An evolutionary perspective on marine faunal connections between southernmost South America and Antarctica

The origins of present day benthic marine faunas from both the Magellan and Antarctic provinces may lie as fat back as the Early Cretaceous (approx. 1 30 Ma). This was the time of the first significant marine incursion across the Gondwana supercontinent and isolation of a high-latitude group of conti nents. It was also the probable time of formation of the temperate. Pacific -margin Weddellian Province, which extended from Patagonia, through Antarct ica and New Zealand, to south-eastern Australia. Both palaeontological and phylogenetic evidence suggest that a number of Living taxa (i.e. genera and families) from both provinces can be traced back to the Late Cretaceous-ea rliest Cenozoic interval. Ar this rime there was no discernible gradient in taxonomic diversity from either southernmost South America or Australasia into Antarctica. The long, essentially temperate, Eocene epoch was followed by a period of major change during the ensuing Oligocene. At some time dur ing this interval the Antarctic circum-polar current was fully formed and t his led to a vicariant event between the Magellan and Antarctic faunas. How ever, it is important to stress that the intensification of circumpolar cir culation also promoted at least some dispersal between various Subantarctic and Antarctic sites. In all probability, it was as late as the late Miocen e (some 10-12 m.y. ago) before an intense pattern of thermal zonation (in b oth horizontal and vertical senses) was established in the world ocean. Thi s may be the true time of full differentiation between the Magellan and Ant arctic provinces. Although certain major groups, such as the bivalve mollus cs and decapod crustaceans, have obviously declined within Antarctic region s through time, others, such as the bryozoans, echinoderms, amphipods and i sopods appear to have flourished. The kev to evolutionary success in cold p olar waters may be not so much resistance to low temperatures, but the abil ity to exploit novel habitats and trophic regimes. Rates of speciation are not necessarily lower in cold, polar waters, or rates of extinction higher. The Antarctic fossil record suggests that there is no simple relationship between the onset of glaciation and the extinction of certain key bivalve a nd decapod groups.