David S. Brown

Academic paper

Revue de Paléobiologie

Vol. Sp&eac

9-16

Recent studies of the cheek region of European Middle and Upper Jurassic plesiosauroid genera indicate that two differing configurations are found. In elasmosaurs the jugal is a horizontal and largely surrounded element, whereas in cryptoclidid plesiosaurs it forms a vertical bar separating the orbit from the infratemporal fossa. These characters are described and correlated with characters of the anterior cervical vertebrae. New diagnoses for the families Elasmosauridae and Cryptoclididae are given, and the likely primitive states of key taxonomic characters are discussed. Tricleidus and Colymbosaurus (formerly elasmosaurs) are now classified as cryptoclidid plesiosaurs. Tricleidus is a primitive genus and is included tentatively on implicit evidence for its cheek structure. Colymbosaurus shows parallel evolution with elasmosaurs, having elongated the neck by an increase in number of vertebrae without acquisition of elasmosaurid cervical characters.

Reptilia, Plesiosauria, Elasmosauridae3 Cryptoclididae.

English