Chilesaurus

Chilesaurus is an extinct genus of herbivorous dinosaur. The type and only species is Chilesaurus diegosuarezi.[1] Chilesaurus lived about 145 million years ago (Mya) in the Late Jurassic period of Chile. Showing a combination of traits from theropods, ornithischians, and sauropodomorphs, this genus has far-reaching implications for the evolution of dinosaurs, such as whether the traditional saurischian-ornithischian split is superior or inferior to the newly proposed group Ornithoscelida.[2]

Chilesaurus
Temporal range: Late Jurassic
~145 Ma
Chilesaurus diegosarezi
Reconstructed skeleton
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Genus: Chilesaurus
Novas et al. 2015
Species:
C. diegosuarezi
Binomial name
Chilesaurus diegosuarezi
Novas et al. 2015

Description

Chilesaurus diagram
Life restoration of Chilesaurus, size based on holotype individual

Chilesaurus measured roughly 3.2 m (10.5 ft) from nose to tail. The holotype is a smaller individual of half that length.

Chilesasaurus
Life restoration of Chilesaurus showing characteristic arm-folding and speculative filaments.

The most unusual feature of Chilesaurus is its spatula-shaped, elongated teeth, obliquely pointing forwards. Such dentition would be unique in the Theropoda, where it has sometimes been recovered, and is typical for a herbivore, indicating Chilesaurus was a plant-eater. Another adaptation for eating plants is the backward-pointing pubic bone in the pelvis, making room for a large gut. Such a pelvic arrangement is typical for the Ornithischia, which some other studies have allied it with, although is also found in some groups of Theropoda. The hind limb of Chilesaurus had become less adapted to running as shown by a small cnemial crest on the front top of the shinbone, and a broad foot with a weight-bearing first toe. Chilesaurus could defend itself with a strong arm, bearing a large first claw which could be extended outwards, just as with the basal Sauropodomorpha.

Discovery and naming

Chilesaurus holotype cast
Cast of the holotype skeleton

Fossils of Chilesaurus, a vertebra and a rib, were first discovered on 4 February 2004 by the seven-year-old Diego Suárez, who together with his parents, geologists Manuel Suárez and Rita de la Cruz, was searching for decorative stones in the Aysén Region. More specimens were present that in 2008 were reported as representing several dinosaurian species.[3] Only later was it realised that these belonged to a single species with a bizarre combination of traits.

In 2015, the type species C. diegosuarezi was named and described by Fernando Emilio Novas, Leonardo Salgado, Manuel Suárez, Federico Lisandro Agnolín, Martín Dario Ezcurra, Nicolás Chimento, Rita de la Cruz, Marcelo Pablo Isasi, Alexander Omar Vargas, and David Rubilar-Rogers. The generic name refers to Chile. The specific name honours Diego Suárez.[1]

The holotype, SNGM-1935, was found in a layer of the Toqui Formation dating from the late Tithonian. It consists of an articulated, rather complete skeleton with skull of a juvenile individual, lacking the feet and most of the tail. Four other partial skeletons (specimens SNGM-1937, SNGM-1936, SNGM-1938, and SNGM-1888) and several single bones (specimens SNGM-1889, SNGM-1895, SNGM-1901, SNGM-1894, SNGM-1898, SNGM-1900, and SNGM-1903) are the paratypes. They represent juvenile and adult individuals.[1]

Classification

Chilesaurus skeleton
Skeletal reconstruction

Chilesaurus was first placed in the theropod group Tetanurae, in a basal position. It shows a confusing mix of traits normally present in Coelurosauria, basal Sauropodomorpha and Ornithischia, but was found in its original description to belong to none of these groups.[1]

The below cladogram illustrates the taxonomy proposed in the original description of Chilesaurus.[1]

