Primeros resultados filogenéticos de los megatherioidea basales (Xenarthra, Folivora) de la Formación Santa Cruz (Mioceno temprano, Patagonia)

FERNICOLA J. C.; RACCO, A.; VIZCAÍNO, S.F.; BARGO, M. S.; TOLEDO N.; DE IULIIS G.

Ciudad Autónoma de Buenos Aires

Jornada; XXX Jornadas Argentinas de Paleontología de Vertebrados; 2016

The Botucaraí Hill outcrop is an important window to the Brazilian Late Triassic, recording a diverse vertebrate fauna that includes several dinosaur remains. Recently, a small dinosaur tibia was collected at the locality and deposited at the Museu Municipal Aristides Carlos Rodrigues (MMACR), at the municipality of Candelária, Rio Grande do Sul state. Specimen MMACR-PV-028-T is particularly notable for its small size (110 cm in proximodistal length). Anatomical comparisons and phylogenetical analyses support a sauropodomorph affinity for the specimen, which shows a subquadrangular distal tibia with marked corners observed in distal view, a condition reminiscent of early dinosaurs. Indeed, the distal tibia is wider lateromedially than craniocaudally, in a rectangular shape that resembles sauropodomorphs like Plateosaurus. Notwithstanding, the fact that the specimen is represented by a single bone precludes a more precise taxonomic assignation than a sauropodomorphs affinity. Histological analysis on diaphyseal transverse sections of the material revealed a very thin cortex (~17% of total midshaft diameter). It is characterized by a woven bone matrix with high vascularization and the complete absence of lines of arrested growth (LAGs). These features suggest that the specimen was undergoing rapid and uninterrupted bone growth prior to death. Additionally, the small size of the specimen, the poor degree of ossification at the extremities, the absence of LAGs and the absence of an outer circumferential layer indicate that the specimen represents a juvenile individual, a record not yet known for the Brazilian Late Triassic dinosaur fauna.