First tetrapod footprints from the Ñirihuau Formation (middle to upper Miocene), North Patagonian Andes of Argentina

HEREDIA, ARTURO M.; SANTONJA, CAMILA; BENAVENTE, CECILIA; BALLESTEROS PRADA, JOHN M.; BECHIS, FLORENCIA; SURIANO, JULIETA; CARABAJAL, ARIANA P.

Khon Kaen

Congreso; 6th International Paleontological Congress; 2022

The Ñirihuau basin is located between 41° and 43° S at the eastern side of the North Patagonian Andes of Argentina. Its filling consists of a thick sequence of Oligocene to Miocene volcanic rocks (Ventana Formation) and Miocene volcaniclastic, clastic and carbonatic rocks deposited mainly in continental environments (Ñirihuau and Collón Curá Formations), which cover the previous ones (Santonja et al. 2021). The Ñirihuau Formation represents alluvial, fluvial, deltaic, and lacustrine environments with limited marine contribution. U-Pb geochronological data has constrained it mainly to the middle to late Miocene (Bechis et al. 2014; Santonja et al. 2021). Tetrapod footprints were recently found in the middle section of the Ñirihuau Formation (~15–13 Ma, Langhian–Serravallian), in outcrops along Las Bayas creek, Río Negro Province. The footprints are registered in relatively low energy lacustrine margin facies and are concentrated in four levels at the lowermost portion of the section with most of the footprints preserved as trampling at the upper one. Although the footprints are generally poorly preserved and also strongly affected by weathering, at least three footprint morphologies were recognised: subcircular, oval and crescent shaped. The small oval footprints (about 10 cm long) could be produced by protherotherids, medium crescent-shaped footprints (about 10 cm long) by megatherioidea or mylodontid, large subcircular footprints (about 15 cm long) by macrauchenids, toxodontids, or gliptodonts andsmall subcircular footprints (about 5 cm long) by typotherid notoungulates. In Argentina, several records of tetrapod footprints are preserved in lower and upper Miocene fluvial (floodplains or abandoned channels), aeolian, and temporary water bodies or shallow evaporitic lake deposits (Krapovickas et al. 2009). However, footprints preserved in both middle Miocene and lake deposits as in the present study are little known and these footprints from the Ñirihuau Formation, are the first and only evidence of mammal occurrence from this unit.