Outcrops of the Cipero Formation that are currently exposed near San Fernando, Trinidad, are described. These contain exceptionally well-preserved planktonic foraminifer assemblages of latest Oligocene and earliest Miocene age. The full range of included species is documented, including some rarely reported forms. Details of foraminifer test construction and wall ultrastructures are examined using scanning electron microscopy of whole and dissected tests. Observations include (i) the presence of micrometer-scale spikes – confirmed by examination of comparable material from the Oligocene of Puerto Rico – on the interior test surface of Cassigerinella chipolensis, (ii) detailed investigation of the calcite crust on tests of Catapsydrax dissimilis and Turborotalita quinqueloba, (iii) well-preserved wall microstructure on tests of “Paragloborotaliapseudokugleri, and (iv) confirmation of a radially crystalline wall in Tenuitella and Globigerinita that compares closely with Miocene Globigerinatella from ODP Site 871 (western Pacific Ocean). Multi-species oxygen and carbon stable isotope measurements divide the assemblage into various ecological groups, including mixed layer, thermocline, and sub-thermocline dwellers. The fossil assemblage and isotope data indicate an oligotrophic open-ocean setting with a well-defined and seasonally stable thermocline with sea surface temperatures of about 27°C (similar to modern) and sub-thermocline waters of about 13°C. The mixed layer was dominated by species of Globigerina, “Paragloborotaliapseudokugleri, Cassigerinella, and Globigerinita. The base of the mixed layer and upper thermocline was inhabited by Turborotalita, Paragloborotalia, Dentoglobigerina larmeui, and Tenuitella. The deep thermocline was inhabited by the large species “Dentoglobigerinavenezuelana and “D.” rohri. Sub-thermocline intermediate water was occupied by Catapsydrax dissimilis.

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