An unusual titanosaur axis from the Upper Cretaceous of Brazil and its significance for sauropod anatomy and systematics.

The Upper Cretaceous São José do Rio Preto Formation (Bauru Group, southeastern Brazil) has yielded a fragmentary but taxonomically diverse record of titanosaur sauropods, although elements from cervical series remain scarce. Here, we describe a nearly complete sauropod axis from the Vila Ventura Paleontological Area, representing an uncommon occurrence within this unit and the Bauru Group as a whole. The Vila Ventura axis exhibits a broad axial intercentrum with paired ventral fossae, an elongated, goblet-shaped axial pleurocentrum that is longer than high, and a tall neural arch. Microcomputed tomography imaging reveals extensive camellate-type internal pneumaticity, including radial and circumferential camellae in the pleurocentrum with incipient development of internal bony plates-traits that are previously found in highly nested titanosaurs, such as saltasaurines. Comparative osteological analysis, however, indicates closer affinities with colossosaurians (e.g., Bonitasaura, Futalognkosaurus, Pitekunsaurus), particularly in centrum proportions and laminar configuration, including an enlarged odontoid, a V-shaped ventral median keel with ventrolateral excavations in the pleurocentrum, a stout prespinal lamina terminating in an anterior spinal projection, and well-developed pneumatopores in the spinodiapophyseal fossae. Additionally, the specimen exhibits unique features, such as a stranded and laterally expanded spinoprezygapophyseal lamina and a transversely broad anterior spinal projection. An evaluation of axis morphology highlights the underrepresentation of phylogenetic characters from this element in current datasets, despite its potential diagnostic value for assessing lower-level relationships within Titanosauria. This discovery increases the diversity of Brazilian titanosaur records and emphasizes the importance of axial elements in understanding sauropod functional morphology and phylogeny.