a subclass of extinct Mesozoic reptiles, which attained their widest distribution in the Jurassic Period. The sole order is Ichthyosauria. The Ichthyopterygia were large (up to 12 m long) predators, the descendants of terrestrial animals that adapted to life in the open sea and underwent a number of changes in structure: the body acquired a fish-like form, the snout became elongated, the neck disappeared, the extremities were converted into paddle-like organs, cutaneous fins developed on the end of the tail and on the back, the vertebrae became biconcave (as in fish), sclerotic plates counteracting water pressure appeared in the eye sockets, and the skin lost its scaly covering.

Owing to the fact that the Ichthyopterygia were exclusively acquatic and the structure of their extremities was such that they were unable to emerge on dry land, they became viviparous. Their principal food was fish and cephalopods. Remains of ichthyopterygia are distributed in Mesozoic marine deposits of the northern hemisphere, including the USSR; the most numerous finds are in Central Europe. The resemblance of these reptiles to sharks and dolphins (animals of different classes of vertebrates), which developed in the process of evolution as a result of similar ways of life, is a classic example of morphological convergence.