Gary Haynes_Ch07.indd

The last important – and possibly the most spectacular – turnover in South American mammal history occurred around the Pleistocene-Holocene boundary, when 100% of megamammal species and about 80% of large mammal species became extinct. In this paper, we consider as “megamammals” those with body mass over 1,000 kg, and “large mammals” those over 44 kg (Tables 7.1 and 7.2). With the exception of a few smaller mammals, no other animal or plant disappeared. Consequently, this extinction event was distinct from mass extinctions (see comments in Cione et al., 2003). South American communities had included at least some megamammals since the latest Paleogene, but gigantism in mammals markedly emerged during the Ensenadan age (early Pleistocene; Fig. 7.1) when many large mammal genera fi rst appeared (Ameghino, 1889; Pascual et al., 1965; Cione and Tonni, 2005). Most genera, with different species, survived during the late Pleistocene when the remarkable fi gure of 37 megamammal species can be documented (Table 7.1). Many appear to have persisted in the early Holocene. However, none is living today. Moreover, presently there are no megammals in the Americas, Europe, and Australia. In Asia today there are only two megamammals and in Africa four megamammal species and the giraffe, close to a ton in body mass. The largest terrestrial mammal in the Neotropics is the tapir Tapirus bairdii, some individuals of which slightly surpass 300 kg (Nowak and Paradiso, 1983). The large mammal extinction of the latest Cenozoic occurred on different continents and islands at different times (Steadman et al., 2005). For explaining these extinctionsseveral hypotheses have been proposed, most related to cold climate, disease, or human activities. We agree with Barnosky et al. (2004) in that the accumulated evidence suggests it is time to move beyond casting the Pleistocene extinction debate as a dichotomy of climate versus humans. In this context, we have proposed what we call the Broken Zig-Zag hypothesis (Cione et al., 2003). During most of the middle and late Pleistocene, dry and cold climate caused open areas to predominate in South America. Nearly all megamammals and large mammals that became extinct were adapted to this kind of environment. The periodic though relatively short interglacial increases in temperature and humidity led to dramatic shrinking of open areas and extreme reduction of the mammalian biomass (albeit not of species richness) adapted to open habitats. However, during the longer glacial periods, open-area mammal populations recovered. This alternation of low and high biomass of mammals from open and closed areas is what we refer to as the Zig-Zag. Remarkably, the extinction rate of large mammals and megamammals during more than a half million years was not high, until rising abruptly during the latest Pleistocene. In this paper, (1) we analyze the biostratigraphic and chronologic pattern of the large continental mammals of South America during the late Pleistocene-earliest Holocene with new evidence, (2) discuss why megamammals and large mammals were more liable to become extinct than those that survived, (3) discuss the timing of human entry into South America, and (4) examine the possible role of humans in this extinction.