FIRST OCCURENCE OF A MAMMAL FROM THE MID-MIOCENE CLARKIA FOSSIL BEDS

The Clarkia lagerstätte of northern Idaho is internationally famous for its fossil flora, many specimens of which still retain original organic material despite their age (16 Ma). Other organisms reported from Clarkia include fungi, insects, and salmonid, cyprinid, and centrarchid fish, providing a uniquely detailed window onto a mid-Miocene paleocommunity. However, to this point, no tetrapods have been described from Clarkia or from any other outcrops of the Latah Formation. Here we report on the first mammal to be uncovered from this lagerstätte. The specimen, preserved as a carbonaceous film with some fragments of dental enamel, was found in the lower, unoxidized zone of Unit 2, a layer that has been dated to 15.7-16 Ma. This means that the fossil dates to the Early Barstovian North American Land Mammal Age and to the height of the mid-Miocene climatic optimum. Incisor morphology clearly indicates that the specimen is a rodent, but the absence of well-preserved cheek teeth make a more precise identification difficult. Its large size is consistent with only three families of Barstovian rodents: Aplodontidae, Castoridae, and Sciuridae. Gross morphology is most consistent with sciurids, and among Barstovian squirrels its size falls within the range of only one described genus: the marmot-like Arctomyoides. The dental fragments associated with the specimen are insufficient to confirm this tentative diagnosis, but ongoing fieldwork is targeted at uncovering more mammal material from Clarkia. For the moment, the expansion of the Clarkia biota to include large sciurids – and indeed, tetrapods in general – will inform research on the taphonomy and paleoecology of one of North America’s most important lagerstätten.