A pervasive view among many paleontologists is that the tropics have for a long time acted as the major evolutionary and dispersal center in both the marine and terrestrial realns. Following the lines of reasoning of earlier workers (Dobzhansky, 1950; Darlington, 1959; Fischer, 1960), it has been widely assumed that the more equable climate and greater habitat stability of tropical regions has promoted faster evolutionary rates and the accumulation of species-rich assemblages. These in turn permit major adaptive thresholds to be crossed in the tropics and then disseminated to higher latitudes (Stehli et al., 1969). In the Southern Hemisphere, in particular, many Mesozoic invertebrate paleontologists have regarded the eastern end of the tropical Tethyan ocean as the most likely source of higher-latitude faunas (Fleming, 1967; Stevens, 1967; Gordon, 1974; Freneix, 1981) and it would seem logical to assume that there has been a constant supply of taxa toward the polar regions right up to the present day (Darlington, 1965; Briggs, 1974). A recent challenge to these established views has come with the discovery of a rich marine invertebrate fauna in the Early Tertiary La Meseta Formation of Seymour Island, Antarctica (Zinsmeister, 1982a). As this fauna was analyzed, it became apparent that it contained a number of elements previously known only from the Late Cenozoic or Recent of much lower latitudes. Initially, Zinsmeister and Feldmann (1984) identified eleven genera (5 bivalves, 1 gastropod, 2 asteroids, 1 crinoid, and 2 decapods) that predated their midlatitude descendants by as much as 40 million years (Ma), but it is now apparent that this list should be expanded to include certain articulate brachiopods, echinoids, and barnacles (Wiedman et al., 1986). It would seem that this exceptionally well preserved fauna may be the record of a major polar diversification. The disposition of continents and ocean currents in the Late CretaceousEarly Tertiary effectively isolated many of the southern Pacific continental margins, and it is thought that a variety of new taxa would have evolved in the comparatively cool (but not glacial) waters bordering western Antarctica. Zinsmeister (1982b) and his co-workers believe that this region acted as a holding tank for these new taxa until such times as conditions were right for them to spread to lower latitudes. The most