The superfamily Plesiosauroidea is one of the two major traditional divisions of plesiosaurs, the group that typically have long necks.
The other major group (superfamily) of plesiosaurs is the Pliosauroidea (often just called pliosaurs).
Tricleidus is a cryptoclidid from the Oxford Clay Formation of the UK. The holotype specimen (NHMUK R 3539) consists of disarticulated elements including most of skull and half the postcranium, from the Kosmoceras jasoni – Peltoceras athleta zones from the lowest deposits of the Oxford Clay Formation.
Thililua has 30 cervical vertebrae. Each cervical has a longitudinal ridge on its lateral surfaces, this is a convergent feature with elasmosaurids. The skull of Thililua is relatively large with a short postorbital region and elongate rostrum.
The holotype of Thalassomedon was discovered by R. L. Landberg in 1939 in Baca County, Colorado. Thalassomedon has a short and deep atlas-axis and 62 cervical vertebrae, and there is no pectoral or pelvic bar in the adult condition (Carpenter 1999).
Plesiosaurus was the first plesiosaur discovered and named (De la Beche and Conybeare 1821). The first 1821 description was based on partial remains so the anatomy of Plesiosaurus – particularly the identity of the isolated bones and how they articulated together – involved some guesswork.
‘Occitanosaurus’ was described by Sciau et al. (1990) and Bardet et al. (1999). The genus name was erected for ‘Plesiosaurus’ tournemirensis by Bardet et al. (1999). The taxon was originally regarded as an early elasmosaurid but is now considered a member of the Microcleididae.
Libonectes was erected for ‘Elasmosaurus morgani’. The pectoral girdle and a forelimb of the holotype are figured (most recently by Welles 1962, fig. 12) but now lost (Carpenter 1999). Libonectes morgani (Welles 1949) is the type and only species.
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.
