Paleoecology of a Neogene footprint assemblage in anastomosed fluvial deposits, Toro Negro Formation, Western Argentina

KRAPOVICKAS, VERONICA; MARSICANO, CLAUDIA; MANGANO, MARIA GABRIELA

Cracovia, Polonia

Congreso; Second international Congress of Ichnology,; 2008

The Toro Negro Formation (Upper Miocene) crops out in La Rioja province of western Argentina. Fragmentary mammalian and avian body fossils are known from this stratigraphic unit. The tetrapod footprint assemblage recorded in this unit suggests the presence of groups that were not documented by body fossils, providing additional information of an Upper Miocene mid-latitude terrestrial paleocommunity. This footprint assemblage is particularly diverse, showing a wide variety of tracks and trackways of members of South American faunas, commonly unrecorded. Trackmakers include several birds (charadrids, perching birds, and relatively large cursorial birds) and mammals (notoungulates, macrauchenids, and tardigrads). In general, there is a wide range in the size of both avian and mammalian tracks, which indicates no preservational bias with regard to the dimensions of the trackmakers. Avian footprints are the most abundant, particularly those assigned to Passeriformes, whereas those produced by notoungulates are the most recurrent within mammalian tracks. In both cases, the smallest producers are the best represented. This is most likely due to the size of the exposed track-bearing surface. In any case, the relative abundance of each type of footprint is interpreted herein as a measure of the activity of the producers and not as an indicator of the number of individuals involved. The track-bearing bed corresponds to inter-channel deposits in an anastomosed fluvial system under a semi-arid climatic regime. This type of depositional system has a high preservation potential for tracks due to the seasonal flooding events, which rapidly bury the footprints. In this context, the variable quality of footprint preservation (mostly controlled by substrate consistency) and the presence of some trampling surfaces do not indicate long periods of exposure. Such 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 confident sample of the original megafauna that inhabited western Argentina during the Late Miocene.