The Ex-Ornithischian Revueltosaurus callenderi

Lukas Panzarin guessed correctly. The reconstruction below is of the Late Triassic pseudosuchian Revueltosaurus callenderi courtesy of Jeff Martz. The new reconstruction is based on new material of this taxon from the Chinle Formation of Petrified Forest National Park.


For those readers not familiar with Revueltosaurus, it was named in 1989 by Adrian Hunt based on isolated teeth from near Revuelto Creek in east-central New Mexico (Bull Canyon Formation of the Dockum Group). The teeth are highly distinctive, leaf-shaped with denticles, and very similar to the teeth of basal ornithischians. Based on these similarities Hunt (1989) tentatively referred Revueltosaurus to the Ornithischia. Below are teeth of Revueltosaurus from Petrified Forest National Park.


However, although no non-dental material of Revueltosaurus was yet known Hunt and Lucas (1994) did consider it to be a bona-fide Ornithischian. Furthermore, these authors erected several other "Ornithischian" taxa from North America based solely on teeth. Heckert (2002) fully redescribed the holotype material and also erected a second species, Revueltosaurus hunti, also based on isolated teeth. As a result, the entire Late Triassic ornithischian record from Narth America and Europe was based solely on isolated teeth. Below is a reconstruction of a hypothesized basal ornithischian.


In 2004 I discovered skeletal material of R. callenderi from the Petrified Forest Member (Chinle Formation) of Petrified Forest National Park. This amazing site (named the Revueltosaurus Quarry) has provided hundreds of bones of numerous individuals of R. callenderi. This was the first recognized non-dental material of this taxon. Below are Randy Irmis, Lori Browne, and Jeff Martz initially excavating the site in 2004.

In 2005 we published a preliminary description of this material. Interestingly the recovered skeletons were not of an ornithischian, or even a dinosaur, but rather from a crocodile-line archosaur (Pseudosuchia). Amazingly, the ornithischian-like dentition was simply a convergence. We argued that none of the North American and European teeth could be unambiguously considered to represent ornithischians. Thus, the entire Late Triassic record of ornithischians from these continents was essentially erased. Below is a cladogram showing the revised position of Revueltosaurus.


A few authors (e.g., Heckert, 2005) have argued against this interpretation, insisting that some of these teeth do represent ornithischians; however, there are no clear characters of these teeth that support this interpretation (Irmis et al, 2007).

In 2006 the Revueltosaurus Quarry yielded a superbly preserved almost complete skeleton of R. callenderi, which I featured at a recent Society of Vertebrate Paleontology meeting. A full description of this specimen (and thus the taxon) is almost complete. One really neat character (I think) apparent in Jeff's reconstruction is the large size of the skull in relation to the postcranial skeleton. The skull is quite a bit longer than the femur, and this is a bit odd considering the skull "accomplishes" this without elongating the forward portion of the skull as in champsosaurids, phytosaurs, and crocodiles.

I have jokingly refered to Revueltosaurus as the "duck-billed platypus" of the Triassic because of its ornithichian-like dentition, aetosaur-like armor carapace, and other characteristics found only in rauisuchians and crocodylomorphs.

Indeed Revueltosaurus belongs to a previously unrecognized clade of archosaur, which hopefully will elucidate phylogentic relationships within Pseudosuchia.

By the way...the PDF of Parker et al (2005) is freely available from the link in the reference below.

REFERENCES

Heckert, A. B. 2002 A revision of the Upper Triassic ornithischian dinosaur Revueltosaurus, with a description of a new species. New Mexico Museum of Natural History and Science Bulletin 21:253–267.

Heckert, A.B. 2005. Krzyzanowskisaurus, a new name for a probable ornithischian dinosaur from the Upper Triassic Chinle Group, Arizona and New Mexico, USA. New Mexico Museum of Natural History and Science Bulletin 29: 77-83.

Hunt, A. P. 1989. A new ?ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. pp. 355–358 in Dawn of the age of dinosaurs in the American Southwest (ed. S. G. Lucas & A. P. Hunt). New Mexico Museum of Natural History, Albuquerque, NM.

Hunt, A. P., and S.G. Lucas. 1994. Ornithischian dinosaurs from the Upper Triassic of the United States. pp. 227–241 in In the Shadows of the Dinosaurs: Early Mesozoic Tetrapods (ed. N. C. Fraser & H.-D. Sues). Cambridge University Press.

Irmis, R.B., Parker, W.G., Nesbitt, S.J., and Liu, J. 2006. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.

Parker, W.G., Irmis, R.B., Nesbitt, S.J., Martz, J.W., and L.S. Browne. 2005. The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society B 272:963-969. doi:10.1098/rspb.2004.3047