October 3. Jurassic plesiosaurs and ichthyosaurs

We talked about marine reptiles, including plesiosaurs and ichthyosaurs last month, since they began during the Triassic. But they really began to grow and to dominate the seas during the Jurassic.

Plesiosaur drawing by Adam Stuart Smith  via Wikimedia, used under Creative Commons license

Ichthyosaurs, “fish lizards,” were shaped like dolphins or tunas and dominated the early Jurassic. The first complete skull of an ichthyosaur was found on the Jurassic Coast of England, a World Heritage Site along the English Channel in Dorset and Devonshire in southwest England. Mary Anning made many fossil discoveries there in the 1820s and 1830s, including the first correctly identified ichthyosaur, which she and her brother found when she was 12 years old. She found and described the first plesiosaurs as well.

Jurassic ichthyosaurs were one to ten meters long, 3 to 30 feet. Ichthyosaurs were completely adapted to aquatic life, and never came onshore. They bore their young alive in the water, and they clearly were predators. One specimen has been found with more than 200 belemnites, squid-like cephalopods, in its stomach.

As ichthyosaurs begin to decline in diversity by middle Jurassic time, they seem to have been replaced by long-necked, seal-like plesiosaurs. There are more than 100 species of plesiosaur known, and they were distributed in seas worldwide. They grew to lengths of 17 meters, more than 50 feet, and they were certainly at the top of the food chain.

Ichthyosaurs survived into the Cretaceous Period but were extinct before the end-Cretaceous extinction. While we don’t know for sure why ichthyosaurs went extinct, competition from plesiosaurs could have been a factor. Plesiosaurs survived until the end of the Cretaceous.

—Richard I. Gibson

Jurassic Coast World Heritage Site

Plesiosaur drawing by Adam Stuart Smith  via Wikimedia, used under Creative Commons license