Tawa hallae

Tawa (named after the Hopi word for the Puebloan sun god) is a genus of theropod dinosaur from the Late Triassic period.[1] The fossil remains of Tawa hallae, the type and only species were found in the Hayden Quarry of Ghost Ranch, New Mexico, US. Its discovery alongside the relatives of Coelophysis and Herrerasaurus supports the hypothesis that the earliest dinosaurs arose in Gondwana during the early Late Triassic period in what is now South America, and radiated from there around the globe.[2] The specific name honours Ruth Hall, founder of the Ghost Ranch Museum of Paleontology.[3]

Tawa hallae
Temporal range: Late Triassic, 215 Ma
Tawa NT small
Life restoration
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Theropoda
Genus: Tawa
Nesbitt et al., 2009
Species:
T. hallae
Binomial name
Tawa hallae
Nesbitt et al., 2009

Description

Tawa hallae SIZE 01
Size comparison between the holotype, an adult specimen, and a human.

Tawa was estimated to have been 2.5 metres (8 ft 2 in) long as an adult, with a weight of 15 kilograms (33 lb).[4] Tawa preserves characters that can be associated with different dinosaur taxa. Its 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 premaxilla. 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 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.[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 ligamental sulcus 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 posterior condyle of proximal tibia, a "step" is present on the ventral surface of the astragalus, metatarsal I is similar in length to the other metatarsals.[3]

Discovery

Fossils now attributed to Tawa were first discovered in 2004. The holotype, a juvenile individual, cataloged GR 241, consists of a mostly complete but disarticulated skull, forelimbs, a partial vertebral column, hindlimbs, 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. 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 a paratype for the genus along with specimens representing a femora, pelvis, and tail (GR 155); and cervical vertebrae (GR 243).[3]

All of these specimens are from the Hayden Quarry, a site in New Mexico which preserves many fossils of early dinosaurs and their close relatives. They were discovered in gray/green siltstone dating to the Norian stage of the Triassic period, approximately 215-213 million years ago.[5] 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.[6]

Based on the study of the overlapping material of Dromomeron romeri and Tawa, S. Christopher Bennett proposed that the two tawa were conspecific, forming a single growth series with D. romeri being the juvenile and Tawa being the adult.[7] However, noting prominent differences between their femurs which cannot be attributed to variation with age, Rodrigo Muller rejected this proposal in 2017. He further noted that, while D. romeri is known from juveniles only, it shares many traits in common with D. gigas, which is known from mature specimens.[5]

Classification

 Dinosauria 

 Ornithischia 

 Saurischia 

Sauropodomorpha 

 Theropoda 

Herrerasauridae 

    

 Eoraptor 

    

Tawa

 Neotheropoda 
    

 Coelophysis 

 Megapnosaurus 

    

 Liliensternus 

 Jurassic theropods 

The type species is Tawa hallae, which was described in 2009 by Nesbitt et al., and considered more basal than Coelophysis, an early theropod from the Late Triassic. In 2009, Mortimer cautioned that the analysis by Nesbitt et 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 be more advanced than the earliest theropod dinosaurs, Eoraptor and Herrerasaurus,[9] and Staurikosaurus.

Sues et al. (2011) considered Tawa a derived early theropod.[10] A cladistic analysis of Tawa and other early theropods indicate that the Coelophysoidea, a group of early dinosaurs, may be an artificial grouping because Tawa combines classic coelophysoid features with features which appear to be ancestral to the neotheropods. Tawa is believed to be the sister taxon of Neotheropoda, a group of carnivorous dinosaurs which largely bore only three functional digits on their feet.[3]

In 2011, Martinez and colleagues concluded that Tawa was the basalmost coelophysoid,[11] while a second 2011 analysis by paleontologists Martin D. Ezcurra and Stephen L. Brusatte, as well as a follow-up analysis modified with additional data by You Hai-Lu and colleagues in 2014, found Tawa to be a primitive theropod.[12][13] This position for Tawa was also recovered in the large analysis of early dinosaurs by Matthew Baron, David B. Norman and Paul Barrett in 2017.[14]

Paleoecology

The Hayden Quarry at Ghost Ranch belongs to the lower portion of the Petrified Forest Member of the Chinle Formation in New Mexico. 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.

