Tawa hallae
- 演化之聲
- 58 minutes ago
- 3 min read
Age
Triassic（Norian）
214 Ma
Taxonomy
Kingdom：Animalia
Phylum：Chordata
Class：Reptilia
Order：Saurischia
Genus：Tawa
Species：Tawa hallae
Morphological description
Tawa hallae was a small early dinosaur with an adult body length of approximately 2.5 meters and a body mass of around 15 kilograms. Its skeletal anatomy displays characteristics linking it to several different dinosaur groups. For example, its skull morphology more closely resembles that of coelophysoids, whereas the structure of the ilium is more similar to that of herrerasaurids.
Similar to coelophysoids, a curved recess is present between the maxilla and premaxilla. The cervical vertebrae of Tawa hallae also exhibit adaptations associated with air sacs, suggesting that cervical air sacs evolved very early in dinosaur evolution. Compared to other early dinosaurs such as Herrerasaurus and Eoraptor, Tawa hallae possessed a far more gracile body build.
Etymology
The generic name Tawa is derived from the Hopi Pueblo name for the sun god.
The specific name hallae honors paleontologist Ruth Hall, who established the Ghost Ranch Paleontological Museum.
Biological interpretation
Fossils of Tawa hallae were first discovered in 2004 and formally named in 2009. The holotype specimen, GR 241, is a juvenile individual consisting of a partial skull, forelimbs, vertebrae, hind limbs, ribs, and gastralia. In addition, at least seven other specimens of Tawa hallae are known. Specimen GR 242 represents an almost complete individual. All known specimens were discovered at Ghost Ranch in New Mexico, a locality famous for yielding numerous early dinosaur fossils.
In 2013, one study proposed that Tawa hallae and Dromomeron were actually the same animal, with Dromomeron representing juvenile individuals and Tawa hallae representing adults. However, a later study published in 2017 conducted a detailed comparison of their fossils and demonstrated that the femora of the two taxa differ significantly, and that these differences cannot be explained by ontogenetic variation. As a result, this hypothesis was rejected.
Phylogenetic analyses indicate that Tawa hallae belongs within Theropoda and occupies a more derived evolutionary position than Eoraptor and Herrerasaurus. It may also represent a sister lineage to Neotheropoda, although its exact phylogenetic placement remains controversial.
During the Late Triassic, the environment inhabited by Tawa hallae was located near the equator and characterized by a warm climate with seasonal rainfall. These conditions caused periodic flooding events in the region. The rich fossil deposits of Ghost Ranch were likely formed by floodwaters transporting and accumulating carcasses. After flooding events, standing water may have persisted for extended periods, allowing buried carcasses to remain in oxygen-poor conditions before final burial by sediment. The relatively good preservation of multiple Tawa hallae specimens was likely the result of these exceptional taphonomic conditions.
（Author: Bai Leng）
Reference
1. Nesbitt, S. J., Smith, N. D., Irmis, R. B., Turner, A. H., Downs, A., Norell, M. A. (2009). A Complete Skeleton of a Late Triassic Saurischian and the Early Evolution of Dinosaurs. Science.
2. Müller, R.T. (2017). Are the dinosauromorph femora from the Upper Triassic of Hayden Quarry (New Mexico) three stages in a growth series of a single taxon?. Anais da Academia Brasileira de Ciências.
3. Bennett, S.C. (2013). A Rebuttal to Nesbitt's and Hone's "An external mandibular fenestra and other archosauriform characteristics in basal pterosaurs". International Symposium on Pterosaurs.
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