Nichollssaura borealis (Druckenmiller and Russell 2008; Early Cretaceous, 110mya) is a traditional relative of Leptocleidus, but here nests between Plesiosaurus + elasmosaurs and Plesiopleurodon + pliosaurs.

Plesiopleurodon wellesi (Carpenter 1996; 71cm skull length; NHMUK R4058; Late Cretaceous) is a traditional enigma, here nesting at the base of the pliosaurids, derived from Nichollsaura. Note the elongate snout and longer premaxilla. The prefrontals contact the premaxilla, separating the nasals from the frontals.

Carpenter K 1996. A review of short-necked plesiosaurs of the Western Interior, North America. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 201(2):259-287.

Druckenmiller PS and Russell AP 2008. Skeletal anatomy of an exceptionally complete specimen of a new genus of plesiosaur from the Early Cretaceous (Early Albian) of northeastern Alberta, Canada. Palaeontographica Abteilung A. 283: 1–33.

wiki/Plesiopleurodon
wiki/Nichollssaura