Significant diachronism in vertebrate burrows: Pleistocene megamammal burrows in Neogene sediments of Central Argentina

This study documents fossil burrows from a new locality of the Upper Miocene-Lower Pliocene Cerro Azul Formation, from La Pampa Province, Central Argentina. This locality is distinguished by the large width of its fossil burrows (up to 2.25 m), in contrast with other tetrapod burrows found in the unit and in the worldwide Miocene record of vertebrate burrows. The 2.15-m-thick local section containing the burrows has a lower palaeosol with alfisol features and an upper calcic palaeosol. The burrows cross-cut carbonate nodules from the upper palaeosol and are easily distinguished by their laminated fill and abundant rhizoliths. Common normal grading in the burrow fill laminae suggest aqueous flows after rains entering the empty excavation and progressively filling it. Preferred occurrence of calcareous rhizoliths in the burrow fills is linked with higher moisture content than the host sediments. Some of the burrows have a horizontal elliptical cross-section which is probably linked to a large mylodontid producer. A single burrow with a subcircular cross section is probably due to roof collapse. We review the criteria for identification of significant diachronism between vertebrate burrows and host sediments and propose a new proxy for identifying potential differences between the age of deposition of the host rock and digging of large Neogene-Quaternary burrows. This proxy is based on the change in horizontal diameter of Cenozoic tetrapod burrows that roughly accompany the body fossil record of Pleistocene megafauna. For the analysed example, the possible time for burrow excavation is Middle to Late Pleistocene.