Alligators swim, and they walk. However, most people who live in coastal areas with American alligators (Alligator mississippiensis) – such as the Georgia barrier islands – also know that alligators can (and do) swim in the open ocean, and that they can (and do) walk long distances overland on beaches and dunes. Still, despite many visits to Georgia coast barrier islands hosting healthy populations of alligators, I have not yet witnessed either behavior. Fortunately, I’m an ichnologist, so I don’t have to just take the word of local residents or actually see these ‘gator behaviors to know they happen. Tracks and other traces are there to inform, letting me know where these alligators go, what they are doing, and when they are doing whatever alligators do when human eyes are not watching.
Fresh tracks and tail-drag traces of a large adult alligator (Alligator mississippiensis) going for a stroll on a beach after an open-ocean swim. Where did it go after crossing the beach? Find out for yourself in the following video. (Yours Trult for scale; photo taken by Ruth Schowalter on Sapelo Island, Georgia.)
Since the end of my academic year in May, I have spent much of the summer at home in Decatur, Georgia writing my next book. I’m pleased to report that I made good progress on that writing, but I really needed a break from it, and one that took me away from home to some other place for a mental shift. That “other place” was Sapelo Island on the Georgia coast, where my wife Ruth and I got in three days of glorious field work. And among the many ichnological and other nature-related wonders we encountered were these alligator tracks.
I’ll let the following video do the talking for me, and I mean that literally, as it is me talking in the video. Because I used my digital camera as the video-recorder, the sound quality isn’t perfect (wind intrudes), but should be 95% understandable. Also, the camera lens had a smudge that I didn’t notice until later, which makes the image a little blurry in spots. So if you can filter out both of these audio and video flaws, you just might enjoy walking with those Sapelo Island alligators, tracking them from the ocean to, well, you’ll have to watch and see.
Writing about a place, its environments, and the plants and animals of those environments is challenging enough in itself. Yet to write about that place and what lives there, but without actually being there, seems almost like a type of fraud. Sure, given a specific place, I could read everything ever published about it, watch documentaries or other videos about it, carefully study 3-D computer-rendered images of its landscapes, interview people who have spent much time there, and otherwise gather information vicariously, all without experiencing it directly. But then is my writing just about the shadows on the wall of the cave?
What do you see in this photo? I see fine quartz and heavy-mineral sand, originally parts of much larger rocks and forming parts of the Appalachian Mountains. I see the sand blowing down a long beach, but pausing to form ripples. I see a river otter galloping alongside the surf, slowing to a lope, then a trot, then back to a lope and a gallop. I see a brief rain shower, only about two hours after the otter has left the beach. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)
This pondering, of course, brings us to river otters. Yesterday, while on the third of a four-day writing retreat to Sapelo Island on the Georgia coast, my wife Ruth and I spent nearly an hour tracking a river otter along a long stretch of beach there. Had I read about river otters and their tracks before then? Yes. Had I watched video footage of river otters? Yes. Had I written about river otters and their tracks before then? Yes. Had I seen and identified their tracks before then? Yes. Had I seen river otters in the wild for myself? Yes, yes, and yes.
But still, this was different. When I first spotted the tracks on the south end of a long stretch of Cabretta Beach on Sapelo, I thought they would be ordinary. Granted, finding otter tracks is always a joy, especially when I’ve seen them on stream banks in the middle of Atlanta, Georgia. (Seriously, folks: river otters live in the middle of Atlanta. How cool is that?) And because Sapelo only has a few humans and is relatively undeveloped, your chances of coming across otter tracks on one of the beaches there isn’t like winning a lottery. But still.
River otter (Lutra canadenis) tracks in what I (and some other trackers) call a “1-2-1” pattern. For gait, that translates into a “lope,” which is typical for an otter. In this pattern, one of the rear feet exceeds the front foot on one side, but the other rear foot ends up beside that same front foot; one front foot is behind. If that second rear foot lags behind the front foot, then it’s a “trot,” but if it exceeds the front foot (both rear feet ahead of both front feet), then that’s a “gallop.” Also, check out the wind ripples beneath the tracks, and raindrop impressions on top of them. (Photo by Anthony Martin, taken on Sapelo Island, Georgia; photo scale in centimeters, with the long bar = 10 cm (4 inches))
What made these tracks different was that they went on, and on, and on. These otter tracks spoke for the otter, saying in no uncertain terms that walking, trotting, loping, and galloping on a beach was the only thing it had on its schedule that morning. For nearly a kilometer (0.6 miles), we followed its tracks in the sandy strip of land between the high-tide line on the right and low coastal dunes on the left.
Follow the river otter tracks for as far as you can in this photo. Then, when you can’t see them any more, decide where it went. Does that sound like a challenge? It probably would be if you’ve only written about tracking otters, but it can be tough for experienced trackers, too. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)
The tracks were only a few meters away from high tide, but sometimes turned that way, vanished, then reappeared further down the beach. This told us the otter was out close to peak tide that morning (between 6-8 a.m.) and was mixing up its exercise regime by occasionally dipping into the surf. Raindrop impressions on top of the tracks confirmed this, as the tracks looked crisp and fresh except for having been pitted by rain. For us, rain started inland and south of there on the island around 10 a.m., but reached the tracks sooner than that. We were there about three hours after then, so the otter was likely long gone, on to another adventure. Nonetheless, we made sure to look up and ahead frequently, just in case the trackmaker decided to come back to the scene of his or her handiwork.