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.[15] Tawa's paleoenvironment included various archosauriforms such as crocodylomorphs, "rauisuchians", phytosaurs, and dinosauriforms like Dromomeron, Chindesaurus, Eucoelophysis, and possibly Coelophysis.[16]

Based on their review of the early carnivorous dinosaur fauna from Ghost Ranch and the Ischigualasto Formation Nesbit et al. (2009) observed that each was descended from a separate lineage, and inferred that the "South American" protocontinent Gondwana was the ancestral range for basal dinosaurs. Nesbit et al. (2009) went on to note that dinosaurs left their ancestral range in Gondwana and 200 million years ago they dispersed across the adjoined continents of Pangea.[3]

Nesbit et al. (2009) noted that repeated flooding events collected vertebrate bones, carcasses, and plant material from the landscape surface, possibly in hyperconcentrated 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. The Tawa specimens were very well preserved which suggests that they were buried extremely soon after dying.[3]

See also

References

  1. ^ Maffly, Brian (December 10, 2009), "New Mexico find sheds light on early dinosaur dispersal", Salt Lake Tribune, archived from the original on December 13, 2009
  2. ^ "New T. Rex Cousin Suggests Dinosaurs Arose in S. America", National Geographic, December 10, 2009
  3. ^ a b c d e f g h 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" (PDF), Science, 326 (5959): 1530–1533, doi:10.1126/science.1180350, PMID 20007898.
  4. ^ Paul, G.S. (2010). "Theropods". The Princeton Field Guide to Dinosaurs. Princeton: Princeton University Press. p. 74. ISBN 9780691167664.
  5. ^ a b 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?" (PDF). Anais da Academia Brasileira de Ciências. 89 (2): 835–839. doi:10.1590/0001-3765201720160583.
  6. ^ Airhart, Marc (December 10, 2009), "New Meat-Eating Dinosaur Alters Evolutionary Tree", Jackson School of Geosciences, archived from the original on March 10, 2010
  7. ^ Bennett, S.C. (2013). "A Rebuttal to Nesbitt's and Hone's "An external mandibular fenestra and other archosauriform characteristics in basal pterosaurs"" (PDF). International Symposium on Pterosaurs: 19–22.
  8. ^ DML: http://dml.cmnh.org/2009Dec/msg00122.html
  9. ^ Harmon, Katherine (December 10, 2009), "Newly Discovered T. Rex Relative Fleshes Out Early Dino Evolution", Scientific American
  10. ^ H.D. Sues, S. J. Nesbitt, D. S. Berman and A. C. Henrici. 2011. "A late-surviving basal theropod dinosaur from the latest Triassic of North America". Proceedings of the Royal Society B 278:3459-3464
  11. ^ Martinez, Sereno, Alcober, Columbi, Renne, Montanez and Currie, 2011. "A basal dinosaur from the dawn of the dinosaur era in Southwestern Pangaea". Science. 331, 206-210.
  12. ^ Ezcurra, M.D.; Brusatte, S.L. (2011). "Taxonomic and phylogenetic reassessment of the early neotheropod dinosaur Camposaurus arizonensis from the Late Triassic of North America". Palaeontology. 54 (4): 763–772. doi:10.1111/j.1475-4983.2011.01069.x.
  13. ^ You, H.-L.; Azuma, Y.; Wang, T.; Wang, Y.-M.; Dong, Z.-M. (2014). "The first well-preserved coelophysoid theropod dinosaur from Asia". Zootaxa. 3873 (3): 233–249. doi:10.11646/zootaxa.3873.3.3.
  14. ^ Baron, M.G.; Norman, D.B.; Barrett, P.M. (2017). "A new hypothesis of dinosaur relationships and early dinosaur evolution". Nature. 543: 501–506. doi:10.1038/nature21700.
  15. ^ Braginetz, Donna. "A new species of dinosauromorph (lower left) was among the mixed assemblage of dinosaurs and dinosauromorphs found at Hayden Quarry in Ghost Ranch, N.M." American Museum of Natural History. Retrieved 31 March 2013.
  16. ^ Irmis et al. 2007

External links

2017 in archosaur paleontology

The year 2017 in archosaur paleontology was eventful. Archosaurs include the only living dinosaur group — birds — and the reptile crocodilians, plus all extinct dinosaurs, extinct crocodilian relatives, and pterosaurs. Archosaur palaeontology is the scientific study of those animals, especially as they existed before the Holocene Epoch began about 11,700 years ago. The year 2017 in paleontology included various significant developments regarding archosaurs.

This article records new taxa of fossil archosaurs of every kind that have been described during the year 2017, as well as other significant discoveries and events related to paleontology of archosaurs that occurred in the year 2017.

