Ten fossil localities were selected (figure 1) in view of their fossil content. The collection of fossil vertebrates was made according to current rules, accompanied by detailed profiles, reporting precise stratigraphic and geographic provenance. This task included sifting through 1 and 2 mm sieves to recover microvertebrates. Fossils collected are housed in the Cátedra de Geología Histórica from the Universidad Nacional del Sur, Bahía Blanca (repository UNSGH and BB). Appendix 1 shows the complete systematic list of the vertebrates found in all localities. For the biostratigraphic analysis, the taxa found in each locality were listed in the profile with their stratigraphic provenance (see figures 3-6). These records determined biostratigraphic units following the Argentine Code of Stratigraphy (Comité Argentino de Estratigrafía, 1992), and chronostratigraphic units, in order to enable the correlation among localities. The defined biozones were set down in a chronostratigraphic chart, which was modified from the one proposed by Zavala and Quattrocchio (2001) according to the new results. This allowed visualization of the space-time correlation of the events. Four Range Zones and two Association Zones were recognized. Biochrons of taxa were taken from literature (compilations of Alberdi et al ., 1995; Cione et al ., 1999 and literature therein), and are shown in Appendix 2. Some of the taxa biochrons had to be analyzed in light of the current knowledge of the geology of their type localities, in order to reduce misinterpretations of stratigraphic provenance. Available absolute dating was also taken into account, resulting in a reliable chronology for the local stratigraphic framework.
The study area is located in the southwest of Buenos Aires Province between 38°30' S lat. and the Mar Argentino, and 61°20'-62°30' W long. (figure 1). The main geographic features are the Sierras Australes and two watercourses, the Sauce Grande river and arroyo Napostá Grande. The regional substratum is formed by reddish-brownish, silty-loessic sediments, informally named ?Sedimentos Pampeanos? (?Pampean Sediments?) that crop out in quarries, riverbanks, and roads cuts. The ?Pampean Sediments? are assigned in this area either to the Monte Hermoso Formation (Lower to Middle Pliocene, Zavala, 1993; Marshall et al ., 1983: 37 and literature therein) or to the Saldungaray-La Toma Formations (Late Pliocene-Early Pleistocene, Furque, 1967; see also Marshall et al ., 1983) depending on the locality. The lithological uniformity of the ?Pampean Sediments? makes them very difficult to correlate even between geographically nearby localities. They span from the Late Miocene to the Middle Pleistocene, from southwest to northeast of the Buenos Aires Province (Fidalgo et al ., 1975; Marshall et al ., 1983). The Sauce Grande river and arroyo Napostá Grande have cut the regional substratum and deposited the fluvial sediments which form the La Delta, San José and Agua Blanca Sequences (assigned to the Early Pleistocene, Early-Middle Pleistocene, and Middle Pleistocene-Holocene respectively (Zavala and Quattrocchio, 2001), and the Chacra La Blanqueada Formation (Late Holocene-Historical Times (Rabassa, 1989; Rabassa et al ., 1991). The eolian sediments are named as the Saavedra and Matadero Saldungaray Formations assigned to the Late Pleistocene-Holocene and Late Holocene- Historical Times respectively (De Francesco, 1970; Rabassa, 1989) (figure 2).
Zavala and Quattrocchio (2001) made the facies and sequence-stratigraphic analysis of the area. They described the depositional sequences related to the valley filling, and interpreted that the sequence was controlled by relative sea level changes, and that these changes were in turn, triggered mainly by paleoclimatic fluctuations, although the authors do not discard neotectonic influence.
The biochron of each taxon was assessed (see Material and methods), as well as the association in which they were recorded (see Appendix 2 and Comments). The ten localities selected for this study encompass the whole lapse represented in the area: Cantera Seminario, Cantera Relleno Sanitario (both near Grünbein), Barrancas de Sarmiento, Las Obscuras, Dique Paso Piedras, Balneario Saldungaray, Bajo San José, Arroyo Napostá Grande, Puesto La Florida and García del Río (figure 1). The first four are formed exclusively by ?Pampean Sediments?. The other six include fluvial and eolian deposits.