Globidonta

Globidonta Temporal range: Late Cretaceous - Present, 83.5–0 Ma PreЄ Є O S D C P T J K Pg N
Skull of Acynodon iberoccitanus, the oldest known globidontan
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Crocodylomorpha
Order:	Crocodilia
Superfamily:	Alligatoroidea
Clade:	Globidonta Brochu, 1999
Families
Alligatoridae

Globidonta is a clade of alligatoroids that includes alligators, caimans, and closely related extinct forms. It is defined as a stem-based clade including Alligator mississippiensis (the American Alligator) and all forms more closely related to it than to Diplocynodon. The group's fossil range extends back into the Late Cretaceous with early alligatoroids such as Albertochampsa and Brachychampsa.^[1] Extinct globidontans were particularly common in North America and Eurasia, and their modern range also includes South America. The oldest known globidontan is Acynodon from France, which is also the most basal.^[2]

Basal globidontans are characterized by their blunt snouts and bulbous teeth. Modern globidontans have flattened snouts and more conical teeth, and are seen as more generalized than earlier globidontans. Generalized forms are usually expected to be ancestral to more specialized forms rather than descendants of them, so it is unusual for basal members of the group to appear specialized. This seems to conflict with the "Law of the Unspecialized" first proposed by Edward Drinker Cope in 1894. Under the Law of the Unspecialized, morphological change is always directed toward specialization, and specialized forms can never become "unspecialized" again. This pattern of change, while not seen in globidontans, can be observed in basal members of Alligatoroidea and Crocodyloidea.^[3]

Alligator sinensis, the Chinese Alligator.

Flat-snouted globidontans occurred two times in the evolution of the clade: once in caimans and once in alligators. Alligator sinensis, the Chinese Alligator, has a snout that is somewhat blunt and could be considered specialized. However, its snout is not nearly as blunt as those of more basal globidontans such as Albertachampsa.^[3]

If the last common ancestor of Diplocynodon and globidontans was more like Diplocynodon, it would have had a generalized snout shape. It is also possible that the generalized form of Diplocynodon may also have arisen from a specialized blunt-snouted ancestor. Baryphracta, a diplocynodontine closely related to Diplocynondon, has a blunt snout and may have been similar in appearance to such an ancestor.^[3]

Phylogenetics

Below is a cladogram modified from Martin (2007):^[2]

Diplocynodontinae  Baryphracta Diplocynodon  Globidonta  Acynodon Stangerochampsa Albertochampsa Brachychampsa  Alligatoridae   Alligatorinae  Ceratosuchus Hassiacosuchus Navajosuchus Allognathosuchus Wilwood alligatorid Procaimanoidea Arambourgia Wannaganosuchus Alligator  Caimaninae  Eocaiman Purussaurus Orthogenysuchus Mourasuchus Caiman Melanosuchus Paleosuchus        Blunt-snouted        Generalized        Duck-like (broad, flat, and elongate)

References

1. Brochu, C.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.
2. Martin, J.E. (2007). "New material of the Late Cretaceous globidontan Acynodon iberoccitanus (Crocodylia) from Southern France". Journal of Vertebrate Paleontology 27 (2): 362–372. doi:10.1671/0272-4634(2007)27[362:NMOTLC]2.0.CO;2.
3. Brochu, C.A. (2001). "Crocodylian snouts in space and time: phylogenetic approaches toward adaptive radiation". American Zoologist 41 (3): 564–585. doi:10.1668/0003-1569(2001)041[0564:CSISAT]2.0.CO;2.