A Remarkable New Mammal from the Lower Chadron of Nebraska

Abstract

While we were examining an unusually fossiliferous exposure in the lower member of the White River beds near Whitney, Dawes County, Nebraska, my wife, Mrs. Margaret C. Cook, discovered tiny, scattered fragments of a lower jaw on the surface of the gray-green member of the 'Chadron formation. While screening the dry surface, using binocular glasses and working into the matrix, she found an articulated skull and cervical vertebrae under where the jaws had weathered out. Near the skull were several disarticulated vertebrae and ribs. These remains were removed in one block. Preparation has been difficult, both because of the nature and condition of the fossils and matrix, and because slippage in seams of the clay-silt matrix has caused many fractures and dislocations. Though the skull is virtually complete, the facial region from the orbits forward has been compressed laterally and twisted to one side. The maxillae and teeth are intact, but much remains to be done before the characters of the skull can be fully determined. The vertebrae also are to be prepared. They appear to be very large and heavy in proportion to the skull. Meanwhile, many of the tiny fragments of the lower jaws and teeth have been fitted together, revealing dentition that is complete except for the lower incisors. The specimen indicates a mammal that is surprisingly primitive for its horizon in the Tertiary, with characters reminiscent of Paleocene and early Eocene mammals. Several of these characters are either homologous with or analogous to those of early creodonts, insectivores and artiodactyls. For the presenit I am referring the creature to the creodonts, though it does not seem to be closely related to any creodont of later than early Eocene age with which I have compared it. A partial summary of these comparisons is presented in the following description.