ichthyosaur

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Ichthyosaurs dominated much of the Mesozoic era; based on fossil evidence, they first emerged around 250 million years ago and survived at least one species until about 90 million years ago into the Late Cretaceous. During the early Triassic period, ichthyosaurs evolved from a group of unidentified land reptiles that returned to the sea in a development similar to how mammalian ancestors of modern dolphins and whales returned millions of years later which they gradually came to resemble through convergent evolution. Ichthyosaurs were particularly abundant in the later Triassic and early Jurassic periods until they were replaced as top aquatic predators by another marine group, Plesiosauria, in the later Jurassic and Cretaceous periods. In the Late Cretaceous, ichthyosaurs suffered a devastating blow from the Cenomanian-Turonian anoxic event. Their last lineage mysteriously became extinct for unknown reasons.