The superfamily Alligatoroidea is thought to have split from the crocodile-gharial lineage in the
late Cretaceous, about 80 million years ago, but possibly as early as 100 million years ago based on
molecular phylogenetics.[1][2][3]Leidyosuchus of
Alberta is the earliest known genus. Fossil alligatoroids have been found throughout Eurasia as land bridges across both the North Atlantic and the
Bering Strait have connected North America to Eurasia during the Cretaceous,
Paleogene, and
Neogene periods. Alligators and caimans split in North America during the early
Tertiary or late Cretaceous (about 53 million[2] to about 65 million years ago[1]) and the latter reached South America by the
Paleogene, before the closure of the
Isthmus of Panama during the Neogene period. The
Chinese alligator split from the
American alligator about 33 million years ago[2] and likely descended from a lineage that crossed
the Bering land bridge during the
Neogene. The modern American alligator is well represented in the fossil record of the
Pleistocene.[4] The alligator's full
mitochondrial genome was sequenced in the 1990s.[5] The full
genome, published in 2014, suggests that the alligator evolved much more slowly than mammals and birds.[6]
Phylogeny
Cladistically, Alligatoroidea is defined as Alligator mississippiensis (the
American alligator) and all
crocodylians more closely related to A. mississippiensis than to either Crocodylus niloticus (the
Nile crocodile) or Gavialis gangeticus (the
gharial).[7] This is a
stem-based definition for
alligators,[8] and is more inclusive than the
crown groupAlligatoridae.[9] As a crown group, Alligatoridae only includes the
last common ancestor of all
extant (living) alligators, caimans, and their descendants (living or
extinct), whereas Alligatoroidea, as a stem group, also includes more
basal extinct alligator ancestors that are more closely related to living alligators than to
crocodiles or
gavialids. When considering only living taxa (
neontology), this makes Alligatoroidea and Alligatoridae
synonymous, and only Alligatoridae is used. Thus, Alligatoroidea is only used in the context of
paleontology.
Traditionally, crocodiles and alligators were considered more closely related and grouped together in the clade
Brevirostres, to the exclusion of the
gharials. This classification was based on
morphological studies primarily focused on analyzing skeletal traits of living and extinct fossil species.[10] However, recent molecular studies using
DNA sequencing have rejected Brevirostres upon finding the crocodiles and gavialids to be more closely related than the alligators.[11][12][13][9][14] The new clade
Longirostres was named by Harshman et al. in 2003.[11]
^Brochu, Christopher A. (1999). "Phylogenetics, Taxonomy, and Historical Biogeography of Alligatoroidea". Society of Vertebrate Paleontology Memoir. 6: 9–100.
doi:
10.2307/3889340.
JSTOR3889340.