Neotropical metatherian diversity around the Eocene-Oligocene Transition: the Shapaja section, Peruvian Amazonia.

STUTZ, N.; ABELLO, M. A.; MARIVAUX, L.; BOIVIN, M.; CUSTÓDIO, M.A.; BENITES-PALOMINO, A.; SALAS-GISMONDI, R.; PUJOS, F.; TEJADA-LARA, J.V.; RIBEIRO, A.M.; ANTOINE, P. O.

Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleonológica Argentina

Metatheria, an infraclass of Mammalia whose living representatives are the marsupials, were remarkable components of the history of South America during the Cenozoic, with greater past diversity and abundance than today. One important chapter of this history is the Eocene-Oligocene transition (EOT), known to be a period of considerable changes around the globe, with marked extinctions and drastic climatic processes (transition from the early Cenozoic ?Greenhouse World? to the post-Eocene ?Icehouse World?). Concerning Metatheria, it is considered to be the major turnover in their evolutionary history in South America, an assumption made based on the fossil record of Patagonian Argentina. However, the EOT is scarcely known at the tropical latitudes of South America, as well as other time intervals, since this region is still poorly understood through a paleontological standpoint. This study aims at partly addressing this problem, by reporting preliminary identifications of metatherians from the late Eocene-earliest Oligocene Shapaja Section, near Tarapoto, in Peruvian Amazonia. Nine fossiliferous outcrops of the Pozo Formation were dated by chemostratigraphy and explored through screen-washing (1 and 2 mm meshes), which allowed recovering small-sized fossils (plants, mollusks, decapods, fishes, amphibians, reptiles, and many mammals). Metatherians were found in most outcrops, except for one, and according to their taxonomic composition, three metatherian assemblages could be recognized throughout the section, a pattern also observed at rodent and fish assemblages. The oldest outcrop (early late Eocene) delivered only one new prepidolopid polydolopimorphian. The second assemblage, latest Eocene in age, encompasses scarce and fragmentary remains of another probable prepidolopid, numerous remains of a small argyrolagid, and teeth of palaeothentoid paucituberculatans (two basal forms plus two palaeothentines). These represent the oldest occurrences of Palaeothentinae and Argyrolagidae, and also the northernmost record of the latter family. Then follows a transitional assemblage (earliest Oligocene), with few fragmented metatherian remains, possibly due to taphonomic biases. Finally, the early Oligocene assemblage includes one larger, less abundant argyrolagid and several palaeothentoids (three basal taxa, three palaeothentids and one abderitid). Thus, in the Shapaja section a period of smaller diversity around the EOT could not be observed, but the changes in the taxonomic composition of the assemblages, as well as other geological and paleontological data, point to two biotic turnovers. The first one, in the late Eocene, has probably been driven by regional processes related to the Andean orogeny, which led to a temporary marine influence in the area. The latter biotic turnover, on the other hand, seems to be related to the EOT global processes, namely the great drop in sea level and the onset of drier climates worldwide, with decreased precipitation and increased seasonality. Indeed, fossil plants from Shapaja indicate the occurrence in the area of multi-stratified rainforests during the latest Eocene and more open, deciduous forests in the earliest Oligocene. Finally, the Shapaja section highlights the importance of fieldwork and research efforts in northern South America, to get better correlations with localities at middle and high latitudes and thus, a refined picture of paleobiodiversity in the whole continent.