Fifty radiolarian events (first or last occurrences) of Quaternary and Neogene age were identified in a transect from 5 drill sites (503, 573, 289, 586, 214) in the equatorial Indian and Pacific Oceans, extending from the Panama basin to the 90°E Ridge. Using an "absolute" chronology for each site based on correlation with the paleomagnetic time scale, we have estimated the degree of synchroneity or time-transgressiveness for each datum. Of those events that appear to be synchronous within the probable uncertainty of our estimate (ca. 0.3 m.y.), 85% are last occurrences. Of those events that are unmistakably time-transgressive (i.e., 1 m.y. or more), 75% are first occurrences in which the taxon evolves earliest in the Indian Ocean and progressively later in the weste n and eastern Pacific. This study, to our knowledge, is the first to document nonsynchroneity of microfossil events within a given latitudinal zone. If this effect proves characteristic of other microfossil groups as well (foraminifera, nannofossils, diatoms), it has important implications for biostratigraphy, geochronology, and plankton evolution. In particular, it suggests a reevaluation of events that presently define zonal boundaries, using precisely dated material, perhaps with the ultimate consequences of favoring last occurrences over first occurrences. The earlier evolution of taxa in the Indian Ocean is particularly unexpected in view of the strong westward zonal flow in low latitudes.

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