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Earth Sciences
Introduction
Plesiosauria de Blainville, 1835 is a clade of reptiles that acquired a series of adaptations to the marine environment. Its biochron spans from the Late Triassic to the end of the Cretaceous, and its distribution is cosmopolitan, being recorded from all continents, including Antarctica (Welles 1952, 1962, del Valle et al. 1977, Brown 1981, Carpenter 1999, Kear 2003, O'Keefe 2004, Sato & Wu 2006, Gasparini et al. 2007, Ketchum & Benson 2010).
In South America, Late Cretaceous plesiosaurs have been known since the mid-nineteenth century, recording both Elasmosauridae (including Aristonectes parvidens Cabrera, 1941) and Polycotylidae (Gay 1848, Gasparini et al. 2003a, 2003b, 2007, Salgado et al. 2007, Otero et al. 2010). In turn, members of both families, including A. parvidens (O'Gorman in press and bibliography cited therein), have been recorded in the Upper Cretaceous of the Antarctic Peninsula.
The phylogenetic relationships of A. parvidens are still debated (Gasparini et al. 2003b, O'Keefe & Street 2009, Ketchum & Benson 2010). In some analyses, A. parvidens is placed within the "Aristonectidae", separated from the Elasmosauridae (O'Keefe & Street 2009), whereas others authors find Aristonectes nested within the Elasmosauridae (Gasparini et al. 2003b, Ketchum & Benson 2010). This instability of phylogenetic position is probably due to the particular combination of characters exhibited by A. parvidens and the insufficient knowledge about its postcranial anatomy. Here we follow the systematic relationships hypothesized by Ketchum & Benson (2010) as theirs is the most comprehensive analysis, however, the revision of the systematic relationships of A. parvidens is not the aim of this work. The Upper Cretaceous record of Patagonian and Antarctic plesiosaurs is dominated by postcranial material, specifically isolated vertebrae (Gasparini et al. 2003b, Salgado et al. 2007, O'Gorman et al. 2011). Among the vertebral cervical morphologies recorded in the Upper Cretaceous of South America, there is one characterized by the absence of neuro-central closure and extremely short vertebral centrum, a morphology that has been traditionally associated with the juvenile stage (Brown 1981, O'Keefe & Hiller 2006). In some cases, the articular faces of this type of vertebrae are "dumbbell-shaped", suggesting a relationship with the Elasmosauridae. In spite of the suggested similarity...