Paleobiology and paleoecology of an arid-semiarid Miocene South American ichnofauna in anastomosed fluvial deposits

Abstract

Miocene deposits of the Toro Negro Formation in La Rioja province, Argentina, host a rich vertebrate and invertebrate ichnofauna. Trace fossils are recorded from the lower part of the Lower Member of the Toro Negro Formation at Quebrada de La Troya. This succession consists of sandstone, intraformational breccia, mudstone and few conglomerate deposited in sandy anastomosingfluvial systems developed under a semi-arid climate. The invertebrate ichnofauna is composed of meniscate trace fossils (Taenidium barretti, Scoyenia gracilis), dwelling structures (Palaeophycus tubularis) and grazing trails (Helminthopsis hieroglyphica). The vertebrate ichnofauna includes avian (Fuscinapeda sirin, incumbent footprint, slender anisodactyl footprint) and mammalian footprints (Macrauchenichnus rector, cf. Venatoripes riojanus, small heteropod footprint, kidney-like footprints, and oval impressions). Tetrapod footprints and rare Palaeophycus and Helminthopsis are preserved on the top of exposed sandbars. Crevasse splay deposits preserve abundant meniscate trace fossils and dwelling tubes (e.g. Taenidium, Scoyenia and Palaeophycus) ascribed to the Scoyenia ichnofacies. Two suites of invertebrate trace fossils can be differentiated: meniscate backfill structures (T. barretti) and other burrows (P. tubularis) without ornamentation, developed in a soft substrate, and striated traces (S. gracilis), that crosscut the former, developed in a firmer substrate. The Toro Negro depositional system displays a high preservation potential for tracks due to seasonal flooding events, which rapidly buried the footprints. The favorable preservational conditions and good exposure of the track-bearing surface enable us to interpret the track assemblage of the Toro Negro Formation as a sample of the original tetrapod fauna that inhabited that region at central-western Argentina during the Miocene. The identification of possible trackmakers is utilized in this work as a useful tool that contributes to our understanding of the ancient environment. The paleoichnocommunity is characterized by the number of trackmakers and the relative abundance of each type of footprints. A new method to measure footprint abundance is proposed; the relative bioturbed area (RBA). This measurement provides numerical characterization of the degree of bioturbation of the whole tracking surface caused by each ichnotaxa. The tetrapod fauna indicated by the footprint record was compared with the body fossil record of the Toro Negro Formation. These records overlap in some ways, and in other ways they are complementary. The presence of large and medium to small ground sloth, proterotheriid litopterns, caviid rodents, and birds is pointed out by both body fossils and ichnofossils. Body fossils include remains attributed to dasypodids, glyptodontids, and abrocomid rodents. Ichnofossils indicate the presence of macraucheniid litopterns and three types of birds: shorebirds, perching birds, and large cursorial birds.