Large mammal burrows in late Miocene calcic paleosols from central Argentina: palaeoenvironment, taphonomy and producers

Large cylindrical structures with a dominantly laminated fill occurring in the loess-paleosol sequence of the late Miocene Cerro Azul Formation (central Argentina) are interpreted as mammal burrows. A total of 115 burrow fills from three localities were measured. The fills typically are shallowly dipping, subcylindrical, unbranched structures with a rounded end and lacking enlargements. The horizontal diameter of the burrows range between 0.15 and 1.50 m, with most of the burrows in the interval of 0.39 to 0.98 m. Geometric morphometric analysis of transverse cross-sections support the distinction of subcircular and elliptical (horizontally flattened) fills. Burrows fills are typically laminated in the lower part and massive in the upper part. The laminated intervals reflect pulses of aqueous currents entering the abandoned burrow during moderate rains, whereas massive intervals reflect en masse input of sediment-water mixtures during heavy rains that produced sheet floods. About 1% of the burrow fills contained fragmentary, disarticulated and weathered mammal bones that were introduced in the open burrow by currents along with other sedimentary particles. Analysis of the tetrapod burrow fossil record suggests that Miocene burrows, including those studied herein; reflect a remarkable increase in the average size of the fossorial fauna. We conclude that large late Miocene mammals dug burrows essentially as a shelter against environmental extremes and also to escape predation. Several mammal groups acquired fossorial habits in response to cold and seasonally dry climatic conditions that prevailed during the late Miocene in southern South America. The ample range of horizontal diameters of the studied burrow fills can be attributed to a variety of producers, including dasypodids, the notoungulate Paedotherium minor , Glyptodontidae and Proscelidodon sp. The simple architecture of the burrows suggests that the producers essentially foraged aboveground.