New specimen of Monosaulax typicus (Mammalia, Castoridae) from the Mascall Formation of Twin Buttes in the Crooked River Basin, Oregon

Abstract

The Mascall formation of Central Oregon is a rock unit that was formed during the middle Miocene (~15 million years ago). The Mascall formation was first described from North Central Oregon, but also outcrops to the south in the Crooked River Basin, where it is much older. While these southern outcrops of the Mascall Formation have been known for some time, recent research shows that they are about a million years older than the outcrops of the same formation in the John Day Basin. This particular formation is known for producing many Barstovian-age mammals. Recent collections by University of Oregon field crews have yielded new material from the Crooked River region. I have diagnosed two new specimens recovered from the Mascall formation from Twin Buttes near Paulina, OR, as Monosaulax typicus from the family Castoridae. One specimen is a fragment of a lower right jaw that contains the p4-m2, while the other is a left jaw fragment. These are the most complete remains of M. typicus from the Mascall formation; other known specimens are isolated teeth. The teeth have all the diagnostic characteristic of other previously identified specimens of M. typicus that were found in Southeast Oregon rocks of similar age. The information of this new specimen will add to the limited knowledge of this species and the environment of the Crooked River Basin (Shotwell, 1968).