ON THE CRANIAL MORPHOLOGY OF ICHTHYOSAURS

摘要: One of the difficulties in study ichthyosaurs has been lack ossification, said to be a common characteristic aquatic groups. This difficulty partially removed by discovery specimen Ophthalmosaurus which is particularly well ossified, revealing clear facets are not usually present. specimen, together with preserved skulls Ichthyosaurus, Leptopterygius and Stenopterygius many other isolated elements fragments Ichthyosaurus s.l. formed basis for interpretation ichthyosaur skull which, literature Triassic forms, led recognition large number ancestral ichthyosaurian features. The principal theories origin have examined light this new evidence shown untenable. In particular, von Huene's Nielsen's direct descent from amphibia criticised it that Romer's suggestions pelycosaur affinities mistaken. relationships then discussed terms fuller knowledge anatomy recent on primitive reptiles amphibia. Ichthyosaurs ancestry chelonians ichthyosaur-chelonian stock considered derived more evolving towards procolophonids or pareiasaurs. It otic conditions superficially similar those captorhinomorphs pelycosaurs but are, fact, “sauropsid” may occur several groups, including ichthyosaurs. evolution temporal region used suggest had their origins Deuterosaurus (Permian) times later probably than close Permian.