| Tawa hallae | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda (?) |
| Genus: | †Tawa Nesbittet al., 2009 |
| Species: | †T. hallae |
| Binomial name | |
| †Tawa hallae Nesbittet al., 2009 | |
Tawa (named after theHopi word for thePuebloansun god) is agenus of possiblebasaltheropoddinosaurs from the LateTriassic period.[1] The fossil remains ofTawa hallae, thetype and only species were found in the Hayden Quarry ofGhost Ranch,New Mexico, US. Its discovery alongside the relatives ofCoelophysisandHerrerasaurus supports the hypothesis that the earliest dinosaurs arose inGondwana during the earlyLate Triassic period in what is now South America, and radiated from there around the globe.[2] Thespecific name honoursRuth Hall, founder of the Ghost Ranch Museum of Paleontology.[3]
Tawa was estimated to have been 2.5 m (8 ft 2 in) long as an adult, with a weight of 15 kg (33 lb).[4]Tawa preserves characters that can be associated with different dinosaur taxa. Its skull morphology resembles that ofcoelophysoids and the ilium approximates that of aherrerasaurid. Like the coelophysoids,Tawa has a kink in its upper jaws, between themaxilla and thepremaxilla. With respect to limb proportion, thefemur is very long compared to thetibia. A neckvertebral adaptation inTawa supports the hypothesis that cervical air sacs antedate the origin of the Neotheropoda and may be ancestral forsaurischians, and also links the dinosaurs with the evolution of birds. Compared to earlier dinosaurs such asHerrerasaurusandEoraptor,Tawa had a relatively slender build.[3]
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group. According to Nesbitt et al. (2009)Tawa can be distinguished based on the following features: the prootic bones meet on the ventral midline of the endocranial cavity, the anterior tympanic recess is greatly enlarged on the anterior surface of the basioccipital and extends onto the prootic and the parabasisphenoid, a deep recess is present on posterodorsal base of the paroccipital process, a sharp ridge extending dorsoventrally on the middle of the posterior face of the basal tuber, an incomplete ligamentalsulcus is present on the posterior side of the femoral head, a semicircular muscle scar/excavation is present on the posterior face of femoral head, a small semicircular excavation on posterior margin of medial posteriorcondyle of proximaltibia, a "step" is present on the ventral surface of theastragalus,metatarsal I is similar in length to the other metatarsals.[3]
Fossils now attributed toTawa were first discovered in 2004. Theholotype, a juvenile individual, catalogedGR 241, consists of a mostly complete, but disarticulated skull, forelimbs, a partialvertebral column, hindlimbs, ribs, andgastralia. The determination was made that this specimen is a juvenile based on the presence of an open braincase and unfused neurocentral sutures. Fossils of at least seven other individuals were also discovered at the site. One of these specimens, cataloged GR 242, is also nearly complete. An isolated femur, GR 244, suggests that adults were at least 30% larger than the juvenile holotype. GR 242 was assigned as aparatype for the genus along with specimens representing a femora, pelvis, and tail (GR 155); and cervicalvertebrae (GR 243).[3]
All of these specimens are from the Hayden Quarry, a site inNew Mexico, which preserves many fossils of early dinosaurs and their close relatives. They were discovered in gray/green siltstone dating to theNorian stage of theTriassic period, about 215-213 million years ago.[5]Tawa was formally described in 2009 by a group of six American researchers led bySterling J. Nesbitt of theAmerican Museum of Natural History.[3] At the time of publication in the journalScience, Nesbitt was a postdoctoral researcher at theUniversity of Texas at Austin'sJackson School of Geosciences.[6]
Based on the study of the overlapping material ofDromomeron romeri andTawa, S. Christopher Bennett proposed that the two taxa were conspecific, forming a single growth series withD. romeri being the juvenile andTawa being the adult.[7] However, noting prominent differences between their femora which cannot be attributed to variation with age, Rodrigo Müller rejected this proposal in 2017. He further noted that whileD. romeri is known from juveniles only, it shares many traits in common withD. gigas, which is known from mature specimens.[5]
Thetype species isTawa hallae, which was described in 2009 by Nesbittet al., and considered more basal thanCoelophysis, an early theropod from the Late Triassic. In 2009, Mortimer cautioned that the analysis by Nesbittet al. was limited because it failed to consider all the characters of the relevant dinosaurs treated by the old analysis (e.g.Guaibasaurus, Panphagia,Sinosaurus,Dracovenator,Lophostropheus, etc.)[8]Tawa was however found to bemore advanced than the earliesttheropod dinosaurs,Eoraptor andHerrerasaurus,[9] andStaurikosaurus.
Sueset al. (2011) consideredTawa aderived earlytheropod.[10] Acladistic analysis ofTawa and other early theropods indicate that theCoelophysoidea, a group of early dinosaurs, may be an artificial grouping becauseTawa combines classic coelophysoid features with features which appear to be ancestral to theneotheropods.Tawa is believed to be thesister taxon ofNeotheropoda, a group of carnivorous dinosaurs which largely bore only three functional digits on their feet.[3]
In 2011, Martinez and colleagues concluded thatTawa was the basalmost coelophysoid,[11] while a second 2011 analysis by paleontologistsMartin D. Ezcurra andStephen L. Brusatte, as well as a follow-up analysis modified with additional data by You Hai-Lu and colleagues in 2014, foundTawa to be aprimitive theropod.[12][13] This position forTawa was also recovered in the large analysis of early dinosaurs by Matthew Baron,David B. Norman and Paul Barrett in 2017.[14]
Cau (2018)[15] and Novaset al. (2021)[16] consideredTawa a non-herrerasaurid herrerasaur, although the first study placed Herrerasauria outside Dinosauria and the second placed it in Saurischia.
The Hayden Quarry atGhost Ranch belongs to the lower portion of the Petrified Forest Member of theChinle Formation inNew Mexico. The discovery ofTawa alongside the relatives ofCoelophysis andCamposaurus supports the hypothesis that the earliest dinosaurs arose inGondwana during the Late Triassic period in what is now South America, and radiated around the globe from there.
Ghost Ranch was located close to the equator 200 million years ago, and had a warm, monsoon-like climate with heavy seasonal precipitation. Hayden Quarry, an excavation site at Ghost Ranch, has yielded a diverse collection of fossil material that included the first evidence of dinosaurs and less-advanced dinosauromorphs from the same time period. The discovery indicates that the two groups lived together during the early Triassic period 235 million years ago.[17]Tawa's paleoenvironment included variousarchosauriforms such ascrocodylomorphs, "rauisuchians",phytosaurs, anddinosauriforms likeDromomeron,Chindesaurus,Eucoelophysis, and possiblyCoelophysis.[18]
Based on their review of the early carnivorous dinosaur fauna from Ghost Ranch and theIschigualasto Formation Nesbit et al. (2009) observed that each was descended from a separate lineage, and inferred that the "South American" protocontinentGondwana was the ancestral range for basal dinosaurs. Nesbit et al. (2009) went on to note that dinosaurs left their ancestral range inGondwana and 200 million years ago they dispersed across the adjoined continents of Pangea.[3]
Nesbitet al. (2009) noted that repeated flooding events collected vertebrate bones, carcasses, and plant material from the landscape surface, possibly inhyperconcentrated flows, and deposited them at what is now Hayden Quarry. It was observed that these flooding events were separated by intervals where there was standing water and weakly developed, poorly drained (hydromorphic) soil formation. TheTawa specimens were very well preserved which suggests that they were buried extremely soon after dying.[3]
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