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Plesiosaurs were another group of marine reptiles which shared the Cretaceous oceans with the mosasaurs. The Plesiosaurs were comprised of two main groups: the Elasmosaurs and the Polycotylids. The major difference between these two groups is in the length of the neck. Elasmosaurs had a much longer neck, which was due to an elongation of the individual cervical vertebrae and an increase in the total number of vertebrae. Elasmosaurs possessed up to 70 cervical vertebrae and had a short skull. However, in the Polycotylids, the skull was elongated and there were only 20 cervical vertebrae creating a shorter neck. In some cases, the skull alone accounted for one quarter of the total length of the reptile!

Plesiosaurs flourished throughout much of the Mesozoic Era, appearing during the Late Triassic Period, reaching their evolutionary pinnacle during the Jurassic, and then slowly decreasing in diversity and population as they approached the great Cretaceous – Paleogene extinction. During the Late Cretaceous Period, the Plesiosaurs were declining upon the arrival of the mosasaurs, although they remained particularly abundant in the cooler waters of the north. The primary food source of the Plesiosaurs was fish.

Plesiosaurs differed in their mode of locomotion from that of the mosasaurs. The steering was left to the short tail, while the main locomotion was conducted by the flippers/paddles. The paddles were generally equal in length, allowing for locomotion patterns similar to that of modern marine turtles.
Rounded and polished stones, called gastroliths, have often been found in the stomach region of some plesiosaur specimens. While plesiosaur teeth were ideal for spearing fish, they were inadequate for chewing and one theory suggests that the gastroliths were swallowed by the animals to grind food in the stomach. Another theory about gastroliths is that they might have also been used as ballast to help control the reptile’s buoyancy.