CONTRIBUTIONS OF THE MEADE BASIN RODENT PROJECT TO THE HISTORY OF BIODIVERSITY ON THE CENTRAL GREAT PLAINS

Building on the stratigraphic and paleontological framework established by the late C. W. Hibbard and his students, through almost 20 years of prospecting, collecting, and laboratory study the Meade Basin Rodent Project has generated a robust species-level database for millions of years of the late Cenozoic in southwestern Kansas and northwestern Oklahoma. Studies of dental morphological and size change in various rodent groups suggest that morphological change within clades combines aspects of both punctuated equilibrium and phyletic evolution. Using electron microscopic imaging of the schmelzmuster, the enamel banding pattern of rodent molars, it is possible for the first time to unequivocally reject phylogenetic relationship at the population level. Patterns of size change, especially in the geomyids (pocket gophers), indicate that identification of character displacement among extant animal species may represent part of a long and complex history of size change that may or may not be mediated by competition. Investigations of diversity dynamics reveal that the Meade Basin rodent community has been in equilibrium for more than 4.5 million years; this is the first confirmation of the Equilibrium Theory of Island Biogeography for continental systems over millions of years. Despite the equilibrium, maintained because species extinctions balance immigrations, elevated pulses of turnover are recognized, correlated with significant climatic change, the introduction of potentially new competitors across Beringia, and the likely influence of widespread, devastating ashfalls such as the Huckleberry Ridge ash (2.11 Ma) and the Lava Creek B ash (0.65 Ma). None of the extinction rates recorded in the Meade Basin system rival extinction levels in modern species due to anthropogenic activities.