Palaeohistological characterisation of the caudal pneumaticity of Rocasaurus muniozi (Sauropoda: Titanosauria)

Abstract

Saltasaurini is a derived clade of titanosaur sauropods whose members are characterised by their relatively small size, dermal ossicles, and a high development of postcranial pneumaticity. Such pneumaticity is generally recognised by osteological correlates, although recently it has been proposed that it can be identified at the histological level through the presence of delicate extrinsic fibres known as pneumosteal tissue, or pneumosteum. This study focuses on the histological correlates of caudal pneumaticity in vertebrae assigned to Rocasaurus muniozi, a saltasaurine from the Late Cretaceous of the Allen Formation of Argentina. Particularly, R. muniozi is one of the sauropod dinosaurs that exhibits the highest degree of pneumaticity in the postcranial skeleton. We perform osteohistological thin sections on six caudal vertebrae. Several occurrences of extrinsic fibres are recorded here, including a possible record of pneumosteal tissue. The latter was recorded in poorly preserved parts of bony trabeculae. Putative pneumosteal tissue is absent in most of the sampled (and well preserved) areas associated with invasion of pneumatic diverticula (e.g. camellae). This contribution reveals that, although endosteal extrinsic fibres can be recognised within pneumatized bones and identified as possible pneumosteonal tissue, this is a rather uncommon feature in Rocasaurus muniozi and possibly other sauropods.