Woodrat (Rodentia: Cricetidae) molars have a number of features that have exhibited morphological change over geological time. These changes have included modification of the occlusal pattern, with a corresponding change in direction of mastication, increasing depth of the reentrant folds, development of dentine tracts, and increasing numbers of anatomical roots. These features and their variations over time can be useful in differentiating between taxa and may have some biostratigraphic and phylogenetic significance as well.

Woodrats have been reported from deposits of Hemphillian age in Texas (Dalquest, 1983) and Kansas (Hibbard, 1967); however, those samples were so small (N = 2 in each case) that there was little basis for comparison with younger specimens. Therefore, this study is based on characters and changes observed among Blancan and younger woodrats. Likewise, this study should be considered preliminary, as many more fossil and extant samples need to be examined in detail. However, I hope the information provided herein will be of benefit by delimiting characters and patterns that can be considered by others in subsequent studies.

Appendix A contains a list of fossil woodrats. Included in this list is the name of the fauna from which the fossil was obtained, the location and age of the fauna, the basis for the age assignment, and a primary reference. I have attempted to list all records of extinct species. Representative occurrences of extant species are taken from Harris (1985) and Kurtén and Anderson (1980).