July 10 , 2016

Anatomical variation in the sacrum is useful for differentiating sauropod lineages. Morphological variation in the sacrum has been underestimated mainly because of its anatomical complexity and uncertain homologies with presacral and postsacral elements. We describe a titanosaurian sauropod pelvis (MLP 46-VIII-21-2) from Plottier (Neuquén Province, Argentina) collected in the 1940s from Coniacian–Santonian (Upper Cretaceous) strata. The degree of bone fusion in the specimen (fused vertebral centra forming a single rod, with neural arches fused to the corresponding centrum and to adjacent neural arches) indicates a late ontogenetic age. The presence of a space between the first vertebra and the anterior margin of the illium, together with a scar located on the anterior part of the preactabular process, suggest the possible presence of a sixth sacral vertebra (putative dorsosacral additional characters present in the ilium point towards affinities with Titanosauria. The completeness and good preservation of the specimen allowed us to track features along the sacral series and to compare characters with other sauropods. Within a phylogenetic context, and based on the pattern present in basal sauropodomorphs, the presence of three sacral elements attached to the rim of the acetabulum in eusauropods opens the possibility for considering such elements as primordial sacrals.

Florencia S. Filippini, Alejandro Otero & Zulma Gasparini (2016)
The phylogenetic relevance of the sacrum among macronarian sauropods: insights from a pelvis from the Upper Cretaceous of Patagonia, Argentina.
Alcheringa 41 (advance online publication)
DOI:10.1080/03115518.2016.1180806