Introduction

Prior to the late Pleistocene, megaherbivores were represented by a wider variety of taxa than occur today, and were present on all continents. Their disappearance from Europe and the Americas took place at the end of the last glacial period of the Pleistocene, around 11,000 years ago, and was synchronous with the extinction of numerous other large mammal forms. The extinctions occurred during a time of rapid climatic change, with associated transformations in habitat conditions. Another important event took place at about the same time: the entry of humans into the Americas, following their expansion through the furthest corners of the Old World. The relative importance of climate and associated habitat changes versus human predation as causal agents in the late Pleistocene extinctions remains an unresolved problem (Martin & Wright 1967; Remmert 1982; Martin & Klein 1984).

Since climatic change and human range expansion are so closely interwoven in time, wider patterns need to be considered in order to understand the causal links in these extinctions. These include the geographic distribution of extinctions, and variations in the incidence of extinctions among genera of differing body size. In the following analysis I focus specifically on large mammalian herbivores, since it is generally accepted that extinctions of carnivores and of large scavenging birds were related to their dependence upon the herbivores as a food source. It is the herbivores that are likely to be most responsive to changing habitats; and it is also such species that were the prime targets as prey for the expanding human population.