Correlation of Pliocene and Pleistocene fossil assemblages from the central and eastern United States: toward a continental rodent biochronology

The ubiquitous presence and rapid evolution of rodents lend this group to the development of a correlational scheme for United States Cenozoic continental fossil deposits, equivalent to the later Cenozoic section of European MN zones. Expanding on earlier work in the Meade Basin of southwestern Kansas and Florida, Pliocene and Pleistocene mammal assemblages from the northern and southern Great Plains and the eastern United States are correlated based on rodent biostratigraphy. Eleven rodent zones (RZs) are constructed for the last 5 million years based on a new database including all rodent species assemblages during the study period. In addition to the 11 refined RZs that aid in temporal sequencing of unknown rodent assemblages, three broader biozones are recognised, including an early period characterised by the <i>Promimomys</i> immigration event and Hemphillian holdovers, a longer, relatively warm Pliocene to early Pleistocene period dominated by primitive cricetids and archaic arvicolids, and a final zone spanning the last two million years defined by the immigration and radiation of hypselodont <i>Microtus</i> and large, dentally advanced species of the cotton rat, <i>Sigmodon</i>. Modern rodent communities are established in the later part of Biozone 3, between about 0.6–0.3 Ma.