PLESIOSAUR

also called Plesiosaurus, plural Plesiosauri (genus Plesiosaurus), any of a group of extinct long-necked, marine reptiles that are found as fossils in rocks of the Late Triassic through the Late Jurassic epochs (230 to 144 million years old). The plesiosaur had a wide distribution throughout the Eastern Hemisphere and also may be represented in South American deposits. Related forms known from North America and elsewhere persisted until the end of the Cretaceous Period (66.4 million years ago). The plesiosaur was about 4.5 m (15 feet) long, with a broad and flat body and relatively short tail. The neck was long and flexible. The nostrils were far back near the eyes, and the palate was almost solid. Long, sharp teeth were present in the jaws; the plesiosaur probably fed by swinging its head from side to side through a school of fish. The plesiosaur apparently was able to employ its large, paddle-like limbs to swim backward or forward or even to rotate itself about the body axis. Early in their evolutionary history, the plesiosaurs split into two main groups: pliosaurs, the short-necked forms, in which the neck was short and the head elongated; and elasmosaurs, in which the head remained relatively small and the neck assumed snakelike proportions and became very flexible. The late evolution of plesiosaurs was marked by a great increase in size. Kronosaurus, an Early Cretaceous pliosaur from Australia, was about 12 m (40 feet) long; the skull alone was about 3.7 m (12 feet) long. Elasmosaurus had as many as 76 vertebrae in its neck and reached a length of about 13 m (43 feet), fully half of which consisted of the head and neck.