For those of you who are intrigued by animal tracks (and why would you not be?), I suggest you try following those made by one animal, and follow it for as long as you can. That way you can learn much more about it as an individual animal, rather than just its species name. In my experience, after tracking an animal for a long time, nuances of its behavior, decisions, and even its personality emerge.
For example, this otter was mostly loping (its normal gait), but once in a while slowed to a walk or trot, or sped up, when it galloped. In short, the tracks showed enough variations to say that the otter was likely reacting to stimuli in its surroundings, and in many different ways. What gave it a reason to slow down? What impelled it to move faster? Why did it jump into the surf when it did, and why did it come out? Or, do otters just want to have fun?
Gallop pattern for a river otter, in which both of its rear feet exceeded the front feet, making a group of four tracks. In this instance, the group defines a “Z” pattern when drawing a line from one track to another, but gallops sometimes also produce “C” patterns. Notice also how the groupings are separated by a space with no tracks. This is also diagnostic of a gallop pattern: the longer the space, the longer the “air time” for the animal, when it was suspended above the ground between when its feet touched the ground. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)
Now I realize that discerning a “personality” and “moods” of a non-human animal based on a series of its tracks might sound like a little too “woo-woo” and “New Agey” for my skeptical scientist friends to accept, followed by jokes about my becoming a pet psychic. As a fellow skeptical scientist, I’m totally OK with that. In fact, I will join them in making fun of people who try to tell us that, say, they know what a Sasquatch was thinking as it strolled through a forest while successfully avoiding all cameras and other means of physical detection.
But here’s what happens when you’ve tracked a lot (which I have) and made lots of mistakes while tracking, but later corrected them (ditto). Intuition kicks in, and it usually works. For instance, at one point in following this otter, I lost its tracks on a patch of hard-packed sand. (Granted, I should have gotten down on my hands and knees to look closer, but was being lazy. Hey, come on, I was on a writer’s retreat.) So I then asked myself, “Where would I (the otter) have gone?” and looked about 10 meters (30+ feet) ahead in what felt like the right place. There they were. This happened three more times, results that led me to conclude this was almost like some repeatable, testable, falsifiable science-like thing happening. So there.
OK, remember when I asked you to follow the river otter tracks for as far as you could in this photo, and when you couldn’t see them any more, decide where it went? If not, go back and re-read it and look at the photo again. If you have, then look at the red arrow, backtrack to the footprints in the foreground of the photo, then go forward. Do you see how the tracks are staying in the subtly lower area, just left of the slightly higher sand piled on the plant debris? Keep picking out those low areas, and you’ll end up where the arrow is pointing. After all, if I were an otter, that’s where I would go. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)
Oh yeah, regarding my main topic sentence: What’s all this have to do with writing about a place? Well, because of that otter and its tracks, I now understand at least one otter much better than before, and feel like I can write with a little more authority about otters in general. You know, like what you just read.
Do you understand this river otter and its place a little better now, thanks to it leaving so many tracks while it enjoyed a morning at the beach, and because I tracked it for such a long time, and then wrote about that experience in that same place? Please say “yes,” as I want to keep writing about stuff like this. P.S. Thanks to Sapelo Island, this river otter, and my wife Ruth for teaching me so much yesterday. (Photo by Anthony Martin, taken on Sapelo Island, Georgia.)
Upon learning that Godzilla would be making its way back onto movie screens this summer, my first thought was not about whether it would it would serve as a powerful allegory exploring the consequences of nuclear power. Nor did I wonder if it would be a metaphor of nature cleansing the world’s ecological ills through the deliberate destruction of humanity. Surprisingly, I didn’t even ponder whether the director of this version (Gareth Edwards) would have our hero incinerate Matthew Broderick with a radioactively fueled exhalation as cinematic penance for the 1998 version of Godzilla.
Instead, my first thought was, “Wow, I’ll bet Godzilla will leave some awesome tracks!” My second thought was, “Wow, I’ll bet Godzilla will leave some awesome bite and claw marks!” My third thought was, “Wow, I’ll bet Godzilla will leave some awesome feces!” All of these musings could be summarized as, “Wow, I’ll bet Godzilla will leave awesome traces, no matter what!”
Godzilla: King of the Tracemakers. (Image and most others here from the movie were taken as screen-capture stills from the official trailer here and modified slightly for your science-learning pleasure.)
So as an ichnologist who is deeply concerned that movie monsters make plenty of tracks and other traces whilst rampaging, I am happy to report that yes, this Godzilla and its kaiju compatriots did indeed make some grand traces. Could they have made traces worthy of ichnological appraisal, ones that could be readily compared to trace fossils made by Godzilla’s ancestors? Yes, but these traces could have been better, and let me explain why.
[Minor spoilers follow, not least of which include the not-surprising news that The King of the Monsters prevails in the end, inevitably setting up a sequel in which I sincerely hope Godzilla and his rivals make more easily defined traces.]
