| Globidonta | |
|---|---|
 | |
| Brachychampsa sp. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Order: | Crocodilia |
| Superfamily: | Alligatoroidea |
| Clade: | Globidonta Brochu, 1999 |
| Subclades[2] | |
| |
Globidonta is aclade ofalligatoroids that includesalligators,caimans, and closely related extinct forms. It is defined as astem-based clade includingAlligator mississippiensis (the American Alligator) and all forms more closely related to it than toDiplocynodon. The group's fossil range extends back into theCampanian stage of theLate Cretaceous with early alligatoroids such asAlbertochampsa andBrachychampsa.[3] Extinct globidontans were particularly common inNorth America andEurasia, and their modern range also includesSouth America.
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 byEdward 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 ofAlligatoroidea andCrocodyloidea.[4]
Flat-snouted globidontans occurred two times in the evolution of the clade: once in caimans and once in alligators.Alligator sinensis, theChinese 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 asAlbertachampsa.[4]
If the last common ancestor ofDiplocynodon and globidontans was more likeDiplocynodon, it would have had a generalized snout shape. It is also possible that the generalized form ofDiplocynodon may also have arisen from a specialized blunt-snouted ancestor.[4]
The belowcladogram shows Globidonta's placement withinCrocodylia, based on a 2018tip dating study by Lee & Yates that simultaneously usedmorphological, molecular (DNA sequencing), andstratigraphic (fossil age) data.[5]
| Crocodylia | |
| (crown group) |
Here is a more detailed cladogram of Globidonta from a 2019 Massonneet al. study, withLeidyosuchus included as a member:[6]
Some studies have alternatively found many members of Globidonta to instead be basal members of thecrown groupAlligatoridae, within either of thestem groupsAlligatorinae orCaimaninae: specificallyNavajosuchus andCeratosuchus asbasal members of Alligatorinae, andStangerochampsa andBrachychampsa as basal members of Caimaninae.[7][8] Adam Cossette and David Tarailo in 2024 recovered a clade withinCaimaninae comprisingBrachychampsa and its close relatives. They named this cladeBrachychampsini, defining it as "the largest clade of alligatorids more closely related toBrachychampsa montana than toCaiman crocodilus (the Spectacled caiman) orAlligator mississippiensis (the American alligator).[1]
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