The atlas-axis complex in the titanosaur Neuquensaurus australis (Dinosauria: Sauropoda)

Saltasaurines are titanosaur sauropods that lived in South America during the Late Cretaceous. Within thisclade, Neuquensaurus australis is a taxon discovered at Patagonia (Argentina) and has been reported sincethe late 19th century. In recent years, new materials (axial and appendicular) have been found, including theatlas and axis. The aims of this study are to describe the anatomy of the atlas and axis of Neuquensaurusaustralis, and compare their anatomy with others titanosaurs in which these elements are known, expandingthe anatomical and palaeobiological knowledge of this taxon. Regarding the anatomy of the atlas, theintercentrum is U-shaped, similar to Rapetosaurus and Quetecsaurus, and also has a subrectangular process atthe proximal end of the intercentrum, a feature that is absent in the other taxa, so it could be anautapomorphy of Neuquensaurus. The neurapophyses are ‘D’ shaped, another possible autapomorphy,since it is only present in Neuquensaurus. Regarding the axis, it presents short vertebral centrum and globoseneural arches. Regarding internal anatomy, the absence of pneumaticity in the atlas, combined with thepresence of a developed pneumatization in the axis, suggests that this is the point where pneumatization ofthe axial postcranium begins, at least in Neuquensaurus.