Early on in the movie – set in 1999 – a surface mine in the Philippines collapses. Drs. Ishiro Serizawa (Ken Watanabe) and Vivienne Graham (Sally Hawkins) are summoned to the site and quickly whisked underground. There they find a spacious chamber containing body fossils – bones or similar endoskeletal parts – of an enormous creature. Instantly, I began yawning. I mean, body fossils: how boring.
A bit of paleontology near the start of Godzilla, in which some of the humans (who are mostly irrelevant) find skeletal remains underneath a surface mine. Little do they know they’re about to undergo enlightenment and become ichnologists.
But then I sat upright in my seat when I realized – along with the screen scientists – that this chamber wasn’t a mere tomb, but also a place of rebirth: it was a hatching chamber. Views from inside and outside of the chamber then revealed the ichnological money shots of the movie, showing first an emergence burrow, then an emergence crater* connecting to a trail, the latter cutting a swath through the forest and leading directly to the sea. This was trace evidence of a yet-unseen monster that was very much alive, and one that was brooded and born in a subterranean terrestrial environment, but then moved to an oceanic environment.
Dr. Serizawa sees light at the end of the tunnel, and it’s not from an oncoming train, but something far worse. Still, it’s a cool example of an emergence burrow, so there was some consolation.
Kaiju emergence burrow connected with a kaiju trail, leading to the sea. So this is definite trace evidence of a heterometabolous animal, with different stages of its metamorphism (terrestrial egg –> marine larva) taking place in different environments. Unlike, you know, Gregor Samsa, who just stayed terrestrial.
A map of seismic signatures shown later in the film denoted where the animal burrowed in the seafloor from the Philippines to Japan, which would have made for one hell of a burrow. Why was this massive animal using so much energy to burrow to Japan? For some radiogenic sustenance, of course, which was conveniently located in a nuclear-power plant there. The “M.U.T.O.,” (= “Massive “Unidentified Terrestrial Object”) then caused a collapse of that power plant, thus qualifying as a feeding trace, rather than plate-tectonic-induced earthquake damage, which is what became the official story. That’s right, geophysicists: you’d better start studying some ichnology if you want to correctly interpret what’s causing those rapid releases of tensional energy that excite you so much. (I’m talking about earthquakes, you perverts.)
Anyway, people die, 15 years pass, families grow apart, blah blah blah, when the action finally returns to something that really matters, like monsters making traces. It turns out the Japanese government had been hiding the truth from the public, which, much like Tom Cruise, can’t handle it. The kaiju not only fed on a nuclear reactor in Japan, but also pupated there. As an example of how gigantic, deadly animal traces can be the real “job creators” in a modern economy, a huge industrial complex with hundreds of Japanese and American employees was monitoring the cocoon, with Drs. Serizawa and Graham as scientific advisors.
Who knew these actors – Ken Watanabe and Vivienne Graham – were actually playing ichnologists in the new Godzilla movie? Just about nobody, including them. (Photograph originally credited to Kimberley French, AP, and much reproduced elsewhere.)
The adult M.U.T.O. that emerged from the cocoon fractured the outer casing, broke through the steel cables that were supposed to restrain it, and immediately started making some tracks. So those are some mighty fine traces, and it was a pleasure watching them get made.
What about its tracks, though? Despite the kaiju’s blend of tetrapod and insect qualities, it had eight appendages and used six while walking – four forelimbs, two of which were wings, and two hindlimbs – making it hexapedal. Moreover, it used an alternating gait, similar to those used by pterosaurs or bats (if they had an extra pair of limbs, that is). Hook-like ends on the forelimbs would have made elongate impressions, and literally impressed a few panicked employees as the monster escaped. On the other hand, er, appendage, the hindlimbs looked as if they were terminated by flat-bottomed hooves. So if one were inclined to track this M.U.T.O, its trackway patterns might have looked like the following:
Hypothesized male (winged) M.U.T.O. trackway pattern, moving from left to right, showing normal walking that ends with take-off. Wing impressions are on the outside and angled, whereas the forelimb tracks are just inside the trackway, and the hindlimb tracks are closest to the midline. Take-off pattern is at the end, with wing impressions forward so that, like a giant pterosaur, it could “pole vault” for its launch. What’s the scale? Really big. (Illustration by Anthony Martin.)
Toward the end of this scene, we find out this kaiju was also flight capable, as it takes off from its former pupation site. Accordingly, it would have made both take-off and landing track patterns, which have been interpreted in the fossil record for pterosaurs and birds, but from nothing nearly as big. (Oh, how I dream of finding Queztalcoatlus take-off or landing tracks some day…) This switch from terrestrial to aerial locomotion is noted in one of the few funny lines uttered in the movie, when U.S. Navy Admiral William Stenz (David Strathairn) first refers to the kaiju as a M.U.T.O., but then updates the status of its behavioral ecology by saying, “It is, however, no longer terrestrial, as it is airborne.”
Later in the movie, another tracemaking M.U.T.O. emerges from its pupation site –a nuclear-waste repository in Yucca Mountain, Nevada – and proceeds to leave a trail of devastation through Las Vegas, which included killing lots of people who probably bet that wouldn’t happen to them.
