Cione et al: Late Cenozoic extinction Rev.in South Mus. America Argentino Cienc. Nat., n.s.1 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. CIONE1, Eduardo P. TONNI1, 2 & Leopoldo SOIBELZON1 1Departamento Científico Paleontología de Vertebrados, 'acultad de Ciencias Naturales y Museo, Paseo del Bosque, 1900 La Plata, Argentina. 2Laboratorio de Tritio y Radiocarbono, LATYR. 'acultad de Ciencias Naturales y Museo, Paseo del Bosque, 1900 La Plata, Argentina. E-mail: [email protected], [email protected], [email protected]. 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- no other metazoan or plant is known to have been mal communities underwent marked changes in affected by the extinction. Consequently, this ex- diversity during the late Cenozoic (Pascual et al., tinction event is different to others such as the K- 1965; Marshall et al., 1984; Tonni et al., 1992; T event. Cione & Tonni, 1995, 2001; see papers in Tonni The late Pleistocene and earliest Holocene & Cione, eds., 1999). The last and most impor- South American mammal fauna was very differ- tant turnover occurred in the late Pleistocene- ent to the Recent not only by the presence of many Holocene, when about 80 % of large mammal spe- very large mammals but also because the glacial cies (weighting over 44 kg) and 100% of climates provoked a different distribution of many megamammal species (weighting over 1,000 kg, species (e.g. Tonni & Cione, 1997; Tonni et al., both herbivore and carnivore; see Owen-Smith, 1999a). In summary, the present composition and 1987) became extinct (Tables 1, 2, 3). South Ameri- distribution is the result of extinction of large can communities had included at least some mammals and the biogeographic shifting and ex- megamammals since the Paleogene but during the tirpation in some areas of most of the biota, caused middle Pleistocene-earliest Holocene the remark- by dramatic climatic changes. able figure of more than 35 megamammal species 'or explaining the latest Cenozoic extinction, was documented. 'or a comparison, only 4 many hypotheses have been proposed, such as megamammal species (and the giraffe, close to a extinction due to climatic change and habitat de- ton in body mass) occur in Africa today and in struction ('iccarelli et al., 1997; Ochsenius, 1997; South America there are no megammal today. The Coltorti et al., 1998), coevolutionary disequili- largest terrestrial mammal in the continent is the brium (Graham & Lundelius, 1984), overkilling tapir Tapirus bairdii (with some individuals by humans (with or without blitzkrieg;» Martin, slightly surpassing 300 kg; Nowak & Paradiso, 1967, 1984; Alroy, 2001), the «keystone herbivore» 1983). A few small mammals also disappeared, but hypothesis (Owen-Smith, 1987), and the infection 2 Revista del Museo Argentino de Ciencias Naturales, n. s. 5 (1), 2003 Table 1. Mammal taxa present in presumed Lujanian (sensu Cione & Tonni, 1999) beds in South America that became extinct. Asteriscs indicate those taxa that occur in archaeological sites. Megamammals Glossotherium tropicorum Megatherium medinae Cuvieronius humboldti * Glyptodon clavipes Mixotoxodon larensis Cuvieronius hyodon Glyptodon perforatus Mylodon darwinii Doedicurus clavicaudatus * Glyptodon reticulatus Mylodon listai * Eremotherium carolinense Glyptodon sp. * (from archaeo- Neothoracophorus depressus Eremotherium laurillardi logical site) Panochthus frenzelianus Eremotherium mirabile Haplomastodon guayasensis Panochthus morenoi Eremotherium rusconii Haplomastodon waringi Panochthus tuberculatus Glossotherium (Oreomylodon) Hemiauchenia paradoxa * Plaxhaplous canaliculatus wagneri Lestodon armatus Stegomastodon platensis Glossotherium lettsomi Lestodon trigonidens Toxodon burmeisteri Glossotherium Macrauchenia patachonica * Toxodon platensis * (Pseudolestodon) myloides