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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. 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. Another species from the Allison Member of the Menefee Formation of the San Juan Basin, B. sealeyi, was discovered in 1996, but was later argued to be synonymous with B. montana by interpreting it as an immature specimen of the latter species. 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.
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, only to be placed outside both Alligatorinae and Alligatoridae (but still within Alligatoroidea) in 1994. Accordingly, studies have shown Brachychampsa as a basal member of Alligatoroidea, within the cladeGlobidonta, as shown in the cladogram below.
Alternatively, other phylogenetic studies have recovered Brachychampsa as an alligatorid, specifically as a stem-caiman, as shown in the cladogram below.
However, Walter et al. (2022) recovered Brachychampsa, Stangerochampsa and Albertochampsa as the basalmost alligatorines based on phylogenetic analysis and claimed that the earliest definitive stem-group caimans are known from the earliest Paleocene.
Palaeoecology
Brachychampsa's dentition was heterodont, with conical teeth at the front and bulbous teeth at the back. It has been regarded as a specialist hunter of turtles, based on its rostral shape, massive dentary, and dental morphology, though this has been challenged over the years. A more generalist diet of small invertebrates and vertebrates has also been proposed. In 2003, Robert M. Sullivan and Spencer G. Lucas observed that most late Cretaceous turtles were probably too big for consumption, and that if they did represent a large portion of Brachychampsa's diet, it would have had to hunt either juveniles or smaller turtle species.
^ Estes, R. (1964). "Fossil vertebrates from the Late Cretaceous Lance Formation, eastern Wyoming". University of California Publications in Geological Sciences. 49: 1–180.
Estes, R.; Berberian, P. (1970). "Paleoecology of a Late Cretaceous vertebrate community from Montana". Breviora. 343: 1–35.
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.
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. ISBN9780813723617.
Averianov, A.; Nessov, L. (1995). "A new Cretaceous mammal from the Campanian of Kazakhstan". Neues Jahrbuch für Geologie und Paläontologie - Monatshefte. 2: 65–74. doi:10.1127/njgpm/1995/1995/65.
Martin, J.E.; Delfino, M. (2010). "Recent advances in the comprehension of the biogeography of Cretaceous European eusuchians". Palaeogeography, Palaeoclimatology, Palaeoecology. 293 (3): 406–418. Bibcode:2010PPP...293..406M. doi:10.1016/j.palaeo.2009.10.021.
^ 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.
Walter J, Darlim G, Massonne T, Aase A, Frey E, Rabi M (2022). "On the origin of Caimaninae: insights from new fossils of Tsoabichi greenriverensis and a review of the evidence". Historical Biology. 34 (4): 580–595. doi:10.1080/08912963.2021.1938563. S2CID238723638.
