Plesiosaurus dolichodeirus (De la Beche and Conybeare 1821) Early Jurassic, ~3 m in length, was one of the first prehistoric animals ever described. Derived from a sister to Pistosaurus, Plesiosaurus was a phylogenetic predecessor to various elasmosaurs (see below) and was a sister to Trinacromerum.
Distinct from Pistosaurus, the skull of Plesiosaurus was relatively smaller and wider, without the elongated premaxilla and fewer premaxillary teeth. In this way the skull of Plesiosaurus was more like that of Lariosaurus and Simosaurus. The parietal was wider, especially posteriorly. The vomer was reduced, as in Simosaurus, but with tiny internal and external nares, as in Pistosaurus. The palate developed suborbital fenestra and enlarged the interpterygoid space. The pterygoid did not have posterior processes, so it was more like that in Simosaurus.
The cervical series was elongated by the adding ten more cervicals. The neural spines were more gracile. The dorsal ribs were less variable in length. The caudals supported a soft tissue vertical fin.
The scapula and clavicle created a 180º arc and the clavicle was as robust as the scapula. The coracoid was twice as large. The humerus was enlarged. The radius and ulna were reduced to near rectangles. The ulna was convex posteriorly. The carpals created an interlocking mosaic. The phalanges were shorter and converged distally forming a more gracile flipper.
The pubis was enlarged. The hind limb matched the forelimb changes in morphology, except the tarsals, which did not form a mosaic.