Rates of evolution in Paleocene-Eocene mammals of the Clarks Fork Basin, Wyoming, and a comparison with Neoghene Siwalik lineages of Pakistan

Rates of size change are quantified in 19 species-level evolutionary lineages of Clarks Fork Basin Paleocene and early Eocene mammals. Intrinsic rates of change in Clarks Fork Basin lineages range from about 10−0.1 to 10−3.4 haldanes (standard deviations per generation), with a median rate of about 10−1.1 haldanes. These values are in the usual range of intrinsic rates commonly seen in other settings. Temporal scaling slopes indicate that seven of the 19 Clarks Fork Basin lineages were stable over time (37%), five were stable with a random component (26%), four were indistinguishable from random (21%), two were directional with a random component (11%), and one lineage cannot be classified. Clarks Fork Basin mammalian species have mean durations of about 0.95 m.y. and median durations of about 0.76 m.y. Intrinsic rates of size change are similar to those for Miocene Siwalik mammals, but the average species duration is less than one-half the average for Siwalik mammals.