Leaving Las Vegas, female M.U.T.O. style, with a well-defined trail in its wake, and perhaps knowing it should have taken a left turn at Albuquerque. Hey, U.S. military: I think it went that way!
This kaiju was female and much larger than the male, thus providing a great example of sexual dimorphism in tracemakers of the same species, as seen in horseshoe crabs (limulids) and many other animals. This meant its trackway width would have been correspondingly wider than that of the male, and its tracks larger. It also lacked wings, with the homologous pair of limbs used instead for walking. Consequently, the kaiju’s locomotion (and hence its tracemaking) was restricted to terrestrial environments, with no take-off or landing tracks. So if any more of these monsters came out of the ground, such ichnological knowledge might come in handy for the U.S. military (or recreational hunters) to know which gender of a M.U.T.O. pair they might be tracking.
Bioerosion trace (boring) made by M.U.T.O. as it encountered a human commerce-generating hive in San Francisco. Unlike most bioeroson structures, this is a locomotion trace, rather than a dwelling or feeding trace.
Other tracemaking done by the M.U.T.O.s included mastication marks on a Russian nuclear submarine and some ICBMs, a little bit of bioerosion when they walked through buildings, and – following some kaiju courtship and sexy time – a nest structure made in San Francisco (no doubt inspiring a new song titled I Left My M.U.T.O. Nest in San Francisco). The nest structure was in the style of those made by many shorebirds, looking like a scratched-out hollow, with the trivial differences of being hundreds of meters across, about a hundred meters deep, and composed of urban debris. The fertilized eggs were in the middle of the structure and attached to an ICBM, like a sort of atomic yolk sac. Overall, it was a tremendous nest structure, dwarfing those likely made by the largest known sea turtle, Archelon from the Late Cretaceous Period, which would have been a mere 10-15 m (33-67 ft) across.
OK, enough about the M.U.T.O. tracemakers. What about our beloved behemoth, The King of the Monsters, The Stomper with the Chompers, Godzilla? The movie – much like this review – held him back until about an hour into the story, only giving us teasing glimpses from photographs over the past 60 years. Sure, this was done deliberately to build suspense, but the title of the movie wasn’t M.U.T.O.s Making Traces (although it could have been, and I would’ve been fine with that). So I was more than ready for Godzilla to leave some tracks, bite marks, and other megatraces that would have made the world’s largest dinosaurs’ traces look puny by comparison.
Tracks on the left are of a sauropod dinosaur trackway in an Early Cretaceous (about 100-million-years-old) limestone bedrock in the Paluxy River of Texas. Tracks on the right are in rocks of same age and area, with left-side front- and rear-foot tracks; the stick is a meter long. For comparision, one Godzilla track would exceed the width of the river. (Both photographs by Anthony Martin, taken in Dinosaur Valley State Park, Texas; to read more about those tracks, go here.)
Did Godzilla leave any clearly defined tracks in the film? Oddly enough, no: imagine my disappointment. Such a glaring ichnological absence led me to believe that Godzilla tracks must not have been a high priority in director Gareth Edwards’s mind while making the film. This is also a rare instance of where the 1998 version of Godzilla surpassed the 2014 one, in that a few nicely outlined tracks were shown in the former.
However, had Edwards decided to add the scientific excitement that would have been induced by overhead views of Godzilla tracks, they would have looked a lot different from the 1998 ones. Although all movie versions of Godzilla have shown it as bipedal on land, the monsters’ feet have been different. For instance, the 1998 Godzilla tracks were definitely modeled after those of theropod dinosaurs, with three separated and forward-pointing toes adorned by sharp claws, albeit greatly up-scaled. According to a reporter in Hawaii who saw one of the Godzilla footprints, he estimated it was about 12 feet long (3.6 m). So using a footprint formula applied to theropod dinosaurs, where the footprint length is multiplied by 4.0, the hip height of that Godzilla would have been 48 feet (14.5 m).
For those of you who have a monster foot fetish, you’re in for a treat. This video shows nothing but close-ups of Godzilla‘s feet landing on and crushing stuff in the 1998 movie.
In contrast, the new Godzilla not only had a pedicure, but also a major foot makeover. Instead of three separate toes, this one has four toes scrunched together into more of an elephantine or sauropod-like configuration. It still has claws, but they look much more robust than those of the previous theropod-like feet of its predecessor, and more like those of a sauropod. Accordingly, Godzilla tracks from the 1998 movie versus the 2014 one would have been way different from one another. This means that a skilled movie-consulting ichnologist could have easily distinguished the two films just by glancing at tracks shown in each. (Mr. Edwards, please do keep me in mind if you need an ichnological advisor for Godzilla 2.)
(Right) Right-foot anatomy of 2014 version of Godzilla, nearly as wide as long and with four digits ending in stout claws. (Left) Hypothesized trackway pattern for present version of Godzilla, using its normal city-destroying gait. Notice its wide stance, like that of a certain retired U.S. senator. A tail drag-mark is not included in this diagram, but probably would have registered once Godzilla stood more upright, such as to kick some M.U.T.U. abdomen. (Both illustrations by Anthony Martin, but foot anatomy is composite drawn freehand from unattributed online photos, such as this one.)
