Species of the early Paleocene (Puercan North American Land Mammal Age) triisodontid Eoconodon were among the first eutherians in the northern Western Interior to evolve body masses significantly larger than those of the latest Cretaceous mammals of the area. Description of additional specimens of Eoconodon nidhoggi from the Tullock Formation, northeastern Montana, sets the stage for formal diagnosis of a new species of Eoconodon, E. ginibitohia sp. nov., in the Betonnie-Tsosie local fauna, Puercan 2, Hemithlaeus kowalevskianu–Taeniolabis taoensis Zone, San Juan Basin, New Mexico. The taxonomic diversity of the species of Eoconodon as well as their stratigraphic and biogeographic ranges support the hypothesis that the first representatives of the clade entered the Western Interior at the beginning of the Puercan as immigrants from currently poorly sampled areas probably in North America or Asia. During the Puercan the clade both diversified and expanded its biogeographic range in the Western Interior.