Introduction

Stratigraphic sections coeval with the Pliocene–Pleistocene boundary section at Vrica have been securely identified in two long terrestrial sequences in the southwestern USA: the San Pedro Valley sequence in Arizona and the Anza-Borrego Badlands sequence in southern California. Those sequences, with excellent chronologic resolution afforded by magnetostratigraphy, biostratigraphy, and isotope dating, provide a framework for interpreting the evolutionary and paleoclimatic changes in North America at the Pliocene–Pleistocene boundary, as well as for interpreting the sequence and effects of intercontinental dispersal events in the late Cenozoic.

Glacial climate change and continental Plio–Pleistocene correlations

In the late 1930s, the so-called Wood Committee (Wood et al., 1941) assigned the Blancan Provincial Land Mammal Age to the late Pliocene and inferred that the Pliocene–Pleistocene boundary was located at the end of the Blancan. Although no formal Pleistocene land mammal age was proposed in the 1941 report, a list of Pleistocene mammals was given, for which the term “Rancholabrean” (characterized by the famous remains from the Rancho La Brea tar pits in Los Angeles) later came into general use. In 1951, D. E. Savage identified the Irvington fauna of the San Francisco Bay area as the type for a new “Irvingtonian” land mammal age, for early Pleistocene, post-Blancan and pre-Rancholabrean fossil mammal faunas. Thus, by the early 1950s, three late Cenozoic land mammal ages (Blancan, Irvingtonian, and Rancholabrean) had been identified in the late Cenozoic of North America, and the Pliocene–Pleistocene boundary (in North American vertebrate paleontology) was placed at the Blancan–Irvingtonian boundary.