structure, which has a burrow entrance surrounded by a conical mound of excavated and pelleted sediment, a vertical shaft connecting to the main burrow network, and branching tunnels that lead to terminal chambers. A burrowing crustacean occupies each chamber, and these are, from left to right: a ghost shrimp (Callichirus major), a mud shrimp (Upogebia pusilla), a ghost crab (Ocypode quadrata), a marine lobster (Homarus gammarus), and a freshwater crayfish (Procambarus clarkii).

Here’s the cool part (or at least I think so): this burrow system also serves as an evolutionary chart – kind of a cladogram – depicting the ancestral relationships of these modern burrowing decapods. For example, lobsters and crayfish are more closely related to one another (share a more recent common ancestor) than lobsters are related to crabs. Mud shrimp are more closely related to crabs than ghost shrimp. Accordingly, the burrow junctions show where these decapod lineages diverged. So the title of the artwork is a double entendre with reference to Darwin’s phrase describing evolution as a process of “descent with modification,” along with burrowing decapods undergoing change through time as they descend in the sediment.

Modern decapod burrows and trace fossils of probable decapod burrows support both the science and the artwork, too. Here are a few examples to whet your ichnological and aesthetic appetites:

Thalassinoides, a trace fossil of horizontally oriented and branching burrow systems made by decapods in Early Cretaceous rocks (about 115 mya) of Victoria, Australia. In this case, these burrows were likely by freshwater decapods, such as crayfish, which had probably diverged from a common ancestor with marine lobsters more than 100 million years before then. Scale = 10 cm (4 in). (Photograph by Anthony Martin.)

Thalassinoides again, but this time in limestones formed originally in marine environments, from the Miocene of Argentina. Note the convergence in forms of the burrows with those of the freshwater crayfish ones in Australia. Think that might be related to some sort of evolutionary heritage? Scale = 15 cm (6 in). (Photograph by Anthony Martin.)

Horizontally oriented burrow junction of a modern ghost shrimp – probably made by a Carolina ghost shrimp (Callichirus major) – exposed along the shoreline of Sapelo Island, Georgia. Note the pelleted exterior, which is also visible on the burrow networks of the fossil ones in the Bahamas, pictured earlier. So if fossilized, this would be classified as the trace fossil Ophiomorpha nodosa. Scale in centimeters. (Photograph by Anthony Martin.)

Two ghost-shrimp burrow entrances on a beach of Sapelo Island, Georgia, with the one on the right showing evidence of its occupant expelling water from its burrow. No scale, but burrow mound on right is about 5 cm (2 in) wide. (Photograph by Anthony Martin.)

Burrow entrance and conical, pelleted mound made by a freshwater crayfish (probably a species of Procambarus) in the interior of Jekyll Island, Georgia. Scale = 1 cm (0.4 in). (Photograph by Anthony Martin.)

So the take-away message of all of these musings and visual depictions is that evolution, faith, science, art, trace fossils, modern burrows, and burrowing decapods can all co-exist and be celebrated, regardless of whether we sing Kumbaya or not. So let’s stop dividing one another, get out there, and learn.

*I’m also proud to say that my post from October 17, 2011, Georgia Life Traces as Art and Science, was nominated for possible inclusion in Open Laboratory 2013. Thank you!

Further Reading

Basan, P.B., and Frey, R.W. 1977. Actual-palaeontology and neoichnology of salt marshes near Sapelo Island, Georgia. In Crimes, T.P., and Harper, J.C. (editors), Trace Fossils 2. Liverpool, Seel House Press: 41-70.

Martin, A.J. In press. Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press, Bloomington, IN: 680 p.

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.

Martin, J.W. 2010. The Prism and the Rainbow: A Christian Explains Why Evolution is Not a Threat. Johns Hopkins University Press, Baltimore, MD: 192 p.

Martin, J.W., and Davis. G.E. 2001. An Updated Classification of the Recent Crustacea, No. 39, Science Series, Natural History Museum of Los Angeles County, Los Angeles, California: 132 p.


