Tawa hallae

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Tawa hallae
Temporal range: Late Triassic, 215  Ma
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Tawa hallae Martz.jpg
Life restoration
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda (?)
Genus: Tawa
Nesbitt et al., 2009
Species:
T. hallae
Binomial name
Tawa hallae
Nesbitt et al., 2009

Tawa (named after the Hopi word for the Puebloan sun god) is a genus of possible basal theropod dinosaurs 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]

Description

Skeletal diagram, with known material in white and unknown in grey Tawa hallae skeleton.png
Skeletal diagram, with known material in white and unknown in grey

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 of coelophysoids and the ilium 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 vertebral adaptation in Tawa supports the hypothesis that cervical air sacs antedate 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 such as 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

Life restoration and size comparison Tawa NT small.jpg
Life restoration and size comparison

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, about 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 taxa 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 femora which cannot be attributed to variation with age, Rodrigo Müller 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  
    
    

  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]

Cau (2018) [15] and Novas et al. (2021) [16] considered Tawa a non-herrerasaurid herrerasaur, although the first study placed Herrerasauria outside Dinosauria and the second placed it in Saurischia.

Paleoecology

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

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 Camposaurus 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. [17] Tawa's paleoenvironment included various archosauriforms such as crocodylomorphs, "rauisuchians", phytosaurs, and dinosauriforms like Dromomeron , Chindesaurus , Eucoelophysis, and possibly Coelophysis . [18]

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

Related Research Articles

<i>Eoraptor</i> Extinct genus of dinosaurs

Eoraptor is a genus of small, lightly built, basal sauropodomorph dinosaur. One of the earliest-known dinosaurs and one of the earliest members of the sauropod family, it lived approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwestern Argentina. The type and only species, Eoraptor lunensis, was first described in 1993, and is known from an almost complete and well-preserved skeleton and several fragmentary ones. Eoraptor had multiple tooth shapes, which suggests that it was omnivorous. Eoraptor was 1.5 feet (0.46 m) tall and 3 feet (0.91 m) long.

<i>Coelophysis</i> Genus of theropod dinosaurs from the late Triassic

Coelophysis is a genus of coelophysid theropod dinosaur that lived approximately 215 to 208.5 million years ago during the Late Triassic period from the middle to late Norian age in what is now the southwestern United States. Megapnosaurus was once considered to be a species within this genus, but this interpretation has been challenged since 2017 and the genus Megapnosaurus is now considered valid.

<span class="mw-page-title-main">Herrerasauridae</span> Extinct family of basal saurischian dinosaurs

Herrerasauridae is a family of carnivorous dinosaurs, possibly basal to either theropods or even all of saurischians, or even their own branching from Dracohors, separate from Dinosauria altogether. They are among the oldest known dinosaurs, first appearing in the fossil record around 233.23 million years ago, before becoming extinct by the end of the Carnian stage. Herrerasaurids were relatively small-sized dinosaurs, normally no more than 4 metres (13 ft) long, although the holotype specimen of "Frenguellisaurus ischigualastensis" is thought to have reached around 6 meters long. The best known representatives of this group are from South America, where they were first discovered in the 1930s in relation to Staurikosaurus and 1960s in relation to Herrerasaurus. A nearly complete skeleton of Herrerasaurus ischigualastensis was discovered in the Ischigualasto Formation in San Juan, Argentina, in 1988. Less complete possible herrerasaurids have been found in North America and Africa, and they may have inhabited other continents as well.

<i>Herrerasaurus</i> Extinct genus of dinosaurs

Herrerasaurus is likely a genus of saurischian dinosaur from the Late Triassic period. This genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 in South America. All known fossils of this carnivore have been discovered in the Ischigualasto Formation of Carnian age in northwestern Argentina. The type species, Herrerasaurus ischigualastensis, was described by Osvaldo Reig in 1963 and is the only species assigned to the genus. Ischisaurus and Frenguellisaurus are synonyms.

<i>Gojirasaurus</i> Extinct genus of dinosaurs

Gojirasaurus is a potentially dubious genus of coelophysoid theropod dinosaur named after the giant monster movie character Godzilla.

