Shuvosaurus and Protoavis: Two Enigmas From the Triassic of Texas

The Late Triassic of the American Southwest was home to a bewildering array of fascinating creatures, completely unlike anything we know of today. Patrolling the forests and riverbanks of the Triassic were fierce rauisuchians, and swift herrerasaurids and coelophysoids. Lounging in the lazy waters were huge crocodile-like phytosaurs and the archaic metoposaurs. Browsing from the thickets of vegetation were armored aetosaurs, the strange tusked dicynodonts, silesaurids, prosauropods, and even a couple of ornithischians. At the same time, all of this was being played out on a smaller scale on the forest floor and canopy, with dog-like crocodilians running around after proconlophids and trilophosaurs. However, the strange creatures of this alien world paled in comparison to two bizarre oddities, Shuvosaurus and Protavis. So yeah, once again I find myself blogging about the archosaurs of Texas.

Shuvosaurus was first described by Dr. Sankhar Chatterjee in 1993. When first discovered, Chatterjee thought that Shuvosaurus appeared to be a Triassic ornithomimosaur. This shocked and bewildered the paleontological community. Not only are ornithomimosaurs some of the most specialized theropods of all time, but they didn't even appear in the fossil record until the Cretaceous (even today, no ornithomimosaur is known from even the Jurassic). Because of the amount of paleontological upheaval Chatterjee's theory would have caused (not to mention the incompleteness of the Shuvosaurus skeleton), many paleontologists viewed Shuvosaurus with skepticism. Since then, there were numerous theories on what exactly was Shuvosaurus. Chatterjee, as far as I know, stood by his ornithomimosaur hypothesis. Dixon, in one of his dinosaur books, suggested that Shuvosaurus was a specialized coelophysoid theropod. Others suggested that Shuvosaurus was a chimera, a fossil "taxon" made of the body parts of more than one animal.

So, it appeared Shuvosaurus was doomed to remain a mystery, an unsolvable enigma of the Triassic. That is, until the discovery of Effigia in 2006. Discovered posing as a coelophysoid theropod in fossils the American Natural History Museum collected from Ghost Ranch, While at first glance Effigia appeared to be a dinosaur, the structure of the hip and ankle told scientists a different story; Effigia was a rauisuchian, distant cousins of modern crocodiles which . And so the pieces clicked together, and the mystery was solved. Shuvosaurus wasn't a dinosaur at all, but a species of beaked, bipedal, herbivorous rauisuchian, one which had evolved conveniently to the oviraptorosaurs and ornithomimosaurs of the Late Triassic. In fact, the discovery of Effigia showed that not only was Shuvosaurus a rauisuchian, but Chatterjea, another odd rauisuchian with suspicious similarities to Shuvosaurus, was actually the same species!

While Shuvosaurus was unusual, it paled in comparison to the controvery stirred up by Chatterjee's other discovery, Protoavis. Protoavis was described as a fossil Triassic bird by its discoverer, a creature about 35 centimeters tall. Even more unusual, several features that Chatterjee described in Protoavis would suggest that it is more advanced than the earliest definite known bird, Archaeopteryx, despite living 75 million years earlier. One of these features is that Protoavis has a very bird-like skull, with teeth even more reduced than in Archaeopteryx. Some scientists have called the validity of Protavis into serious question, but Chatterjee has stood by his claim.

Enter the drepanosaurids. Drepanosaurids, more informally known as monkey lizards, were a group of arboreal reptiles that have been found across the world during the Triassic period (for more info on drepanosaurs see the excellent post at the Hairy Museum of Natural History here http://www.hmnh.org/galleries/monkeylizards/index.html). Some species appear to have taken the place of squirrels or primates, others may have lived a life like the modern tamandua, and still others may have even been aquatic or flying squirrel-like animals. But most importantly, these animals had a very bird-like skull, to the point where one of their "other" names is avicephalans (bird heads). The best specimens of these animals have been found in the Eastern U.S. (Hyperonector) or Italy (Drepanosaurus and Megalancosaurus), but there is a species of drepanosaurid known from the American Southwest, Dolabrosaurus, found in the sediments of Petrified Forest National Park.

So it appears quite likely that Protoavis is a paleontological chimera, a fossil made up of the dislocated remains of a coelophysoid theropod and a drepanosaur. There are other features which seem to suggest this. The quarry where Protoavis was found suggests that it was the remnants of a group of flash flood victims, as the animals are all jumbled up and disassembled. In addition, several of the alleged Protoavis vertebrae appear to be rather similar to the vertebrae of the drepanosaur Megalancosaurus. And of course the bones of Protoavis are very badly preserved in the first place.

Well, there you have it...the two weirdest fossils from the New World of the Triassic (I won't say the entire Triassic, as Sharovipteryx and Longisquama may give these two a run for their money). Two Texas enigmas, one vindicated by further paleontological discoveries and has emerged as one of the most unique animals in the Triassic world, the other a chimera made of two parts juvenile coelophysoid, and one part drepanosaur (mix well). I hope you enjoyed this little presentation, and stay tuned to Metazoica!

References

Jacobs, L. 1995. Lone Star Dinosaurs. Texas A&M University Press, College Station, Texas

Fraser, N. 2006. Dawn of the Dinosaurs, Life in the Triassic. Indiana University Press

http://www.hmnh.org/galleries/monkeylizards/index.html