Triassic Bites and a Carnivore Conundrum

The Triassic was one of the strangest times in the history of the planet. Rebounding from the worst mass extinction of all time, life flourished into startling new varieties, including the first dinosaurs, weird marine reptiles, and croc-line critters that came in forms like “armadillodiles” and huge, jagged-toothed carnivores, to name just a few Triassic stars. But the strange nature of the Triassic extends beyond the odd anatomy of the creatures that evolved during the period.

While paleontologists are still piecing together the details of Triassic life for the early parts of the period, researchers know that the landscapes of the Late Triassic were dominated by carnivores. At many classic Late Triassic localities – such as those in Petrified Forest National Park and Ghost Ranch, New Mexico – flesh-eaters outnumber herbivores in both abundance and species diversity. This doesn’t fit with the classic ecosystem pyramid we learn in grade school, with a greater number of herbivores providing fodder for a small number of carnivores. No, something strange was going on during the Late Triassic, and a pair of damaged leg bones may hint at why the Late Triassic held an embarrassment of predators.

The two bones, studied by paleontologists Stephanie Drumheller, Michelle Stocker, and Sterling Nesbitt, were found in different localities at different times. But they have three important features in common. They both belong to large Late Triassic carnivores called paracrocodylomorphs, both are upper leg bones called femora, and both are pocked by bitemarks from different carnivores.

There isn’t any animal quite like a paracrocodylomorph alive today. These were archaic cousins of today’s alligators on crocodiles, but vastly different in shape, with legs held beneath their bodies and boxy skulls full of blade-like teeth. Their ranks included the largest terrestrial predators of their time, but, as the bite marks attest, paracrocs could still end up as nourishment for their carnivorous neighbors.

The smaller of the two bones was found in the 218 million year old rock around Fort Wingate, New Mexico. Exactly what species the bone represents isn’t clear, but another animal chomped onto the paracroc’s upper leg around the time of the animal’s death.

The femur of the larger paracroc, showing multiple bites. "A" points to the embedded tooth and a healed puncture. From Drumheller et al. 2014.
The femur of the larger paracroc, showing multiple bites. “A” points to the embedded tooth and a healed puncture. From Drumheller et al. 2014.

The larger femur tells a more complex tale. Found somewhere in the vicinity of Ghost Ranch, New Mexico, this bone came from an approximately 27-foot-long paracroc that was savagely bitten on the upper leg. Healed bite wounds and an embedded tooth, itself surrounded by repaired tissue, show that the victim survived the assault, but, in time, the reptile still became a meal. A second set of bite marks on the same bone show no indication of healing – the sign of a successful predator or a snacking scavenger.

What was attacking and eating these apex predators? The broken tooth tip identifies the initial attacker of the larger paracroc as a phytosaur – one of the long-snouted, crocodile-like ambush predators that inhabited the rivers and lakes of the Late Triassic. In particular, the attacker was likely smaller than the victim. Drumheller, Stocker, and Nesbit estimate the biting phytosaur was about 20-feet-long. And while whatever bit the smaller paracroc isn’t as clear, as several Triassic carnivores could have left such marks, phytosaurs are among the possible culprits.

This particular phytosaur (right) is about to chomp a Placerias (left), a protomammal cousin of ours. Mount at the New Mexico Museum of Natural History and Science, photo by Brian Switek.
This particular phytosaur (right) is about to chomp a Placerias (left), a protomammal cousin of ours. Mount at the New Mexico Museum of Natural History and Science, photo by Brian Switek.
Protomammal Placerias (left) is about to get chomped by the phytosaur Smilosuchus (right). Mount at the New Mexico Museum of Natural History and Science.

Petrified punctures can only take us so far in reconstructing these scenes. The unhealed bite marks could have been made by carnivores that killed the paracrocs, or by other individuals who came to feed on the carrion and wrench the meaty legs off the carcasses. And even in the case of the healed wounds, it’s impossible to say whether the phytosaur was trying to prey on the paracroc, the paracroc botched an attempt to feed on the phytosaur, or the two got into a fight for some other reason.

All the same, the two battered bones definitively show that Triassic bodies did not go to waste. More than that, the researchers behind the study note, the bones show a connection between the terrestrial and aquatic realms. It wasn’t as if the paracrocs ruled the land and the phytosaurs kept to themselves in the water. Fresh meat was exchanged across the boundary of the terrestrial and aquatic realms, with phytosaurs perhaps being both predators and part of the carcass clean-up crew. In short, the Late Triassic was a carnivore-eat-carnivore world.


