LIFE UNDERGROUND IN THE NORTH AMERICAN MIOCENE: INTERPRETING THE BURROWS OF SMALL TERRESTRIAL MAMMALS

Terrestrial vertebrate ichnofossils provide information that is not usually obtainable from skeletal remains, such as digging behavior or ecological niche. Aside from the Daemonelix (“devil’s corkscrew”) burrows of extinct fossorial beavers, fossilized rodent tunnels remain largely undescribed. The upper portion of the Harrison Formation, in which beaver Daemonelix occur, preserves evidence of other burrowing mammals with different digging habits and lifestyles. Sinuous tunnels of c. 7 cm diameter show diagnostic paired grooves from rodent incisors on the ceiling, and claw marks on the floor and around bends. Rambling architecture and irregularly branching tunnels over large areas imply foraging behavior, and possibly a solitary lifestyle. Overlapping networks of tunnels around fossilized roots may indicate directed search for subterranean tubers as a food source.  These burrows may be those of Gregorymys, a small, gopher-like rodent that occurs abundantly in these beds. Like the beavers that excavated Daemonelix, these rodents dug tunnels primarily with their incisors, possibly because the abrasive volcanic sands of the formation would create excessive wear on the foreclaws. In this respect, rodents of the Harrison Formation were similar to extant incisor-diggers, such as the blind mole-rat Spalax, which digs efficiently through abrasive soils but cannot navigate in loose sand.

Contrasting with burrows in the Harrison Formation are younger (Miocene: late Barstovian) burrows from the Pawnee Creek Formation, Colorado, which are attributed to the mylagaulid rodent Pterogaulus laevis. Burrows from both formations share certain features, such as knob-like, scratched-out areas on or near the ceiling (possibly indicating where tubers or stones were removed), and rounded or bilobate terminal chambers. The mylagaulid burrows have sinuous architecture, but are covered exclusively with claw marks that match the long foreclaws of P. laevis. The relatively soft sands and silts of the Pawnee Creek Formation may have necessitated specializations for claw-digging in this mylagaulid, and restricted its burrowing niche. Comparative analyses of rodent ichnofossils can thus supplement osteological evidence for functional morphology and ecological data on subterranean dwellers in ancient ecosystems.