Article: The ichthyosaurian tail: sharks do not provide an appropriate analogue

The lunate caudal fin, characteristic of post-Triassic ichthyosaurs, is often interpreted as functioning as a reversed shark's tail, generating vertical as well as horizontal components. The functional morphology of the shark's tail is discussed, and considered far from well understood. Although the ichthyosaurian tail is superficially similar to a reversed heterocercal tail, the two structures are not strictly analogous and there are functional grounds why the ichthyosaurian tail should not generate vertical forces. The selachian tail is therefore unsatisfactory, and alternate models are sought among cetaceans and scombroid fishes.Bone density in ichthyosaurs appears low, as in cetaceans, suggesting they were positively buoyant, at least while close to the surface. Diving may have been initiated by a downward flexing of the body, as Taylor (1987) suggested, and the increasing hydrostatic pressure, by reducing lung volume, would probably soon have eliminated positive buoyancy. The pectoral fins were probably used as inclined planes, adjusting swimming levels. The pelvic fins may have functioned as stabilizers, though their effects may have been marginal because of their small size. That ichthyosaurs evolved a downturned rather than an upturned tail may have been entirely fortuitous.