<span class="mw-page-title-main">Ghost Ranch</span> Place in New Mexico, United States

Ghost Ranch is a 21,000-acre (85 km2) retreat and education center located close to the village of Abiquiú in Rio Arriba County in north central New Mexico, United States. It served as the summer home and studio of Georgia O'Keeffe in the later 20th century, as well as the subject of many of her paintings.

<i>Effigia</i> Extinct genus of Archosaurs

Effigia was an extinct genus of shuvosaurid known from the Late Triassic of New Mexico, south-western USA. With a bipedal stance, long neck, and a toothless beaked skull, Effigia and other shuvosaurids bore a resemblance to the ornithomimid dinosaurs of the Cretaceous Period. However, shuvosaurids were not dinosaurs, but were instead a specialized family of poposauroid pseudosuchians, meaning that their closest living relatives are crocodilians.

<span class="mw-page-title-main">Coelophysidae</span> Extinct family of dinosaurs

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<i>Camposaurus</i> Extinct genus of dinosaurs

Camposaurus is a coelophysid dinosaur genus from the Norian stage of the Late Triassic period of North America. The pertinent fossil remains date back to the early to middle Norian stage, and is widely regarded as the oldest known neotheropod.

<i>Chindesaurus</i> Extinct genus of dinosaurs

Chindesaurus is an extinct genus of basal saurischian dinosaur from the Late Triassic of the southwestern United States. It is known from a single species, C. bryansmalli, based on a partial skeleton recovered from Petrified Forest National Park in Arizona. The original specimen was nicknamed "Gertie", and generated much publicity for the park upon its discovery in 1984 and airlift out of the park in 1985. Other fragmentary referred specimens have been found in Late Triassic sediments throughout Arizona, New Mexico, and Texas, but these may not belong to the genus. Chindesaurus was a bipedal carnivore, approximately as large as a wolf.

<i>Nyasasaurus</i> Extinct genus of reptiles

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<i>Dromomeron</i> Extinct genus of reptiles

Dromomeron is a genus of lagerpetid avemetatarsalian which 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.

<span class="mw-page-title-main">Lagerpetidae</span> Extinct family of reptiles

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<i>Eodromaeus</i> Extinct genus of dinosaurs

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<i>Daemonosaurus</i> Genus of reptiles (fossil)

Daemonosaurus is an extinct genus of possible theropod dinosaur from the Late Triassic of New Mexico. The only known fossil is a skull and neck fragments from deposits of the latest Triassic Chinle Formation at Ghost Ranch. Daemonosaurus was an unusual dinosaur with a short skull and large, fang-like teeth. It lived alongside early neotheropods such as Coelophysis, which would have been among the most common dinosaurs by the end of the Triassic. However, Daemonosaurus retains several plesiomorphic ("primitive") traits of the snout, and it likely lies outside the clade Neotheropoda. It may be considered a late-surviving basal theropod or non-theropod basal saurischian, possibly allied to other early predatory dinosaurs such as herrerasaurids or Tawa.

<i>Megapnosaurus</i> Extinct genus of dinosaur

Megapnosaurus is an extinct genus of coelophysid theropod dinosaur that lived approximately 188 million years ago during the early part of the Jurassic Period in what is now Africa. The species was a small to medium-sized, lightly built, ground-dwelling, bipedal carnivore, that could grow up to 2.2 m (7.2 ft) long and weigh up to 13 kg (29 lb). It was originally given the genus name Syntarsus, but that name was later determined to be preoccupied by a beetle. The species was subsequently given a new genus name, Megapnosaurus, by Ivie, Ślipiński & Węgrzynowicz in 2001. Some studies have classified it as a species within the genus Coelophysis, but this interpretation has been challenged by more subsequent studies and the genus Megapnosaurus is now considered valid.

<span class="mw-page-title-main">Timeline of coelophysoid research</span>

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.

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<i>Pendraig</i> Extinct genus of dinosaurs

Pendraig is a genus of coelophysoid theropod dinosaur from South Wales. It contains one species, Pendraig milnerae, named after Angela Milner. The specimen was discovered in the Pant-y-Ffynnon quarry. In life it would have measured 1 m (3.3 ft) in length.

References

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