Cione et al: Late Cenozoic extinction in South America 1 Rev. Mus. Argentino Cienc. Nat., n.s. 5(1): 000, 2003 Buenos Aires. ISSN 1514-5158 The Broken Zig-Zag: Late Cenozoic large mammal and tortoise extinction in South America Alberto L. CIONE 1 , Eduardo P. TONNI 1, 2 & Leopoldo SOIBELZON 1 1 Departamento Científico Paleontología de Vertebrados, Facultad de Ciencias Naturales y Museo, Paseo del Bosque, 1900 La Plata, Argentina. 2 Laboratorio de Tritio y Radiocarbono, LATYR. Facultad de Ciencias Naturales y Museo, Paseo del Bosque, 1900 La Plata, Argentina. E-mail: acione@museo.fcnym.unlp.edu.ar, eptonni@museo.fcnym.unlp.edu.ar, lsoibelzon@museo.fcnym.unlp.edu.ar. Corresponding author: Alberto L. CIONE Abstract: During the latest Pleistocene-earliest Holocene, South American terrestrial vertebrate faunas suffered one of the largest (and probably the youngest) extinction in the world for this lapse. Megamammals, most of the large mammals and a giant terrestrial tortoise became extinct in the continent, and several complete ecological guilds and their predators disappeared. This mammal extinction had been attributed mainly to overkill, climatic change or a combination of both. We agree with the idea that human overhunting was the main cause of the extinction in South America. However, according to our interpretation, the slaughtering of mammals was accom- plished in a particular climatic, ecological and biogeographical frame. During most of the middle and late Pleis- tocene, dry and cold climate and open areas predominated in South America. Nearly all of those megamammals and large mammals that became extinct were adapted to this kind of environments. The periodic, though rela- tively short, interglacial increases in temperature and humidity may have provoked the dramatic shrinking of open areas and extreme reduction of the biomass (albeit not in diversity) of mammals adapted to open habitats. Many populations were surely close to a minimum level of population viability. During the longer glacial periods, mammals 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. During the present interglacial, humans entered South America and broke the Zig-Zag when killed all the megamammals and almost all the large mammals during their less favourable periodic lapse. Key words: Extinction, South America, Pleistocene, Holocene, Mammalia, Man, Climate. ____________ It is well known that South American mam- mal communities underwent marked changes in diversity during the late Cenozoic (Pascual et al., 1965; Marshall et al., 1984; Tonni et al., 1992; Cione & Tonni, 1995, 2001; see papers in Tonni & Cione, eds., 1999). The last and most impor- tant turnover occurred in the late Pleistocene- Holocene, when about 80 % of large mammal spe- cies (weighting over 44 kg) and 100% of megamammal species (weighting over 1,000 kg, both herbivore and carnivore; see Owen-Smith, 1987) became extinct (Tables 1, 2, 3). South Ameri- can communities had included at least some megamammals since the Paleogene but during the middle Pleistocene-earliest Holocene the remark- able figure of more than 35 megamammal species was documented. For a comparison, only 4 megamammal species (and the giraffe, close to a ton in body mass) occur in Africa today and in South America there are no megammal today. The largest terrestrial mammal in the continent is the tapir Tapirus bairdii (with some individuals slightly surpassing 300 kg; Nowak & Paradiso, 1983). A few small mammals also disappeared, but no other metazoan or plant is known to have been affected by the extinction. Consequently, this ex- tinction event is different to others such as the K- T event. The late Pleistocene and earliest Holocene South American mammal fauna was very differ- ent to the Recent not only by the presence of many very large mammals but also because the glacial climates provoked a different distribution of many species (e.g. Tonni & Cione, 1997; Tonni et al., 1999a). In summary, the present composition and distribution is the result of extinction of large mammals and the biogeographic shifting and ex- tirpation in some areas of most of the biota, caused by dramatic climatic changes. For explaining the latest Cenozoic extinction, many hypotheses have been proposed, such as extinction due to climatic change and habitat de- struction (Ficcarelli et al., 1997; Ochsenius, 1997; Coltorti et al., 1998), coevolutionary disequili- brium (Graham & Lundelius, 1984), overkilling by humans (with or without blitzkrieg;» Martin, 1967, 1984; Alroy, 2001), the «keystone herbivore» hypothesis (Owen-Smith, 1987), and the infection