Recently published diatom biochronologies provide accurate (to 0.1 m.y.) determination of the ages of appearances and disappearances of planktonic diatoms during the past 18 m.y. in the equatorial Pacific, North Pacific, and Southern Ocean. Comparisons of these records reveal the age of evolutionary appearance and extinction of species and their region of origin. Extinct planktonic diatom species have a mean longevity of 3.4 ?? 2.8 m.y. (SD, n = 53) in the equatorial Pacific, 2.5 ?? 2.1 m.y. (n = 52) in the North Pacific, and 2.9 ?? 2.3 m.y. (n = 38) in the Southern Ocean. The relatively large standard deviations are likely due to the inclusion of taxa that probably could be subdivided into two or more species. In the equatorial Pacific, evolutionary turnover of diatom species was relatively high between 18.0 and 6.0 Ma compared with the period after 6.0 Ma, presumably reflecting changing oceanic circulation and evolving water masses. In the North Pacific, evolutionary turnover peaked between 10.0 and 4.5 Ma, with increasing high-latitude cooling and enhanced provincialism. In the Southern Ocean, evolutionary turnover of endemic diatoms was greatest between 5.0 and 1.6 Ma, which provides evidence for the strong provincial character of Pliocene diatom assemblages. Taken as a whole, oceanic diatom assemblages became increasingly provincial in character during the late Miocene and Pliocene, as pole-to-equator thermal gradients increased and oceanic frontal systems were strengthened.