Foraminifer count and multispecies isotopic data from Ocean Drilling Program (ODP) Sites 806 (0 ¿N, 165 ¿E) and 847 (0 ¿N, 95 ¿W) are presented for the last 6.4 m.y. Faunal evidence shows a decline of taxa of Miocene origin between 4.5 and 4.0 Ma at both sites. Prior to this decline, Globoturborotalita species and Neogloboquadrina acostaensis dominated Sites 806 and 847 assemblages, respectively. Globoturborotalitids were not replaced by other thermocline dwellers after 4.2 Ma, which suggests a deepening of the thermocline and thickening of the warm pool at Site 806. At Site 847 in the eastern cold tongue, N. acostaensis was gradually replaced by N. dutertrei. After 4.2 Ma, there was also a large increase (from 0.7 to 1.3?) in the difference between the Δ18O values of Globigerinoides sacculifer, a surface dweller, and Neogloboquadrina dutertrei, a seasonal thermocline dweller at Site 847; these data, in conjunction with faunal evidence, suggest that the temperature gradient in the upper photic zone increased significantly, largely because of subsurface cooling. The events in planktonic foraminifer evolution and ecology suggest that between 4.5 and 4.0 Ma a significant step was made toward development of the modern east-west gradient in surface hydrography. The timing of these events suggests that they were related to the closing of the Central American Seaway and subsequent changes in meridional temperature gradients and/or changes in air-sea interactions that modified tropical winds. ¿ 2000 American Geophysical Union