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Chindesaurus

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Chindesaurus
Temporal range: Late Triassic, 235–210 Ma
Restoration
Life restoration as a herrerasaurid
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Genus: †Chindesaurus
Long & Murry, 1995
Species:
†C. bryansmalli
Binomial name
†Chindesaurus bryansmalli
Long & Murry, 1995
Synonyms

Chindesaurus (/ˌtʃɪndɪˈsɔːrəs/ CHIN-di-SAWR-əs) is a genus of saurischian dinosaur that lived approximately 235-210 million years ago during the latter part of the Triassic Period in what is now the Southwestern United States. Chindesaurus was a small, bipedal carnivore that could grow up to 2 to 2.3 m (6.6 to 7.5 ft) long. Its classification is controversial and various papers have had different conclusions on its affinities. Its fossils were originally considered to belong to "prosauropods" (basal sauropodomorphs), but numerous subsequent papers argued that it was a herrerasaurid.[1][2][3] A 2019 redescription of its holotype considered it a theropod closely related to Tawa.[4]

Discovery

Chindesaurus is known from six incomplete specimens (seven if Caseosaurus is included). Of these, the type specimen PEFO 10395 is the most complete. This was a partial skeleton, discovered in the Petrified Forest National Park member of the Chinle Formation in Apache County, Arizona. Its remains were discovered by Bryan Small in 1984, in blue mudstone which was deposited during the Norian stage of the Triassic, approximately 228-208 million years ago.[5][6] It consists of a single tooth, a fragmentary cervical (neck) vertebra, fragmentary dorsal (back) vertebrae, several rib fragments, two complete sacral (hip) vertebrae, fragmentary caudal (tail) vertebrae, a chevron, several fragmentary hip bones, a complete left femur and a fragmentary right femur, a fragmentary right tibia, and a right astragalus bone of the ankle.[1]

Several more incomplete specimens have been referred to the genus. Specimen PEFO 4849 consists of a single dorsal vertebra collected in Norian-aged sediments in a different part of the Upper Petrified Forest member. Specimen PEFO 33982, which consists of nine vertebrae, an ilium fragment, a proximal femur, and other bone fragments was also collected in Norian-aged sediments of Upper Petrified Forest member. These two specimens are housed in the collection of the University of California Museum of Paleontology in Berkeley, California.

Specimens NMMNH P16656 and NMMNH P17325 were discovered in the Bull Canyon Formation of New Mexico, sediments deposited during the Norian stage of the Late Triassic. The assignment of these two specimens to Chindesaurus has now come under question.[7] These specimens are housed in the collection of the New Mexico Museum of Natural History and Science in Albuquerque, New Mexico.

Additional material referable to Chindesaurus was discovered in 2006, in gray/green siltstone at the Hayden Quarry of the Petrified Forest Member of the Chinle Formation in New Mexico. Another specimen, TMM 31100-523 which consists of a proximal femur, was discovered in the Colorado City Formation in Texas, in mudstone considered to be from the Carnian stage of the Triassic, approximately 235-228 million years ago. It is currently housed in the collection of the Texas Memorial Museum in Austin, Texas.

The genus name Chindesaurus is derived from the Navajo word chindi meaning "ghost" or "evil spirit" and the Greek word "sauros" (σαυρος) meaning "lizard";[8] thus, "ghost lizard" or "Lizard from Chinde Point". The specific name, bryansmalli, honors the discoverer, Bryan Small. Chindesaurus was described and named by R.A. Long and P.A. Murry in 1995; the type species is Chindesaurus bryansmalli. When this specimen was first discovered it was nicknamed "Gertie" after Gertie the Dinosaur and received much publicity.[1]

Description

The size of Chindesaurus (interpreted as a herrerasaurid) compared to a human

The type and paratype specimens were approximately 2 to 2.3 metres (6.6 to 7.5 ft) in length.[9] Some estimates suggest that Chindesaurus weighed 50 kilograms (110.2 pounds) at most.[10]

Distinguishing anatomical features

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. (2007), Chindesaurus can be distinguished based on the following characteristics:

  • a triangular rugosity is present on the postacetabular process
  • the proximal tibial intercondylar groove is strongly situated medially
  • the posterior edge of the fibular condyle of the proximal tibia is straight when examined in proximal view
  • the presence of a ventral cleft on the astragalus

Classification

Restoration as a theropod, from Petrified Forest National Park

Chindesaurus has been difficult to classify, and has been recovered in several different positions at the base of the saurischian family tree. When it was first discovered in 1984, the fossil specimen which would eventually be named Chindesaurus was thought to be a prosauropod.[11] When it was finally described and named a decade after its discovery by Long and Murry, they regarded it as a herrerasaurid, an opinion that has been followed by most paleontologists since.[1] Most phylogenetic analyses published through 2007 continued to find it to fall among the herrerasaurids,[2] though a few studies cast doubt on this, including one in 2007 by Irmis, Nesbitt and colleagues which found Chindesaurus to be a probable basal saurischian dinosaur, and noted that it shares a wide range of characteristics with several lineages of basal saurischians, making any classification problematic.[12] Rauhut (2003) noted that the medially expanded brevis shelf of Chindesaurus resembles that of crurotarsans, and not that of most dinosaurs, which is usually laterally expanded.[13] Benson and Brussatte (2012) found that Chindesaurus was a basal saurischian related to Herrerasaurus.[9]

