LATE MIOCENE (HEMPHILLIAN) RODENT BURROWS FROM EAST-CENTRAL NEBRASKA, USA: A SNAPSHOT OF BEHAVIOR UNDER THE EMERGING GREAT PLAINS

The Happy Jack Mine near Scotia, Nebraska, near the eastern edge of the Ogallala Group outcrop, exposes dozens of fossil rodent burrow systems in late Miocene (Hemphillian) lacustrine calcareous diatomites and overlying fluvial sandstones. Burrow systems include tunnels and large, elongate to roughly ovoid, underground chambers that attain diameters in excess of 500 mm. One connected pair of such chambers exceeds 2.3 m in length. Tunnels in these burrow systems average 89.8 mm in diameter, a value similar to the body and burrow-tunnel diameters attributed to some modern ground squirrels (Otospermophilus, Poliocitellus, Urocitellus) and pocket gophers (Geomys, Pappogeomys). There are as many as four entrance or exit tunnels, in many cases near-vertical shafts, connected to a chamber in any single burrow system. Most burrow systems are exposed in the walls of the mine, but the ceiling also exposes plan views of a few systems consisting of horizontal tunnels branching in V-, Y-, and T-shapes and also having short, blind laterals. Paired incisor grooves exist on the surfaces of many of the tunnels and chambers in the mine. Isolated teeth from the extinct pocket gopher Pliogeomys as well as a partial upper incisor from an ancient ground squirrel have been screened from the sediment fills of some of the burrows at Happy Jack Mine. Overall, we find more similarities between the fossil burrows and those of the extant ground squirrels of the Tribe Marmotini than we do with those of extant pocket gophers (Family Geomyidae), although representatives of both groups lived around the site during the late Miocene. There is no convincing evidence for shallow, near-surface fossil foraging burrows like those made by extant geomyids. We stress that fossil rodent burrows provide broad and largely unrealized insights into the behaviors and life histories of ancient rodents over time and into the paleoecology of evolving Cenozoic grasslands.