Large paleoburrow diversity and paleoecology in Buenos Aires Plio-Pleistocene vertebrate digging assemblages

TOLEDO, N.; MUÑOZ, N. A.; BARGO M. S.; TAGLIORETTI, M. L.; PÉREZ, L. M.; ZÁRATE, M. A.; VIZCAÍNO S. F.; BOSCAINI, A.; KRAPOVICKAS, V.

Congreso; XVIII Reunión Argentina de Sedimentología - IX Congreso Latinoamericano de Sedimentología; 2023

Upper Cenozoic Sedimentary sequences cropping out along the Atlantic coastal cliffs of Buenos Aires province contain abundant biogenic structures (paleoburrows) of various sizes attributed to burrowing mammals. Recent field studies have focused on paleoburrows from eight localities from proximities of Miramar to the northern area of Mar del Plata, covering the Pliocene to Upper Pleistocene, aiming to elucidate patterns of size variation. The minimum width of each paleoburrow was analyzed as an indicator of its actual transverse diameter. Available allometric equations for extant vertebrates were used to estimate the potential body size of the producers based on this measurement. In the Pliocene levels, small burrows (~15 cm diameter), related to rodents, typotheres, and small armadillos are very abundant, while less abundant medium-sized caves (~90 cm), attributed to large armadillos and pampatheres, are recorded. Conversely, in the late Pleistocene levels, small burrows are very scarce, medium-sized ones are abundant, and there are many giant burrows (over 150 cm in diameter, attributed to ground sloths). The high-level taxonomic composition is similar in both Pliocene and Pleistocene times, including small rodents and typotheres, armadillos of different sizes, pampatheres, and digging ground sloths. This may suggest that large ground sloths did not burrow during the Pliocene although they built galleries during the Pleistocene. Here we discuss hypotheses on the increase of burrowing behaviour by ground sloths throughout the Pleistocene. A fossorial lifestyle has advantages for large-sized herbivores since burrows are microhabitats for resting, hibernating, and rearing offspring. Burrows are more humid and thermostable than open spaces, which is important for low-metabolism herbivorous mammals such as ground sloths and offer protection from predators. Burrowing also enhances access to food sources such as tubers, roots, and soil invertebrates. Disadvantages related to digging extensive burrows include a significant expenditure of energy, limitation to evaporative cooling, reduced visibility, and risk of trapping during floods. The increment of digging behaviour in ground sloths can be discussed considering paleoclimatic and paleoenvironmental differences between Pliocene and Pleistocene environments. Although showing deteriorating conditions after the Pliocene Climate Optimum (~4.4 million years), the climate during most of the Pliocene was still humid and warm. However, climatic conditions became colder and more arid during the last part of the Pliocene and through the Pleistocene. The presence, during the Pleistocene, of large predators such as sabre-toothed cats and bears could have increased the pressure on large herbivores, promoting digging behaviour in those functionally apt for that. In this sense, a fossorial lifestyle could have provided ground sloths protection from cold and dry above-ground conditions, as well as access to additional food resources when vegetation changed due to the expansion of steppe conditions. This wide exploitation of the fossorial niche is unique worldwide, and it could help to explain the coexistence of the diverse megafauna during the Pleistocene.