Daemonosaurus

Daemonosaurus (pron.:"DAY-mow-no-SORE-us") is an extinct genus of theropod dinosaur from the Late Triassic of New Mexico. Fossils have been found from deposits in the Chinle Formation, which is latest Triassic in age. While theropods had diversified into several specialized groups by this time, Daemonosaurus is a basal theropod that lies outside the clade Neotheropoda. Daemonosaurus is unusual among early theropods in that it had a short skull and long protruding teeth.

Dromomeron

Dromomeron (meaning "running femur") is a genus of lagerpetonid dinosauromorph archosaur that lived around 220 to 211.9 ± 0.7 million years ago. The genus contains species known from Late Triassic-age rocks of the southwestern United States and northwestern Argentina. It is described as most closely related to the earlier Lagerpeton of Argentina, but was found among remains of true dinosaurs like Chindesaurus, indicating that the first dinosaurs did not immediately replace related groups.Based on the study of the overlapping material of Dromomeron and Tawa hallae, Christopher Bennett proposed that the two taxa were conspecific, forming a single growth series of Dromomeron. However, noting prominent differences between their femurs which cannot be attributed to variation with age, Rodrigo Muller rejected this proposal in 2017. He further noted that, while D. romeri is known from juveniles only, it shares many traits in common with D. gigas, which is known from mature specimens.

List of the Mesozoic life of New Mexico

This list of the Mesozoic life of New Mexico contains the various prehistoric life-forms whose fossilized remains have been reported from within the US state of New Mexico and are between 252.17 and 66 million years of age.

List of the prehistoric life of New Mexico

This list of the prehistoric life of New Mexico contains the various prehistoric life-forms whose fossilized remains have been reported from within the US state of New Mexico.

Nyasasaurus

Nyasasaurus (meaning "Lake Nyasa lizard") is an extinct genus of dinosauriform reptile from the Middle Triassic Manda Formation of Tanzania that appears to be the earliest known dinosaur. The type species Nyasasaurus parringtoni was first described in 1956 in the doctoral thesis of English paleontologist Alan J. Charig, but it was not formally described until 2013. Previously, the oldest record of dinosaurs was from Argentina and dated back to the late Carnian stage, about 231.4 million years ago. Nyasasaurus comes from a deposit that dates back to the Anisian stage, meaning that it predates other early dinosaurs by about 12 million years.

Timeline of coelophysoid research

This timeline of coelophysoid research is a chronological listing of events in the history of paleontology focused on the coelophysoids, a group of primitive theropod dinosaurs that were among Earth's dominant predators during the Late Triassic and Early Jurassic epochs. Although formally trained scientists didn't discover coelophysoid fossils until the late 19th century, Native Americans of the modern southwestern United States may have already encountered their fossils. Navajo creation mythology describes the early Earth as being inhabited by a variety of different kinds of monsters who hunted humans for food. These monsters were killed by storms and the heroic Monster Slayers, leaving behind their bones. As these tales were told in New Mexico not far from bonebeds of Coelophysis, this dinosaur's remains may have been among the fossil remains that inspired the story.The first scientifically documented coelophysoid taxon was Coelophysis bauri itself. However, when the species was first described by Edward Drinker Cope in 1887, it was thought to belong to a genus of small carnivorous dinosaurs called Coelurus. Later that same year Cope changed his mind and transferred it to the genus Tanystropheus. Tanystropheus turned out to be a long-necked reptile not regarded by scientists as a true dinosaur. As such, "Tanystrophaeus" bauri was soon given its own genus, Coelophysis in 1889. Over the ensuing decades, many new coelophysoids would be discovered, like Podokesaurus, Procompsognathus, and Segisaurus.In 1947, a paleontological team led by Edwin Colbert made a major discovery in New Mexico. While on an expedition to Petrified Forest National Park in Arizona, he made a detour to Ghost Ranch, New Mexico where many phytosaur fossils had been found. There they discovered a massive bonebed preserving hundreds of Coelophysis, many of which were complete and articulated. The find has been considered the most significant Triassic fossil discovery in North America. Later, other coelophysoids and even other bonebeds would be discovered. Notable coelophysoids discovered during the mid to late 20th century include Syntarsus (now Megapnosaurus) and Gojirasaurus. Despite this extensive history of research, the formal recognition of the Coelophysoidea as a distinct group of dinosaurs is relatively recent and the group would not be formally named until a 1994 by Thomas Holtz.

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