The Oligocene-Miocene Transition (OMT) was a time of significant oceanic, climatic, and biotic change, but there is still a great deal we do not understand about its effects, particularly in terms of ocean circulation. The Central American Seaway (CAS) was an important ocean gateway at this time; recent fully coupled modeling results have suggested a possible temporary reversal of surface flow, from westward to eastward, during the OMT. Such a flow reversal would have altered numerous oceanographic properties and the dispersal of marine taxa. Here, we find a mismatch in the timing of the Atlantic vs. Pacific first appearances of the tropical mixed layer planktic foraminifer Paragloborotalia kugleri, a key zonal marker for the OMT. The first appearance ages for P. kugleri from fourteen ocean drilling sites vary from ∼23.2–23.05 Ma in the Pacific to ∼23.05–22.7 Ma in the Atlantic. Key requirements for including a site in this compilation are: 1) sampling resolution; 2) independent non-biostratigraphic chronology, such as magnetostratigraphy or orbital tuning; and 3) a preference for shore-based biostratigraphic analyses rather than shipboard estimates. Although we explore alternative explanations, we conclude that, given the restricted nature of the CAS gateway, timing of dispersal, and results from previous modeling efforts, CAS flow reversal is the most parsimonious explanation for the delayed first appearance of P. kugleri in the Atlantic relative to the Pacific. We suggest that after originating in the tropical Pacific, P. kugleri was initially blocked from dispersal into the Atlantic by westward surface circulation through the CAS during the latest Oligocene. During the OMT, circulation reversed and Pacific surface water flowed through the CAS into the Atlantic, allowing P. kugleri to disperse into the Atlantic. Previously published ocean-climate simulations suggest that the cause of this reversed flow may be related to the progressive constriction of Tethys and opening of the Drake Passage at the time of the OMT, compounded by a short-lived glaciation event in Antarctica and possible change in meridional temperature gradient and prevailing wind patterns in the tropics.

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