Origin Ichthyosaur

grippia longirostris triassic of spitsbergen well-adapted aquatic lifestyle.

the origin of ichthyosaurs contentious. clear transitional forms land-dwelling vertebrate groups have not yet been found, earliest known species of ichthyosaur lineage being aquatic. in 1937, friedrich von huene hypothesised ichthyosaurs not reptiles, instead represented lineage separately developed amphibians. today, notion has been discarded , consensus exists ichthyosaurs amniota , descended terrestrial egg-laying tetrapoda during late permian or earliest triassic. however, has proven difficult establish position within amniote evolutionary tree. there several conflicting hypotheses on subject. in second half of twentieth century, had been usual assume ichthyosaurs anapsida, seen branch of primitive reptiles. explain appearance of ichthyosaurs in fossil record, , lack of clear affinities other reptile groups, anapsids supposed little specialised. hypothesis has become unpopular being inherently vague because anapsida unnatural paraphyletic group. modern exact quantitative cladistic analyses consistently indicate ichthyosaurs diapsida. studies showed basal, or low, position in diapsid tree. more analyses result in being neodiapsida, derived diapsid subgroup. of these, majority show them basal lepidosauromorpha; minority show them basal archosauromorpha.

until 1980s, close relationship assumed between ichthyosauria , sauropterygia, marine reptile group, within overarching euryapsida. today, however, rejected, euryapsida being seen unnatural polyphyletic assemblage of reptiles happen share adaptations swimming lifestyle. exception study in 1997 john merck, showed monophyletic archosauromorph euryapsids.