Shuvosaurus: Prehistoric Animal of the Week

This week we shall be checking out one of the animals that Gary and I had the joy of digging up this past summer in New Mexico!  Check out Shuvosaurus inexpectatus.  Shuvosaurus lived in what is now North America during the Triassic Period between 215 and 200 million years ago.  It was about ten feet long from snout to tail and would have coexisted with Postosuchus. 

When Shuvosaurus was first discovered in the early 90s, scientists thought it was a kind of dinosaur that was related to the ornithomimid family like Struthiomimus ("ostrich dinosaurs").  Therefore, this was considered a really important find since ornithomimids were only ever known from the late Cretaceous, over 100 million years later than Shuvosaurus!  It's not hard to see why they thought this way.  Just look at the little guy.  It would have ran on two slender, powerful legs, had a long neck, a small head, large, round eyes and a toothless beak.  Looks like an ornithomimid to me!

Shuvosaurus life reconstruction by Christopher DiPiaza.

As it turns out, Shuvosaurus wasn't an ornithomimid at all.   In fact, it wasn't even a dinosaur!  The answer to this creature's identity lies in its ankle bones.  You see, all true dinosaurs, including birds, have an ankle joint that only rotates one way, much like the hinges on a door.  Other animals can swivel their ankles around.  Well, Shuvosaurus had a swiveling ankle and therefore wasn't a dinosaur.  Its hip bones also weren't totally consistent with those of dinosaurs either.   Scientists later realized all of this thanks to the discovery of more bones from an animal belonging to the same family as Shuvosaurus called Effigia.

So what was Shuvosaurus if it wasn't a dinosaur?  It was assigned to a branch of reptiles called Poposauroidae.  These creatures were still related to dinosaurs but weren't quite dinosaurs themselves.  They were actually more closely related to crocodiles and other crocodile-relatives like Desmatosuchus and Postosuchus (which also was bipedal!).  During the Triassic a lot of archosaurs (group of reptiles that includes dinosaurs and crocodiles) were evolving the same sort of lightweight, fully erect, bipedal body design.  At the end of the Triassic, however, the dinosaurs were the only ones left standing (get it...standing?  tee-hee!) with this design.

Shuvosaurus skull

Shuvosaurus, despite its resemblance to ornithomimid dinosaurs, was just an example of convergent evolution after all.  It's strange to think of an animal related to crocodiles that had no teeth!  Scientists are unsure as to what exactly Shuvosaurus was eating but it probably wasn't a serious predator.  It may have consumed vegetation and/or small animals like invertebrates. 

That's all for this week!  As always feel free to comment below or on our facebook page!

References

Chatterjee, S. (1991) An unusual toothless archosaur from the Triassic of Texas: the world's oldest ostrich dinosaur? Abstract, Journal of Vertebrate Paleontology, 8(3): 11A.

Rauhut, O. W. M. (1997). "On the cranial anatomy of Shuvosaurus inexpectatus (Dinosauria: Theropoda)." In: Sachs, S., Rauhut, O. W. M. & Weigert, A. (eds) 1. Treffen der deutschsprachigen Palaeoherpetologen, Düsseldorf, 21.-23.02.1997; Extended Abstracts. Terra Nostra 7/97, pp. 17-21.

Nesbitt, S. (2007). "The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa." Bulletin of the American Museum of Natural History, 302: 84 pp.