The Plesiosauria (Reptilia: Sauropterygia) is a group of Mesozoic marine reptiles known from abundant material, with specimens described from all continents. The group originated very near the Triassic–Jurassic boundary and persisted to the end- Cretaceous mass extinction. This study describes the results of a specimen-based cladistic study of the Plesiosauria, based on examination of 34 taxa scored for 166 morphological characters. The Pliosauroidea is found to by polyphyletic due to the inclusion of the Polycotylidae; this second clade is instead a member of the Plesiosauroidea, and thus more closely related to elasmosaurs than to other ‘pliosaurs’. Characters of body proportion such as neck length and head size are very labile, with the ‘pliosauromorph’ body plan evolving three times, while extremely long necks evolved in two clades. Characters from the entire skeleton support these relationships, although characters of the skull roof and palate are especially useful. Lastly, a new genus and species, Hauffiosaurus zanoni, is named based on German material of Toarcian age.
Frank Robin O’Keefe (2001). A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). ). Acta Zoologica Fennica 213: 1-63.