The Oligocene rodent Ischyromys of the Great Plains; replacement mistaken for anagenesis

ABSTRACr-Ischyromys, the most abundant early Oligocene rodent from the Great Plains, has been considered by some workers to represent a single gradually evolving lineage comprising three or more chronospecies. Statistical investigation of large samples suggest instead that two closely related species coexisted, and the shift in mean size that was thought to represent anagenesis actually represents replacement. In Nebraska and eastern Wyoming both I. parvidens (small) and I. typus (large) were rare but of equal abundance in the Chadronian, I. parvidens was more prevalent in the early Orellan, and I. typus was more prevalent in the middle and late Orellan. In northeastern Colorado, northern South Dakota, and North Dakota I. typus is the only species of Ischyromys found in Orellan deposits, thus showing that the two species had differing geographic ranges. The mean size of I. typus does increase up section at all localities, but this change is minor and not deserving of chronospecies recognition. Much of this change occurred in the latest Orellan and earliest Whitneyan as I. typus approached extinction, and it was accomplished mostly by loss of small individuals rather than a shift of the entire distribution. Rate of evolutionary change in Ischyromys is found to be inversely correlated with population size, and no new species arose during the Orellan when Ischyromys