The resolution of the problem of ichthyosaurian affinities has been complicated and prolonged by an emphasis on the morphology of Jurassic ichthyosaurs, some 30 to 40 m.y. younger than the earliest known species. Morphology of the skull and axial skeleton of Triassic ichthyosaurs is the key to resolving the problem. The ichthyosaurs are included here in the Subclass Diapsida on the basis of skull roof and palatal features. Within the Diapsida, ichthyosaurs appear to be most closely related to the Younginiformes of the Infraclass Lepidosauromorpha, with which they may have shared a common ancestor.
Triassic ichthyosaurs were considerably different ecologically from their Jurassic descendants. Many Triassic ichthyosaurs were longer bodied and had long, broad tails in contrast to the deep, streamlined bodies and lunate tails of Jurassic species. As a result, Triassic ichthyosaurs probably tended to be ambush predators, whereas Jurassic ichthyosaurs were mainly pursuit predators. Early ichthyosaurs may have been more generalized in their prey preference, as suggested by heterodonty in many Early and Middle Triassic species.