New information on the Late Cretaceous Argentinean sauropod dinosaur Mendozasaurus and its implications for basal titanosaur relationships.

ORTIZ DAVID, LEONARDO; GONZÁLEZ RIGA, BERNARDO J.; MANNION, PHILIP D.; POROPAT, STEPHEN F.; CORIA, JUAN PEDRO

Salt Lake City, Utah

Congreso; 76th Annual Meeting for the Society of Vertebrate Paleontology; 2016

Society of Vertebrate Paleontology

The sauropod dinosaur Mendozasaurus neguyelap is represented by several partial skeletons from a single locality within the Coniacian Sierra Barrosa Formation in the south of Mendoza Province, northern Neuquén Basin, Argentina. A detailed revision of Mendozasaurus, including previously undocumented remains, allows us to more firmly establish its position within Titanosauria, as well as enabling an emended diagnosis of this taxon. Two cervical vertebrae, five metacarpals, a complete femur, an astragalus, six metatarsals, three phalanges and one ungual phalanx are the new bones recovered from the holotype site. Characters of the emended diagnosis include: a) middle and posterior cervical vertebrae with tall and transversely expanded neural spines that are wider than the centra, formed laterally by spinodiapophyseal laminae that are not connected with the pre- or postzygapophyses; b) anterior caudal vertebrae (excluding anteriormost) with ventrolateral ridge-like expansion of prezygapophyses; c) humerus with divided lateral distal condyle on anterior surface. Comparative studies of articulated pedes of other taxa allow us to interpret that the pedal formula was 2-2-2-2-0, based on disarticulated bones that form a right hind foot. Mendozasaurus was incorporated into an expanded version of a titanosauriform-focussed phylogenetic data matrix, along with several other contemporaneous South American titanosaurs. The resultant data matrix comprises 75 taxa scored for 403 characters and our analyses recover Mendozasaurus as the sister taxon to Futalognkosaurus, supporting previous analyses that resolve a monophyletic Lognkosauria. Lognkosauria forms a clade with the rinconsaurian Muyelensaurus + (Notocolossus + Pitekunsaurus). Novel to our study, Epachthosaurus is recovered just outside of this grouping. A basal lithostrotian position for this South American clade is well supported, contrasting with some analyses that have placed these taxa outside of Lithostrotia or closer to Saltasauridae. Unlike many previous studies, we find that Aeolosaurus is more closely related to Saltasauridae than to these taxa. Our new data on Mendozasaurus and related forms provides a platform for elucidating the interrelationships of derived titanosaurs.