Merychippus Isonesus (Cope) from the Later Tertiary of the Crooked River Basin, Oregon

Abstract

Although Tertiary deposits, presumed to be the correlative of the Mascall and Rattlesnake formations of the John Day basin, are known to occur in the Crooked River basin south of the Ochoco Mountains in central Oregon, no vertebrate fossils of diagnostic value have been obtained from these beds. Merriam and Sinclair record the finding of a caniniform premolar of a merycoidodont in Mascall beds exposed on the divide between Camp Creek and the Crooked River. Recently R. W. Chaney has reviewed the evidence on which is based the recognition of the Mascall horizon in the Crooked River region. In the course of palreontological investigations conducted in central Oregon under the leadership of Dr. Chester Stock, an upper tooth of Merychippus was collected in deposits, regarded as Mascall in age, along the Crooked River between Post and Paulina. In view of the opportunity to compare the stage of evolution of this specimen with that represented by the dentition of the merychippine forms from the type Mascall, a description seems desirable. The writer wishes to acknowledge the guidance of Dr. Stock during the progress of this study.