ichthyosaurs

Home > Plants and Animals > Zoology and Veterinary Medicine > Zoology: General

ichthyosaurs Ichthyosaurs (fish-reptiles) were among the most specialized of the marine reptiles and show very well the changes that had to be made in animals that became secondarily aquatic. The reptile lungs were retained, requiring the animals to breathe at the surface, as whales do now; the legs and feet were transformed into paddles; and tail fins similar to those of fish were developed to propel the animals through the water. As the reproductive strategy of laying eggs in water had already been abandoned by reptiles, those aquatic reptiles that could not come out on land to lay eggs developed live birth.

Ichthyosaurs first appeared in the Early Triassic as fully fledged aquatic animals with no obvious ancestral forms. It is, however, clear that their ancestry must lie among the diapsids, the major group of reptiles that also includes the crocodiles, dinosaurs, lizards, and snakes. These most specialized of aquatic reptiles were essentially like large fish in their way of life and show similar specializations for fast swimming. The typical genus Ichthyosaurus was a fish-like animal up to 3 m in length with a large elongated head and enormous eyes. The jaws were long and narrow and lined with small sharp teeth, and the nostrils were positioned on top of the head above the eyes. The head merged with the body to produce a tapering streamlined shape, indicating that this animal was a powerful swimmer. Well-preserved fossils from marine black shales sometimes shows the body outline of these animals, and from this material it has been possible to reconstruct the large dorsal fin and the tail, which was a large lunate structure like that of a tuna. The four limbs had been altered to paddles by a shortening of the limb bones and a great increase in the number of finger and toe bones, which formed flattened hexagonal elements pressed closely to each other. It is clear that the animal moved by lateral undulations of the body, as a fish does, and used its paddles to steer with. The ichthyosaurs were active hunters, tracking their prey by sight, and stomach contents show that they fed on squids and probably also on marine molluscs such as the ammonites. Some specimens from black shales in Germany have unborn embryos within the body cavities of adults, showing that these animals bore living young.

The largest ichthyosaurs occur in the Triassic; Shonisaurus from Nevada reaches a length of almost 15 m. Later ichthyosaurs were smaller but were very widespread, occurring in all the Mesozoic oceans. Two main groups of ichthyosaurs can be distinguished; the latipinnate (wide paddle) and longipinnate (long paddle) forms. The latipinnate forms were abundant during the Jurassic but became extinct before the end of that period; longipinnate forms lasted into the Late Cretaceous. Despite their specializations for a marine existence, this entire group became extinct at the end of the Creataceous, as did the other main group of marine reptiles, the plesiosaurs, and the dinosaurs, as well as many invertebrate groups.

David K. Elliott

ichthyosaurs See ICHTHYOSAURIA.