Something important to also note about these trackways is the lack of any tail drag marks. This is because both the 1998 and 2014 Godzillas kept their tails off the ground, which aligns with modern interpretations of how theropod dinosaurs walked. The original Godzilla – and many sequels after it – showed it dragging a weighty tail behind it. This behavior would have left a deep groove in the middle of the trackway, perhaps with a slight undulating pattern caused by side-by-side movement. This would have looked sort of like an alligator or crocodile trackway, but with only right-left tracks, because Godzilla was walking more like some guy wearing a rubber suit.
Still taken from original 1954 Godzilla (Gojira), showing a bipedal trackway going from a terrestrial to marine environment. But also check out the prominent groove in the middle of the trackway, caused by a tail dragging behind it, and four forward-pointing toes on each foot.
What other traces would I have really liked to see Godzilla make, ones that would have made me stand up in the theater and scream “Ichnology for the win”? My #1 and # 2 choices, in that order, would have been urination marks and feces. In my latest book, Dinosaurs Without Bones(2014, Pegasus Books), I’ve written about trace fossils linked with dinosaur urination and defecation; dinosaur coprolites in particular are great trace fossils for showing what dinosaurs had for lunch millions of years ago. Alas, Godzilla performed neither excretory behavior in the movie, but that didn’t stop at least one scientist from speculating on how much urine this Godzilla would have produced.
So for my upcoming post, I’ll explore the possibility of a Godzilla urination trace. What mark would Godzilla have left if he got really pissed? Tune in next week, and in the meantime, enjoy seeing the movie. but now with an added ichnological perspective.
*Just as a cool astronomical-geological-ichnological-cultural aside, indigenous Australians first interpreted a meteorite impact structure in Wolfe Creek Crater National Park of Western Australia as an emergence crater made a great, burrowing snake. Some stories that involve traces seem to repeat themselves in our human history.
Life can certainly imitate art, as can life traces. I was reminded of this last week while doing field work on St. Catherines Island (Georgia), and after encountering traces made by two very different animals, alligators and fiddler crabs. What was unexpected about these traces, though, was how they intersected one another in a way that, for me, evoked scenes from the recent blockbuster summer movie, Pacific Rim.
Could these be the tracks of a kaiju, making landfall on the shores of Georgia? Sorry to disappoint you, but they’re just the right-side and very large tracks of an American alligator (Alligator mississippiensis), accompanied by its tail drag-mark, left on a sandy area next to a salt marsh. Note the scale impressions in its rear-foot track, a symbol of the awesome reptilian awesomeness of its tracemaker. But wait: what nefarious nonsense is happening to the tail drag-mark, which is being covered by tiny balls of sand? Who made that hole next to the drag-mark? And what the heck was a raccoon (Procyon lotor) doing in the neighborhood, leaving its track on the tail drag-mark? With such a monster on the loose, shouldn’t that raccoon be hiding in the forest? (Photo by Anthony Martin, taken on St. Catherines Island; scale in centimeters.)
For anyone who has not seen Pacific Rim, you can read what I wrote about its distinctive fictional ichnology here. But what came to my mind while I was doing field work was one of the themes expressed early on in the film: how quickly humanity returned to normalcy following a lull in attacks by gigantic monsters (kaiju) that emerged from the ocean, destroyed major cities, and killed millions of people. It reminded me of how horrific hurricanes can strike a coast, such as the 1893 Sea Islands Hurricane that hit Georgia, but because no hurricane like it has happened there since, coastal developers think it’s hunky-dory to start building on salt marshes.
But enough about malevolent evil as exemplified by kaiju and coastal developers: let’s get back to traces. Last week, I was on St. Catherines Island for a few days with my wife (Ruth) and an undergraduate student (Meredith) to do some field reconnaissance of my student’s proposed study area. The area was covered by storm-washover fans; these are wide, flat, lobe-shaped sandy deposits made by storm waves, which span from the shoreline to more inland on barrier islands. We were trying to find out what traces had been left on these fans – tracks, burrows, scrapings, feces, and so on – which would tell us more about the distribution and behaviors of animals living in and around the washover fans.
Part of a storm washover fan on St. Catherines Island (Georgia), with the sea to the left and salt marsh (with a patch of forest) in the background. Say, I wonder what made those tracks coming out of the tidal creek and toward the viewer? (Photograph by Anthony Martin.)
It didn’t take long for us to get surprised. Within our first half hour of walking on a washover fan and looking at its traces, we found a trackway left by a huge alligator, split in half by a wavy tail-drag mark. I recognized this alligator from its tracks, as I had seen them in almost exactly the same place more than a year before. Besides their size, though, what was remarkable about these tracks was their closeness to a salt marsh behind the washover fan. When we looked closer, we could see long-established trails cutting through the salt-marsh vegetation, which were the width of a large adult alligator.
That ain’t no skink: the distinctive tracks and tail drag-mark of a large alligator, strolling through a storm-washover fan and next to a salt marsh. You think these animals are “freshwater only”? Traces disagree. Scale = 10 cm (4 in). (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)
Alligator trail cutting through a salt marsh. Trail width was about 45-50 cm (18-20 in), which is about twice as wide as a raccoon trail. And it wasn’t made by deer or feral hogs either, because, you know, alligators. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)
So although the conventional wisdom about alligators is that these are “freshwater-only” animals, their traces keep contradicting this assumption. Sure enough, in the next few days, we saw more alligator tracks of varying sizes going into and out of tidal creeks, salt marshes, and beaches.
