QUATERNARY COLONIZATION OR PALEOGENE PERSISTENCE - HISTORICAL BIOGEOGRAPHY OF SKATES (CHONDRICHTHYES, RAJIDAE) IN THE ANTARCTIC ICHTHYOFAUNA

Seven endemic species of skates (Chondrichthyes: Rajidae) represent th e only family of elasmobranchs currently known to live in Antarctic co ntinental waters. Many previous authors believed skates colonized Anta rctic waters from Patagonia during interglacial periods in the Quatern ary. However, recent fossil material collected from the middle Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula, indicates that they may have persisted in Antarctic waters since the Paleogene. Additionally, oceanographic barriers present in the Neogene and Quater nary would have prevented dispersal from southern continents to Antarc tica. A revised dispersal scenario, based on skate fossils, biology, p aleogeography, and present centers of skate diversity, suggests that s kates evolved in the western Tethys and North Boreal seas of western E urope in the Late Cretaceous and early Paleogene and emigrated into An tarctica during the early to middle Eocene via a dispersal corridor al ong the continental margins of the western Atlantic Ocean. Skates prob ably populated the Pacific Basin by passing from this dispersal corrid or through the Arctic Ocean. Vicariant events, such as opening of the Drake Passage, the development of the Circum-Antarctic Current, and fo rmation of deep and wide basins around Antarctica in the late Paleogen e, created barriers that isolated some species of skates in Antarctica and prevented movement of other species of skates into Antarctica fro m northern areas. Skates are the only group of fishes known to have su rvived the Oligocene cooling of Antarctica that killed or extirpated t he Paleogene ichthyofauna; they persisted by a combination of cold-tol erance, generalized diet, and unspecialized bathymetric and habitat pr eferences.