Speciation models for marine plankton commonly assume that tectonic barriers and water mass fronts act as potent isolating mechanisms. Here we present evidence indicating that tectonic and water mass barriers to dispersal are, in some cases, very weak. We identify a transient occurrence (19 k.y. duration) of the planktic foraminifer Truncorotalia truncatulinoides in the Atlantic Ocean 500 k.y. before its generally accepted first appearance anywhere outside the southwest Pacific. This finding provides strong evidence for long-distance dispersal of plankton, enabling them to opportunistically colonize normally inhospitable environments as soon as they become favorable. Our newly described appearances of T. truncatulinoides in the Atlantic are restricted to the onset of the first very severe glacial stage (marine isotope stage 100) at 2.54 Ma, approximately 200 k.y. after the global evolution of the species in the southwest Pacific. This ability of plankton to disperse long distances, combined with recent evidence for high gene flow throughout the extra-polar oceans, suggests that it may be very difficult, even impossible, to geographically isolate pelagic populations for extended periods of time, a key component in vicariant and allopatric speciation models. Instead, high dispersal capability favors sympatric or parapatric processes as the prevalent modes of marine speciation.