Aristonectes

Aristonectes (meaning 'best swimmer') is an extinct genus of plesiosaur from the Late Cretaceous Paso del Sapo Formation of what is now Argentina, the Quiriquina Formation of Chile and the Lopez de Bertodano Formation of Antarctica. The type species is "Aristonectes parvidens", first named by Cabrera in 1941.
Classification.
"Aristonectes" was classified variously since its original 1941 description, but a 2003 review of plesiosaurs from Patagonia conducted by Gasparini "et al." (2003) found that "Aristonectes" was most closely related to elasmosaurid plesiosaurs like "Elasmosaurus". The authors also considered "Morturneria" a junior synonym of "Aristonectes" because the former's holotype has unfused neural arches of the vertebrae indicative of juvenile status. Subsequent study, however, revalidated "Morturneria" based on non-ontogenetic differences from "Aristonectes".
"Aristonectes" was placed within its own family, Aristonectidae, along with "Tatenectes", "Kaiwhekea", and "Kimmerosaurus", by O'Keefe and Street (2009), as sister family of the polycotylid cryptoclidoids. However, subsequent studies (Otero "et al.", 2014) returned "Aristonectes" to Elasmosauridae, recovering the genus as a derived elasmosaurid and therefore relegating Aristonectidae to a subfamily of Elasmosauridae, as Aristonectinae.