Orthogenysuchus

Orthogenysuchus is an extinct genus of caimanine alligatorids. Fossils have been found from the Wasatch Beds of the Willwood Formation of Wyoming, deposited during the early Eocene. The type species is O. olseni. The holotype, known as AMNH 5178, is the only known specimen belonging to the genus and consists of a skull lacking the lower jaws. The braincase is filled in by the matrix and most of the suture lines between bones are indiscernible, making comparisons with other eusuchian material difficult.[1]

Orthogenysuchus
Temporal range: Early Eocene
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Family: Alligatoridae
Subfamily: Caimaninae
Genus: †Orthogenysuchus
Mook, 1924
Type species
†Orthogenysuchus olseni
Mook, 1924

Phylogeny

Orthogenysuchus was first named in 1924 by Charles C. Mook and was referred to as a eusuchian, although not to any particular eusuchian group known at the time.[2] Later publications assigned the genus to the Crocodylidae,[3] but more recent analyses propose that it is a pristichampsid or even a synonym of Pristichampsus.[4][5] In 1999, Orthogenysuchus was placed within a new clade containing the Miocene caimanines Purussaurus and Mourasuchus. Orthogenysuchus antedates these genera by around 30 million years, suggesting that they both had significant ghost lineages.[1][6]

Furthermore, the 1999 study proposed a clade containing only Orthogenysuchus and Mourasuchus. This clade is similar to the family Nettosuchidae, which was originally constructed in 1965 for Mourasuchus and the newly described Nettosuchus (later shown to be a junior synonym of the genus).[7][8] However, the clade is based primarily on ambiguous characters as a result of the poor preservation of the holotype of Orthogenysuchus, thus has not yet been formally described. All unambiguous synapomorphies are based on the nasal region of the skull where the individual bones are easily distinguished. Orthogenysuchus shares with Mourasuchus a characteristically long, broad snout, extremely wide external nares consisting of the nasal aperture and dorsal fossa, and many small maxillary alveoli.[1]

Paleobiology

Orthogenysuchus is representative of the diverse Wasatchian faunas that occurred during the early Eocene in North America. These faunas are also characterized by the appearance or diversification of many chelonians such as emydids and testudinids, as well as the occurrence of rhineurid amphisbaenians. This followed a major faunal turnover at the Clarkforkian-Wasatchian boundary which resulted in the regional disappearance of champsosaurs and the extinction of the alligatorine Ceratosuchus.[9]

The presence of Orthogenysuchus in North America during the Eocene suggests that a dispersal event occurred of caimans into the continent from South America after the original spread of early alligatorines and caimanines into South America that occurred during the Cretaceous–Paleogene boundary (Cretaceous–Tertiary boundary).[1]

References
  1. Brochu, C. A. (1999). "Phylogenetics, taxonomy, and historical biogeography of Alligatoroidea". Society of Vertebrate Paleontology Memoir. 6: 9–100. doi:10.2307/3889340. JSTOR 3889340.
  2. Mook, C. C. (1924). "A new crocodilian from the Wasatch Beds". American Museum Novitates (137): 1–4.
  3. Steel, R. (1973). "Crocodylia". In Kuhn, O. (ed.). Handbuch der Palaoherpetologie (16 ed.). Stuttgart: G. Fischer Verlag. pp. 1–116.
  4. Rossman, T. (1998). "Studien an känozoischen Krokodilen: 2. Taxonomische Revision der Familie Pris- tichampsidae Efimov (Crocodilia: Eusuchia)". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 210: 85–128. doi:10.1127/njgpa/210/1998/85.
  5. Gunnel, G. E.; Bartels, W. S.; Gingerich, P. D.; Torres, V. (1992). "Wapiti Valley Faunas: Early and Middle Eocene fossil vertebrates from the North Fork of the Shoshone River, Park County, Wyoming". Contributions from the Museum of Paleontology, University of Michigan. 28: 247–287.
  6. Brochu, C. A. (2001). "Crocodylian snouts in space and time: Phylogenetic approaches toward adaptive radiation". American Zoologist. 41 (3): 564–585. CiteSeerX 10.1.1.554.231. doi:10.1668/0003-1569(2001)041[0564:CSISAT]2.0.CO;2. S2CID 198156966.
  7. Langston, W. (1965). Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences. Vol. 52. University of California Press. pp. 1–152.
  8. Langston, W. (1966). "Mourasuchus Price, Nettosuchus Langston, and the family Nettosuchidae (Reptilia: Crocodilia)". Copeia. 1966 (4): 882–885. doi:10.2307/1441424. JSTOR 1441424.
  9. Bartels, W. S. (1983). "A transitional Paleocene-Eocene reptile fauna from the Bighorn Basin, Wyoming". Herpetologica. 39 (4): 359–374.
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