Ichthyosaurs had a very wide geographic distribution, and their fossil remains span almost the entire Mesozoic Era (about 248 million to 65 million years ago); but they were most abundant and diverse during the Triassic (Triassic Period) and Jurassic periods (Jurassic Period) (248 million to 144 million years ago). Excellent fossil specimens occur in the fine-grained Early Jurassic shales of southern Germany. In one specimen, the entire outline of the body is preserved, including the outline of a well-developed, fleshy dorsal fin. Several specimens are known in which the skeletal remains of small, immature ichthyosaurs are fossilized within the bodies of larger individuals, even within the birth canal.

Ichthyosaurus, a representative genus from which the larger group takes its name, was about 3 metres (10 feet) long and was probably able to move through the water at high speeds. Very fishlike in appearance, it is especially well known from Early Jurassic deposits in England. The body was streamlined; no distinct neck was present, and the head blended smoothly into the body. The limbs were modified into paddlelike appendages used to steer the animal. It propelled itself by using a well-developed fishlike tail and by undulating the body. The vertebral column, which was formed from disklike structures, bent downward into the lower lobe of the caudal, or tail, fin; the upper lobe was unsupported by bone. Early reconstructions of ichthyosaurs showed them with the spinal column straightened, and it was not until well-preserved evidence was found that the bent condition of the backbone became apparent. The skull and jaws of Ichthyosaurus were long and contained numerous sharp teeth. The eyes were very large, and the nostrils were positioned far back on the top of the skull (another specialized adaptation to an aquatic existence). They probably fed largely upon fish as well as other marine animals. It is unlikely that they ventured onto land, and they certainly reproduced in the water. If stranded ashore, they would have been as helpless as beached whales (whale).

Ichthyosaurs are first known from the Triassic Period of Asia, where they began as long-bodied, undulating swimmers without many of the specializations seen in later species. By the Late Triassic some lineages had achieved great size. Fossils from the western United States and Canada indicate that some ichthyosaurs could exceed 13 metres (43 feet) in length. Deep-bodied and with long fins, these appear to have been ambush predators that fed on fishes. The typical ichthyosaur form was fully realized by the Early Jurassic, when the tunalike body plan suggestive of high-speed pursuit and great mobility asserted itself. By this time, however, the other lineages of ichthyosaurs had become extinct. Ichthyosaurs persisted into Late Cretaceous times and may have been well adapted for deep diving as well as near-shore predation, but all species became extinct well before the end of the Cretaceous Period.