Observations on dental variation in Microtus from the Cudahy Ash Pit Fauna, Meade County, Kansas and implications for Irvingtonian microtine rodent biochronology

A thorough re-examination of the Microtus remains from the Cudahy Ash Pit f auna in Meade County, Kansas revealed a greater degree of morphological var iation than previously reported for this fauna. Morphological variants of t he mi sample include typical M. paroperarius morphotypes with only four clo sed triangles, morphotypes with five closed triangles, and specimens with i ntermediate morphologies in which well-developed fifth triangles show varia ble degrees of partial closure. Morphological variants of the M2 include sp ecimens with a well-developed posterolingual dentine field often considered to be indicative of the presence of M. pennsylvanicus. A survey of six add itional species of North American Microtus revealed that the posterolingual dentine field is present as a regular feature in several species and canno t be used to confidently identify M. pennsylvanicus. Current North American microtine rodent biochronologies utilize the first appearance of M. pennsy lvanicus to define the youngest microtine rodent division of the Irvingtoni an (Irvingtonian III). The atypical morphotypes in the Cudahy fauna are not identified to species, but the presence of specimens indistinguishable fro m M. pennsylvanicus in a fauna at least 660,000 years old, combined with ch anges in the known temporal distribution of other microtine rodent tare, de stabilizes the lower boundary definition of the Irvingtonian III and its co ntinued use can no longer be supported. This results in the recognition of only two microtine rodent divisions of the Irvingtonian and lessens the tem poral resolution currently obtainable from microtine rodents in the middle Pleistocene, but more accurately reflects our knowledge of the evolution an d temporal occurrence of microtine rodents on the North American continent.