Tawa (dinosaur)

Tawa (pron.:"Ta-WAH"; named after the Hopi word for the Puebloan sun god) is a genus of theropod dinosaur from the Late Triassic period.[1] The discovery of Tawa alongside the relatives of Coelophysis and Herrerasaurus supports the hypothesis that the earliest dinosaurs arose in Gondwana during the Late Triassic period in what is now South America, and radiated around the globe from there.[2] The specific name honours Ruth Hall, founder of the Ghost Ranch Museum of Paleontology.

Description
Tawa was estimated to have been 2 m (6.5 ft) long, almost 2.5 feet at the hips. Other estimates suggest that Tawa weighed 40 kilograms (88 pounds) at most.[4] The holotype, a juvenile individual, cataloged GR 241, consists of a mostly complete but disarticulated skull, forelimbs, the pectoral girdle, a partial vertebral column, hindlimbs, the pelvic girdle, ribs, and gastralia. The determination was made that this specimen is a juvenile based on the presence of an open braincase and unfused neurocentral sutures.[3] An isolated femur GR 244 suggests that adults were at least 30% larger than the juvenile holotype. Tawa preserves characters that can be associated with different dinosaur taxa. It's skull morphology resembles that of coelophysoids and the illium approximates that of a herrerasaurid. Like the coelophysoids, Tawa has a kink in its upper jaws, between the maxilla and the premaxillae. With respect to limb proportion, the femur is very long compared to the tibia. A neck vertebrae adaptation in Tawa supports the hypothesis that cervical air sacs predate the origin of the taxon Neotheropoda and may be ancestral for Saurischians, and also links the dinosaurs with the evolution of birds. Compared to earlier dinosaurs like Herrerasaurus and Eoraptor, Tawa had a relatively slender build.

The following were assigned as paratypes: a nearly complete skeleton (GR 242); femora, pelvis and tail (GR 155); and cervical vertebrae (GR 243).

Discovery
Fossils now attributed to Tawa were first discovered in 2004. Fossils of at least seven other individuals were also discovered at the site. One of these specimens, cataloged GR 242, is also nearly complete. All of these specimens are from the Hayden Quarry and date to between 215 to 213 million years old. Tawa was formally described in 2009 by a group of six American researchers led by Sterling J. Nesbitt of the American Museum of Natural History.[3] At the time of publication in the journal Science, Nesbitt was a postdoctoral researcher at the University of Texas at Austin's Jackson School of Geosciences.