The Last Resting Place of Decuriasuchus

While making the rounds promoting his new book Boneheads, art dealer and author Richard Polsky stopped by NPR last Sunday to talk about his personal quest to find a Tyrannosaurus rex. The romance of what paleontologist Bob Bakker has called “the big game hunt in Deep Time” drew him in. Having a pet Tyrannosaurus is impossible, but, if you know where to look, you can find the bones of this celebrated dinosaur, and Polsky wanted to do just that.

Polsky’s goal was not to aid science. He doesn’t seem to have cared a whit about it. Books about dinosaurs by paleontologists are “boring”, says Polsky, and the objective of his brief jaunt into the field was to do nothing more or less than find a dinosaur. To track down. To capture. To own. Not to understand. For Polsky, just being present as a Tyrannosaurus scrap was turned up in the field and having it named in his honor appears to have been enough.

Finding a dinosaur in the field is thrilling, especially when you’re hot, dirty, tired, and have nearly given up hope on discovering anything. I was elated when I spotted the tip of a tooth from a large theropod dinosaur – likely a tyrannosaur, given its Late Cretaceous age and context in an outcrop near Elk Ridge, Wyoming – but finding that fragment, by itself, did not satisfy me. I wanted to know the species the tooth once belonged to, how old the individual animal was, what sort of environment it lived in, how the tooth came to rest on the surface, and whether there might be more of it laying about. A piece of a tyrant dinosaur is not a collectible. It is a clue.

Unfortunately for me, there was no other sign of the dinosaur or even the rest of the tooth. Fossil skeleton fragments littered the ground around my feet, but they were coming from a layer of rock that represented a high-energy channel or stream that busted up most of the bones. My chances of finding whole bones, much less articulated parts of a skeleton, were vanishingly small.

Covered in chalky white enamel flecked with splotches of blue, the coarsely-serrated tooth tip now sits on my desk. It reminds me of the importance of context in paleontology. By itself, the fragment is a natural curiosity of passing interest – a small connection to a species that has been extinct for over 65 million years. When viewed against the background in which it was found, however, it becomes part of a larger story. The fragment is a small starting point that leads me to wonder about an ecosystem that I will never see.

It is not paleontology’s aim to simply fill museums with the inhabitants of lost worlds or create static menageries of ancient monsters. The goal of this science is to understand prehistoric life, and this requires that we pay careful attention to the context and associations of bones. Carelessly rip a specimen out of the rocks, and you lose a world of information – a simple rule beautifully expressed by a spectacular fossil find recently made in Brazil.

A skeletal reconstruction of Decuriasuchus. Note the rows of osteoderms of the back and on the underside of the tail. From França et al., 2011.

There is no common name for the creature that Marco Aurélio França, Jorge Ferigolo, and Max Langer describe as Decuriasuchus quartacolonia. The closest thing might be “land croc”, but this wouldn’t be quite right or informative enough.

Found in the 228-237 million year old rock of southern Brazil’s Santa Maria Formation, Decuriasuchus belonged to a group of extinct vertebrates called “rauisuchians.” The quotation marks denote uncertainty, not sarcasm. Paleontologists are agreed that these animals were archosaurs – the diverse and disparate group of “ruling reptiles” that encompasses crocodiles and birds (read: dinosaurs) today – and rauisuchians fell along the crocodile side of a deep split that divided the ancestors and close relatives of dinosaurs from the ancestors and close relatives of crocodiles.

So far, so good, but rauisuchians are so poorly known that it is unclear whether they formed a natural evolutionary group or not. So few informative fossils are known that relationships between animals identified as rauisuchians often shift around, and the discovery of beaked, bipedal forms such as Effigia and semi-aquatic species like Qianosuchus have significantly expanded the range of variation seen within the group. (Recent studies have found the rauisuchia to be a natural group, but more fossils and further studies will be needed to better resolve the relationships of rauisuchians to each other.) Regardless, Decuriasuchus was quite similar to well-known species like Prestosuchus and Postosuchus that embody the traditional rauisuchian archetype. Imagine adapting a crocodile to have a more greyhound-like build with a narrower, deeper skull full of knife-blade teeth, and you will have the general picture of what these Triassic predators were like.

An illustration of the Decuriasuchus bonebed in Brazil. Skeletons are labeled a-i. From França et al, 2011.

