Timothy H. Heaton
Department of Earth Sciences/Physics
University of South Dakota
Vermillion, South Dakota 57069
The Big Badlands of southwestern South Dakota have yielded a superb Oligocene age mammalian fauna and have been a prime collecting site for museums for 150 years. The most abundant rodent in these collections is Ischyromys, a primitive squirrel-like animal. The type specimen of Ischyromys, as well as types of several species named for this genus, come from the Big Badlands, but none of these type specimens have good locality or stratigraphic data.
Oligocene sediments along Pine Ridge in northwestern Nebraska and eastern Wyoming have yielded two coexisting species of Ischyromys, one large and one small (Heaton, 1993). These have typically been called I. typus and I. parvidens, respectively, after a large skull and a small lower jaw from the Big Badlands. Fossils of these two species can be distinguished only by size and characters allometrically related to size, and there is considerable overlap. Oligocene sediments in northeastern Colorado, northwestern South Dakota, and southwestern North Dakota appear to contain only the larger of these two species.
The presence of two coexisting species of Ischyromys in Nebraska and Wyoming was demonstrated statistically by highly skewed and bimodal size distributions of lower jaws (Heaton, 1993). A similar plot of 817 jaws from the Big Badlands forms a perfectly normal distribution with a mean value close to the mean for I. typus in the Nebraska samples. The type specimen of I. parvidens is part of this plot but is one of the smallest jaws in the extreme tail of the distribution.
The logical interpretation of this data is that I. typus is the only species of Ischyromys from the Big Badlands: that the smaller Nebraskan species never lived in South Dakota or did so only during an interval that is not recorded. If true, this would mean that I. parvidens is a junior synonym of I. typus, thus throwing into question the use of I. parvidens as a valid name for the smaller species of Ischyromys in Nebraska and Wyoming. Heaton, T. H. 1993. The Oligocene rodent Ischyromys of the Great Plains: replacement mistaken for anagenesis. J. Paleontol. 67:297-308.