Megalonyx sp. Glossotherium robustum * Megatherium americanum * Large mammals Holmesina paulacoutoi Pampatherium typum Antifer niemeyeri Hoplophorus euphractus Paraceros fragilis Arctotherium bonariense Lama gracilis * Parapanochthus jaguaribensis Arctotherium brasiliense Morenelaphus lujanensis Propraopus grandis Arctotherium tarijense Mylodopsis ibseni Propraopus humboldti Brasiliochoerus stenocephalus Neochoerus aesopy Propraopus magnus Equus (A.) andium Neochoerus sirasakae Scelidodon cuvieri Equus (A.) insulatus Neuryurus n. sp. Scelidodon chiliensis Equus (A.) lasallei Nothropus priscus Scelidodon reyesi Equus (A.) neogeus * Nothrotherium roverei Scelidotherium leptocephalum Equus (A.) santa-elenae Ocnopus gracilis Sclerocalyptus sp. * Eulamaops paralellus Ocnotherium giganteum Smilodon populator Eutatus seguini * Onohippidion saldiasi * Tapirus cristatellus Glyptotherium sp. Palaeolama niedae Trigonodops lopesi Hippidion principale * Palaeolama weddelli Xenorhinotherium bahiense Holmesina occidentalis Pampatherium humboldti Medium sized mammals Protocyon orcesi Protopithecus brasiliensis Canis dirus Protocyon troglodytes Valgipes deformis Canis avus Small mammals Eligmodontia n. sp. Microcavia robusta Desmodus draculae hypothesis ('erigolo, 1999). Both more arid tain that there are few bones of extinct mammals (Ochsenius, 1997) and more humid (Oliveira, 1999) in the early archaeological sites (Politis et al., 1995). conditions were found to be important for explain- With respect to the infection hypothesis, immi- ing the extinction. Opponents of the theory that grants that could carry infection vectors to the lo- large mammal extinctions were caused by climatic cal fauna have been entering South America since change state that there were many other similar the beginning of the Great American Interchange and periodic climatic changes during the late Pleis- (ca. 3,000,000 YBP) and thus cannot readily explain tocene that did not triggered massive extinctions a much later, short event. (see Roy, 2001). Besides, counters of the blitzkrieg It is not our goal to discuss the other hypoth- hypothesis carried out by human immigrants main- eses (there is abundant casuistic about it; see Cione et al: Late Cenozoic extinction in South America 3 Table 2. Present large mammals in South America (Nowak & Paradiso, 1983). Blastocerus dichotomus Lama vicugna Tapirus terrestris Catagonus wagneri Lama guanicoe Tapirus pinchaque Hippocamelus antisensis Panthera onca Tapirus bairdii Hippocamelus bisulcus Priodontes maximus Tremarctos ornatus Hydrochoerus hydrochaeris Puma concolor Table 3. Present large mammals in North America (Nowak & Paradiso, 1983). Alces alces Odocoileus hemonius Panthera onca Antilocapra americana Odocoileus virginianus Puma concolor Bison bison Oreamnos americanus Rangifer tarandua Canis lupus Ovibos moschatus Ursus americanus Cervus elaphus Ovis canadensis Ursus maritimus Enhydra latris Ovis dalli Ursus arctos above) but proposing a new one, the Broken Zig- Zag hypothesis. In this paper, we analyze the bio- stratigraphic pattern of the large continental mammals and tortoises in South America during the late Pleistocene-earliest Holocene, and sug- gest that the extinction was caused by human hunting activities, but within a particular envi- ronmental framework. METHODS We rely heavily on the middle-late Pleistocene- Holocene biostratigraphic record (and radiomet- ric dating) of the Pampean area of central Argen- tina, because it is the best known in South America (see articles in Tonni & Cione, 1999, eds.; 'igure 1). Most of the previous analyses took the «Lujanian» mammals in toto for the analysis of the extinction process (see Marshall et al., 1984) and considered the Lujanian South American Land Mammal «Age» as including the Bonaerian and Lujanian sensu