Summary
Squamata (skwae'meitə, Latin squamatus, 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Members of the order are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. Squamates are the most variably sized living reptiles, ranging from the dwarf gecko (Sphaerodactylus ariasae) to the Reticulated python (Malayopython reticulatus). The now-extinct mosasaurs reached lengths over . Among other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the clade Lepidosauria. Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The only surviving member of the Rhynchocephalia is the tuatara. Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to the Archosauria, the clade that contains crocodiles and birds, and their extinct relatives. Fossils of rhynchocephalians first appear in the Early Triassic, meaning that the lineage leading to squamates must have also existed at the time. Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record, The first fossils of geckos, skinks, and snakes appear in the Middle Jurassic. and their overall diversity was established during the mid-Jurassic, with further diversity expansions being mostly the result of added species. Other groups like iguanians and varanoids appeared in the Cretaceous.
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