Architecture of fossil rodent burrows from a Holocene paleosol of La Pampa, Argentina

CRISTINA CARDONATTO, MARÍA; RICARDO NESTOR MELCHOR; CLAUDIA MONTALVO

Colonia del Sacramento

Simposio; Tercer Simposio Latinoamericano de Icnología (SLIC2015); 2015

Facultad de Ciencias, Universidad de la República

Sandy eolian deposits of late Pleistocene to Holocene age from Cantera de Lorda (36° 40? 50?S; 64° 24? 27?W), Toay, La Pampa province contains abundant fossil vertebrate burrows. The local section comprises late Miocene siltstone of the Cerro Azul Formation overlain by up to 10 m thick well-sorted, fine-grained sand that ends in a poorly developed paleosol, which is covered by 1-4 m thick cross-bedded sand. Megatherium remains found at the base of the sandy section suggest a late Pleistocene age, whereas deposition of the upper part of the sand below the paleosol took place at about 3.8 ± 0.4 ka BP (OSL). The rodent burrows start in the top of the paleosol and are associated with invertebrate trace fossils (Feoichnus, Skolithos, and ant burrows), root traces and fossil remains of mammals found scattered in the sand. The latter includes the marsupial Lestodelphys halli and the rodents Ctenomys sp. (including one articulated specimen) and Eligmodontia sp. Rodent burrow commonly display a massive fill that is darker or coarser grained than the host. The structural plan of the rodent burrow system consist of an up to 50 cm long oblique to vertical entrance tunnel (31?86°), which can be enlarged (diameter: 8.3?10 cm). The entrance tunnel continues with the main gallery that is connected with side tunnels and/or chambers. The main gallery is straight to slightly sinuous, displays a lower slope than the entrance tunnel (35?45°), and an oval to circular cross-section (vertical diameter: 4.5-9 cm, horizontal diameter: 4.5?8 cm, average vertical/horizontal diameter ratio: 1.22). The main gallery reaches a maximum depth of 122 to 170 cm from the top of the paleosol. Side tunnels or lateral chambers form nearly orthogonal angles with the main gallery (60°?85°) and are spaced between 75 and 105 cm. An ovoid lateral chamber is 15 cm high and 20 cm wide. The terminal chambers are irregularly-shaped to ovoid (13.4 cm high and 21.4 cm long) and have a laminated fill. Burrow walls exhibit a surface texture consisting of 1.5 cm wide sets of four parallel grooves at a high angle with the tunnel axis. Considering its architecture (penetration angle, side tunnels, and chambers), these burrows can be assigned to caviomorph rodents. According to the cross-sectional area of the burrows the body mass of the producer was 80?140 g, but no adequate candidate can be found in the modern fauna and fossil remains. Absence of superposition of tunnels and chambers, tunnel diameter and cross-section and surface texture match those of extant Ctenomys. However, the main sloping gallery (instead of a shallow horizontal burrow system typical of Ctenomys) and the oblique to vertical entrance tunnels suggest assignation to older representatives of Ctenomyidae.