linguistic:translation, structural:summarization
Relationships

In modern phylogeny, clades are defined groups that contain all species belonging to a certain branch of the evolutionary tree. One way to define a clade is to let it consist of the last common ancestor of two such species and all its descendants. Such a clade is called a " node clade ". In 2008, Patrick Druckenmiller and Anthony Russell in this way defined Plesiosauria as the group consisting of the last common ancestor of Plesiosaurus dolichocheirus and Peloneustes philarchus and all its descendants. Plesiosaurus and Peloneustes represented the main subgroups of the Plesiosauroidea and the Pliosauroidea and were chosen for historical reasons; any other species from these groups would have sufficed Plesiosaurs were related to the earlier Nothosauridae, that had a more crocodile-like body. The flipper arrangement is unusual for aquatic animals in that probably all four limbs were used to propel the animal through the water by up-and-down movements.

Another way to define a clade is to let it consist of all species more closely related to a certain species that one in any case wishes to include in the clade than to another species that one to the contrary desires to exclude. Such a clade is called a " stem clade ". Such a definition has the advantage that it is easier to include all species with a certain morphology. Plesiosauria was in 2010 by Hillary Ketchum and Roger Benson defined as such a stem-based taxon: "all taxa more closely related to Plesiosaurus dolichodeirus and Pliosaurus brachydeirus than to Augustasaurus hagdorni ". Ketchum and Benson (2010) also coined a new clade Neoplesiosauria, a node-based taxon that was defined by as " Plesiosaurus dolichodeirus, Pliosaurus brachydeirus, their most recent common ancestor and all of its descendants". The clade Neoplesiosauria very likely is materially identical to Plesiosauria sensu Druckenmiller & Russell, thus would designate exactly the same species, and the term was meant to be a replacement of this concept.

Benson et al. (2012) found the traditional Pliosauroidea to be paraphyletic in relation to Plesiosauroidea. Rhomaleosauridae was found to be outside Neoplesiosauria, but still within Plesiosauria. The early Carnian pistosaur Bobosaurus was found to be one step more advanced than Augustasaurus in relation to the Plesiosauria and therefore it represented by definition the basalmost known plesiosaur. This analysis focused on basal plesiosaurs and therefore only one derived pliosaurid and one cryptoclidian were included, while elasmosaurids were not included at all. A more detailed analysis published by both Benson and Druckenmiller in 2014 was not able to resolve the relationships among the lineages at the base of Plesiosauria.

The following cladogram follows an analysis by Benson & Druckenmiller (2014).