Early and Middle Pleistocene of North America

Abstract

Biochronologic division and subdivision of the Pleistocene in North America is based entirely on mammalian taxa. The North American Land Mammal Age system is based upon patterns of evolution, immigration, and extinction of mammals on the continent and has been formally recognized since the 1950s as a means to organize faunas into a relative temporal sequence throughout the Pleistocene. The Blancan and Irvingtonian Land Mammal Ages accommodate faunas from the Early and Middle Pleistocene. Faunas of both ages are concentrated in the Great Plains, southwestern United States, and Florida. Blancan fauna are rare in the east outside of Florida and are essentially unknown in cave deposits, but many of the important Irvingtonian faunas are preserved in caves. The lowering of the base of the Quaternary and Pleistocene, formerly placed at ∼1.77Ma, to 2.588Ma places the later portion of the Blancan within the Early Pleistocene. The start of the Blancan is defined by the first appearance of the primitive arvicoline Mimomys south of 55° N latitude. The Irvingtonian Land Mammal Age corresponds temporally to the Middle and Early Late Pleistocene. We review two definitions for the Irvingtonian, one based on the occurrence of the genus Mammuthus, the other on the occurrence of the Microtus-like voles Allophaiomys. The definition utilized has significant implications for the recognition and duration of the Irvingtonian, as well as the preceding Blancan mammal age. Early and Middle Pleistocene records of birds, fish, and, to some extent, herpetofauna, are far less well documented and understood than the mammals. The rich record of localities, faunal remains, and geochronologic data for the Pleistocene in North America, however, contains the resources necessary to shed new light on those understudied groups. Many long-standing questions persist, including those related to the dynamics of faunal change and the extent of faunal provinciality, our ability to recognize provinciality through time, and to utilize it for finer-scale correlations. Improving our understanding in those areas requires renewed focus on fundamental aspects of the fossil record and its interpretation, specifically geochronology, reliable specimen identification, taxon occurrences, and climatic proxies that are independent of faunas. These are fertile areas for future research.