José Luis Carballido’s research while affiliated with National Scientific and Technical Research Council and other places

The Upper Cretaceous fossil vertebrate fauna of the Bajo de la Carpa Formation (Santonian), derived from several different locations in the Neuquén Basin, is relatively abundant and diverse, comprising mainly small to medium-sized reptiles, birds and fishes. The Cerro Overo – La Invernada locality (CO – LI), in the north of the basin, records a similar faunal component for that age, but stands out for its relatively greater abundance and diversity of titanosaur sauropods. The study of the materials presented here allows us to establish the presence of specimens of sauropods other than Overosaurus, the only titanosaur recorded so far in the area, the coexistence of small–medium sized forms with larger forms, which in turn represent some forms with more basal characteristics and others with more derived features. The abundance of the record of sauropods in CO – LI shows both morphological and evolutionary variations, so the discovery of more complete specimens will allow paleofaunistic, paleobiological, and paleoecological studies to be carried out. This will provide a better understanding of the role of titanosaurian sauropods in Late Cretaceous ecosystems.

Rinconsaurus caudamirus, from the Bajo de la Carpa Formation (Santonian), Río Negro, Argentina, is represented by several axial and appendicular elements from, at least, four specimens. The axial skeleton of this taxon was described in detail in a recent contribution, pending a complete analysis and description of its appendicular skeleton. This contribution focuses on the description of the appendicular skeleton of Rinconsaurus and its phylogenetic relationships, considering the new information provided here. Rinconsaurus clearly differs from other titanosaurs for the presence of several autapomorphic characters and for a unique association of characters, some of which are also present in lognkosaurians, aeolosaurines and saltasaurines titanosaurs, and by having a scapula with a scapular blade angled 65° with respect to the coracoid articulation, similar to that of Bellusaurus, Dreadnoughtus and Muyelensaurus. Equations for estimating body mass in sauropods based on long bone circumference suggest a body mass of at least 3-5 tonnes for the largest individuals of Rinconsaurus, being lighter than saltasaurines, but heavier than aeolosaurines. Rinconsaurus was incorporated into an expanded version of a phylogenetic data matrix along with several ontemporary South American titanosaurs. The resulting data matrix comprises 102 taxa scored for 431 characters, and our phylogenetic analysis retrieves Rinconsaurus as a member of the clade Rinconsauria. For its part, the clade Rinconsauria, in which Aeolosaurini is nested, is recovered within a diverse Lognkosauria. When the resulting trees are time calibrated and taking into account the position of Ninjatitan within Rinconsauria, there results that the possible origin of the clades Lognkosauria and Rinconsauria (among other titanosaur clades) could have occurred towards the beginning of the Early Cretaceous.