The superfamily Alligatoroidea is thought to have split from the crocodile-gharial lineage in thelate Cretaceous, about 80 million years ago, but possibly as early as 100 million years ago based onmolecular phylogenetics.[1][2][3]Leidyosuchus ofAlberta is the earliest known genus. Although, a 2025 study considers it andDeinosuchus to be non-crocodylianeusuchians closely related tocrocodylians.[4] Fossil alligatoroids have been found throughout Eurasia as land bridges across both the North Atlantic and theBering Strait have connected North America to Eurasia during the Cretaceous,Paleogene, andNeogene periods. Alligators and caimans split in North America during the earlyTertiary or late Cretaceous (about 53 million[2] to about 65 million years ago[1]) and the latter reached South America by thePaleogene, before the closure of theIsthmus of Panama during the Neogene period. TheChinese alligator split from theAmerican alligator about 33 million years ago[2] and likely descended from a lineage that crossedthe Bering land bridge during theNeogene. The modern American alligator is well represented in the fossil record of thePleistocene.[5] The alligator's fullmitochondrial genome was sequenced in the 1990s.[6] The fullgenome, published in 2014, suggests that the alligator evolved much more slowly than mammals and birds.[7]
Cladistically, Alligatoroidea is defined asAlligator mississippiensis (theAmerican alligator) and allcrocodylians more closely related toA. mississippiensis than to eitherCrocodylus niloticus (theNile crocodile) orGavialis gangeticus (thegharial).[8] This is astem-based definition foralligators,[9] and is more inclusive than thecrown groupAlligatoridae.[10] As a crown group, Alligatoridae only includes thelast common ancestor of allextant (living) alligators, caimans, and their descendants (living orextinct), whereas Alligatoroidea, as a stem group, also includes morebasal extinct alligator ancestors that are more closely related to living alligators than tocrocodiles orgavialids. When considering only living taxa (neontology), this makes Alligatoroidea and Alligatoridaesynonymous, and only Alligatoridae is used. Thus, Alligatoroidea is only used in the context ofpaleontology.
Traditionally, crocodiles and alligators were considered more closely related and grouped together in the cladeBrevirostres, to the exclusion of thegharials. This classification was based onmorphological studies primarily focused on analyzing skeletal traits of living and extinct fossil species.[11] However, recent molecular studies usingDNA sequencing have rejected Brevirostres upon finding the crocodiles and gavialids to be more closely related than the alligators.[12][13][14][10][15] The new cladeLongirostres was named by Harshmanet al. in 2003.[12]
^Brochu, Christopher A. (1999). "Phylogenetics, Taxonomy, and Historical Biogeography of Alligatoroidea".Society of Vertebrate Paleontology Memoir.6:9–100.doi:10.2307/3889340.JSTOR3889340.
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