Description
The typical plesiosaur had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. Plesiosaurs evolved from the earlier nothosaurs, that had a more crocodile-like body; major types of plesiosaur are primarily distinguished by head and neck size. Members of Plesiosauroidea, such as Cryptoclididae, Elasmosauridae, and Plesiosauridae, had long necks and may have been 'bottom-feeders', in shallow waters. Most pliosaurs and rhomaleosaurs, however, had shorter necks with a large, elongated head and may have been at home in deeper waters.
All plesiosaurs had four paddle-shaped 'flipper' limbs. This is an unusual arrangement in aquatic animals and it is thought that they were used to propel the animal through the water by a combination of rowing movements and up-and-down movements. There appears to have been no tail fin and the tail was most likely used for helping in directional control. This arrangement is in contrast to that of the later mosasaurs and the earlier ichthyosaurs. There may be similarities with the method of swimming used by penguins and turtles, which respectively have two and four flipper-like limbs.
In general, plesiosaurians are among the largest marine predators in the fossil record, with even the smallest species around 2 m (6.5 ft) long. The largest pliosaurs were up to 15 m long, while the plesiosaurs grew even longer; Mauisaurus was 20 meters long. However, the late Triassic ichthyosaurs, such as shastasaurids, are known to have reached 21 m in length, and modern large marine animals, such as the sperm whale (20 m), and especially the blue whale (~30 m) are much larger than any currently known plesiosaurians.
The anteriorly placed internal nostrils have palatal grooves to channel water, the flow of which would be maintained by hydrodynamic pressure over the posteriorly placed external nares during locomotion. During its passage through the nasal ducts, the water would have been 'tasted' by olfactory epithelia.
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