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The Raben Ranch Local Fauna of northwestern Nebraska is the largest and most diverse Early Oligocene (Chadronian) microfauna recovered from the High Plains. Within the microfauna is the largest accumulation of teeth referable to the Multituberculata yet recovered from rocks of Early Oligocene age. The age of the fauna is Middle Chadronian and, along with fossils from Flagstaff Rim in Wyoming, may represent the youngest known occurrence of the Allotheria. Forty-seven specimens referred to the genus Ectypodus are described, and comparisons are made with previously described teeth from the Chadronian of Wyoming and Saskatchewan. The data presented by the Raben Ranch sample indicate the presence of only one species of Ectypodus. Eight problematical teeth are also described and questionably referred to the Multituberculata. In light of the geologic range extension of the Allotheria by the presence of teeth in Early Oligocene rocks, the theory of competitive inferiority is challenged as being the sole reason for the extinction of the group. It is hypothesized that a combination of factors caused the final extinction of the sub-class.