Drumheller, S., Stocker, M., Nesbitt, S. 2014. Direct evidence of trophic interactions among apex predators in the Late Triassic of western North America. Naturwissenschaften. doi: 10.1007/s00114-014-1238-3

Tracks Hint at the Social Life of Tyrant Dinosaurs

What’s scarier than a tyrannosaur? Three tyrannosaurs. That’s simple, undeniable math. The question is whether or not the tyrant dinosaurs ever prowled together in real life. Up until now, the evidence has been equivocal. But a trackway found in British Columbia finally provides firmer ground for speculation on the social lives of these celebrated carnivores.

The idea that large tyrannosaurs – like Albertosaurus and the mighty Tyrannosaurus itself – worked together to bring down prey isn’t new. University of Alberta paleontologist Philip Currie has been championing the hypothesis for over a decade, citing bonebeds that contain multiple individuals of the Late Cretaceous tyrannosaurs Albertosaurus and Daspletosaurus. And three years ago Currie went to the public with his idea in the form of a book and documentary called Dino Gangs. “If dinosaurs hadn’t become extinct, would gangs of killer tyrannosaurs now rule the world?”, the hyperbolic documentary asked.

At the time, I wasn’t sold on tyrannosaurs being communal carnivores. Just because a whole bunch of dinosaurs ended up buried together doesn’t mean that they actually lived and hunted together. A mass of dinosaur skeletons – such as an Albertosaurus bonebed containing 12 individuals – can contain elements from animals that died at different times or were brought into a small area by unusual circumstances. Accumulations of dinosaur  bones represent the circumstances of death and burial more than clues about life.

If evidence for social tyrannosaurs exists, it has to be in a less ambiguous form. Tracks hold the most potential.

Tracks and other traces are signs of prehistoric behavior and biology. For example, we know tyrannosaurs fought by biting each other on the face from healed wounds on their skulls. Whether tyrannosaurs lived in groups, though, requires something more. While an Albertosaurus bonebed is ambiguous evidence for social behavior, a trackway showing that tyrannosaurs walked together would be a much clearer sign of social tyrants.

This is what happened with “raptors.” The pack-hunting idea played out in Jurassic Park was based on a Deinonychus quarry that contained multiple predators as well as the apparent prey, but the site only really shows that several of the sickle-clawed predators died at the site. It has only been more recently, with the discovery of distinctive, two-toed tracks that paleontologists have been able to confirm that raptors at least sometimes walked together.

The problem for tyrannosaurs is that their footprint record is very poor. Some supposed tyrannosaur tracks – such as a big, three-toed prints found in a Utah coal mine – turned out to be prints left by shovel-beaked hadrosaurs. Other, authentic tyrannosaur tracks are isolated specimens, recording a single footfall on the Cretaceous ground and nothing more. This historic dearth of trace evidence is what makes the tyrannosaur trackway discovered in Canada so important.

A diagram of the tyrannosaur trackways found in British Columbia. From McCrea et al., 2014.
A diagram of the tyrannosaur trackways found in British Columbia. From McCrea et al., 2014.

In October 2011, a few months after Dino Gangs debuted, local outfitter Aaron Fredlund discovered two dinosaur tracks in the roughly 75 million year old rock of northeastern British Columbia’s Wapiti Formation. An excavation by the Peace Region Palaeontology Research Centere turned up a third track in the sequence, but the best find didn’t come until the following summer. In August 2012 paleontologists and volunteers found two more sets of tracks right next to each other.

Unfortunately for paleontologists, the dinosaurs who left the footprints did not die in their tracks. But, as reported by Peace Region Palaeontology Research Centre researcher Richard McCrea and colleagues in a new PLoS One paper, there was only one type of dinosaur alive in the area that could have left such prints. All of the tracks are big – with a length of over 19 and a half inches – and were made by a dinosaur with three forward-pointed toes that ended in sharp claws. The tracks had to be made by a tyrannosaur.

Detail of Trackway A. From McCrea et al., 2014.
Detail of Trackway A. From McCrea et al., 2014.

Which species of tyrannosaur created the tracks is unknown. Three large tyrannosaurs – Albertosaurus, Gorgosaurus, and Daspletosaurus – all lived in western Canada at the time the tracks were made. But the tracks are distinctive enough that, following trace fossil convention, McCrea and coauthors have given them their own name. The tracks have hence been labeled Bellatoripes fredlundi, an homage to the “warlike foot” shapes of the tracks found by Fredlund.

