Rodents and Lagomorphs of the Upper Sespe

Abstract

In contrast with the rodent faunules of the middle Sespe, that of the upper Sespe is represented by exceedingly few and fragmentary specimens. Even this scant assembly, however, indicates clearly, as do the larger mammals, that a much younger stage in geological time is recorded, namely early Miocene, possibly in part late Oligocene, rather than late Eocene. Some of the species were reported in an earlier paper (Wilson, 1934). In the present paper, all known specimens are discussed, and the stratigraphic value of the material is indicated. Although only 9 rodent and lagomorph specimens are known from the upper Sespe, these represent apparently 6 families. Seven rodent specimens are available, which fall into 5 families. Further, of the 7 known families from the Arikareean, including the John Day, 4 are recorded from the upper Sespe. Those absent are the Castoridae, Aplodontidae, and Mylagaulidae. The absence of the Aplodontidae may possibly be accounted for on the basis of geographic distribution. A member of the lschyromyidae is also present in the Sespe. Representatives of this family are common in older deposits, but so far they have remained unrecorded from the Arikareean stage.