What makes Decuriasuchus special is not its anatomy, but the way the first specimens were found. The remains of nine individuals were uncovered in a single bonebed – including several nearly-complete skeletons – with skull fragments of a tenth found nearby. This was not a jumble of isolated bones, but a collection of articulated body parts and skeletons. What could have caused the bodies of so many predators to come to rest in the same grave?

There are many ways to make a bone bed. Each one tells a different story, and context is everything. One of the possibilities considered by França, Ferigolo, and Langer is that the Decuriasuchus became mired in a pit of sucking mud. Paleontologists have seen this sort of situation before, but, for that reason, the authors rule out death by quicksand for their specimens. Had the Decuriasuchus become trapped in muck, their struggles to get out would have left their bodies straining upwards, and there would have been a preservation bias for the rear parts of the skeletons. The fact that the bodies were found on their sides – virtually wrapped around each other – suggests that some other cause created the assemblage.

A more likely scenario, França and colleagues propose, is that the bodies were washed together by local flooding. Since this environment alternated between dry and wet seasons during this part of the Triassic, the return of wet weather might have caused rapid waterflows over the prehistoric floodplain where the Decuriasuchus were buried. The completeness and orientation of the skeletons suggest that they were rapidly buried at or shortly after the time of death. This bonebed did not come together over a long span of time, but may instead be a clue to a brief event that occurred over 228 million years ago.

But even if França and co-authors are correct, the question of why so many Decuriasuchus specimens were in the same area at the same time remains unresolved. As soon as I saw the figure of the bonebed I could not help myself from imagining packs of Decuriasuchus stalking the Triassic floodplain, but just because these animals were buried together does not mean that they lived together in a group. The animals may have been in the same general area when the flood struck, with their bodies eventually brought together by the current flow. Both scenarios propose that the animals were in close proximity when they died, but the authors of the new paper don’t think that the assemblage represents any kind of complex, pack-hunting behavior. “There is no evidence that [the interaction between individuals] was more complex than that seen among extant crocodiles,” they write.

What might have brought to many Decuriasuchus into such close company is a mystery. França and co-authors do not speculate on this point, but, if these animals were not moving together in a single group, I could not help but wonder about reasons for them to congregate.

Perhaps a carcass – or group of carcasses – drew multiple predators to the area. Among other potential prey animals, Decuriasuchus shared its habitat with a tubby, eight-foot-long herbivore called Dinodontosaurus. Don’t let the name fool you. This creature was more closely related to us than to reptiles, and maybe a previous flood had left a wealth of rotting Dinodontosaurus carcasses on the floodplain for opportunistic Decuriasuchus to exploit. A second local flood could have killed the scavengers and aggregated their bodies, but, if this were so, then we might expect to find scraps of other animals washed in among the Decuriasuchus bodies. Only the bones of Decuriasuchus were found at the site.

Another possibility is that the Decuriasuchus were gathering to breed, much like modern American alligators do. Perhaps female Decuriasuchus moved through territory held by large males, with unestablished males waiting in the wings for the opportunity to sneak a brief copulation when the dominant males weren’t looking. If multiple individuals came to the same area to mate, a small-scale flood could have created the bone bed, but this is pure speculation on my part, based upon behaviors of animals we will never be able to observe in life. The Decuriasuchus bodies can act as a window into their prehistoric world, but the view will always remain at least partly obscured.

Top Image: A restoration of Prestosuchus – a typical, predatory rauisuchian. Illustration by Dmitry Bogdanov and from Wikipedia.

References:

Brusatte, S.; Benton, M.; Desojo, J.; Langer, M. (2010). The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida) Journal of Systematic Palaeontology, 8 (1), 3-47 : 10.1080/14772010903537732

Davis, L., Glenn, T., Elsey, R., Dessauer, H., & Sawyer, R. (2001). Multiple paternity and mating patterns in the American alligator, Alligator mississippiensis Molecular Ecology, 10 (4), 1011-1024 DOI: 10.1046/j.1365-294X.2001.01241.x

França, M., Ferigolo, J., & Langer, M. (2011). Associated skeletons of a new middle Triassic “Rauisuchia” from Brazil Naturwissenschaften DOI: 10.1007/s00114-011-0782-3

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