Mesozoic marine reptiles noneed a range of swimming abilities. Jurassic and Cretaceous ichthyosaurs, with their deep, streamlined body, narrow caudal peduncle, and lunate tail, were probably efficient, fast sustained swimmers. The elongated bodies and long, broad tails of the early ichthyosaurs and mosasaurs (and marine crocodiles) suggest that they were adapted for rapid acceleration and were probably ambush predators. More problematic are the plesiosaurs, which utilized subaqueous flight using two pairs of wing-shaped appendages. The long-necked plesiosauroids were probably slow swimmers and relied on a sneak attack to capture prey. The pliosauroids, with shorter necks, more compact bodies, and larger, broader limbs were probably faster swimmers than the plesiosauroids.

By estimating the total drag and the amount of energy available for locomotion, relative sustained swimming capabilities can be estimated for reptiles of the same mass but different body forms. Calculations suggest that ichthyosaurs were faster than either plesiosaurs or mosasaurs, and plesiosaurs were slightly faster than mosasaurs. If the results are scaled to body length rather than mass, the differences in sustained swimming speeds are even more pronounced. Differences arise from estimates of hydrodynamic efficiency, shape, and metabolic rate for each kind of reptile, the latter being the most important factor.