Early-Pleistocene Glaciations in Argentina (South America) and the Response of Mammals: The Case of the Pampean Region Esteban Soibelzon and Eduardo P. Tonni ➤ Keywords: Mammalian Biocenoses, Paleoclimate, Pampean Region, South America The Pampean region of Argentina (approximately between 33° 22′ and 38° 22′ S, and 65° 45′ and 56° 45′ W) at present has a temperate-humid climate with predominant subtropical faunal elements. Some authors have pointed out that most of the present-day mammal biocenoses and climatic conditions in this region seem to have been recently established (Deschamps et al. 2003; Tonni et al. 1999). Climate during most of the Pleistocene was more arid than at present, and fauna of the Pampean region was adapted to these conditions, with an abundance of megaherbivores (body mass more than 1 ton; see Cione et al. 2008; Tonni et al. 1999). In the Pleistocene sequences of Argentina, loesses and paleosols show fluctuations between glacial and interglacial cycles that occurred during the Pleistocene. Loess (or loessoid) levels were deposited during glacial periods, and paleosols developed during interglacial periods (Bidegain et al. 2007; Nabel et al. 2000; Pecsi 1990). These climatic changes affected the biota, modifying speciation and extinction rates, mainly as a result of habitat frag- mentation (Barnosky 2005; Cione et al. 2008; Réale et al. 2003). More than 15 glacial cycles have been detected in the Pleistocene sequences of Patagonia (Argentina); among these, a major one occurred around 1 million years ago (Ma) and is known as the “Great Patagonian Glaciation” (GPG) (Fig. 6 in Rabassa et al. 2005; Singer et al. 2004; Fig. 3 in Soibelzon et al. 2006). After the GPG another important glacial event is recorded in Patagonia around the Matuyama/Brunhes boundary (0.78 Mya) (Rabassa et al. 2005). These glacial events produced significant environmental changes in the Pam- pean region, affecting not only the distribution and composition of its fauna, but also increasing continentality as a consequence of the drop in sea level that occurred during the Last Glacial Maximum (Tonni et al. 1999). In the Pam- pean region the GPG and post-GPG events are correlated with the arid events registered around the Matuyama/Brunhes polarity change (Bidegain et al. 2005; Nabel et al. 2000), based on the characteristics of glass shards recorded in the loess. The base of the Brunhes polarity zone shows a peak of glass shards that suggests that the Matuyama/Brunhes turnover coincided with intense volcanic activity in the Cordillera de Los Andes. Nabel et al. (2000) have Esteban Soibelzon and Eduardo P. Tonni, División Paleontología Vertebrados, Museo de La Plata, Facultad de Ciencias Naturales y Museo, UNLP, Paseo del Bosque s/n, B1900FWA La Plata, Argentina; e-mails: esoibelzon@fcnym.unlp.edu.ar eptonni@museo.fcnym.unlp.edu.ar CRP 26, 2009 SOIBELZON/TONNI 175