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Phosphatic remains of marine vertebrates recovered from five fossil sites of the Pisco Formation ranging from the latest Middle/earliest Late Miocene (Ca 11-13 Ma) to the Early Pliocene (Ca 3.5 Ma) have been analysed for their oxygen isotope compositions (delta O-18(p)). Coexisting seals, dolphins, whales, penguins and sharks from each locality have distinct delta O-18(p) values reflecting ecology and physiology differences, ranging from 18.2%. to 21.4%. for marine mammals, from 19.5%. to 21.5%. for marine birds and from 20.9%. to 23.1%. for sharks. Systematic offsets observed between dolphin teeth and bones as well as between dolphin and whale bones indicate that the fractionation equation established by using data from extant cetaceans may not be directly applicable to Miocene cetaceans in order to estimate water 6180, values. Assuming that polar ice-caps were not totally developed during this time interval, marine palaeotemperatures ranging from 13.0 +/- 1.3 degrees C to 17.2 +/- 1.3 degrees C were estimated. Comparison of our results with those obtained in other World’s areas suggests that the oxygen isotope ratios of Pisco vertebrates reflect the influence of both global and local events, such as the setting of the Atacama Desert, the cold Humboldt Current or the global phases of ice-cap growth and decay. (c) 2008 Elsevier B.V. All rights reserved.