Possible burrows of mylagaulids (Rodentia: Aplodontoidea: Mylagaulidae) from the late Miocene (Barstovian) Pawnee Creek Formation of northeastern Colorado

The massive siltstone beds of the late Miocene (Barstovian) Pawnee Creek Formation in Logan County, northeastern Colorado, contain abundant terrestrial vertebrate burrows. Among these are distinctive claw-marked burrows attributable to the extinct mylagaulid rodent Pterogaulus [= Mylagaulus] laevis Korth, 2000. These burrows differ in several respects from the spiraling Daemonelix burrows of fossorial beavers. The Pawnee Creek burrows are slightly ovate in cross-section, tubular, and sinuous rather than helical. They consist of primary tunnels of up to at least 7 m long and range from 11 to 18 cm in diameter. They occasionally branch into secondary tunnels. Burrow walls and terminations show prominent, parallel ridges in sets of two to three, up to 9.3 mm deep and 3.7 mm wide. The burrows are too large to have been made by anything but a vertebrate, and their considerable length and complexity implies a mammalian excavator. The meandering, branching morphology resembles the burrow systems of extant fossorial rodents, such as pocket gophers (Geomys) and mole rats (Spalax, Myospalax). Skeletal remains of P. laevis are abundant at sites where the burrows occur, with at least eight individuals represented by humeri in a collected sample. Although mylagaulid remains are not unequivocally associated with fossil burrows, the curved, laterally compressed claws and phalangeal arrangement of mylagaulids produce marks very similar in size and morphology to the ridges on the burrows. Position and pattern of the ridges indicate downward-and-backward motions of the mani, possibly supplemented by motions of the pedes to kick loosened soil backward. A new ichnogenus, Alezichnos, and new ichnospecies, chelecharatos, are erected to describe these burrows.