Darwin, Worm Grunters, and Menacing Moles

In my most recent previous post, I teased readers with the promise of revealing how Charles Darwin used a piano as a scientific tool for studying the behavior of earthworms. Regardless of whether or not you already looked up the answer through The Google, by reading Darwin’s last book (The Formation of Vegetable Mould through the Action of Worms with Observations on Their Habits), or other means, I will now gladly make connections between the seemingly disparate subjects of Darwin’s musically inclined experimentation, earthworm behavior, and fishermen of the southeastern U.S. catching earthworms as bait.

What makes this earthworm (Diplocardia) run away as fast as its little chetae, mucus, and peristalic movement can carry it through the soil? Let’s just say it’s not picking up good vibrations. Photograph by Bruce A. Snyder, from here, from www.discoverlife.org.

In writing about earthworms and their traces in my upcoming book, I devoted several pages to Mr. Darwin’s fascination with earthworms. In this exploration, I tell how Darwin was on to something when he tried applying sound – which included those made by playing musical instruments – to earthworms he had gathered from the English countryside. These musical performances were not an instance of Darwin trying to entertain these worms, boost their self esteem, or otherwise help them get in touch with their emotions. Rather, he was simply testing whether worms reacted to sound. What happened? Well, instead of me describing his results, I’ll let Darwin’s words inform you directly:

Worms do not possess any sense of hearing. They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest tones of a bassoon. They were indifferent to shouts, if care was taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet.

Charles Darwin, The Formation of Vegetable Mould through the Action of Worms with Observations on Their Habits (1881), p. 27.

Hence it was with deep appreciation last month when I gazed at the piano in the drawing room of Down House, the former Darwin family home, and thought about these experiments. Smiling, I imagined Darwin carefully watching a container of worms while he or someone else in his family forcefully banged on the keys of this piano. Of course, you also can’t help but wonder what was played “as loudly as possible.” Were these single, random notes, chords, or actual musical compositions? If the last of these, what pieces were played? Ideally, I like to think Mr. Darwin or one of his family members played a sea shanty learned during his days on The Beagle (or perhaps even songs learned from pirates), rather than just pounded random notes up or down a scale.

As conclusive as Darwin’s paragraph might seem about the lack of earthworm reactions to sound, he, like any good storyteller, then injected a dramatic twist when reporting his results. He followed up the preceding paragraph with one describing how earthworms, although deaf, are extremely sensitive to vibrations transmitted through solid media. Here he revealed exactly which notes were played (C on the bass clef, G in the treble clef, C in the treble clef) while two worms were in pots placed on top of the piano.

The vibrations transmitted through solid media – not air – caused the worms to withdraw from the soil surface, presumably hiding from the source of the vibrations. As an extension of this experiment, Darwin also used a fork to agitate the soil underneath other worms, which then provoked them to move up to the surface. Darwin correctly surmised that this stirring activity, like sound, also sent vibrations through the soil, which likewise produced aversive reactions in the earthworms.

These responses made sense in an evolutionary way, and show how Mr. Darwin was applying his principle of natural selection to the predator-prey relationships that had evolved between earthworms and moles. The behaviors he observed would have favored the survival of earthworms that associated vibrations with their most feared predators, and reacting accordingly, which is to say, fleeing in terror. And just what were their aversion-inducing predators? They were not robins or other species of birds – early, punctual, or otherwise timed – but the earthworm version of graboids: burrowing moles.

Eastern mole (Scalopus aquaticus) emerging from its burrow, seeking earthworms and other fresh food. Photograph by Kenneth Catania, from Fairfax County Schools.

Graboid emerging from its burrow, seeking humans and other prey. Note the eerie resemblance of its behavior to that of an eastern mole, albeit orders of magnitude larger and accompanied by a keen interest in large, surface-dwelling, bipedal prey. Photo from Wikipedia, but originally taken from the greatest ichnologically inspired horror film of all time, Tremors.

So you didn’t know about graboids, those burrowing predators of the underworld? Fortunately, this educational video provides all of the details you need to know. But if you’re interested in studying their neoichnology, be careful, and stay on the pavement.

