New collections record the presence of two species of Eoconodon, E. nidhoggi, and E. hutchinsoni, in the Pu2/Pu3 fauna of northeastern Montana. These northern species differ from the three species of Eoconodon previously known from the San Juan Basin. Their pattern of distribution supports the hypothesis of biogeographic provinciality in the Western Interior during the Puercan North American Land Mammal Age.
In comparison to the tribosphenic dentitions of primitive eutherians, the dentition of Eoconodon and later "triisodontids" is distinctly modified with relatively blunt cusps suggesting emphasis on chopping and crushing functions. By the middle and late Puercan, if not earlier, Hunter-Schreger bands had evolved in the enamel of cheek teeth of Eoconodon increasing their resistance to fracturing. Evolution of this derived microstructural pattern could well be associated with increase in body size and adaptations to a more predaceous and/or scavenging mode of feeding.
Particularly in the northern Western Interior, the absence of Eoconodon or closely related sister groups from well sampled Lancian faunas suggests that "triisodontids" were not present in this area in the latest Cretaceous. The earliest occurrences of Eoconodon are documented in the Mantua local fauna, E. copanus, and a second, smaller species in a Pu1 local fauna in the Hanna Basin. This pattern of first occurrences is in line with the hypothesis proposed by
Lillegraven and Eberle (1999) and others that many taxa particularly archaic ungulates, first recorded in the Pu1 record of the Western Interior appear to be immigrants. During the Pu2 and Pu3 interval zones the species of Eoconodon ranged through the Western Interior and differentiated producing distinct groups of northern and southern species. In the late Puercan (Pu3) a second genus of "triisodontid," Goniacodon, appeared in the Western Interior. Samples of early Torrejonian (To1) local faunas are limited but document the first occurrence of Triisodon and suggest further diversification of the species of Eoconodon.