[2] Pelagosaurus was originally described from a specimen from Normandy, but the holotype for P. typus was discovered north of the town of Ilminster in Somerset, England.

[4][5][6][7][8] Its position as a basal metriorhynchid was suggested by Eric Buffetaut in 1980.,[9][10] Phylogenetic analyses from the 1980s, 1990s and early 2000s found Pelagosaurus to be the sister taxon to both Teleosauridae and Metriorhynchidae.

[17][18] Pelagosaurus was well adapted to aquatic life; it had developed a long, streamlined snout, a tail with fin-like attributes and paddle-like limbs for swimming in the warm, shallow waters of its time.

Pelagosaurus had 30 teeth suitable for hunting and grasping fish, crustaceans and insects whilst swimming; indeed, one fossil specimen was found with a Leptolepis— an early teleost fish— in its stomach contents.

Its forward-facing eyes and streamlined body suggest that Pelagosaurus was a pursuit predator, rather than a scavenger or ambush hunter.