Haplomylus specimens from Lower Eocene rocks of the Powder River Basin can be assigned to four species based on morphology. One of these is the generotypic species, H. speirianus, and a second is a similar-sized, but morphologically distinct new species, H. bozemanesnsis. These two species occur together, in apparent stasis, for over 120 meters of section. Two other species replace them after an interval represented by approximately 20 meters of deposits. These latter species are larger, but each resembles one of the earlier forms. The co-occurrence of two closely related species with total overlap in metric characteristics shows the need for caution in using an exclusively measurement-based stratophenetic approach to lineage evaluation through time.

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