Theropoda

Herrerasauridae

Neotheropoda

Coelophysoidea

Averostra

Ceratosauria

Tetanurae

Chilesaurus

Megalosauroidea

Allosauroidea

Coelurosauria

Chilesaurus NT small
Restoration

Alternatively, Baron & Barrett proposed in 2017 that Chilesaurus may be a basal ornithischian, with orithischians being closer to theropods than sauropodomorphs as a part of the Ornithoscelida.[4][5] Less than a year later, Müller et al. (2018) published a reply to Baron & Barrett (2017), arguing that their phylogenetic dataset actually suggested that Chilesaurus was a basal sauropodomorph rather than an ornithischian.[6] Baron & Barrett reached out to Müller et al. to inform them that they had accidentally published a faulty early version of their dataset with many traits scored incorrectly, and that their original results were based on an edited final dataset. They corrected their original publication and supplied the final dataset to Müller et al., who agreed that it supported the placement of Chilesaurus in Ornithischia as Baron & Barrett (2017) argued.[6][2] However, Müller et al. also noted that Baron & Barrett did not test the original proposal of Chilesuaurus as a theropod, and that its classification was still uncertain.[6] Baron & Barrett disagreed, stating how Chilesaurus was recovered as an ornithischian regardless of the parameters used in analyses, and that the uncertainty claimed by Müller et al. was a result of the erroneous dataset being supplied to those authors. Baron & Barrett also note that Chilesaurus is crucial to their finding of Ornithoscelida, a new hypothesis of dinosaur evolution which places theropods with ornithischians instead of sauropodomorphs.[2] Below is a cladogram illustrating the classification proposed by Baron & Barrett (2017), which places Chilesaurus as the most basal ornithischian.[4]

Dinosauria
Saurischia

Herrerasauridae

Sauropodomorpha

Ornithoscelida

Theropoda

Ornithischia

Chilesaurus

Pisanosaurus

Neornithischia

Thyreophora

References

  1. ^ a b c d e Novas, F. E.; Salgado, L.; Suárez, M.; Agnolín, F. L.; Ezcurra, M. N. D.; Chimento, N. S. R.; de la Cruz, R.; Isasi, M. P.; Vargas, A. O.; Rubilar-Rogers, D. (2015). "An enigmatic plant-eating theropod from the Late Jurassic period of Chile". Nature. 522 (7556): 331–334. doi:10.1038/nature14307. PMID 25915021.
  2. ^ a b c Baron, Matthew G.; Barrett, Paul M. (2018-03-01). "Support for the placement of Chilesaurus within Ornithischia: a reply to Müller et al". Biology Letters. 14 (3): 20180002. doi:10.1098/rsbl.2018.0002. ISSN 1744-9561. PMC 5897612. PMID 29593075.
  3. ^ Salgado, L.; De La Cruz, R.; Suárez, M.; Gasparini, Z.; Fernández, M. (2008). "First Late Jurassic dinosaur bones from Chile". Journal of Vertebrate Paleontology. 28 (2): 529–534. doi:10.1671/0272-4634(2008)28[529:fljdbf]2.0.co;2.
  4. ^ a b Baron, Matthew G.; Barrett, Paul M. (2017). "A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs". Biology Letters. 13 (8): 20170220. doi:10.1098/rsbl.2017.0220. PMC 5582101. PMID 28814574.
  5. ^ https://www.cam.ac.uk/research/news/study-identifies-dinosaur-missing-link
  6. ^ a b c Müller, Rodrigo Temp; Pretto, Flávio Augusto; Kerber, Leonardo; Silva-Neves, Eduardo; Dias-da-Silva, Sérgio (2018-03-01). "Comment on 'A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs'". Biology Letters. 14 (3): 20170581. doi:10.1098/rsbl.2017.0581. ISSN 1744-9561. PMC 5897605. PMID 29593074.

External links

Averostra

Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.

Avetheropoda

Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.

Cerapoda

Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.

Coihaique Group

The Coihaique Group is a group of geological formations in northwestern Patagonia. From top to bottom the formations that make the group are Apeleg, Katterfeld and Toqui. The contact between the formations of the group are diachronous with Katterfeld Formation interfingering with the formations on top and below it. The lower and upper boundaries of the group are unconformities formed by erosion. The older parts of Coihaique Group represent a marine transgression while the younger parts evidences a return to non-marine conditions.

Dinosauriformes

Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.

Fernando Novas

Fernando Emilio Novas is an Argentine paleontologist working for the Comparative Anatomy Department of the Bernardino Rivadavia Natural Sciences Museum in Buenos Aires, Argentina. Novas holds a PhD in Natural sciences.

