Field of Science

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.

15 comments:

  1. Another truly awesome fine, great to see (although as usual incredibly frustrating that the Journal's length limits mean that there is only one sentence on the postcranial skeletal pneumaticity).

    Some of the media reports are describing Tawa as a feathered theropod -- the Guardian for example. There doesn't seem to be anything about this in the paper -- do you know where it comes from?

    Anyway, many congratulations to Sterling and co.

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  2. Er. Awesome find. Yeah. That's what I said.

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  3. Mike,

    Probably comes from the fact that our artistic reconstructions show it with a covering of protofeathers.

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  4. Can't wait to read the full paper! Hopefully there will be a monograph on it too soon! :)

    Regardless: awesome find!!

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  5. Neat discovery. It's nice to see another nice, short genus name. Some are so long it's crazy. It took me years to pronounce Opisthocoelicaudia.

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  6. Nifty find. I'm not surprised about Eoraptor and herrerasaurids being theropds, since the analyses like Langer's that have them as outside Eusaurischia don't include many of their theropod-like characters. I'm going to remain skeptical of a paraphyletic Coelophysoidea though, given the apparent branch support of 1 step for excluding Liliensternus and 1 more step for excluding Zupaysaurus. I'd also like to see how many of Tykoski's (2005) characters were included, as those tend to increase coelophysoid and ceratosaur sensu lato support.

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  7. Oh, too cool! But I'm confused--it occupies an area between herrerasaurids and coelophyoids or just all theropods that aren't herrerasaurids?

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  8. Mickey - A number of constraint analyses are presented in the Supplementary Information. Although it only takes one additional step to place Liliensternus within Coelophysoidea, a monophyletic Coelophysoidea sensu lato takes five additional steps, which I think is a fairly robust result. If nothing else, the character combination in Tawa definitely weakens support for a monophyletic Coelophysoidea.

    Zach - As you can see in the cladogram that Bill posted, Tawa occupies a phylogenetic position between Eoraptor and neotheropods (including coelophysids). Herrerasaurids are one branch below Eoraptor.

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  9. I'm happy to hear constraint analyses were used. One new feature of my website I've been preparing is taking theropod analyses and trying different constraints on them to see how well they support or reject various hypotheses. My preliminary conclusion is it's usually pretty easy to get any plausible rearrangement in less than ten steps. And we all know how many characters there are that one author will include but another won't.

    I think too many people, especially amateurs, see a new cladogram and very qualitatively think that it's now "the best" and other arrangements are rejected. In reality it's better to ask just how much better supported the new hypothesis is. In the case of Liliensternus, based on subadult specimens which have not been described since the 30's, I'd say chances are great that more than one currently unused character exists to tie it to Coelophysis. Of course, the same goes for tying them to Dilophosaurus or more derived theropods, so I'm not saying the Tawa result is wrong or unlikely. I'm certainly not insulting the phylogenetic work in the paper, as I haven't even read it yet. I'm just adding a cautionary note for interpreting any phylogeny of anything anywhere.

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  10. Sure, Liliensternus hasn't been described since the 1930s, but two of the authors on the paper have studied and coded the material first hand. I think that is one of the benefits of our "cladogram du jour": not only do we have the largest basal dino dataset to date (both in terms of taxa and characters), but 99% of the specimens coded have been examined first-hand (see Table S2 in the Suppl Info). I'm not saying our phylogenetic hypothesis is necessarily all correct, but I do think it is a step in the right direction.

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  11. What do you think about the cladistic ipotesis for Tawa hallae, proposed by Andrea Cau on his blog (Theropoda)?

    Valery

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  12. Andrea Cau has admitted that his post was premature.

    See his comment here:

    http://whenpigsfly-returns.blogspot.com/2009/12/hey-look-new-dinosaur.html

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  13. I'm happy to hear Liliensternus was examined personally. Now someone just needs to pay your team to redescribe it. ;) I hope I didn't come off as too harsh, but as a perfect example of what I was saying, just look at Tom Holtz's title for his post today on Tawa- "Tawa hallae: everything you know about basal saurischians is wrong...‏" *sigh* But keep up the good work.

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