EVOLUTION OF TITANOSAURID SAUROPODS. II: THE CRANIAL EVIDENCE

Abstract

"The titanosaurid skull is interpreted as Camarasaurus-Iike, but with ""peg-like"" teeth restricted to the extremity of the jaws, which exhibit wear facets sharply inclined with respect to the labio-lingual axis. The last character is shared with Brachiosaurus Riggs and Pleurocoelus Marsh, and it is considered a probable synapomorphy of Titanosauriformes. Several cranial characters are considered synapomorphies of Titanosauria or a less inclusive group (long recurved paraoccipital processes, becoming slender downwards; reduced, narrow supratemporal fenestra), or synapomorphies within Titanosauridae (""peg-Iike"" teeth; teeth restricted ro the anterior region of the snout and mandibular symphysis perpendicular to the long axis of the lower jaw). Several characters, such as ""peg-like"" teeth restricted to the anterior region of the snout, wear facets sharply inclined with respect to the labiolingual tooth axis, and mandibular symphysis perpendicular to the long axis of the lower jaw, suggest that Nemegtosaurus mongoliensis Nowinski and Quaesitosaurus orientalis Kurzanov and Banikov are related to the Titanosauridae. ""Pleurocoelus"" sp., from the Lower Cretaceous of Texas and Utah, is considered a basal titanosaur by having procoelous anterior caudals, teeth with an intermediate morphology between Brachiosaurus and titanosaurids, and dorsal verterbrae with centro-parapophyseallamina and ventrally widened, sligthly forked infradiapophyseal Iarnina. Basal titanosaurs and other titanosaur-related sauropods had a wide distribution during the Early Cretaceous. The hypothesis that Alamosaurus sanjuanensis Gilmore is a Late Cretaceous inmigrant from South America is consistent with its phylogenetic position. The Saltasaurinae, in turn, represent an endemic group of South America. "