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.