The influence of caudofemoral musculature on the titanosaurian (Saurischia: Sauropoda) tail skeleton: morphological and phylogenetic implications

Abstract

Due to their abundance, taxonomic and morphological diversity, wide range of body sizes and broad geographical distribution, titanosaurian sauropods were one of the most important Cretaceous herbivorous dinosaur groups. Consequently, titanosaurs constitute one of the best samples in which to evaluate the relationship between bony structures and unpreserved soft-tissues within Sauropoda. We reconstruct the morphology and interpret the implications of selected soft-tissues associated with the titanosaurian caudal skeleton. These tissues, especially the M. caudofemoralis longus (CFL), exerted a considerable influence on the anatomy of the caudal vertebrae and haemal arches. In all studied titanosaurian taxa, the reconstructed caudofemoral musculature corresponds to one of three principal morphotypes that accord with previously recognised phylogenetic patterns within the clade. Basal titanosaurians had an elongate M. CFL that extended for much of the proximal half of the tail; in saltasaurines, this muscle was much shorter. Non-saltasaurine lithostrotians exhibited an intermediate condition. Furthermore, the differing position of the fourth trochanter, and therefore, the insertion of the caudofemoral muscles, among various titanosaurian taxa suggests distinctions in the locomotor function of these animals.

Keywords::

• Sauropoda
• Titanosauria
• caudal vertebrae
• soft-tissues
• caudofemoral musculature

Acknowledgements

We thank Rubén Martínez and Gabriel Casal (UNPSJB), Ignacio Cerda (MCS), Rodolfo García and Carlos Muñoz (MPCA), and Juan Porfiri and Jorge Calvo (MUC) for granting access to titanosaurian fossils in their care. Likewise, we thank Rubén Martínez and Ignacio Cerda for allowing us to include photographs of specimens of Epachthosaurus and Neuquensaurus, respectively, in Figures 4, 5 and 7. We also thank Jason Poole (Academy of Natural Sciences, Philadelphia, USA) for illustrations in Figures 3 and 6 and for facilitating access to dissection specimens. This work benefitted from discussions with Peter Dodson (University of Pennsylvania, Philadelphia, USA), William Gallagher (Rider University, Lawrenceville, USA) and James Spotila (Drexel University, Philadelphia, USA). Jerry Harris (Dixie State College, St. George, USA) provided access to relevant literature. English translations of several works cited herein are freely available from the Polyglot Paleontologist website (http://www.paleoglot.org). An earlier version of the manuscript benefitted greatly from constructive comments by the editor Gareth Dyke (University of Southampton, Southampton, UK) and reviews by Alejandro Otero (Museo de La Plata, La Plata, Argentina) and two anonymous reviewers. This research was supported by the Office of Graduate Studies at Drexel University (grant to LMI).