TAXONOMIC AND ECOLOGICAL CHANGES IN RODENT FAUNAS OF THE LATE OLIGOCENE CABBAGE PATCH BEDS OF WESTERN MONTANA: RESPONSE TO ENVIRONMENTAL CHANGES?

Studies of extant mammalian communities where environmental disturbances have altered habitat and vegetation predict temporally correlated changes in mammalian taxonomic and ecological composition. I analyzed the sequence of rodent faunas preserved in the Cabbage Patch (CP) beds of Montana to test if and how rodents responded to the climatic and environmental perturbations of the late Oligocene. I used over 650 jaws and teeth, representing over 20 genera and seven families, from isotaphonomic localities in stratigraphic context to study the taxonomic and ecological make-up of rodent faunas over three million years through the opening of the environment in the northern Rocky Mountain region.

There is no significant change in rodent richness throughout the CP beds despite turnover in select taxa. The relative abundance of rodent taxa, however, varies greatly. These changes are better understood when considering the ecomorphology of the relevant taxa. Thus, I estimated body mass, locomotion, and diet for each taxon found within the CP beds. There is a significant increase in average body mass through time across the rodent fauna and within some taxa (such as castorids, geomyids, and aplodontids). The proportion of ground-dwelling (including some eomyids and cricetids) and arboreal taxa (including some aplodontids) decreases over time whereas that of burrowing taxa (some aplodontids and entoptychine geomyids) increases. Overall, there is an increase in the relative abundance of taxa with open-habitat affinities through time. Other mammalian orders also display losses of forest dwelling taxa including the leptomerycid Pronodens.

Coeval faunas of the Great Plains where the spread of grasslands was already underway show a high proportion of larger-bodied taxa, some of which were burrowing forms (mylagaulids, entoptychines, and palaeocastorines). In the John Day Basin – an area that was likely more forested at the time — at least one microfauna includes few fossorial and few large-bodied taxa. The ecomorphological composition of late Oligocene rodent faunas and its changes may suggest a response to environmental disturbances across the northwestern United States. This hypothesized link will be further tested in the CP beds by studying the vegetation from the same horizons as fossil mammals using phytoliths.