Vertebrate fossils have been collected since 1989 from the lower part of the Chadron Formation, in the Medicine Pole Hills, seven miles south of Rhame, Bowman County, southwestern North Dakota. Although the Chadron Formation is exposed in other areas in the state, this is the first report of an abundant vertebrate local fauna of significant diversity. A preliminary faunal list is given in Table 1. On the basis of a brontotheriid, the Medicine Pole Hills Local Fauna is interpreted as Chadronian in age.

Table 1:

Vertebrate Taxa of the Medicine Pole Hills Local Fauna

Outcrops of the Chadron Formation in the Medicine Pole Hills are few and restricted to butte tops. The thickness of these strata varies from a few centimeters to as much as 5.2 m (17 ft), as measured in drill holes (E.C. Murphy, North Dakota Geological Survey, written comm., 1992). The deposits are composed of a brownish orange, medium- to fine-grained, unconsolidated arkosic sandstone, which overlies a thin conglomeratic layer. The base of this gravel lies unconformably on a cemented sandstone that is interpreted as part of the Bullion Creek Formation.

Fossils were recovered from the brownish orange sandstones by surface prospecting and through the dry screening of bulk-sampled matrix. Isolated fragments of large bones (probably brontotheriid) were found in gravel deposits, but most are waterworn and heavily abraded. About 1800 specimens have been collected to date from the Medicine Pole Hills localities. The fauna is notable because of 1) its high diversity, especially in mammalian taxa; and 2) the appearance of crocodilians and tapirs, rarely found in Chadronian faunas. The nine orders represented by 18 mammalian taxa, so far identified, represent a highly diverse fauna when compared to other Chadronian localities in North America. Continuing study of the Medicine Pole Hills fauna, by members of the Pioneer Trails Museum, will undoubtedly add to our knowledge of the Chadronian fossil record in North Dakota.