Based on a few traits of these big tracks, such as their crisp outlines (including scale impressions), the alligator had probably walked through this place just after the tide had dropped, only a couple of hours before we got there. But when we looked closer at some of the tracks along the trackway, we were astonished to see that something other than the tides had started to erase them, causing these big footprints to get fuzzy and almost unrecognizable.
The culprits were sand fiddler crabs (Uca pugilator), which are exceedingly abundant at the edge of the storm-washover fans closest to the salt marshes. These crabs are industrious burrowers, making J-shaped burrows with circular outlines corresponding to their body widths. They also scrape the sandy surfaces outside of their burrows to eat algae in the sand, then roll up that sand into little balls, which they deposit on the surface.
In this instance, after this massive alligator had stomped through their neighborhood, they immediately got back to work: digging burrows, scraping the surface, and making sand balls. Within just a few hours, parts of the alligator trackway was obscured. If these parts had been seen in isolation, not connected to the clear tracks and tail drag mark, I doubt we would have identified these slight depressions as large archosaur tracks.
Hey, what’s going on here? Who would dare to erase and fill in giant alligator tracks? Don’t they know who they’re dealing with? (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)
Going, going, gone: alligator tracks nearly obliterated by burrowing, surface scraping, and sand balls caused by feeding of sand fiddler crabs (Uca pugilator). (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)
What was even neater, though, was how some of the fiddler crabs took advantage of homes newly created by this alligator. In at least a few tracks, we could see where fiddler crabs had taken over the holes caused by alligator claw marks. In other words, fiddler crabs saw these, said, “Hey, free hole!”, and moved in, not caring what made them.
Don’t believe me about fiddler crabs moving into alligator claw marks? OK, then what’s that I see poking out of that alligator claw mark (red square)? (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)
Why, it’s a small sand fiddler crab! Does it care that its new home is an alligator claw mark? Nope. Does ichnology rule? Yup. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia.)
Need a free burrow? Then why start digging a new one when alligator claw marks (arrow) gives you a nice “starter burrow”? Notice the sculpted, round outline, showing the claw mark was modified by a crab. Also check out the sand balls left outside of the other claw marks, meaning these have probably been occupied and mined for food by fiddler crabs, too. (Photograph by Anthony Martin, taken on St. Catherines Island, Georgia; scale in centimeters.)
As a paleontologist, the main lesson learned from this modern example that can be applied to fossil tracks, is this: any tracks made in the same places as small, burrowing invertebrates – especially in intertidal areas – might have been destroyed or otherwise modified immediately by the burrowing and feeding activities of those much smaller animals. The secondary lesson is on how large vertebrate tracks can influence the behaviors of smaller invertebrates, resulting in their traces interacting and blending with one another.
More symbolically, though, these alligator tracks and their erasure by fiddler crabs also conjured thoughts of fictional and real analogues: Pacific Rim and coastal development, respectively. With regard to the latter, it felt too much like how, as soon as a hurricane (a meteorological “monster”) passes through a coastal area, we begin to talk about rebuilding in a way that, on the surface, wipes out all evidence that a hurricane ever happened.
Yet unlike fiddler crabs, we have memories, we have records – including the plotted “tracks” of hurricanes – and thanks to science, we can predict the arrival of future “monsters.” So the preceding little ichnological story also felt like a cautionary tale: pay attention to the tracks while they are still fresh, and be wary of those that vanish too quickly.
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.
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.)
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.
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.
Last week, while in Athens, Georgia, I found myself musing about footprints from the barrier islands of Georgia and the Cretaceous rocks of Australia, despite their separation by half a world and more than 100 million years. These seemingly random thoughts came to me during a visit to the Department of Geology at the University of Georgia to give a lecture in their departmental seminar series.
It was a pleasure speaking at the geology department for many reasons, but perhaps the most gratifying was how it was also a homecoming. I had worked on my Ph.D. there in the late 1980’s, and in 1988-1989 had taught introductory-geology classes in the very same lecture hall where I gave my presentation. Several of my former professors, who were junior faculty then, are still there and now comprise a distinguished senior faculty. So seeing them there now, their smiling faces in the audience along with the latest generation of undergraduate and graduate students, generated all sorts of warm-and-fuzzy feelings.
But enough about the present: let’s go back about 100 million years to the Cretaceous Period, which was the subject of my talk. I had actually asked to speak about the modern Georgia barrier islands and their traces: you know, the main theme of this blog and my upcoming book of the same title (Life Traces of the Georgia Coast, just in case you need reminding). Nonetheless, my host and valued friend, paleontologist Dr.Sally Walker, figured that a summary of my latest research on the Cretaceous trace fossils of Victoria, Australia would bring in a wider audience, especially if I used the magical word “dinosaur” in the title (which I did).
For my talk at the UGA Department of Geology, I could have talked about this place – St. Catherines Island, Georgia – and it’s modern traces. After all, it’s only about a four-hour drive and short boat ride from Athens, Georgia.
