Aristonectes parvidens Cabrera, 1941, and Morturneria seymourensis (Chatterjee and Small, 1989), two plesiosaurs from the Maastrichtian of South America and Antarctica whose phylogenetical position is controversial, are reviewed and found to be congeneric and conspecific. Most differences between the two type specimens are interpreted as related to ontogenetic growth: Morturneria is based on an immature, whereas Aristonectes is based on an adult, probably an old individual. Aristonectes exhibits an unique set of characters among Plesiosauria: a low and wide ogival skull, a paired vomero-nasal fenestra, a mandible high anteriorly with a very short and high symphysis, a homodont dentition composed of numerous, strongly outwardly directed and poorly ornamented teeth (dental formula: 10–13 premaxillary, at least 51 maxillary and probably 60–65 dentary teeth, depending upon individual ontogeny). Moreover, Aristonectes shares several synapomorphies with the elasmosaurid clade, mainly strongly binocular-shaped and platycoelous cervical centra with lateral ridges. In contrast to cryptoclidids, it retains some plesiomorphic characters (e.g., horizontal jugal and poorly ventrally excavated cheek, glenoid fossa at about the same level as the alveolar row). The dental morphology and peculiar occlusal pattern, forming an interlocking trap, suggest that Aristonectes strained a diet of small, soft organisms from the water.