Evolution Squamata

slavoia darevskii, fossil squamate

squamates monophyletic sister group tuatara. squamates , tuatara sister group crocodiles , birds, extant archosaurs. fossils of squamate sister group, rhynchocephalia, appear in triassic, meaning lineage leading squamates must have existed well. modern squamates originated in mid jurassic, when fossil relatives of geckos , skinks , snakes appear; other groups, including iguanians , varanoids, first appear in cretaceous period. appearing in cretaceous polyglyphanodonts, lizard group of uncertain affinities, , mosasaurs, group of predatory, marine lizards grew enormous sizes. @ end of cretaceous, squamates suffered major extinction @ k-t boundary, wiped out polyglyphanodonts, mosasaurs, , number of other groups.

the relationships of squamates have been debated. although many of groups recognized on basis of morphology still accepted, our understanding of relationships each other has changed radically result of studying dna. morphological data, iguanians long thought ancient branch of tree; however, studies of dna suggest geckos represent ancient branch. iguanians united snakes , anguimorphs in group called toxicofera. dna suggests various limbless groups—snakes, amphisbaenians, , dibamids—are unrelated, , instead arose independently lizards.