But instead I talked about this place – coastal Victoria, Australia – and its trace fossils from more than 100 million years ago. Which wasn’t such a bad thing.
In retrospect, she was right, and I thoroughly enjoyed putting together an informative and (I thought) entertaining presentation that shared highlights of fossil discoveries from that part of Australia during the past five years. For the benefit of the students in the audience, basic geology was woven throughout the talk, as I included facets of sedimentology, stratigraphy, geochemistry, paleobotany, paleoclimatology, plate tectonics, evolution, history of science, field methods, and oh yes, dinosaurs. (If you are interested in hearing more about the science and personal experiences behind these recent findings in Australia, these are related in another blog of mine written previous to this one, The Great Cretaceous Walk.)
So how do the barrier islands of the Georgia coast and their animal traces relate to the Cretaceous of Australia? I mentioned the main reason briefly in my talk, but will elaborate more here: I likely owed one of my most important fossil discoveries in Australia to track-imprinted memories gained from field work on the Georgia coast. The fossil find, which happened in June 2010, was of about two dozen thin-toed theropod dinosaur tracks in Cretaceous rocks along the Victoria coast. These tracks represent the best assemblage of dinosaur tracks found thus far in southern Australia, and the largest collection of polar-dinosaur tracks in the Southern Hemisphere. Moreover, some of these tracks just happened to be about the same size and forms of footprints made by sandhill cranes (Grus canadensis).
Comparison between the footprint of a sandhill crane (Grus canadensis), made in moist sand next to a freshwater pond, St. Catherines Island, Georgia (top), and a footprint made by a theropod dinosaur about 105 million years ago on a river floodplain, Victoria, Australia (bottom). Notice the resemblance?
Sandhill cranes do not normally live on the Georgia barrier islands, and nearly all of them simply fly over or stop briefly during their annual migrations from south of Georgia to the Great Plains, or vice versa. However, at least a few have settled on St. Catherines Island, the same place on the Georgia coast where I recently studied gopher tortoise burrows. According to Jen Hilburn, the island ornithologist, some of these cranes found life so comfortable on the island that they stayed. This turned out to be fortunate for me, as I became familiar with their tracks after repeated visits to St. Catherines. Even though these tall, beautiful, and majestic birds restrict themselves to just one island year-round, St. Catherines is big enough to hold a wide variety of habitats and substrates, so I have seen their tracks in salt marshes, next to fresh-water ponds, and along dusty roads throughout the entire length of the island.
Who are you calling a “dinosaur”? A sandhill crane on St. Catherines Island graciously poses for its portrait, helping this ichnologist get a better idea of what an anatomically similar tracemaker might have looked like more than 100 million years ago.
Sandhill-crane trackway on the sandy substrate of a high salt marsh, St. Catherines Island, Georgia. In this environment, its tracks are accompanied by fiddler-crab burrows and feeding pellets, as well as the tracks and dig marks of raccoons hunting the fiddler crabs. Scale (toward the top of the photo) in centimeters.
So to make a long story short, while walking along the Victoria coast last year, I also carried with me mental picture of these tracks in Georgia. These images, I am sure, contributed to my stopping to look at a rock surface that held faint but nearly identical impressions made by dinosaurian feet on the once-soft sediments of a river floodplain. This is how ichnology is supposed to work, and it did.
A comparison between sandhill-crane tracks on the Georgia barrier islands and those of Cretaceous dinosaurs in Australia is actually not as far-fetched as one might think at first. For one, we now know that birds are dinosaurs, evolutionarily speaking. This formerly vague hypothesis is now a certainty, and is based on an ever-improving fossil record of feathered theropod dinosaurs, as well as studies from modern biology that show genetic and developmental affinities between modern birds and theropods. Even so, this idea is not new, either. For example, evolutionary biologist Thomas Huxley (1825-1895), friend and noted proponent of Charles Darwin, readily connected Archaeopteryx, the Late Jurassic bird (or dinosaur, depending on evolutionary perspective) with theropod dinosaurs.
Preceding Huxley, though, was one of the first scientists to formally apply ichnology to fossilized dinosaur tracks, Edward Hitchcock (1793-1864). Hitchcock interpreted the abundant dinosaur tracks of the Connecticut River Valley – many made by theropods – as those of large, flightless birds that lived before humans. Although he never made the evolutionary connection between dinosaurs and birds, his hypothesis reflected anatomical similarities between their feet.
A close-up look at sandhill crane feet while it takes a step. Notice the left foot has a little toe facing backwards, but off the ground. This is the equivalent of our “big toe,” also known as digit I, and it rarely registers in their tracks unless a crane walks in soft mud or sand. Instead, you will see impressions of the other three toes with clawmarks, and the middle toe normally makes the deepest mark.
