HEMPHILLIAN RODENTS FROM NORTHERN OREGON AND THEIR BIOSTRATIGRAPHIC IMPLICATIONS

Hemphillian localities south of the Columbia River in northern Oregon have produced thousands of small mammalian fossils. Castorids, geomyids, cricetids, and a murid described herein were derived from the Arlington, Ordnance, and McKay Reservoir localities. Large sample sizes allow a greater understanding of the range of variation and ontogenetic changes, as well as paleoecology and biostratigraphic distribution. Two new taxa are described, Parapliosaccomys wittei sp. nov., a geomyine, and Paronychomys woodburnei, a hypsodont cricetid. Castor, Dipoides, and Parapliosaccomys indicate the assemblages are late Hemphillian in age, although certain taxa indicate temporal differences among the localities. The lower Arlington and Ordnance localities are the oldest, Arlington 15 is medial, and McKay Reservoir is the youngest of the assemblages. A CastorParapliosaccomys Concurrent Range Biozone is named for the interval represented by the assemblages from northern Oregon and is correlated to Christmas Valley, OR, possibly Churchhill Butte and Hoye Canyon, NV, as well as with assemblages from the Great Plains at Pliohippus Draw, ZX Bar, Mailbox, and Honey Creek, NE. The appearances of Prosomys and Spermophilus (Spermophilus) at the McKay Reservoir Locality provide another important datum indicating latest Hemphillian time.