Brachychampsa

Brachychampsa is an extinct genus of alligatoroid, possibly a basal caiman.[4] Specimens have been reported from New Mexico, Colorado, Wyoming,[5] Montana,[2][6][7] North and South Dakota,[8] New Jersey, and Saskatchewan, though only those from Montana, Utah, and New Mexico are based on material sufficient to justify the referral. One specimen has been reported from the Darbasa Formation of Kazakhstan, although the species status is indeterminate 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 late Maastrichtian stage of the Cretaceous (Lancian North American Land Mammal "Age"). Brachychampsa is distinguished by an enlarged fifth maxillary tooth in the upper jaw.

Life reconstruction of Brachychampsa montana
Skull

Brachychampsa
Temporal range: Late Cretaceous - Early Paleocene,
Brachychampsa sp.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Family: Alligatoridae
Subfamily: Caimaninae
Genus: †Brachychampsa
Gilmore, 1911[2]
Species
  • †B. montana Gilmore, 1911 (type)[2]
  • †B. perrugosus (Cope, 1875)
  • †?B. sealeyi Williamson, 1996[3]

Species

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.[2] 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.[9] Another species from the Allison Member of the Menefee Formation of the San Juan Basin, B. sealeyi, was discovered in 1996,[3] but was later argued to be synonymous with B. montana by interpreting it as an immature specimen of the latter species.[10] However, other studies have shown that some of the variation seen between the two species, such as the orientation of the maxillary tooth row, are not ontogenic, thus making B. sealeyi a valid taxon.[11]

Phylogeny

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,[5] only to be placed outside both Alligatorinae and Alligatoridae (but still within Alligatoroidea) in 1994.[9] Accordingly, studies have shown Brachychampsa as a basal member of Alligatoroidea, within the clade Globidonta,[12][13] as shown in the cladogram below.[13]

Alligatoroidea

Diplocynodontinae†

Globidonta

Leidyosuchus†

Deinosuchus†

Navajosuchus†

Ceratosuchus†

Stangerochampsa†

Brachychampsa sealeyi†

Brachychampsa montana†

Orientalosuchina†

Eoalligator†

Jiangxisuchus†

Protoalligator†

Orientalosuchus†

Krabisuchus†

Alligatoridae

Alligatorinae

Caimaninae

Alternatively, other phylogenetic studies have recovered Brachychampsa as an alligatorid, specifically as a stem-caiman,[14][15] as shown in the cladogram below.[14]

Alligatoroidea

Leidyosuchus†

Diplocynodon†

Alligatoridae

Alligatorinae (stem-based group)

Caimaninae

Stangerochampsa†

Albertochampsa†

Brachychampsa†

Protocaiman†

Gnatusuchus†

Globidentosuchus†

Eocaiman†

Notocaiman†

Kuttanacaiman†

Purussaurus†

Mourasuchus†

crown Caimaninae

(stem-based group)
(crown group)
(stem-based group)

References
  1. Rio, Jonathan P.; Mannion, Philip D. (6 September 2021). "Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem". PeerJ. 9: e12094. doi:10.7717/peerj.12094. PMC 8428266. PMID 34567843.
  2. 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.
  3. 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.
  4. Rio, Jonathan P.; Mannion, Philip D. (2021-09-06). "Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem". PeerJ. 9: e12094. doi:10.7717/peerj.12094. ISSN 2167-8359. PMC 8428266. PMID 34567843.
  5. Estes, R. (1964). "Fossil vertebrates from the Late Cretaceous Lance Formation, eastern Wyoming". University of California Publications in Geological Sciences. 49: 1–180.
  6. Estes, R.; Berberian, P. (1970). "Paleoecology of a Late Cretaceous vertebrate community from Montana". Breviora. 343: 1–35.
  7. 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.
  8. 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. Vol. 361. Geological Society of America. pp. 145–167. doi:10.1130/0-8137-2361-2.145. ISBN 9780813723617.
  9. 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.
  10. 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. S2CID 140631483.
  11. 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. S2CID 85050852.
  12. Michael S. Y. Lee; Adam M. Yates (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". Proceedings of the Royal Society B. 285 (1881). doi:10.1098/rspb.2018.1071. PMC 6030529. PMID 30051855.
  13. Tobias Massonne; Davit Vasilyan; Márton Rabi; Madelaine Böhme (2019). "A new alligatoroid from the Eocene of Vietnam highlights an extinct Asian clade independent from extant Alligator sinensis". PeerJ. 7: e7562. doi:10.7717/peerj.7562. PMC 6839522. PMID 31720094.
  14. 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.
  15. Adam P. Cossette; Christopher A. Brochu (2020). "A systematic review of the giant alligatoroid Deinosuchus from the Campanian of North America and its implications for the relationships at the root of Crocodylia". Journal of Vertebrate Paleontology. 40: e1767638. Bibcode:2020JVPal..40E7638C. doi:10.1080/02724634.2020.1767638.
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