Working for the CONICET, he described or co-described many dinosaurs, among them Abelisaurus, Aniksosaurus, Aragosaurus, Austroraptor, Chilesaurus, Megaraptor, Neuquenraptor, Orkoraptor, Patagonykus, Unenlagia, Araucanoraptor, Skorpiovenator, Tyrannotitan, Talenkauen, and Puertasaurus, most from the Patagonia region of Argentina. Chilesaurus diegosuarezi made the cover of Nature magazine on June 18, 2015.

Haya griva

Haya is an extinct genus of basal neornithischian dinosaur known from Mongolia.

Jeholosauridae

Jeholosaurids were herbivorous neornithischian dinosaurs from the Cretaceous Period (Aptian - Santonian, with a possible Campanian record) of Asia. The family was first proposed by Han et al. in 2012. The jeholosaurids were defined as those ornithischians more closely related to Jeholosaurus shangyuanensis than to Hypsilophodon foxii, Iguanodon bernissartensis, Protoceratops andrewsi, Pachycephalosaurus wyomingensis, or Thescelosaurus neglectus. The Jeholosauridae includes the type genus Jeholosaurus and Yueosaurus.

Jingshanosaurus

Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.

Melanorosauridae

The Melanorosauridae were a family of sauropodomorph dinosaurs which lived during the Late Triassic and Early Jurassic. The name Melanorosauridae was first coined by Friedrich von Huene in 1929. Huene assigned several families of dinosaurs to the infraorder "Prosauropoda": the Anchisauridae, the Plateosauridae, the Thecodontosauridae, and the Melanorosauridae. Since then, these families have undergone numerous revisions. Galton and Upchurch (2004) considered Camelotia, Lessemsaurus, and Melanorosaurus members of the family Melanorosauridae. A more recent study by Yates (2007) indicates that the melanorosaurids were instead early sauropods.

Neotheropoda

Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.

Orionides

Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.

Ornithischia

Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure similar to that of birds. The name Ornithischia, or "bird-hipped", reflects this similarity and is derived from the Greek stem ornith- (ὀρνιθ-), meaning "of a bird", and ischion (ἴσχιον), plural ischia, meaning "hip joint". However, birds are only distantly related to this group as birds are theropod dinosaurs.Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, pachycephalosaurids and the ornithopods. There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there is much debate over whether these filaments found in specimens of Tianyulong, Psittacosaurus, and Kulindadromeus may have been primitive feathers.

Orodrominae

Orodrominae is a subfamily of parksosaurid dinosaurs from the Cretaceous of North America and Asia.

Raeticodactylidae

Raeticodactylidae is a family of eudimorphodontoid eopterosaurian pterosaurs that lived in Switzerland during the Late Triassic. The family includes Caviramus, and the type genus Raeticodactylus, which are both known from the Kössen Formation, around 205 mya. Raeticodactylidae was first used in 2014 by Andres et al., as a group of all pterosaurs closer to Raeticodactylus than Eudimorphodon. The following phylogenetic analysis follows the topology of Andres et al. (2014).

Riojasauridae

Riojasauridae is a family of sauropod-like dinosaurs from the Upper Triassic. It is known primarily from the genera Riojasaurus and Eucnemesaurus. Sites containing Riojasauridae include the Lower Elliot Formation of Orange Free State, South Africa (where fossils of Eucnemesaurus have been found), and Ischigualasto, in La Rioja Province, Argentina ( where fossils of Riojasaurus have been recovered).

Toqui Formation

The Toqui Formation is a geological formation in the Aysén Region of southern Chile. It has been dated to the Tithonian stage of the Late Jurassic by uranium–lead dating of zircons, providing an age of 147 ± 0.1 Ma. It consists of an sequence of clastic sedimentary sandstones and conglomerates, interbedded with volcanic tuffs and ignimbrite. The dinosaurs Chilesaurus and indeterminate diplodocids are known from the formation. The formation was deposited in a fluvio-deltaic environment.

Xixiposaurus

Xixiposaurus is a genus of prosauropod dinosaur which existed in what is now Lower Lufeng Formation, China during the lower Jurassic period. It was first named by Sekiya Toru in 2010 and the type species is Xixiposaurus suni.

Yueosaurus

Yueosaurus is an extinct genus of basal ornithopod dinosaur known from Zhejiang Province, China.

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