Ichthyiosaurs  resembled modern day porpoises, with the body streamlined, with the tail specialized into a propulsive fin, with the legs flattened into paddles for balancing, with the jaws elongated and equipped with many teeth for fish catching, and large eyes used to see in the dark waters.

The first icthyosaurs resembled lizards with fins and probably swam like eels. They were highly specialized and the only respects in which they were more primitive than their descendants are in certain details of the skull and in the fact that the tail fin was not quite as highly developed for swimming as the ichthyosaurs of jurassic age.