As yet another example of ‘backyard science,” Darwin observed many traces of the European mole (Talpa europaea) in the fields just outside Down House, most of which were their mounds, or “molehills.” Indeed, last month as I admired one of Darwin’s original wormstones in the pasture behind Down House, I also noticed a good number of molehills on the grounds. Rather stupidly, I neglected to take a photo of one of these. (I mean, how cool would it have been to share images of the traces of moles that descended from those whose traces Darwin noticed?) Nonetheless, some of my photos of the grassy area near the wormstone show 20-30 cm wide bare patches in this otherwise meticulously maintained lawn. These spots, I suspect, are traces of the Down House groundskeepers, who probably level molehills as quickly as they appear, an ichnological version of “whack a mole.”

The pasture just behind Down House (Charles Darwin’s former home), with a “wormstone” in the lower right, and a few bare patches of ground just to the left. Could the latter mark recent sites of mole tunnels and molehills leveled by Down House groundskeepers, or are these just places where grass did not grow, and hence the products of an ichnologist’s overactive imagination? Anyway, I did see molehills out there, but don’t blame y’all for being a bunch of skeptical scientists and wanting more evidence than my just saying so.

OK, now how does all of this wonderfully elucidated Victorian-era science relate to the ecosystems and biota of the southeastern United States? Enter the “worm grunters.” Worm grunters are people who, independently of Darwin, figured out the same adaptive responses of earthworms to underground vibrations. Through their own experiments, worm grunters, who were interested in efficiently gathering many worms in a short time for putting on fishhooks (or making money selling earthworms to people who put them on hooks), rubbed steel slabs across the top of wooden posts stuck in the ground. Much later, researchers interested in finding out how this technique worked calculated frequencies of the seismic vibrations that caused earthworms to flee upward away from perceived predators.

The southeastern U.S., including the Georgia barrier islands, not only has its own species of earthworms (Diplocardia mississippiensis), but also has its own species of moles: the eastern mole (Scalopus aquaticus) and the less common star-nosed mole (Condylura cristata). Both types of moles no doubt strike fear in the multiple hearts of earthworms, and natural selection being how it is, the fastest burrowing moles (who are most likely to catch worms) also cause considerable vibrations from their digging. This accordingly means the earthworms that detect and escape these vibrations live long enough to reproduce and pass on whatever genes that aided in such perceptions.

In getting caught by this mole, this earthworm may have just won the worm equivalent of a Darwin Award, depending on whether it had reproduced or not. (Which it probably did, considering earthworm hermaphroditism means they are at least twice as likely to get lucky.) Photo from University of Illinois Extension; Home, Yard, and Garden Pests Newsletter, here.

Thus a visit to Down House in southern England and consideration of Darwin’s contributions to ichnology and behavioral ecology are not so far removed conceptually from the practical knowledge gained by some people in parts of the southeastern U.S. Moreover, many of these same people are of English, Irish, or Scottish descent, and effectively applied the same knowledge surmised by Darwin about worms and moles, which is kind of neat in a heritage sort of way.

Would all of these findings count as applied science, despite its historical lack of Ph.D.-bearing investigators, grant funding, publications, and press conferences announcing the results? Yup. After all, science is about its methods.

So next week, we’ll take a closer look at the traces moles make on the Georgia barrier islands. Do these moles just go after earthworms in the forests and meadows of those islands? Nope. After all, science is not just about its methods, but also surprises.

Further Reading

Darwin, C. 1881. The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits. John Murray, London, U.K.: 326 p.

Edwards, C.A., and Bohlen, P.J. 1996. Biology and Ecology of Earthworms (3rd Edition). Springer, Berlin: 426 p.

Gorman, M.L., and Stone, R.D. 1990. The Natural History of Moles. University of Chicago Press, Chicago, Illinois: 138 p.

Hendrix, P.F. 1995. Earthworm Ecology and Biogeography in North America. CRC Press, Boca Raton, Florida: 244 p.

Mitra, O., Callaham, M.A., Jr., and Yack, J.E. 2009. Grunting for worms: seismic vibrations cause Diplocardia earthworms to emerge from the soil. Biology Letters, 2009: 16-19.