ICHTHYOSAUR

any member of an extinct group of aquatic reptiles that were very porpoise-like in appearance and habits; the ichthyosaurs were the most highly specialized reptiles adapted to aquatic environments. The ichthyosaurs had a very wide geographic range and span almost the entire Mesozoic Era (between 66,400,000 and 245,000,000 years ago), but they were most abundant and diverse during the Jurassic Period (208,000,000 to 144,000,000 years ago). Excellent fossil specimens occur in the fine-grained shales of Early Jurassic age of southern Germany, in which the outline of the body, including a well-developed fleshy dorsal fin, is preserved. A representative genus is Ichthyosaurus, from which the group takes its name. Ichthyosaurus, very fishlike in appearance, is especially well known from Jurassic deposits. The body was streamlined; no distinct neck was present, and the head blended smoothly into the body. The limbs were modified into paddle-like organs used to steer the animal. The animal propelled itself by means of a well-developed, fish-like caudal fin and by undulations of the body. The vertebral column, which is formed from disklike elements, 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 material was found that the bent nature of the backbone became apparent. The skull of Ichthyosaurus was long; the jaws were greatly lengthened and armed with numerous sharp teeth. The eyes were very large, and the nostrils were positioned far back on the top of the skull, another adaption to an aquatic existence. Ichthyosaurus was about 3 metres (10 feet) long and was probably able to move through the water at high speeds. It probably fed largely on fish and other marine animals. The ichthyosaurs were so highly specialized for an aquatic existence that it is unlikely they ventured upon the land; if stranded upon a shore, they must have been as helpless as modern beached whales and porpoises are. It is apparent that the ichthyosaurs must have evolved a mechanism by which they could reproduce in the water. Several specimens are known in which the skeletal remains of small, immature ichthyosaurs are preserved within the bodies of larger individuals. Although it has been suggested that these smaller ichthyosaurs had been eaten by larger ones, it is more likely that the specimens are females and offspring that died during parturition. The ancestry of the ichthyosaurs is uncertain at best. Certain Early Mesozoic representatives already possessed most of the specializations that characterize the group. Ichthyosaurs became adapted for different sorts of habits. One form fed on mollusks; its teeth were modified into flat, crushing structures. In another the lower jaw was shortened, while the upper jaw was long and pointed in a structure similar to that of the modern swordfish. The ichthyosaurs became progressively rarer toward the end of the Mesozoic and were extinct before the era ended.