Lower Cretaceous plesiosaurs are rare, so Leptocleidus is important because it fills a gap in the fossil record of plesiosaurians. Leptocleidus was once considered to be a late surviving member of the family Rhomaleosauridae but it has recently been reidentified as a close relative of polycotylids. The fossils of all known species of Leptocleidus were preserved in inshore brackish or freshwater deposits. This led Cruickshank (1997) to speculate that the large pliosaurids that appeared during the Middle and Late Jurassic may have outcompeted the rhomaleosaurids in the open ocean and forced them to exploit near-shore niches instead.
The genus Leptocleidus and the type species L. superstes was established by Andrews (1922b) for a substantially complete specimen with a skull from the Weald Clay Formation of Sussex, England. There are presently two other valid species of Leptocleidus: L. capensis, and L. clemai. The genus ‘Peyerus’ was proposed for Leptocleidus capensis by Stromer (1935) but that name is now considered a junior synonym of Leptocleidus (Cruickshank 1997).
