, 2002, Kershaw et al., 2003 and Wroe et al., 2004). Climate change proponents argue

that only a small number of extinct megafauna have been demonstrated to overlap with humans and that the bulk of extinctions occurred prior to human arrival, questioning Roberts et al.’s (2001) terminal extinction date (Field et al., 2008). In the Americas and Eurasia, warming at the end of the Last Glacial Maximum (LGM, ca. Gemcitabine ic50 18,000 years ago) resulted in rapid changes to climate and vegetation communities during the Pleistocene–Holocene transition, creating a set of environmental changes to which megafauna were unable to adapt (Graham and Grimm, 1990, Guthrie, 2003 and Guthrie, 2006). Extinctions in the New World may have been further affected by the onset of the learn more Younger Dryas, a 1000-year cooling event, which exacerbated shifts in vegetation communities. Much of the climate change model hinges on dietary assumptions about Pleistocene herbivores, and to some degree, carnivores. A variety

of new studies are testing these assumptions using genetic (mtDNA), morphologic, and isotopic (δ 13C and δ 15N) data. North American proboscideans (e.g., mammoths, mastodons) and camelids had very different and specialized diets that may have made them vulnerable to rapid climate change and vegetation shifts, for example, but carbon isotope studies of tooth enamel suggest that C4 grasslands that supported large herbivores generally remained intact during glacial to interglacial transitions (Connin et al., 1998, Koch et al., 1994, Koch et al., 1998 and Koch et al., 2004). Patterns of specialization Uroporphyrinogen III synthase have also been found with North American carnivore species. The species with the greatest extinction vulnerability tended to be the largest and most carnivorous of their families (e.g., dire wolves, saber-tooth cats, short-faced bears). The smaller, more generalized species (e.g., gray wolves, puma and bobcats, and black and brown bears) survived into the Holocene (Leonard et al.,

2007 and Van Valkenburgh and Hertel, 1993). Other studies of environmental changes across the Pleistocene–Holocene transition have suggested that climate change is not a sufficient explanation for megafaunal extinctions. Martínez-Meyer et al. (2004) found, for example, that the reduction of habitable niches for eight megafauna taxa in North America is insufficient to explain their extinction. Pollen records further show that megafaunal extinctions in Eurasia and the Americas coincided with rapid vegetational shifts, but the link between vegetation changes and extinctions in Australia is much less clear (Barnosky et al., 2004). Although comprehensive studies are needed, current pollen records also suggest that Pleistocene–Holocene changes in vegetation were not substantially different from previous glacial–interglacial cycles (Koch and Barnosky, 2006:225–226; also see Robinson et al., 2005).