Brachychampsa

Brachychampsa Temporal range: Late Cretaceous, 72.1–66 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Brachychampsa sp.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Order:	Crocodilia
Clade:	Globidonta
Family:	Alligatoridae
Genus:	†Brachychampsa Gilmore, 1911[2]
Species
†B. montana Gilmore, 1911 (type)[2] †B. perrugosus (Cope, 1875) †?B. sealeyi Williamson, 1996[3]

Brachychampsa is an extinct genus of alligatorid, 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. Some specimens have been reported from the Campanian-aged deposits of Central Asia (Chimkent of Kazakhstan and Kirkuduk of Tajikistan), although the species status is indeterminate for these fossils.^[9][10] 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

Taxonomy

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.^[11] 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.^[12] 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.^[13]

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.^[11] Accordingly, studies have shown Brachychampsa as a basal member of Alligatoroidea, within the clade Globidonta,^[14][15] as shown in the cladogram below.^[15]

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,^[16][17] as shown in the cladogram below.^[16]

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)

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.^[18]

Palaeoecology

Brachychampsa's dentition was heterodont, with conical teeth at the front and bulbous teeth at the back.^[19] It has been regarded as a specialist hunter of turtles, based on its rostral shape, massive dentary, and dental morphology,^[20] though this has been challenged over the years. A more generalist diet of small invertebrates and vertebrates has also been proposed.^[21] 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.^[22]

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. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
17. 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.
18. 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. S2CID 238723638.
19. Melstrom, Keegan M.; Irmis, Randall B. (2019). "Repeated Evolution of Herbivorous Crocodyliforms during the Age of Dinosaurs". Current Biology. 29 (14) – via CellPress.
20. Carpenter, Kenneth; Lindsey, D. (1980). "The dentary of Brachychampsa montana Gilmore (Alligatorinae; Crocodylidae), a Late Cretaceous turtle-eating alligator". Journal of Paleontology. 54 (6): 1213–1217.
21. Bartels, William S. (1984). "Osteology and Systematic Affinities of the Horned Alligator Ceratosuchus (Reptilia, Crocodilia)". Journal of Paleontology. 58 (6): 1347–1353. ISSN 0022-3360.
22. Sullivan, Robert M.; Lucas, Spencer G. (2003-12-24). "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. ISSN 0272-4634.

External links

• Brachychampsa in the Paleobiology Database