Although the order Crocodylia dates back at least 200 million years to the Age of Reptiles, its living members, including those of the family Alligatoridae, can hardly be described as primitive. Instead, they survived the mass extinction 65 million years ago that ended the dinosaurs' reign and evolved over the centuries into animals well suited to their current place in the natural world. Like other members of the order, the family Alligatoridae are the descendants of the Archo-sauria, or "ruling lizards," which included the dinosaurs. A defining characteristic of these animals is a diapsid skull, which has two temporal openings. Turtles, by comparison, have anapsid skulls with no temporal openings.
Within the crocodilians, the family Alligatoridae can be followed as far back as the Paleocene (57-65 million years ago), when caiman ancestors are thought to have roamed the earth. Ancestors of other species, including the American alligator (Alligator mississippiensis) and Chinese alligator (Alligator sinensis), date back to the Miocene and Pleistocene, respectively. The alligatorids are separated into two major groups: the alligators (subfamily Alligatorinae) and the caimans (subfamily Caimaninae). The former group has two living representatives in the Alligator genus. The other six species of alligatorids fall under three genera within the caimans. (Some systematists list only five caimans, with the Yacaré as a subspecies of the common caiman.) In all, the eight species are as follows:
• American alligator, Alligator mississippiensis
• Chinese alligator, A. sinensis
• common caiman, Caiman crocodilus
• broad-snouted caiman, C. latirostris
• black caiman, Melanosuchus niger
• Cuvier's dwarf caiman, Paleosuchus palpebrosus
• smooth-fronted caiman, P. trigonatus
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