The hypothesized step cycle for the tyrannosaurs that made the trackways. From McCrea et al., 2014.
The hypothesized step cycle for the tyrannosaurs that made the trackways. From McCrea et al., 2014.

While they might superficially look like trident-shaped potholes, the tracks provide a wealth of information about the animals that made them. The animal that made the first-discovered trackway, for example, was apparently missing the end part of the second toe of the left foot, leaving impressions of the nub as it walked. And the details of the footprints showed that these tyrannosaurs walked in a different way than some other carnivorous dinosaurs. Trackways made by predatory dinosaurs typically show that the dinosaur put their foot down and lifted their foot out in a forward motion, while the tyrannosaur tracks suggest that these animals lifted their feet out in a backwards motion.

But the arrangement of the tracks is what has gained the most attention and plays into speculations of tyrannosaur packs. All three trackways face the same direction and were made in close proximity, with 18 feet between Trackway A and B, and eight feet between B and C. While there’s a chance that the trackways were made by three individuals that traveled the same area at different times, McCrea and colleagues consider this unlikely. Tyrannosaurs were a rare part of the fauna, there is no sign of an obstacle that would create a bottleneck requiring tyrannosaurs to walk the same path, and the detail of the tracks – down to foot scales on some – hint that they were all made around the same time, while substrate conditions were consistent. That two, if not three, tyrannosaurs were walking together is the simplest explanation of the pattern.

So did tyrannosaurs hunt in packs? Maybe. The tracks, stunning as they are, can only take us so far.

There remains a shred of doubt that these footprints were made by a single social group. If the tracks showed some sign of interaction between the animals – like raptor footprints that show one adjusting its course to move out of the way of another – then we could be sure. In fact, I’ve encountered this problem out in the field while looking for tyrannosaurs and their Mesozoic kin. Sometimes I’ll find myself closely following the tracks of another fossil hunter, even though they passed by minutes or hours before I did, and I’ll leave my own trail right alongside theirs.

But let’s say the site really does record a social group, as it seems to. We don’t know why the tyrannosaurs were walking together. Was this a family? A hunting party? Suitors trying to follow a mate? Nor do we know how long the band remained together. The tracks only record a few brief moments in Late Cretaceous time when, if even for a moment, tyrants flocked together.


McCrea, R., Buckley, L., Farlow, J., Lockley, M., Currie, P., Matthews, N., Pemberton, S. 2014. A ‘terror of tyrannosaurs’: The first trackways of tyrannosaurids and evidence of gregariousness and pathology in tyrannosauridae. PLoS ONE. 9, 7: e103613. doi:10.1371/journal.pone.0103613

Prehistoric Hippo Potholes are the First Fossil Mammal Swim Traces

When Charles Darwin was composing On the Origin of Species, he had to face an extremely frustrating problem. The fossil record, despite initially inspiring some of his evolutionary notions, was largely a mystery. Part of the problem stemmed from the gaps and missing chapters in life’s history. But there was also a more hopeful aspect of the argument that Darwin hasted to point out as he wrote new editions of his most famous book. Despite the fossil record’s unavoidable incompleteness, paleontologists of Darwin’s day had only just begun to discover the vestiges of prehistoric life that the world’s rocks contained. There was far more to find than had been turned up. And that’s still true. Case in point – paleontologists have only just now uncovered the first known fossilized traces of a mammal as it swam.

Paleontologists found the traces at a site best-known for different fossils. Koobi Fora, on the east shore of Kenya’s Lake Turkana, has yielded multiple fossils of early humans that lived in the region between ~4.3 million years ago and 600,000 years ago. The footfalls of prehistoric people are known from the site, too, but the more numerous tracks of other creatures fill out what the environment was like when Homo erectus was ambling around. Excavations conducted over three decades ago found footprints of hippos and a wading bird that slogged through the shallows 1.4 million years ago, for example, and upon revisiting the track surface in 2008, researchers found additional, crude divots in the ancient stone. These newly-found tracks – described by paleontologists Matthew Bennett, Sarita Morse, and Peter Falkingham – were made by swimming hippos.

Fantasia to the contrary, hippos are not very graceful on land. They can swing their legs in a quick run, true, but most of the time they have to keep three legs on the ground at any point in the step cycle because of their girth. But a submerged hippo is an elegant beast. They punt off the bottom with their four-toed feet and glide through the water, quite at home under the surface. And it’s that kind of “bottom-walking” behavior that created the newly-described Koobi Fora tracks.

