CLAW-MARKED BURROWS IN THE MIOCENE OF COLORADO: INDICATIONS OF MYLAGAULID (MAMMALIA:RODENTIA) PALEOECOLOGY

The Pawnee Creek Formation of northeastern Colorado contains massive, sandy silt beds in which numerous vertebrate burrows occur. The fine sediments, localized channel deposits, and a high density of root casts (rhizoconcretions) suggest a flood plain environment with abundant trees and other vegetation. The burrows, unofficially named “Maeandrichnia,” are tubular, sinuous natural casts that preserve the inner wall surfaces of the original tunnels. They show prominent digging marks representing narrow, laterally compressed claws, and contain other clues to the paleobiology and ecology of the excavator and inhabitant(s).

The most likely excavator of Maeandrichnia is Pterogaulus laevis, a mylagaulid rodent which is abundant in the burrow layers and other localities in the area. The long, recurved foreclaws of this rodent match the marks occurring on the tunnel walls. In addition, the complex branching patterns in Maeandrichnia tunnels suggest a mammalian, rather than a reptilian (tortoise) excavator. Cross-sectioned burrows show a relatively resistant outer fill layer of ca. 4 cm thickness, containing root casts and similar in aspect to the sediments in which the burrows occur. This layer forms a concentric wall around the tunnel, and shows no meniscate fill structures, indicating that it filled the entire tunnel at one time.

A second, inner tunnel fill consists of unconsolidated sediment with abundant hackberry (Celtis sp.) seed husks and spheres of ca. 1 cm diameter, which may be anhydrite mineral crystals. This inner fill is easily removed, revealing a cavity of ca. 5.5 cm diameter. Molds of the boundary between this cavity and the outer fill layer may show trace marks, and help determine whether the cavity represents a secondary tunnel, excavated within the first after the original tunnel filled with sediment. Phytolith assemblages from outer vs. inner fill layers, as well as from end-chambers, distinguish concentrations of plants inside tunnels as possible nests or food caches.