Glenn W. Storrs

Book

Ancient Marine Reptiles

Academic Press

Jack M. Callaway and Elizabeth L. Nicholls

145-190

Plesiosaurs (Diapsida: Sauropterygia: Plesiosauria) are extinct Mesozoic reptiles comprising one of the most successful and widely distributed groups of marine tetrapods. They developed a world wide range early in their history and some representatives of the clade survived into the Maastrichtian, becoming extinct perhaps only at the terminal Cretaceous. The evolutionary and systematic relationships of the Plesiosauria, however, are almost entirely unknown. The group appears as isolated bones and associated partial skeletons in the Middle Triassic (Anisian) of Germany, but the first unambiguous, fully articulated specimens occur in the uppermost Triassic and Lower Jurassic (Liassic) of England. By Liassic times the plesiosaurs were particularly diverse, already fully marine, and highly modified from the presumed terrestrial condition of their forebears. The adoption of limb-dominated locomotion through two symmetrical sets of hyperphalangic appendages was a unique functional response by the plesiosaurs to a secondary invasion of the sea (Storrs, 1993a). In the Lias also, the group had already begun to exhibit specializations that led to several distinct lineages in the later Mesozoic, although the origins of this lineage diversification are obscure. The Lower Jurassic plesiosaurs, because of their often excellent state of preservation and well constrained stratigraphic positions, currently provide the best potential for elucidation of early plesiosaur diversification and testing of postulated lineage monophyly.

English