Burrows of a gopher-like rodent, possibly Gregorymys (Geomyoidea: Geomyidae: Entoptychtinae), from the early Miocene Harrison Formation, Nebraska

This study describes the architectural and surface morphology of small, 5.6−6.7 cm diameter burrows that occur in close proximity to the Daemonelix burrows of palaeocastorin beavers in the early Miocene upper Harrison Formation, Nebraska. The small-diameter burrows are sinuous and rambling or weakly helical in morphology. Distinctive paired grooves from rodent incisors, measuring 4.1−4.3 mm wide, occur on the ceiling and sides of the burrows, whereas ridge-like claw marks of 4.1 mm width occur low on walls and floors. Scratched-out, chamber-like areas on the sides of tunnel shafts are covered on all sides with claw marks, whereas tooth marks are mostly confined to main tunnel shafts. Both types of marks are abundant, indicating a mixed tooth-and-claw digging strategy that appears to have evolved in palaeocastorin beavers as well. Similar digging methods in two unrelated Harrison rodents indicates the relative hardness of the volcanic ash-rich soils, which necessitated digging with teeth in addition to less resistant keratin claws. The new ichnotaxon Alezichnos trogodont is erected to describe these ichnofossils. A possible excavator is Gregorymys sp., an extinct, gopher-like entoptychine rodent that occurs in these and correlative Miocene units in the Great Plains. The width of Gregorymys incisors matches the width of incisor marks on A. trogodont. These burrows may represent the first recorded evidence of subterranean habitat and digging behavior in an otherwise common fossil rodent. The excavator of A. trogodont had a different burrow system, and possibly different foraging behavior, than the contemporaneous beavers, providing new evidence for diversity in a subterranean Miocene ecosystem.