Theropod dinosaurs, like many modern birds, mostly made three-toed tracks, a condition also called tridactyl. Although theropod tracks are occasionally confused with similar tracks made by ornithopod dinosaurs, they have the following traits: (1) three prominent, forward-facing digit impressions; (2) a footprint that is longer than wide; (3) angles of less than 90° between the outermost digits; and (4) well-defined clawmarks. One of the many changes that happened to bird feet as they evolved from non-avian theropods was the dropping of and rearward projection of their first digit (equivalent to our big toe). This condition was a great adaptation for grasping branches in trees and otherwise getting around off the ground. Bird tracks from the Cretaceous Period also tend to be wider than long, a function of the angles between the outermost toes becoming greater than 90°, and most of these also show the impression of a backward-pointing toe. Sandhill-crane footprint made in firm sand of a high salt marsh, St. Catherines Island, Georgia. Like many bird tracks, this one is wider than it is long, which is unlike most theropod dinosaur tracks. Still, these are very similar to tracks made by certain types of thin-toed theropod dinosaurs during the Cretaceous Period. Scale in centimeters.
Much later in their evolutionary history, though, some lineages of birds became either flightless or otherwise spent more time on the ground than in the trees, such as wading birds and shorebirds. These circumstances resulted in their first digit becoming reduced or absent, or vestigial. Violá, the tridactyl theropod-dinosaur footprint came back in style, so to speak, and now dinosaur ichnologists regularly study the tracks and behaviors of birds with such feet to better understand how their theropod relatives may have moved during the Mesozoic Era.
Comparison of a track made by a greater rhea (Rhea americana, right), which is a large flightless bird native to Argentina, to that of an equivalent-sized theropod dinosaur track (right). Both tracks have three forward-facing digits ending with sharp clawmarks and are longer than wide. Scale = 15 cm (6 in). The dinosaur track is a replica of an Early Jurassic theropod (from about 200 million years ago) from the western U.S. Photograph of the rhea track is by Anthony Martin, and of the dinosaur-track replica is by Ty Butler of Tylight™. Scale in the photo to the left = 15 cm (6 in).
Thus while writing the research paper on the dinosaur tracks, I kept in mind the comparison between sandhill-crane footprints in Georgia and the Australian dinosaur tracks. I also recalled how paleontologists had previously measured theropod skeletons – feet and rear limbs, specifically – and proposed a relationship between foot length and probable hip height.
Based on these studies, you can take a theropod track, multiply it by 4.0, and you get the approximate hip height of its trackmaker. When I applied this calculation to the Australian tracks, their hip heights ranged from about 25 to 60 centimeters (10-23 inches). The smallest of these dinosaurs I imagined as chicken-sized; perhaps these were juveniles of the larger ones. But what might be living today that would compare to the largest of the trackmakers? Immediately I thought of herons, but then it struck me that sandhill cranes provided a more apt analogy.
So I think you know where this is going. Adult sandhill-crane tracks are about 12 centimeters (4.7 inches) long, so if you apply the same formula for theropod-dinosaur tracks to them, their hip heights should be 48 centimeters (19 inches). Would this relationship also hold up on a modern dinosaur, such as a sandhill crane?
Just to satisfy my curiosity, I wrote to Jen Hilburn (St. Catherines Island) and asked her to do me a little favor: could she measure the hip height of a living, adult sandhill crane for me? Fortunately, Jen carried out my unusual request (she said it was not easy, so I definitely owe her), and she wrote back with an answer: 58 centimeters (22 inches). This wasn’t a perfect fit with regard to the footprint formula, but it certainly worked for the size of the Australian dinosaurs I had in mind as trackmakers. Based on my study of the Australian tracks, they were made by small ornithomimids, which likewise made thin-toed tridactyl tracks.
After thanking Jen, I delighted in explaining how her measurement of a Georgia-island-dwelling sandhill crane related to a dinosaur-track discovery on the other side of the world. Furthermore, in the Emory University press release that accompanied the publication of the dinosaur-track discovery in August 2011, the reporter (Carol Clark) used my analogy of the trackmakers as “…theropods ranging in size from a chicken to a large crane.”
Sandhill crane walking down a sand pile next to a fresh-water pond and maritime forest on St. Catherines Island, Georgia, and leaving lovely tracks for an ichnologist to study and keep in mind while tracking non-avian theropod dinosaurs.
Artist conception of Struthiomimus, a Late Cretaceous non-avian theropod dinosaur from western North America. Although not a perfect fit, the tracks of cranes and other similarly sized birds can be compared to those of ornithomimid dinosaurs to better discern the presence and behaviors of these dinosaurs. Artwork by Nobu Tamura and from Wikipedia Commons.
What other modern traces from the Georgia coast will contribute to our better understanding the fossil record? Time will tell, and I hope some day to again share those thoughts at my former home – the Department of Geology at the University of Georgia – with friends, students, and colleagues, new and old.
Further Reading
Elbroch, M., and Marks, E. 2001. Bird Tracks and Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, PA: 456 p.
Forsberg, M. 2005. On Ancient Wings: The Sandhill Cranes of North America. Michael Foreberg Photography: 168 p.
Henderson, D.M. 2003. Footprints, trackways, and hip heights of bipedal dinosaurs: testing hip height predictions with computer models. Ichnos, 10: 99–114.
Johnsgard, P.A. 2011. Sandhill and Whooping Cranes: Ancient Voices over America’s Wetlands. University of Nebraska Press, Lincoln, NB: 184 p.
Lockley, M.G. 1991. Tracking Dinosaurs: A New Look at an Ancient World. Cambridge University Press, Cambridge, UK: 264 p.
