Spinosuchus Caseanus

''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 it represents an advanced trilophosaurid very closely related to ''Trilophosaurus''.

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).Case, E.C. (1922). New reptiles and stegocephalians from the Upper Triassic of western Texas. ''Carnegie Institution of Washington Publication'' 321. 84 p. He considered it to be about 2.5 meters (8.5 ft) long. Additional material was referred to it, including a femur (UMMP 3396), an ilium (UMMP 8870), and a basicranium (UMMP 7473).Case, E.C. (1927). The vertebral column of ''Coelophysis'' Cope. ''Contributions from the Museum of Paleontology, University of Michigan'' 2:209-222. 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'',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. the ilium to a herrerasaurid, either ''Chindesaurus'' or ''Caseosaurus'', depending on the taxonomic authority,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. and the basicranium to the rauisuchian ''Postosuchus''.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.

Friedrich von Huene recognized it as a new genus in 1932, and named it in honor of Case.von Huene, F. (1932). Die fossile Reptil-Ordnung Saurischia, ihte Entwicklung und Geschichte. ''Monographien zur Geologie und Palaeontologie'' 1(4). 361 p. erman/ref> He considered it to be a " podokesaurid". Since the 1970s, though, it has been considered a nondinosaurian.Zhang, F-K. (1975). A new thecodont ''Lotosaurus'', from Middle Triassic of Hunan. ''Vertebrata PalAsiatica'' 8(3):144-147.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. 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. 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. 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.Richards, H.R. (1999a). Is ''Spinosuchus'' a dinosaur? ''Journal of Vertebrate Paleontology'' 19(Supplement to 3), Abstracts of Papers, Fifty-Ninth Annual Meeting, 70A.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. 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. Justin Spielmann and colleagues published a redescription in 2009 that concluded ''Spinosuchus'' was a trilophosaurid closely related to ''Trilophosaurus''.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.

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 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.

References

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.

External links

Case, 1927

{{Taxonbar, from=Q3080413

Allokotosaurs
Prehistoric reptile genera
Late Triassic reptiles of North America
Triassic geology of Texas
Paleontology in Texas
Fossil taxa described in 1932
Taxa named by Friedrich von Huene