722 Mammals (Late Quaternary), Extinctions of Figure Late Quaternary extinction and prehistoric human dispersal; for numerical sequence and estimates of extinction timing, see the legend to Figure (Reproduced from Martin PS and Steadman DW (1999) Prehistoric extinctions on islands and continents In: MacPhee R (ed.) Extinctions in Near Time, pp 17–55 New York: Kluwer/Plenum, with permission from Springer.) million years (Ma), with the late Quaternary representing the past 200,000 years In North America the late Quaternary includes the Rancholabrean mammalian age stage, marked by the appearance of the genus Bison With the possible exception of Australasia, most late Quaternary extinctions fall within the reach of radiocarbon assay, essentially the past 40,000 years Fossils uncontaminated by groundwater and yielding bone collagen or gelatin are suitable for dating Environments favorable for the preservation of bone collagen include dry caves of arid regions, the frozen ground of high latitudes, lacustrine deposits, and saline or petrolic sediments such as those at Rancho la Brea, California Burned bone and associated charcoal are ideal for dating, although charcoal may prove intrusive A variety of geochemical dating methods, especially radiocarbon dating, allow biogeographers to compare both the rate of extinction within a landmass and the timing of extinctions between landmasses A robust chronology allows intercontinental and insular comparisons of considerable value in inferring cause of the extinctions Although details regarding just how the extinctions were triggered remain speculative, it is increasingly clear that the two major contending explanations of what forced LQEEF that is, climatic changes and cultural impactsFcan now be evaluated globally When approached on a comparative basis, human expansion and climatic change track a punctuated extinction episode in North America and perhaps in South America, whereas climatic change is less clearly involved in Australia or Madagascar and appears to be of no significance in New Zealand and other remote Pacific islands North America Focusing on the past million years only (the Pliocene and the Quaternary), it is possible to evaluate the LQEE using agestage divisions of biostratigraphers (Table 1) The overwhelming importance of extinctions of North American large mammals at the end of the Quaternary, shown in Figures and and in Table 1, has been evident for at least the past 50 years The widespread adoption of screen washing of fossil deposits has vastly enriched the fossil record of small vertebrates The harvest of identified small mammals is vital in supporting the conclusion that no appreciable extinctions of small mammals occurred (Figure 3)Fcertainly not matching the extinction spasm of large mammals approximately 11,000 radiocarbon years ago (Figure 4) The fossil record is based on a much larger sample of late Pleistocene than early Pleistocene and Pliocene faunas (Table and Figures and 4) Presumably this accounts at least in part for the increase in number of genera of both large and small mammals in the later part of the record Although Mammuthus, Equus, and extinct Bison included multiple species, most of the large genera listed in Table are monotypic (represented by one species only) Although a few survived on other continents, most large genera totally vanished (Table 1) In North America north of Mexico, 33 genera of large mammals disappeared in the late Quaternary, which constitutes two-thirds of the total late Rancholabrean fauna of 48 genera and more than all the generic extinctions recorded during the previous million years prior to LQEE (Table 1) Events that might trigger regional or global extinctions in the early part of the past million years include intercontinental suturing of the Americas with faunal intermingling, extraterrestrial accidents such as the impact of the Eltanan bolide, and the initiation of continental glaciation However, only at the end of the Quaternary is there a major extinction spasmFone that impoverished America’s large mammalian faunas (Figure 4) Unlike the case of the large mammals, a much larger number of small genera (48) vanished in the past million years Within the minifauna (rat, mouse, or shrew-size mammals), which make up the majority of the 75 living genera of small mammals, no generic extinctions are known in the late Quaternary Among the medium-sized mammals, three genera were lost: the antilocaprid Stockoceros, the skunk Brachypotoma, and the rabbit Aztlanolagus The difference deserves close inspection Although small mammals escaped virtually unscathed, the LQEE (late Rancholabrean losses) blighted all terrestrial orders of large land mammals and eliminated one, the Proboscidea (Table 1)