Brachychampsa

Brachychampsa is an extinct genus of alligatoroid. Specimens have been found from New Mexico, Colorado, Wyoming,[3] Montana,[1][4][5] North and South Dakota,[6] New Jersey, and Saskatchewan. One specimen has been found from the Darbasa Formation of Kazakhstan, although the species status is indeterminant for the fossil. The genus first appeared during the late Campanian stage of the Late Cretaceous (Judithian North American stage) and became extinct during the early Danian stage of the Paleocene (Puercan North American stage), a few million years after the Cretaceous–Paleogene extinction event. Brachychampsa is distinguished by an enlarged fourth maxillary tooth in the upper jaw.

Life reconstruction of Brachychampsa montana

Brachychampsa
Temporal range: 83.5–63.3 Ma Late Cretaceous - Early Paleocene
Brachychampsa sp.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Clade: Globidonta
Genus: †Brachychampsa
Gilmore, 1911[1]
Species
  • †B. montana Gilmore, 1911 (type)[1]
  • †B. perrugosus (Cope, 1875)
  • †?B. sealeyi Williamson, 1996[2]

Brachychampsa's position within Alligatoroidea has undergone many revisions since it was first named. Originally it was placed within Alligatoridae, and was later refined to the Alligatorinae in 1964,[3] only to be placed outside both Alligatorinae and Alligatoridae (but still within Alligatoroidea) in 1994.[7] On the other hand, in the phylogenetic analysis conducted by Bona et al. (2018) Brachychampsa was recovered as an alligatorid, specifically as a stem-caiman.[8]

Species
Skull

The type species of Brachychampsa is B. montana, first discovered from the Hell Creek Formation of Montana and described by Charles W. Gilmore in a paper in 1911. In that same paper, Gilmore recombined Bottosaurus perrugosus as a new species of Brachychampsa, called B. perrugosus.[1] The holotype specimen of B. perrugosus went missing as the paper was being written, but it was later rediscovered and soon afterward designated as a nomen dubium due to a lack of diagnostic features that distinguish it from other alligatorids discovered since the paper was published.[7] Another species from the Allison Member of the Menefee Formation of the San Juan Basin, B. sealeyi, was discovered in 1996,[2] but was later argued to be synonymous with B. montana by interpreting it as an immature specimen of the latter species.[9] However, other studies claim that some of the variation seen between the two species, such as the orientation of the maxillary tooth row, may not be ontogenic, thus making B. sealeyi a valid taxon.[10]

References
  1. Gilmore, Charles W. (1911). "A new fossil alligator from the Hell Creek Beds of Montana" (PDF). Proceedings of the United States National Museum. 41 (1860): 297–302. doi:10.5479/si.00963801.41-1860.297.
  2. Williamson, Thomas E. (1996). "?Brachychampsa sealeyi, sp. nov., (Crocodylia, Alligatoridea) from the Upper Cretaceous (Lower Campanian) Menefee Formation, northwestern New Mexico". Journal of Vertebrate Paleontology. 16 (3): 421–431. doi:10.1080/02724634.1996.10011331. JSTOR 4523734.
  3. Estes, R. (1964). "Fossil vertebrates from the Late Cretaceous Lance Formation, eastern Wyoming". University of California Publications in Geological Sciences. 49: 1–180.
  4. Estes, R.; Berberian, P. (1970). "Paleoecology of a Late Cretaceous vertebrate community from Montana". Breviora. 343: 1–35.
  5. Bryant, L. J. (1989). "Non-dinosaurian lower vertebrates across the Cretaceous-Tertiary boundary in northeastern Montana". University of California Publications in Geological Sciences. 134: 1–107.
  6. Pearson, Dean A.; Schaefer, Terry; Johnson, Kirk R.; Nichols, Douglas J.; Hunter, John P. (2002). "Vertebrate biostratigraphy of the Hell Creek Formation in southwestern North Dakota and northwestern South Dakota". In Hartman, Joseph Herbert; Johnson, Kirk. R.; Nichols, Douglas J. (eds.). The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains. An Integrated Continental Record of the End of the Cretaceous. 361. Geological Society of America. pp. 145–167. doi:10.1130/0-8137-2361-2.145. ISBN 9780813723617.
  7. Norell, M. A.; Clark, J. M.; Hutchison, J. H. (1994). "The Late Cretaceous alligatoroid Brachychampsa montana (Crocodylia): new material and putative relationships". American Museum Novitates. 3116: 1–26.
  8. Paula Bona; Martín D. Ezcurra; Francisco Barrios; María V. Fernandez Blanco (2018). "A new Palaeocene crocodylian from southern Argentina sheds light on the early history of caimanines". Proceedings of the Royal Society B: Biological Sciences. 285 (1885): 20180843. doi:10.1098/rspb.2018.0843. PMC 6125902. PMID 30135152.
  9. Sullivan, Robert M.; Lucas, Spencer G. (2003). "Brachychampsa montana Gilmore (Crocodylia, Alligatoroidea) from the Kirtland Formation (upper Campanian), San Juan Basin, New Mexico". Journal of Vertebrate Paleontology. 23 (4): 832–841. doi:10.1671/A1082-8. JSTOR 4524385.
  10. Brochu, Christopher A. (2004). "Alligatorine phylogeny and the status of Allognathosuchus Mook, 1921". Journal of Vertebrate Paleontology. 24 (4): 857–873. doi:10.1671/0272-4634(2004)024[0857:APATSO]2.0.CO;2. JSTOR 4524781.
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