The 240 traces Bennett and coauthors describe roughly fit into five different types, ranging from tracks with well-defined toes and nails to “oval- or tear-shaped prod-like impressions” and messy tracks on top of tracks. These could have been made by adults and juveniles of the large Hippopotamus gorgops, or the small tracks could be attributed to a smaller species that lived in the area named Hippopotamus aethiopicus.

Unfortunately the tracks don’t offer the clues to tell the difference between the species, not to mention that the fore- and hindfeet of hippos are so similar that distinguishing which foot made which track isn’t easy. But despite these complications, the preserved anatomy shows that the traces are distinctively hippo and record the underwater excursions of several individuals. The fact that there are many tracks but there don’t seem to be any clear trackways indicates that the water horses where floating along beneath the surface and occasionally giving themselves a push off the lakebottom. Of course, the water had to be fairly deep to allow hippos to swim, so this hints that the Homo erectus tracks found in the same level may be a sign of the people wading into the lake.

And this discovery is more than a fossil first. Bennett and colleagues tie the discovery to long-running debates over swimming sauropod dinosaurs.

Despite the swampbound “Brontosaurus I saw in books and movies as a kid, there is no good evidence for swimming sauropods. The few trackways that seemed to show dinosaurs like Brachiosaurus walking themselves along the bottom, their hind legs and tails buoyed up in the water, have turned out to be undertracks of the front limbs that preserved more deeply than the hind footprints. The only definitive dinosaur swim tracks paleontologists know of so far were made by carnivorous theropods – the sort of dinosaur paleontologists used to think of as aquaphobic – and these are usually long scratches made by claws on three-toed feet that pushed the little predators along.

The hippo tracks could help paleontologists come up with a better idea of what to look for while pondering swimming sauropods. While hippos are adapted to an amphibious life and sauropods were terrestrial animals that were probably really lousy at swimming, there’s no reason to think that Diplodocus and company totally avoided water. When they crossed lakes, rivers, or lagoons, what sort of tracks did they make? If the dinosaurs pushed off with their tippy toes like hippos, they’d probably leave weird divots and semi-complete tracks on the bottom in a pattern that would be relatively difficult to follow as a trackway (unless the dinosaur was pushing off at at regular interval along a particular path).

There are two complications with using hippos as guides, though. First, Camarasaurus and kin had smaller, crescent-shaped front feet and much larger, broader hind feet that bore big claws. In general shape, elephants may offer better approximations of sauropod feet. If anyone happens across swim tracks made by prehistoric elephants – as they must have created as they swam to some of the islands where dwarfed species evolved – those prints may refine the search image for swimming sauropod clues.

Then again, sauropods had something that hippos and elephants don’t. While dinosaurs like Apatosaurus were large and had column-like limbs, much like elephants, the front half of their bodies was invaded by air sacs that extended from the lungs. This would have made them relatively more buoyant and affected their swimming technique. And even though birds have these air sacs, too, they’re so anatomically different from sauropods that avian dinosaurs aren’t good models for how Sauroposeidon might have swum. It’s really difficult to find a workable sauropod substitute.

Nevertheless, paleontology is a comparative science. Finding fossils and properly interpreting them relies on mental images of what to look for in the first place. Having swim tracks to look at will help scientists pick out additional specimens. Now that Bennett, Morse, and Falkingham have given us the first-known fossil impressions of a big swimming mammal, who knows what other traces will suddenly snap into focus as paleontologists continue to scour the fossil record?


Bennett, M., Morse, S., Falkingham, P. 2014. Tracks made by swimming Hippopotami: An example from Koobi Fora (Turkana Basin, Kenya). Palaeogeographic, Palaeoclimatology, Palaeoecology. 409: 9-23

Swim Tracks Undermine Dinosaur Stampede

In 2004, the Australian government established Dinosaur Stampede National Monument in central Queensland. At this one place, preserved in the 95 million year old stone of the Lark Quarry, over 3,000 small dinosaur tracks are scattered across the rock surface. The tracks have traditionally been thought to be the only existing evidence of a dinosaur stampede. Australia’s ABC Television reconstructed the event like this:

But, paleontologists have found, there was no mixed herd, no marauding predator, no lake, and, in fact, no stampede. This exceptionally-rich, scientifically-important site has been drastically misinterpreted. If University of Queensland paleoichnologist Anthony Romilio and coauthors are correct, the park should be renamed something along the lines of “Swimming Dinosaur National Monument.”