Ursidae (Mammalia, Carnivora) from the Quaternary of western Rio Grande do Sul, Brazil: Taxonomical, stratigraphic, and chronological aspects

Abstract

All known Pleistocene South American fossil ursids belong to Arctotherium, an extinct genus of tremarctine bears. Its occurrence encompasses many countries in Central and South America, with findings mainly from Argentina. In Brazil, the genus is recorded in eight states, highlighting Rio Grande do Sul due to its provenance from fluvial deposits. In contrast, all other Brazilian records come from caves or fossil tanks. The Arctotherium material from Rio Grande do Sul comes from the Passo do Juquiry site, located on the banks of the Quaraí River, which marks the frontier between Brazil and Uruguay. The fossils are MCN-PV 1940/A-K, a set of postcranial bones: first to sixth lumbar vertebrae, sacrum, left scapula, left humerus, right portion of the pelvic girdle, and left femur. The anatomic description of these bones confirmed their belonging to Arctotherium sp. due to their size, the form of the vertebral processes, the presence of the entepicondylar foramen of the humerus, and the morphology of the trochlea and capitulum. Body mass estimates resulted in values mostly above 400 kg. The first absolute age from Passo do Juquiry indicated 40,000 years BP. The stratigraphic framework of the outcrop conformed to a floodplain fines facies association, a succession of mudstones with carbonate nodules on top. This facies association has lithological correlations to the Brazilian Touro Passo Formation, the Uruguayan Sopas Formation, and the Argentinean Toropí/Yupoí Formation. While having similar features, the ages of the lithologically correlated facies associations do not always line up, indicating that the evolution of the Uruguay River basin throughout the Late Pleistocene followed a pattern, but not synchronously. The presence of carbonate levels suggests that the climate during the time of deposition was drier than the present one, with the carbonate-less middle layer representing a more humid period.