Key points of the paper:
• Tawa possesses a mix of “coelophysoid” (Neotheropoda) and herrerasaurid characters that help clarify basal saurischian relationships.
- Herrerasaurids (Herrerasaurus, Chindesaurus, Staurikosaurus) and Eoraptor are unambiguously nested within Theropoda.
- Tawa is the sister taxon to Neotheropoda (see phylogenetic reconstruction artwork and tree below).
- Coelophysoidea is found to be paraphyletic, with the hypothesis that this clade had been serving as a “phylogenetic vacuum cleaner”, picking up deep theropod synapomorphies and ceratosaur/tetanuran reversals and treating them as coelophysoid synapomorphies.
• The presence of cervical pleurocoels in Tawa supports the hypothesis that cervical air sacs predate the origin of Neotheropoda and may be ancestral for Saurischia.
• The age of the Hayden Quarry (~215-213) and comparison with the Ischigualasto fauna supports the hypothesis of the diachronous evolution of Triassic dinosaur faunas.
• The presence of Dromomeron, Chindesaurus, Tawa, and a basal neotheropod in the Hayden Quarry suggests that the North American theropod fauna was not endemic, an was the result of several dispersals of taxa from South America.
• This prevalence of dispersal among early dinosaurs and other Triassic reptile groups indicates that physical barriers did not prevent these groups from moving around Pangaea - specifically into North America. Thus the question is why didn't sauropodomorphs and ornithischians make it to North America during the Triassic? Nesbitt et al. hypothesize that they were excluded by climate.
Reconstruction of the head of Tawa hallae.
Articulated manus of Tawa hallae
In-situ articulated limbs of Tawa hallae
Associated dentary and maxilla of Tawa hallae
Block containing skull and other bones.
Thanks to Randy Irmis and Sterling Nesbitt for providing the photos and reconstructions used here. The reconstructions are by Jorge Gonzalez.
Link to National Science Foundation special report with lots of additional information on Tawa, its discovery, and its finders.
Irmis, R. B, Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H, Woody, D., and A. Downs. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358–361.
Nesbitt, S. J., Smith, N. D., Irmis, R. B., Turner, A. H., Downs, A., and M. A. Norell. 2009. A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326:1530-1533.