THE BASAL TITANOSAURIAN RUKWATITAN BISEPULTUS (DINOSAURIA, SAUROPODA) FROM THE MIDDLE CRETACEOUS GALULA FORMATION, RUKWA RIFT BASIN, SOUTHWESTERN TANZANIA

摘要

Whereas titanosaurians represent the most diverse and cosmopolitan clade of Cretaceous sauropod dinosaurs, they remain rare components of Cretaceous African faunas. Currently recognized continental African titanosaurians include Aegyptosaurus baharijensis and Paralititan stromeri from early Upper Cretaceous deposits near Bahariya Oasis, Egypt, and Malawisaurus dixeyi and Karongasaurus gittelmani from the Lower Cretaceous (~Aptian) Dinosaur Beds of Malawi, in addition to several undesignated and fragmentary forms across the continent. Here, we describe a new titanosaurian taxon, Rukwatitan bisepultus, on the basis of a partial, semiarticulated postcranial skeleton recovered from the middle Cretaceous Galula Formation in southwestern Tanzania. Unique to Rukwatitan are carotid processes on posterior cervical vertebrae, a deep coracobrachialis fossa and subquadrangular cross-section of the humerus, and a slender, curved, teardrop-shaped pubic peduncle on the ilium. Parsimony and Bayesian phylogenetic analyses of 35 sauropod taxa congruently place Rukwatitan as a non-lithostrotian titanosaurian, a relationship supported by cervical vertebrae with undivided pleurocoels and strongly procoelous anterior caudal vertebrae. Rukwatitan differs from the potentially penecontemporaneous and geographically proximate Malawisaurus by exhibiting weakly developed chevron articulations and posteriorly inclined neural spines on the middle caudal vertebrae, a proximally robust and distally unexpanded humerus, and an anteroventrally elongated coracoid. Similar to biogeographic patterns identified in certain crocodyliform clades (e.g., small-bodied notosuchians), titanosaurians on continental Africa appear to exhibit a regional (e.g., southern versus northern Africa), rather than a continental- or supercontinental-level signal.