Pachypleurosaurus edwarsi (Carroll and Gaskill 1985) Late Permian ~260 mya, 1 m in length, was derived from a sister to Anarosaurus and phylogenetically preceded Cartorhynchus, Qianxisaurus and the Eusauropterygia, which includes the giant plesiosaurs.
Distinct from Anarosaurus, the reduced skull of Pachypleurosaurus was lower and longer. The increase in elongation occurred in the premaxilla, which extended beyond the naris. The upper temporal fenestra were reduced nearly to the size of the parietal opening. The nares were dorsolaterally oriented at mid rostrum. The frontal had three anterior processes and two posterior processes surrounded by other bones. The quadratojugal was absent.
The cervical count was raised to 18, pushing the presacral count to 37. The sacral count was raised to 4. The chevrons were expanded ventrally.
The pectoral girdle was greatly reduced. The interclavicle underlapped the clavicles, which is not the pattern in other reptiles. The coracoids were reduced from discs to diagonal bars representing the posterior rims of the former discs. The fingers were slender, with metacarpal III the longest. The distal carpals were poorly ossified.
The joints between the pelvis elements were not firmly sutured. The distal tarsals were absent. The unguals were reduced to absent, lowering the phalangeal count.
Pachypleurosaursand ichythyosaurs gave birth without laying eggs. By inference, their last common ancestors, Claudiosaurus and Stereosternum likely did so too. Metatherian and eutherian mammals are the only other archosauromorphs known to produce live birth.
The family tree of the Enaliosauria is here. The complete reptile family tree is here.