Temporal range: Middle Jurassic, Callovian
The head of Cryptoclidus was rather flattened, with eyes facing upward. The skull was broad and light, with jaws lined with about a hundred long, fine teeth, ideal for catching fish and squid. The internal nares were set forward, and the nostrils were relatively small. At up to 8 m (26 ft) long and 8 metric tonnes (7.9 long tons, 8.8 short tons) Cryptoclidus was a medium-sized plesiosaur. 
It had a neck that was up to 2 metres (6.6 ft) long that does not seem to have been very flexible. This probably kept its bulky body away from its small head so as not to alarm potential prey. It had four broad paddle-shaped limbs, with which it either "flew" through the water in wave-like undulating movements, or swam like a porpoise by moving upwards on two flippers and gliding back down again on the other two.
Due to their seal-like body plan, small plesiosaurs such as Cryptoclidus have been depicted as amphibious animals instead of fully marine reptiles. Despite looking clumsy and cumbersome, in water it would have been relatively graceful, using all four limbs as paddles, to swim and hunt its prey. It may have laid eggs in sand, but this is conjectural.
The fragile build of the head and teeth preclude any grappling with prey, and suggest a diet of small, soft-bodied animals such as squid and shoaling fish. Cryptoclidus may have used its long, intermeshing teeth to strain small prey from the water, or perhaps sift through sediment for buried animals.
The size and shape of the nares and nasal openings have led Brown and Cruickshank (1994) to argue that they were used to sample seawater for smells and chemical traces.
Cryptoclidus is a plesiosaur whose specimens include adult and juvenile skeletons, and remains which have been found in various degrees of preservation in England, Northern France, Russia, and South America. Its name, meaning "hidden clavicles", refer to its small, practically invisible clavicles buried in its front limb girdle.
The type species was initially described as Plesiosaurus eurymerus by Phillips (1871). The species name "wide femur" refers to the forelimb, which was mistaken for a hindlimb at the time.
The cladogram below follows the topology from Benson et al. (2012) analysis.
- Brown, David S., and Arthur RI Cruickshank. The skull of the Callovian plesiosaur Cryptoclidus eurymerus, and the sauropterygian cheek. Palaeontology 37.4 (1994): 941.
- Info ABC.net.au. Retrieved February 4, 2011 Archived from the original on July 13, 2012
- Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 75. ISBN 1-84028-152-9.
- Brown and Cruickshank, 1994
- Benson, R. B. J.; Evans, M.; Druckenmiller, P. S. (2012). Lalueza-Fox, Carles, ed. "High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary". PLoS ONE 7 (3): e31838. doi:10.1371/journal.pone.0031838. PMC 3306369. PMID 22438869.