Marine Reptiles II: Plesiosaurs

A second group of marine reptiles is the Plesiosaurs, usually translated as "near lizards" or "ribbon reptiles," so-called because of their long, thin necks. The plesiosaurs had euryapsid skulls (see my article "Reptile Skulls" for more information), a diapsid skull that is specially modified for life underwater.

All plesiosaurs had short tails, squat bodies, and flippers that they used for propulsion. They flourished from the end of the Triassic Period to the end of the Cretaceous Period. There were two main groups of plesiosaurs: the long-necked with a short head Plesiosauroidea (usually just called Plesiosaurs), and the short-necked with a long head Pliosaurs.

The long-necked Plesiosaurs had sharp teeth for catching fish. The shape of their flippers and shoulders indicated that they could manoever rapidly, twisting and turning in the water. Their necks became very long: Elasmosaurus, a genus found in the Upper Cretaceous Period, had over 70 neck vertebrae! The plesiosaurs probably lived near the surface of the water and could have lifted their necks and heads out of the water to better see fish.

The short-necked Pliosaurs had massive teeth that were probably used for crushing the shells of cephalopods rather than catching fish. Paleontologists have found the remains of deep water cephalopods in the stomach cavities of Pliosaur specimens, telling us that they probably hunted in deep water. Their necks became even shorter, with only twenty neck vertebrae, and their heads became even longer as time passed. Some species of Pliosaur had heads that were so long that one-fourth of their body length was the length of the skull! The shape of their flippers and shoulders indicate that they would not have been able to manoever as easily as the long-necked Plesiosaurs. Instead, they had a powerful swimming stroke; they were built for power and sustained speed rather than quick turns and quick bursts of speed.

Both groups of Plesiosaurs were successful throughout the Mesozoic Era. They were able to do this partly because the two types occupied such different physical niches in the ocean ecology and fed on different prey animals. So, they would not have been in direct competition with each other.

Both groups probably did compete with the Mosasaurs, who fed on both fish and cephalopods. (See my article "Marine Reptiles III: Mosasaurs".

The first plesiosaur specimen was found in 1828 by Mary Anning (see my article "Mary Anning"). After initially doubting the validity of her find, Georges Cuvier studied and named the specimen.

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