The Descanso-Yauri is an intra-mountain basin located in the southern Peru, between the Altiplan and the Western Cordillera of the Central Andes, over 4,000 meters of altitude. The sedimentary fill of the basin is approximately of 1,500 m thick, with ages from the Lower Miocene to Pliocene. In this study, taxonomic and morphometric geometric analysis of ostracods, unknowed until now, from various sections in the basin allowing to identify 4 genera and 11 species: Cyprideis (8 species, 4 described as new); Limnocythere (1 species?), Alicenula (1 species) and Heterocypris (1 species). In the northern of the section, greater abundance and diversity of Cyprideis represented by endemic species, make the term Cyprideis flock of El Descanso Basin apropriated, as proposed here. The endemism of Cyprideis is typical in long-lived lake (ancient lake). The analysis of sieve pores in Cyprideis indicates that the conditions of the ancient lake were freshwater. The occurrence of Limnocythere and Alicenula to the top of this section may indicate depth variation of this paleolake. In the central and southern sections, the presence of genus Heterocypris is associated to temporary lakes and ponds. The possible relationship between the changes in the ostracofauna with paleogeographic and/or paleoclimatic implications is discussed. Recent palynological and isotopic researches suggest that the rise of Andean Plateu causes abrupt climate changes in the Upper Miocene to Pliocene in the studied area. This assemblage of stable and temporary lakes and the radiation event of Cyprideis in the basin are probably related to these climatic changes. The occurrence of the flock of Cyprideis species of El Descanso Basin and the coincidence with the decline of the Miocene Cyprideis flock of western Amazon seems to correspond to these tectonic and climatic events.
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