Brachychampsa Explained

Brachychampsa is an extinct genus of alligatorid, possibly a basal caiman.[1] Specimens have been reported from New Mexico, Colorado, Wyoming,[2] Montana,[3] [4] [5] North and South Dakota,[6] 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.[7] [8] 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.

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.[3] 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,[9] 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

Brachychampsas 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.[12] Accordingly, studies have shown Brachychampsa as a basal member of Alligatoroidea, within the clade Globidonta,[13] [14] as shown in the cladogram below.[14]

Alternatively, other phylogenetic studies have recovered Brachychampsa as an alligatorid, specifically as a stem-caiman,[15] [16] as shown in the cladogram below.[15]

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.[17]

External links

Notes and References

  1. 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. 10.7717/peerj.12094. 34567843. 8428266. 2167-8359 . free .
  2. Estes . R. . 1964 . Fossil vertebrates from the Late Cretaceous Lance Formation, eastern Wyoming . University of California Publications in Geological Sciences . 49 . 1–180 .
  3. Gilmore . Charles W. . Charles W. Gilmore . 1911 . A new fossil alligator from the Hell Creek Beds of Montana . Proceedings of the United States National Museum . 41 . 1860. 297–302 . 10.5479/si.00963801.41-1860.297.
  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. Book: 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 . [{{google books | id = cHvcIeh2f84C| page = 145 | plainurl = yes }} The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains ]. An Integrated Continental Record of the End of the Cretaceous . Joseph Herbert . Hartman . Kirk. R. . Johnson . Douglas J. . Nichols . Geological Society of America . 361 . 145–167 . 10.1130/0-8137-2361-2.145 . 9780813723617 .
  7. Averianov, A.. Nessov, L.. 1995. A new Cretaceous mammal from the Campanian of Kazakhstan. Neues Jahrbuch für Geologie und Paläontologie - Monatshefte. 2. 65–74. 10.1127/njgpm/1995/1995/65.
  8. 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 . 10.1016/j.palaeo.2009.10.021. 2010PPP...293..406M .
  9. 10.1080/02724634.1996.10011331 . Williamson . Thomas E. . 1996 . ?Brachychampsa sealeyi, sp. nov., (Crocodylia, Alligatoridea) from the Upper Cretaceous (Lower Campanian) Menefee Formation, northwestern New Mexico . . 16 . 3. 421–431 . 4523734 .
  10. 10.1671/A1082-8 . Sullivan . Robert M. . Lucas . Spencer G. . 2003 . Brachychampsa montana Gilmore (Crocodylia, Alligatoroidea) from the Kirtland Formation (upper Campanian), San Juan Basin, New Mexico . . 23 . 4. 832–841 . 4524385 . 140631483 .
  11. 10.1671/0272-4634(2004)024[0857:APATSO]2.0.CO;2 . Brochu . Christopher A. . 2004 . Alligatorine phylogeny and the status of Allognathosuchus Mook, 1921 . . 24 . 4. 857–873 . 4524781 . 85050852 .
  12. 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 .
  13. 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 . . 285 . 1881 . 10.1098/rspb.2018.1071 . 30051855 . 6030529 . free.
  14. 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 . 10.7717/peerj.7562 . 31720094 . 6839522 . free .
  15. 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 . 10.1098/rspb.2018.0843 . 30135152 . 6125902 .
  16. 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 . 10.1080/02724634.2020.1767638 . 2020JVPal..40E7638C . free .
  17. 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 . 10.1080/08912963.2021.1938563 . 238723638 .

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