The Choristodera represent a distinct diapsida clade that shows specialized aquatic and semi-aquatic adaptations in Mesozoic through Late Cenozoic terrestrial ecosystems. The clade Choristodera has a long geologic range, from Late Triassic to middle Miocene, and a wide biogeographic distribution, including the North American Western Interior, Europe and East Asia. Choristoderes once reached north above the Arctic Circle, and south to the 36 degrees north latitude. Through their evolutionary history, choristoderes showed a considerable phylogenetic diversity with 24 valid nominal species in 11 genera and six families; other taxa appear to belong to the Choristodera but have no proper familial assignment. Besides their phylgenetic diversity, choristoderes showed a considerable degree of ecomorphic diversification, including three basic ecomorphic types: a long-snouted gavial type, a short-snouted crocodile type, and a long-necked ecomorphic type showing homoplastic similarities to plesiosaurs, Mesozoic marine reptiles. The phylogenetic diversity, geologic range and biogeographic distribution, and ecomorphic diversification of choristoderes reveal important evidence reflecting the evolution history of this distinct diapsida clade.
