Pleniglacial Desertization, Large-Animal Mass Extinction and Pleistocene-Holocene Boundary in South America

Resumo

The drastic qualitative and quantitative impoverishment, represented by the extinction of the megafauna biomass constitutes one o{ the most notable events in the South American Quaternary. Extensive savanna and steppe biomes which today cover 2/3 of the continent are actually in terms of herbivorous megafauna biological deserts (e.g. Fuego- Patagonian, Pampas and Altiplano steppes, Llanos del Orinoco, Cerrados, Gran Chaco, Gran Sabana and Bogotá savannas as well as the Caatinga scrubs).

The supraregional scope of this phenomenon and its concurrence with geoscientific data lead for the first time to the hypothesis that it was caused by the radical increase of aridity during the Late Pleistocene (12,000-18,000 y.B.P.), especially in the tropics, and not by cultural reasons which had only a very limited impacto In {act, an area with about 11.5 x 106 km2 suffered long interannual droughts during this period being an unsurmontable  hysiological barrier for the large mammals. A first extinction waveaffected particularlythe tropicaland subtropicalecosystems of the Atlantic side (regions of the Orinoco, NE of Brazil and the Pampasia of Argentine and Uruguay) between 12,000-18,000 y. B.P. A retarded wave badly affected all the Andean and peri-Andean ecosystems between 10,000-9,000 y.B.P. (regions of the Gran Chaco, western Fuego-Patagonia, Altiplano of Bolivia and the present Atacama Desert). This delay or did ferential extinction on the South American scale must be associated with the glaciation that affected the Andean Cordillera until the close of the Wisconsin.

To the total surface of arid land that included at least the implantation of four continental deserts along the whole tropical and subtropical zone, we must add the marginal deserts of the emerged marine platforms, as well as a discontinous belt of desert-islands for the megafauna impossed by glaciers and mountains (ea. 1.2 x 106 km2). In this way, and as indicated by the zoo-and geodata compiled in the last ten years, the transition Pleistocene-Holocene was brutal. In fact, post-glacial fossiliferous beds (less than or equivalent to 10,000 y.B.P.) lack the characteristical giant forms of the last glacial). Mastodons, megatheres, toxodons, palaeolamas, glyptodons, bears, etc. are totally absent. Independent of their biochron, ancestry or geographic origin, only the smaller species survived, leading at the beginning of the Holocene to a ciear inversion of the founal character, -that is the  placement of a dominant megafauna by a microand mesofauna. With few exception (Camelidae, Cervidae) the whole megafauna became extinct from the Straight of Magellan up to the Lesser Antilles. This supports the fact that the present zoogeographical arrangement of South America constitutes a modern evento Finally it cannot be disregarded that a similar aridity during the Late Pleistocene was responsible for the large-animal extinctions in other southern continents, such as Australia and Africa.