One specimen originally assigned to Chindesaurus, from the Tecovas Formation of Texas, was later placed in its own genus and species, Caseosaurus crosbyensis.[14] Subsequent research has shown that this separation was probably in error, and that the two forms represent the same species.[15] Nesbitt, Irmis and Parker agreed in a 2007 paper that there is little reason to separate Caseosaurus from Chindesaurus, and the two even share some unique characteristics not found in similar species. Nesbitt and colleagues suggested that any differences between the two were probably related to differences in size. However, since both species are so fragmentary, they decided not to formally make them synonyms.[7]

The following is a cladogram based on the phylogenetic analysis by Hans-Dieter Sues, Sterling J. Nesbitt, David S Berman and Amy C. Henrici, in 2011, which indicated that Chindesaurus is a herrerasaurid.[16] A similar position for Chindesaurus was recovered by the analyses by Baron, Norman & Barrett and Baron & Williams.[17][3]

  Dinosauria  

In 2019, the holotype specimen of Chindesaurus was redescribed by Adam D. Marsh, William G. Parker, Max C. Langer, and Sterling J. Nesbitt. They included a phylogenetic analysis which placed Chindesaurus as a basal theropod. It was specifically found to be the sister taxon of Tawa hallae, a theropod known from Ghost Ranch in New Mexico.[4]

Paleoecology

The Upper Petrified Forest National Park member of the Chinle Formation was an ancient floodplain where phytosaurs, rauisuchids, archosaurs, pseudosuchians, and other tetrapods lived and competed with the dinosaur Chindesaurus and its relative Coelophysis for resources. This paleoenvironment also had abundant lungfish and clams.

References

  1. ^ a b c d Long and Murry, (1995). "Late Triassic (Carnian and Norian) tetrapods from the Southwestern United States." New Mexico Museum Natural History Science Bulletin, 4: 1-254.
  2. ^ a b Bittencourt and Kellner, (2004). "The phylogenetic position of Staurikosaurus pricei Colbert, 1970 from the Triassic of Brazil." Journal of Vertebrate Paleontology, 24(3):.
  3. ^ a b Matthew G. Baron; Megan E. Williams (2018). "A re-evaluation of the enigmatic dinosauriform Caseosaurus crosbyensis from the Late Triassic of Texas, USA and its implications for early dinosaur evolution". Acta Palaeontologica Polonica. 63. doi:10.4202/app.00372.2017.
  4. ^ a b Marsh, Adam D.; Parker, William G.; Langer, Max C.; Nesbitt, Sterling J. (2019-05-04). "Redescription of the holotype specimen of Chindesaurus bryansmalli Long and Murry, 1995 (Dinosauria, Theropoda), from Petrified Forest National Park, Arizona" (PDF). Journal of Vertebrate Paleontology. 39 (3): e1645682. doi:10.1080/02724634.2019.1645682. ISSN 0272-4634.
  5. ^ Parker, W. G., R. B. Irmis, and S. J. Nesbitt. 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park . Pages 160-161 in Parker, W. G., S. R. Ash, and R. B. Irmis, editors. A century of research at Petrified Forest National Park: geology and paleontology. Museum of Northern Arizona, Flagstaff, Arizona. Bulletin 62.
  6. ^ Litwin, R.J., Traverse, A., and Ash, S.R., 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Paleobotany and Palynology, v. 77, p. 269-287.
  7. ^ a b Nesbitt, Irmis and Parker, (2007). "A critical re-evaluation of the Late Triassic dinosaur taxa of North America." Journal of Systematic Palaeontology, 5(2): 209–243.
  8. ^ Liddell, Henry George; Robert Scott (1980). A Greek-English Lexicon (Abridged Edition). United Kingdom: Oxford University Press. ISBN 978-0-19-910207-5. {{cite book}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
  9. ^ a b Benson, R.B.J.; Brussatte, S. (2012). Prehistoric Life. London: Dorling Kindersley. p. 217. ISBN 978-0-7566-9910-9. {{cite book}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
  10. ^ "Chindesaurus". Hostgator.com. Retrieved 31 May 2013.
  11. ^ Meyer, (1986). "D-Day on the Painted Desert." Arizona Highways, 62(7): 3-13.
  12. ^ Irmis, R.B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D.T., and Downs, A. (2007). "A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs". Science 317: 358-361
  13. ^ Rauhut, 2003. The interrelationship and evolution of basal theropod dinosaurs. Special Papers in Palaeontology. 69, 1-215.
  14. ^ Hunt, Lucas, Heckert, Sullivan and Lockley, (1998). "Late Triassic Dinosaurs from the Western United States." Geobios, 31(4): 511-531.
  15. ^ Langer, (2004). "Basal Saurischia." In Weishampel, Dodson and Osmolska (eds.). The Dinosauria Second Edition. University of California Press. 861 pp.
  16. ^ Hans-Dieter Sues; Sterling J. Nesbitt; David S Berman; Amy C. Henrici (2011). "A late-surviving basal theropod dinosaur from the latest Triassic of North America". Proceedings of the Royal Society B. 278 (1723): 3459–64. doi:10.1098/rspb.2011.0410. PMC 3177637. PMID 21490016.
  17. ^ Baron, M.G., Norman, D.B., and Barrett, P.M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543: 501–506. doi:10.1038/nature21700
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