Elasmosaurs (Sauropterygia, Plesiosauria) from the Santa Marta Formation (Santonian-lower Campanian), James Ross Island, Antarctica

JOSE PATRICIO OGORMAN; RODOLFO A. CORIA; LEONARDO SALGADO; ARIANA PAULINA CARABAJAL; IGNACIO A. CERDA; JUAN J. MOLY

General Roca, Río Negro.

Jornada; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina?, Noviembre de 2012; 2012

APA

The Elasmosauridae, one of the most diverse group of Late Cretaceous plesiosaurs, are characterized by their long necks with more than 46 cervical vertebrae. Two patterns of neck elongation have been identified among these marine reptiles. The ?elongate? group formed by Elasmosaurus Cope, 1869 and Styxosaurus Welles, 1943 is characterized by an average VLI (Vertebral Length Index) higher than 125, in contrast with the ?not elongate? group (Hydralmosaurus, Welles, 1943; Thalassomedon Welles, 1943). Interestingly, the ?elongate group? appears to have been recorded only during the Santonian-Campanian interval, and are restricted to the Western Interior Sea, USA. Until now, this biogeographical pattern has been tested in only one elasmosaur from that age in the Southern Hemisphere (Mauisaurus Hector, 1874). Here we report an elasmosaur specimen (MLP 11-II-20-6) collected from the lower Campanian section of the Santa Marta Formation (Beta Member), James Ross Island, Antarctica. This specimen is the first articulated elasmosaur collected in this formation where the Elasmosauridae were previously represented only by isolated vertebrae. The new specimen allows testing the biogeographical pattern above mentioned. Only the mid to posterior section of the neck section of the MLP 11-II-20-6 has been collected. It consists in a articulated series of eighteen vertebrae. The fusion between neural arches, cervical ribs and vertebral centra suggests an adult ontogenic stage. The mean VLI of this cervical section (108.37) and the standard deviation (4.82) indicate that this specimen belongs to the ?non elongate? group, supporting the inferences about the restricted geographical distribution of the ?elongate? group.