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

Publisher Website
Google Scholar
Abstract
A thorough re-examination of the Microtus remains from the Cudahy Ash Pit fauna in Meade County, Kansas revealed a greater degree of morphological variation than previously reported for this fauna. Morphological variants of the ml sample include typical M. paroperarius morphotypes with only four closed triangles, morphotypes with five closed triangles, and specimens with intermediate morphologies in which well-developed fifth triangles show variable degrees of partial closure. Morphological variants of the M2 include specimens with a well-developed posterolingual dentine field often considered to be indicative of the presence of M. pennsylvanicus. A survey of six additional species of North American Microtus revealed that the posterolingual dentine field is present as a regular feature in several species and cannot be used to confidently identify M. pennsylvanicus. Current North American microtine rodent biochronologies utilize the first appearance of M. pennsylvanicus to define the youngest microtine rodent division of the Irvingtonian (Irvingtonian III). The atypical morphotypes in the Cudahy fauna are not identified to species, but the presence of specimens indistinguishable from M. pennsylvanicus in a fauna at least 660,000 years old, combined with changes in the known temporal distribution of other microtine rodent taxa, destabilizes the lower boundary definition of the Irvingtonian III and its continued use can no longer be supported. This results in the recognition of only two microtine rodent divisions of the Irvingtonian and lessens the temporal resolution currently obtainable from microtine rodents in the middle Pleistocene, but more accurately reflects our knowledge of the evolution and temporal occurrence of microtine rodents on the North American continent.