A postcranial skeleton of an elasmosaurid plesiosaur from the Maastrichtian of central Chile, with comments on the affinities of Late Cretaceous plesiosauroids from the Weddellian Biogeographic Province

An almost complete postcranial skeleton recovered from late Maastrichtian beds of central Chile, which can be confidently referred to the clade Elasmosauridae (Sauropterygia: Plesiosauroidea), is described. The material displays diagnostic characters separately observed in several known Late Cretaceous plesiosaurians from the Southern Hemisphere, particularly from the Weddellian Biogeographic Province. The phylogenetic analysis indicates that the material studied has close affinities with the genus Aristonectes, while supports the position of the Late Cretaceous plesiosaurian Kaiwhekea katiki as well as the inclusion of Aristonectes parvidens (=“Morturneria seymourensis”) in a new clade (Aristonectinae) within the Elasmosauridae. This indicates that cryptoclidians are still restricted to the Middle-Upper Jurassic, and verifies that the family Aristonectidae is polyphyletic. The material studied represents a valuable source for comparison of the morphologic characters present in elasmosaurids from the Weddellian Province towards the end of the Cretaceous, shedding light on the evolution of derived members of this clade.