Articulated Skeletons Give a New Look at “Armadillodiles”

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Early dinosaurs and other Triassic creatures have been in the news quite a bit lately. From a new review of the origin of dinosaurs to the recognition of a mistaken dinosaur and the discovery of the skeleton of a fearsome predator closely related to crocodiles, some of the most interesting recent paleo news has focused on the time when dinosaurs and their ancestors were small animals in a world dominated by an array of strange reptiles. A new paper just published in the Journal of Vertebrate Paleontology adds to this wave of Triassic research, and it gives paleontologists a more complete look at one of the oddest vertebrates to have ever evolved.

Despite a superficial resemblance, the Triassic aetosaurs were not close relatives of the well-armored ankylosaurs. Instead aetosaurs were more closely related to crocodiles, and their extra armor plating has caused them to be informally called "armadillodiles" by many. As with most fossil vertebrates, though, fragments of aetosaurs have been found more often than complete skeletons, and reconstructing what they would have looked like in life has been made all the more difficult by the fact that their armor often became scattered after death. Nevertheless, several exquisitely preserved aetosaur skeletons have been found, and the new report provides the long-awaited details of two articulated aetosaur skeletons discovered in the 228- to 199-million-year-old rock of New Mexico.

Both specimens are of the aetosaur Typothorax coccinarum. The first skeleton, collected from the Badlands Ranch, is mostly complete except for some of the front parts of the body. The authors hypothesize that it was scavenged before it was completely buried, but despite this loss the specimen is remarkable because the skeletal parts are very close to their arrangement in life. The second skeleton is even more complete. Called the Revuelto Creek specimen, this skeleton preserves almost the entire body from the tip of the tail to the end of the snout—and both of these skeletons held a few surprises.

Aetosaurs were so well-armored that they had osteoderms around the orifice situated behind their hips called the cloacal vent, but in Typothorax the armor in this area of the body was unlike any described before. Rather than just have flat bits of armor, Typothorax had eight outward-curving spikes around its cloacal vent, leading the researchers to wonder what the function of these spikes might be. It is impossible to tell at the moment, but it certainly would have made mating an prickly prospect for these animals.

Of further interest were the limbs of Typothorax. The anatomy of the forelimbs was consistent with the hypothesis that some aetosaurs may have dug in the soil for food, and the hind feet of the specimens appeared to match Triassic-age trackways given the designation Brachychirotherium. It is always tricky matching a particular animal to a trackway (unless an animal literally dies in its tracks), but the correspondence between the Typothorax feet and Brachychirotherium tracks supports the idea that these tracks were made by an aetosaur.

All of this presents a finer picture of Typothorax than has previously been available. Not only will they allow scientists to better understand the arrangement of armor on these armadillodiles, but their skeletons may provide important clues to their paleobiology, as well.

For more on this new research, see the posts by Matt Celeskey (one of the paper's co-authors), Bill Parker, and Jeff Martz.

Andrew B. Heckert; Spencer G. Lucas; Larry F. Rinehart; Matthew D. Celeskey; Justin A. Spielmann;Adrian P. Hunt (2010). Articulated skeletons of the aetosaur Typothorax coccinarum Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA Journal of Verterbrate Paleontology, 30 (3), 619-642 : 10.1080/02724631003763524

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