A new specimen with skull and vertebrae of Najash rionegrina (Lepidosauria: Ophidia) from the early Late Cretaceous of Patagonia

The limbed snake Najash rionegrina from the Cenomanian (early Late Cretaceous) of the La Buitrera Palaeontological Area (LBPA), northern Patagonia is a key taxon in any study of the origin and early evolution of snakes. The original concept of the taxon was based on the holotype and a number of referred specimens including an isolated partial skull; a conservative rebuttal argued this concept was too broad due to the lack of association between the holotype elements and the referred specimens. Here we describe a new snake specimen consisting of a partial skull and closely associated vertebrae from the La Buitrera Locality, one of many productive snake localities within the LBPA. The analysis of the vertebrae of the new specimen identifies distinct features shared with the vertebrae of the N. rionegrina holotype; the partial skull of the new specimen is also identical to the partial skull that formed part of the original concept of N. rionegrina. Phylogenetic analysis, using data from the type and referred specimens, and new materials described here, reconstructs N. rionegrina as a basal snake in a ‘madtsoiid’ clade outside of crown-group Serpentes. These new morphologies, and autapomorphies recognized in character analyses, permit the construction of a new and expanded diagnosis of N. rionegrina.