Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina

PÉREZ, MARÍA ENCARNACIÓN; VIZCAÍNO, SERGIO F.; MONSALVO, EDUARDO S.; ARAMENDÍA, INÉS; KAY, RICHARD; VLACHOS, EVANGELOS; BARGO, SUSANA; CUITIÑO, JOSÉ; NORIEGA, JORGE IGNACIO

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Lugar: Santiago de Chile; Año: 2022 vol. 49 p. 382 - 422

0718-7092

The early–middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The basal sector of the CBF consists of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork produced fossil vertebrates from the lower levels of the CBF with an estimated age between ~16.5 Ma and 15.1 Ma (late Burdigalian-early Langhian). The section temporally overlaps with the middle or upper sections of the SCF in the Austral–Magallanes Basin of southern Patagonia, the Río Frias Formations in Chile, and the lower Collón-Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal differences between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías and Collón-Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both trees and open environments for the CBF, but it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.