A New Late Triassic Theropod, Tawa hallae, and its Implications for Early Dinosaur Evolution

A new Late Triassic basal theropod, Tawa hallae, is described by Nesbitt et al. (2009) in a new paper out today in Science. Tawa is known from two relatively complete skeletons and other isolated material from the Hayden Quarry at Ghost Ranch New Mexico [another link]. The Hayden Quarry is in the Petrified Forest Member of the Chinle Formation.

This quarry is an absolutely spectacular site that is proving to be the most diverse locality in the Chinle Formation, even more than the famed Coelophysis Quarry (which is close geographically but higher stratigraphically). Other taxa found at this site include Dromomeron romeri, Chindesaurus bryansmalli, Eucoelophysis baldwini, a coelophysoid theropod, and a shuvosaurid as well as various aetosaurs, crocodylomorphs, rauisuchians, phytosaurs, drepanosaurs, metoposaurs, and fish (Irmis et al. 2007).

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.

Dentary of Tawa hallae


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.

Links to some early news articles [here] [here] [here]

Link to National Science Foundation special report with lots of additional information on Tawa, its discovery, and its finders.

REFERENCES

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.

The Coelophysis Quarry at Ghost Ranch

I'm back from a couple of days at the Hayden Quarry near Ghost Ranch, New Mexico. As usual it was a great experience with great people and absolutely amazing fossils. My student interns were able to meet quite a few paleontologists including Sterling Nesbitt, Randy Irmis, Nate Smith, Alan Turner, Alex Downs, Mike Getty, and Mark Loewen. Plus they got to get really dirty and find fossils. A great experience for them.

Another perk is that we took some time off to go visit the classic Coelophysis Quarry. For those of you who have never seen it....here it is in all of its glory. The main bone horizon is now a big flat area where the footprints going up end. This is the siltstone member of the Chinle Formation and the sandstone blocks are fallen blocks from the overlying Entrada Sandstone (Jurassic).

This is the famous "Barney-rock" that overlooks the quarry and all workers were always afraid of it possibly coming down on their heads while they were digging.

Here are the interns (Chuck, Adrian, and Rachel) relaxing after the hike up to the quarry and enjoying the view.

We're Off to the Hayden Quarry

Every year I treat my student interns to a weekend working somewhere else in the Chinle Formation with groups from other institutions. For example in 2002 we worked with Sterling Nesbitt in the Moenkopi Formation excavating an excellent skeleton of Arizonasaurus and in 2003 we joined the Yale Peabody Museum crew in Escalante National Monument in Utah and helped with their Poposaurus excavation (also see this link). I feel that it is important to give my students the opportunity to network with other researchers and to see how others work and collect data. This weekend we are headed out to Ghost Ranch, New Mexico to work in the Hayden Quarry (the type locality of Dromomeron romeri and location of a portion of the Dinosaurs Alive IMAX film) with Sterling Nesbitt, Randy Irmis, and others. This is our third summer there and it is always a great experience.

Photos are from past seasons and were lifted from here and here.

Alan Turner preparing his afternoon siesta spot.

Amazing views from the quarry.

Another photo that proves that from the quarry you really can see scenes like the last one.


A bone that I wish that I had found instead of my usual scrap.

Randy protecting his lunch from other ravenous quarry workers.

Dromomeron, the Hayden Quarry, and Cool Jobs!

These have been out for awhile, but I just became aware of them a little bit ago. Anyhow for those of you who are interested and haven't seen these, here are my Triassic colleagues Randy Irmis, Sterling Nesbitt, and Alex Downs discussing The Skinny on Naming a New Dinosaur, Fossil Hunters Roll the Bones for Clues, and Cool Jobs: Fossil Hunters. These clips are from the Discovery Channel and I found them here. Enjoy!