Tonnoidean gastropods have planktotrophic larval lives of up to a year and are widely dispersed in ocean currents; the larvae maintain genetic exchange between adult populations. They therefore are expected to respond rapidly to new geographic barriers by either extinction or speciation. Fossil tonnoideans on the opposite coast of the Americas from their present-day range demonstrate that larval transport still was possible through Central America at the time of deposition of the fossils. Early Miocene occurrences of Cypraecassis tenuis (now eastern Pacific) in the Caribbean probably indicate that constriction of the Central American seaway had commenced by Middle Miocene time. Pliocene larval transport through the seaway is demonstrated by Bursa rugosa (now eastern Pacific) in Caribbean Miocene-latest Pliocene/Early Pleistocene rocks; Crossata ventricosa (eastern Pacific) in late Pliocene rocks of Atlantic Panama; Distorsio clathrata (western Atlantic) in middle Pliocene rocks of Ecuador; Cymatium wiegmanni (eastern Pacific) in middle Pliocene rocks of Atlantic Costa Rica; Sconsia sublaevigata (western Atlantic) in Pliocene rocks of Darien, Pacific Panama; and Distorsio constricta (eastern Pacific) in latest Pliocene-Early Pleistocene rocks of Atlantic Costa Rica. Continued Early or middle Pleistocene connections are demonstrated by Cymatium cingulatum (now Atlantic) in the Armuelles Formation of Pacific Panama. Tonnoideans indicate that the Central American seaway began to be constricted after early Miocene time, and some larval transport through the seaway was possible throughout Pliocene time. Intermittent marine connections were maintained at least during late Pliocene to early Pleistocene interglacial periods of high sea-level, and alternated with a land bridge during glacial periods of low sea-level.