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.
There are presently three known valid species of Leptocleidus: L. superstes (the type species), 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).






Leptocleidus superstes
Species
L. superstes
Author
Andrews, 1922
Classification
Sauropterygia
Eosauropterygia
Eusauropterygia
Pistosauroidea
Pistosauria
Plesiosauria
Plesiosauroidea
Leptocleididae
Leptocleidus
Age
Barremian, Lower Cretaceous
Type location
Sussex, England
Type specimen
NHMUK R4828, skull and partial skeleton
Referred material
To compile
Leptocleidus capensis
Species
L. capensis
Author
(Andrews, 1911)
Classification
Sauropterygia
Eosauropterygia
Eusauropterygia
Pistosauroidea
Pistosauria
Plesiosauria
Plesiosauroidea
Leptocleididae
Leptocleidus
Age
Uppermost Valanginian, Early Cretaceous
Type location
Picnic Bush site, overlooking Redhouse Farm, Swartkops River Valley, Cape Province, South Africa
Type specimen
SAM-K5822, a complete skull and partial lower jaw
Referred material
None
Leptocleidus clemai
Species
L. clemai
Author
Cruickshank and Long, 1997
Classification
Sauropterygia
Eosauropterygia
Eusauropterygia
Pistosauroidea
Pistosauria
Plesiosauria
Plesiosauroidea
Leptocleididae
Leptocleidus
Age
Barremian, Early Cretaceous
Type location
Near Kalbarri, Carnarvon Basin, Western Australia
Type specimen
WAM 92.8.1-1 to 68, a partial skeleton
Referred material
WAM 94.1.6-1 to 100, a partial skeleton less well preserved than the holotype