Timothy H. Heaton
Dept. of Earth Sciences
Univ. of South Dakota
Vermillion, SD 57069
There has long been debate whether Titanotheriomys, first named from the Chadronian of Pipestone Springs, Montana, is a distinct genus or a junior synonym of Ischyromys, first named from the Orellan of the Big Badlands of South Dakota. Albert E. Wood has argued throughout his career that the two genera are distinct and that each contains several species. Craig C. Black (1968. Ann. Carnegie Mus. 38:1-48) declared the two genera to be synonymous, and he even went so far as to consider the large and small forms from the Great Plains and Pipestone Springs to the be the same two species, I. typus and I. veterior respectively, despite differences in their age and location. The one thing these two respectful but opposing workers agreed upon was that the two forms could not be distinguished on the basis of lower jaws and teeth, the most commonly found elements.
An extensive morphometric study of Ischyromyids now demonstrates that the Great Plains and Montana forms can in fact be distinguished based on lower jaws and teeth. The most distinct difference is in M/3, which is relatively shorter in the Montana forms. Discriminant analysis can distinguish individual specimens with better than 90% accuracy, and group mean values cluster by age and locality using cluster analysis and multidimensional scaling. This suggests that Ischyromys and Titanotheriomys are distinct. When specimens from Wyoming are considered, the distinction is not as good but still seems valid.