Large samples of Lancian local faunas from the northern Western Interior obtained by surface collecting and screen washing techniques lack records of Eoconodon, other "triisodontids," or closely related sister groups. Although significantly smaller, samples of Lancian local faunas from the southern Western Interior also lack similar records. Together their absence is in accord with the hypothesis that "triisodontids" or closely related sister groups were not present in the sampled areas of the Western Interior in the latest Cretaceous.
The oldest records of Eoconodon are limited to occurrences in two Pu1 local faunas in Wyoming. These specimens document the presence of two species, which indicates some diversification of "triisodontids" during the Cretaceous in areas that have yet to be sampled. Eoconodon or other "triisodontids" are absent from the very large samples of Pu1 local faunas and the time-averaged Bug Creek Assemblage localities in the northeastern Montana. Probably Eoconodon had not spread throughout the northern Western Interior in the Pu1 interval zone. Contemporaneous local faunas have yet to be discovered in the southern Western Interior.
Large samples of Pu2, Pu3, and Pu2/Pu3 undifferentiated local faunas are available from both the northern and southern Western Interior. During the Pu2 and Pu3 interval zones Eoconodon spread its range throughout the Western Interior and differentiated producing distinct groups of northern and southern species. A second genus of "triisodontid," Goniacodon made its appearance in the Pu3 Wagonroad local fauna of Utah. The limited available samples of To1 local faunas document the first occurrence of Triisodon and suggest further diversification of the species of Eoconodon.