PLESIOSAURS WERE MARINE REPTILES.

They belonged to the same group as nothosaurs and placodonts, the Sauropterygia. All plesiosaurs had four winglike flippers and many pointed teeth. They probably used their flippers to “fly” underwater in a similar way to marine turtles or penguins. While many plesiosaurs had long necks and small skulls, others, the pliosaurs, were short-necked and had enormous skulls. The biggest pliosaurs, such as Liopleurodon and Kronosaurus, had skulls 10 ft (3 m) long with huge pointed teeth. It seems likely that pliosaurs were predators that fed on other marine reptiles. This is confirmed by long-necked plesiosaur skeletons that bear pliosaur tooth marks.

Plesiosaurs have been discovered with fossils of belemnites (squid-like animals), and ammonites (giant nautilus-like molluscs) associated with their stomachs. They had powerful jaws, probably strong enough to bite through the hard shells of their prey. The bony fish (Osteichthyes), started to spread in the Jurassic, and were likely prey as well. Recent evidence seems to indicate that some plesiosaurs may have, in fact, been bottom feeders.

It had been theorised that smaller plesiosaurs may have crawled up on a beach to lay their eggs, like the modern leatherback turtle, but it is now clear plesiosaurs gave birth to live young: The fossil of a pregnant plesiosaur Polycotylus latippinus shows that these animals gave birth to one large juvenile and probably invested parental care in their offspring, similar to modern whales.

Another curiosity is their four-flippered design. No modern animals have this swimming adaptation (sea turtles only swim with their front flippers), so there is considerable speculation about what kind of stroke they used. While the short-necked pliosauroids (e.g. Liopleurodon) may have been fast swimmers, the long-necked varieties were built more for maneuverability than for speed. Skeletons have also been discovered with gastroliths in their stomachs, though whether to help break down food in a muscular gizzard, or to help with buoyancy, or both, has not been established.

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