Mimomys in North America

The earliest record of the microtine rodent genus Mimomys is from West Siberia, where, over 5 million years ago, it evolved from Promimomys. From western Asia, early Mimomys dispersed both east and west (to North America and to Europe), arriving at about the same time in both areas as a slightly larger and more evolved form that is essentially identical in both continents. Different species names are applied, as they obviously represent the beginnings of geographically separate lineages. The immigration of Mimomys to North America marks the beginning of the Blancan Mammal Age by original definition. The earliest dated North American record is the Upper Alturas Fauna of California, which is well dated by paleomagnetic stratigraphy, tephra ‘‘fingerprinting’’, and potassium–argon dating at 4.8 Ma. Lindsay et al. (1999) have supported this age by paleomagnetic stratigraphy in two areas of Nevada. One other known fauna, the Maxum fauna of Contra Costa County, California, contains immigrant Mimomys that may be slightly older, but is not dated. Dispersal of the temperate-climate genus Mimomys between Eurasia and North America was through the Beringian Faunal Region, near the latitude of the Arctic Circle, and later intercontinental continuity of the genus was severely restricted by climate, not water. Other microtine rodents, more tolerant of cooler latitudes, were able to cross the Bering land area and thus integrate the biochronologies of the two continents, but the genus Mimomys is of little use in intercontinental correlation during most of the Pliocene. Eurasia and North America contain many faunal regions, and the temperate United States comprises at least two, separated by the Rocky Mountains. Each faunal region has, to an extent limited by its isolation, unique faunas and a different historic biochronology. Although the first Mimomys immigrants to North America and Europe were very similar, the genus evolved independently on the separate continents for the next 3.4 m.y. , producing species, subgenera, and ultimately, new genera dramatically different in the two areas. During this isolation, separate lineages in the United States resulted in the subgenus M. (Cosomys) and the genus Ophiomys in the Western Faunal Region and in the subgenus M. (Ogmodontomys) and the genus Hibbardomys in the Eastern Faunal Region. Although also climatically constrained, the barrier to dispersal between these two faunal regions of the United States was much less rigorous than in Beringia. Several times taxa dispersed along two documented routes between the Eastern and Western Faunal Regions of the United States: through Yellowstone Pass in western Wyoming and around the southern end of the Rocky Mountains through Arizona and New Mexico. These faunal interchanges are useful in correlating the two regional biochronologies, and times of exchange correlate with the climate changes that made the Rocky Mountains, or the region to the south of it, habitable by temperate Mimomys. About 1.3 Ma, global warming again permitted Mimomys to disperse through Beringia. At or slightly before this second immigration of Mimomys, the descendants of the first immigration had become extinct or had evolved into forms no longer conforming to the diagnosis of Mimomys. The new immigrants are assigned to the Eurasian subgenus Mimomys (Cromeromys) and they are known to have lived well into the Irvingtonian mammal age, less than 0.78 Ma. Present records thus indicate that the genus Mimomys and derived forms lived in the United States for over four million years before they became extinct. 1 Denver Museum of Nature & Science, Denver, CO 80205-5798. 470 NO. 279 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY