An historical indeterminate elasmosaurid specimen CAMSM X50356 (CAMSM = Sedgwick Museum) collected during the 19th century is described. The specimen comes from the Cenomanian Cambridge Greensand although the possibility of an Albian–lower Cenomanian age is discussed. CAMSM X50356 is recovered within the Elasmosauridae. Our phylogenetic analysis indicates that elongated cervical centra with absence of the lateral ridge are the basal condition of elasmosaurids. The lateral ridge is present in almost all other elasmosaurids with the exception of some taxa that have secondarily shortened the cervical centra such as the aristonectines and Nakonanectes bradti. The lack of correlation between cervical elongation and the lateral ridge is thus recorded here for the first time as CAMSM X50356 is the only elasmosaurid with cervical centra longer than high but without the lateral ridge.

Key words: Sauropterygia, Elasmosauridae, Cambridge Greensand, Albian, Cenomanian, Cretaceous.

Jose P.O’Gorman [joseogorman@fcnym.unlp.edu.ar; ORCID: https://orcid.org/0000-0001-9279-6314], División Paleontología Vertebrados, Museo de La Plata, Universidad Nacional de La Plata, La Plata, Argentina. CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina. Roger B.J. Benson [rbenson@amnh.org; ORCID: https://orcid.org/0000-0001-8244-6177], American Museum of Natural History, New York, New York, USA. Department of Earth Sciences, University of Oxford, Oxford, UK.