Spinosuchus

Spinosuchus (meaning "spined crocodile") is an extinct genus of trilophosaurid allokotosaur from the Late Triassic of Texas, southern United States. It has been assigned to a variety of groups over its history, from coelophysid dinosaur to pseudosuchian to uncertain theropod dinosaur and to Proterosuchidae. This uncertainty is not unusual, given that it was only known from a poorly preserved, wall-mounted, partial vertebral column of an animal that lived in a time of diverse, poorly known reptile groups. However, newly collected material and recent phylogenetic studies of early archosauromorphs suggest that is represents an advance trilophosaurid very closely related to Trilophosaurus.[1][2]

Spinosuchus
Temporal range: Late Triassic, Carnian–Norian
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: †Allokotosauria
Order: †Trilophosauria
Family: †Trilophosauridae
Genus: †Spinosuchus
von Huene, 1932
Species
  • †S. caseanus von Huene, 1932 (type)
Synonyms

History

In 1922, Ermine Cowles Case described a partial vertebral column (UMMP 7507) he'd discovered in 1921 from the Tecovas Member of the Carnian-age Upper Triassic Dockum Formation of Crosby County, Texas, as Coelophysis sp. (Coelophysis at that time also being poorly known).[3] He considered it to be about 2.5 meters (8.5 ft) long.[3] Additional material was referred to it, including a femur (UMMP 3396),[3] an ilium (UMMP 8870), and a basicranium (UMMP 7473).[4] These additional remains have since been recognized as belonging to a variety of other Triassic animals, all of which were poorly known or unknown at the time: the femur to an aetosaur, possibly Desmatosuchus,[5] the ilium to a herrerasaurid, either Chindesaurus or Caseosaurus, depending on the taxonomic authority,[6] and the basicranium to the rauisuchian Postosuchus.[7]

Friedrich von Huene recognized it as a new genus in 1932, and named it in honor of Case.[8] He considered it to be a "podokesaurid".[8] Since the 1970s, though, it has been considered a nondinosaurian.[9][10][11] However, a review by Hunt et al. in 1998 suggested that it was a theropod, possibly a herrerasaurid, citing its hollow centra as evidence for dinosaurian affinities.[5] In an abstract for the 1999 Society of Vertebrate Paleontology meeting and his unpublished thesis, Richards recognized that it had a variety of characters that are apomorphic for various dinosaur subgroups, but that these are also found in different basal archosaur groups, and that the poorly preserved, distorted, and reconstructed vertebrae offer no evidence for assignment to any major archosaur group; it does, though, show some resemblances to the trilophosaurs.[12][13] Further review, as part of a larger series of papers on the evolution of dinosaurs in the Late Triassic by Sterling Nesbitt, Randall Irmis, and William Parker, found Spinosuchus to be a valid genus. However, the authors could not classify it beyond Archosauriformes, and disagreed with Richards' trilophosaur hypothesis.[14] Justin Spielmann and colleagues published a redescription in 2009 that concluded Spinosuchus was a trilophosaurid closely related to Trilophosaurus.[1]

Nesbitt et al. (2015) performed a phylogenetic analysis focusing on relations within Allokotosauria and recovered Trilophosaurus jacobsi to be more closely related to S. caseanus than to the type species of Trilophosaurus, Trilophosaurus buettneri. To further test this possibility, the types of S. caseanus and T. jacobsi were scored separately from the referred the Kahle Trilophosaurus Quarry elements (referred to T. jacobsi by Spielmann et al. (2008) or to S. caseanus by Spielmann et al. (2009)). A phylogenetic analysis recovered the three in a monophyletic clade to the exclusion of T. buettneri based on a single autapomorphy. Furthermore, the types of S. caseanus and T. jacobsi as well as the Kahle Quarry material all scored identically, suggesting that T. jacobsi not only should be reassigned to Spinosuchus, but in fact represents the junior synonym of its type and only species (S. caseanus). Nesbitt et al. (2015) refrained from officially synonymizing the two taxa pending further study of other advanced trilophosaurids.[2]

References
  1. Spielmann, J.A., Lucas, S.G., Heckert, A.B., Rinehart, L.F., and Richards, H.R., III. 2009. Redescription of Spinosuchus caseanus (Archosauromorpha: Trilophosauridae) from the Upper Triassic of North America. Palaeodiversity 2:283-313.
  2. Nesbitt, S.J.; Flynn, J.J.; Pritchard, A.C.; Parrish, M.J.; Ranivoharimanana, L.; Wyss, A.R. (2015). "Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles". Bulletin of the American Museum of Natural History (398): 1–126. doi:10.5531/sd.sp.15. hdl:2246/6624. ISSN 0003-0090.
  3. Case, E.C. (1922). New reptiles and stegocephalians from the Upper Triassic of western Texas. Carnegie Institution of Washington Publication 321. 84 p.
  4. Case, E.C. (1927). The vertebral column of Coelophysis Cope. Contributions from the Museum of Paleontology, University of Michigan 2:209-222.
  5. Hunt, A.P, Lucas, S.G., Heckert, A.B., Sullivan, R.M., and Lockley, M.G. (1998). Late Triassic dinosaurs from the western United States. Geobios 31(4):511-531.
  6. Long, R.A., and Murry, P.A. (1995). Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4:1-154.
  7. Chatterjee, S. (1985). Postosuchus, a new thecodontian reptile from the Triassic of Texas and the origin of tyrannosaurs . Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences 309(1139):395-460.
  8. von Huene, F. (1932). Die fossile Reptil-Ordnung Saurischia, ihte Entwicklung und Geschichte. Monographien zur Geologie und Palaeontologie 1(4). 361 p. [German]
  9. Zhang, F-K. (1975). A new thecodont Lotosaurus, from Middle Triassic of Hunan. Vertebrata PalAsiatica 8(3):144-147.
  10. Padian, K. (1986). On the type material of Coelophysis Cope (Saurischia: Theropoda) and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation). In: Padian, K. (ed.). The Beginning of the Age of Dinosaurs. Cambridge University Press: Cambridge. 45-60. ISBN 0-521-36779-4
  11. Murry, P.A., and Long, R.A. (1989). Geology and paleontology of the Chinle Formation, Petrified Forest National Park and vicinity, Arizona and a discussion of vertebrate fossils of the southwestern Upper Triassic. In: Lucas, S.G., and Hunt, A.P. (eds.). Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum of Natural History: Albuquerque, New Mexico. 29-64. ISBN 0-12-226810-5
  12. Richards, H.R. (1999a). Is Spinosuchus a dinosaur? Journal of Vertebrate Paleontology 19(Supplement to 3), Abstracts of Papers, Fifty-Ninth Annual Meeting, 70A.
  13. Richards, H.R. M.S. (1999b). Osteology and relationships of Spinosuchus caseanus Huene, 1932 from Texas (Dockum Group, Upper Triassic): a new interpretation. December 17, 1999. Richard J. Zakrzewski, Thesis Advisor. Fort Hays University, Department of Geosciences.
  14. Nesbitt, Sterling J.; Irmis, Randall B.; Parker, William G. (2007). "A critical re-evaluation of the Late Triassic dinosaur taxa of North America". Journal of Systematic Palaeontology. 5 (2): 209–243. doi:10.1017/S1477201907002040.

Sources
  • Glut, D.F. (2002). Is Spinosuchus a theropod? In: Glut, D.F. Dinosaurs: The Encyclopedia. 2nd Supplement. McFarland & Company, Inc.:Jefferson, North Carolina, 46-48. ISBN 0-7864-1166-X
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