The ichthyosaurs were the most specialized of marine reptiles. Their name means "fish lizards," and describes them well, since they ate fish, and were shaped like fish (though, of course, they were air-breathing reptiles). In fact, their overall body shape was like that of a modern mackerel or tuna — a highly streamlined form that allows these living fish to reach swimming speeds of over 25 mph/40 kmph.
Unlike their contemporaries, the plesiosaurs (see pp. 74-77), ichthyosaurs did not rely on their paddlelike flippers for swimming. Instead, they developed a fishlike tail, whose lateral movements provided the main propulsive force, just like the tail of a modern shark or tuna.
So fully adapted were the ichthyosaurs to a marine life that they could no longer come ashore to lay eggs, as did the sea turtles and plesiosaurs. Ichthyosaurs gave birth to live young at sea. In fact, some of the most remarkable of fossil finds show adult female ichthyosaurs with young babies just emerging from their bodies — preserved forever in the process of birth.
As a group, the ichthyosaurs occupied the ecological niche of today's dolphins. They were wide-ranging and highly successful for about 100 million years. They cruised the open seas of the world from Early Triassic times, and reached their peak of diversity in the Jurassic. Thereafter, they declined until, by the mid-Cretaceous, they were all iiut extinct. Their demise could be related to competition from the advanced sharks, which had evolved into their modern form by this time, and were the dominant carnivores of the Late Mesozoic seas (see pp. 26—29).
The origin of the ichthyosaurs is not known. The only certainty is the fact that they descended from some terrestrial group of reptiles, rather than evolving from one of the known types of aquatic reptile.
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