Evolution of the caudal vertebral series in macronarian sauropod dinosaurs

Macronaria is one of the most abundant and most widely accepted phylogenetic sauropod group. One of the most common preserved parts are the caudal vertebrae which displayed enough morphological diversity in order to infer the morphology, phylogenetic, morphofunctional and paleobiological aspects of that group of sauropods. I analyzed all these aspects by performing a morpho-evolutionary and phylogenetic study, based on caudal vertebrae of macronarian sauropods. This comparative anatomical study of macronarian caudal series documented evolutionary caudal trends within the clade in greater detail. Titanosauria is the most abundant and taxonomic diverse group within Macronaria. An example of this is the putative new taxon, MPM-PV 1156 from the Pari Aike Formation, Upper Cretaceous, Patagonia. This new specimen, among other parts, includes a partially articulated caudal sequence as well as several related and isolated caudal elements. Because MPM-PV 1156, represents one of the large titanosaurian and the caudal section is the best represented within large titanosaurids, the complete analysis of these caudal elements enhanced our understanding of the caudal morphological diversity of this sauropod group. Thus, osteological caudal undocumented characters were determined. The well known caudal sequences of Andesaurus delgadoi, Epachthosaurus sciuttoi, MPM PV 1156 and Neuquensaurus australis represent excellent taxa for the reconstruction of soft tissue and their implications for the morphology, phylogeny and morpho-functional patterns of titanosaurids. All these aspects were analyzed and inferred for these particular sauropods as well as for Titanosauria as a whole. Therefore, based on the soft tissue implications, three morpho-type were proposed and phylogenetic and morpho-functional patterns were inferred and proposed for Titanosauria. Furthermore, new and previously discovered sauropod dinosaur materials from the Upper Cretaceous Bajo Barreal Formation, central Patagonia, were described. The remains consist of associated and isolated axial skeletal elements recovered from three separate localities, and are herein assigned to the morphologically aberrant basal diplodocoid clade Rebbachisauridae Furthermore, the Bajo Barreal fossils constitute the southernmost known occurrences of Diplodocoidea. We propose that, at least in southern South America, a reduction in land area resulting from early Late Cretaceous paleogeographic changes was a causal factor in the extinction of Rebbachisauridae and therefore of Diplodocoidea.