adopted a similar classification in 1961. Welles took issue with White's classification in his 1943 revision of plesiosaurs, noting that White's characteristics are influenced by both preservation and ontogeny. He divided plesiosaurs into two superfamilies, the Plesiosauroidea and Pliosauroidea, based on neck length, head size, ischium length, and the slenderness of the humerus and femur (the propodialia). Each superfamily was further subdivided by the number of heads on the ribs, and the proportions of the epipodialia. Thus, elasmosaurids had long necks, small heads, short ischia, stocky propodialia, single-headed ribs, and short epipodialia. Pierre deSaint-Seine in 1955 and Alfred Romer in 1956 both adopted Welles' classification. In 1962 Welles further subdivided elasmosaurids based on whether they possessed pelvic bars formed from the fusion of the ischia, with Elasmosaurus and Brancasaurus being united in the subfamily Elasmosaurinae by their sharing of completely closed pelvic bars. Persson, however, considered Welles' classification too simplistic, noting in 1963 that it would, in his opinion, erroneously assign Cryptoclidus, Muraenosaurus, Picrocleidus, and Tricleidus to the Elasmosauridae. Persson refined the Elasmosauridae to include traits such as the crests on the sides of the neck vertebrae; the hatchet-shaped neck ribs at the front of the neck; the fused clavicles; the separation of the coracoids at the rear; and the rounded, plate-like pubis. He also retained the Cimoliasauridae as separate from the Elasmosauridae, and suggested, based on comparisons of vertebral lengths, that they diverged from the Plesiosauridae in the Late Jurassic or Early Cretaceous. However, D.S. Brown noted in 1981 that the variability of neck length in plesiosaurs made Persson's argument unfeasible, and moved the aforementioned genera back into the Elasmosauridae; he similarly criticized Welles' subdivision of elasmosaurids based on the pelvic bar. Brown's diagnosis of elasmosaurids included the presence of five premaxillary teeth; the ornamentation of teeth by longitudinal ridges; the presence of grooves surrounding the occipital condyles; and the broad-bodied scapulae meeting at the midline. Modern phylogenetic context Carpenter's 1997 phylogenetic analysis of plesiosaurs challenged the traditional subdivision of plesiosaurs based on neck length. He found that Libonectes and Dolichorhynchops shared characteristics such as an opening on the palate for the vomeronasal organ, the plate-like expansions of the pterygoid bones, and the loss of the pineal foramen on the top of the skull, differing from the pliosaurs. While polycotylids had previously been part of the Pliosauroidea, Carpenter moved polycotylids to become the sister group of the elasmosaurids based