Current knowledge of the break-up of Gondwana during the Tertiary indicates that shallow water marine habitats may have been present continuously, and on occasions were considerably more extensive than at present. Although direct fossil evidence is sparse after the Eocene, geophysical evidence suggests that shallow waters have been present since the late Mesozoic, and possibly much longer. The break-up of Gondwana was accompanied by a more or less steady lowering of both surface and bottom temperatures in the Southern Ocean from about 15°C in the Late Cretaceous to the present range of roughly +2 to −1.8°C. Microfossils in deep-sea drilling cores indicate that temperature drops were particularly sharp in the early Oligocene ( c. 38 Ma), mid-Miocene (10–14 Ma) and Pliocene ( c. 4 Ma BP). Geological evidence suggests that the Drake Passage opened, and the present oceanographic regime established, about 25–30 Ma BP. This is now known to be about the time of full-scale development of the East Antarctic ice cap. Subsequently ice sheets extended across, and deeply eroded, the continental shelves but the effects of these glacial maxima on the marine biota are not fully understood. Late Cretaceous/early Tertiary marine fossils from the James Ross Island group indicate a diverse shallow water marine fauna, including two groups notably lacking in diversity in the living fauna: decapods and teleost fish. In several genera occurrences in this fauna predate first occurrences in lower latitudes by as much as 40 Ma, suggesting the possibility that a number of groups originated at high southern latitudes. The living fauna exhibits a high biomass in many areas, and within-site diversity can be as high as anywhere in the world. Some individual taxonomic groups, however, (notably bivalves and gastropods) have a lower diversity than in the tropics, supporting the concept of a latitudinal cline in diversity. Studies of physiological adaptation to temperature suggest that the decline in seawater temperature during the Cenozoic has not presented a particularly severe evolutionary problem. The reasons for the absence of large decapods and the low diversity of fish in the present fauna are unclear. Most of the biological features of the modern fauna are more likely a response to the seasonality of the ecosystem rather than low temperature per se . Overall the evidence suggests that the present Southern Ocean shallow water marine fauna largely evolved in situ, having been present since at least the Late Cretaceous, and possibly much longer. Some groups have invaded, for example along the Scotia arc, but the isolation of the Southern Ocean by the present oceanographic regime and the limited dispersal ability of many forms means that exchange with lower latitudes is very slow.

Large‐scale biogeographic patterns in marine systems are considerably less well documented and understood than those in terrestrial systems. Here, we synthesize recent evidence on latitudinal and bathymetric gradients of species diversity in benthic mollusks, one of the most diverse and intensively studied marine taxa. Latitudinal gradients in coastal faunas show poleward declines in diversity, but the patterns are highly asymmetrical between hemispheres, and irregular both within and among regions. The extensive fossil record of mollusks reveals that latitudinal gradients have become steeper during the Neogene, partly because of a rapid diversification in tropical coral reefs and their associated biotas. Much of the inter‐regional variation in contemporary latitudinal trends depends on the longitudinal distribution of reefs and major Neogene vicariant events. Thus, coastal faunas reveal a strong evolutionary–historical legacy. Bathymetric and latitudinal gradients in the deep ocean suggest that molluscan diversity is a function of the rate of nutrient input from surface production. Diversity may be depressed at abyssal depths because of extremely low rates of organic carbon flux, and at upper bathyal depths and high latitudes by pulsed nutrient loading. While the deep‐sea environment is not conducive to fossilization, relationships between local and regional diversity, and the distribution and age of higher taxa indicate an evolutionary signal in present‐day diversity gradients. Marine invertebrate communities offer tremendous potential to determine the relative importance of history and ecological opportunity in shaping large‐scale patterns of species diversity.

One of the most important outcrops of uppermost Cretaceous (Campanian-Maastrichtian)

The aim of this study was to use data for gastropod and bivalve molluscs to determine whether the fauna of the Southern Ocean is sufficiently well known to establish robust biogeographical and macroecological patterns. We chose molluscs for this work because they have been collected by almost every biological expedition to Antarctica, and are relatively well known taxonomically. Sampling of the continental shelf fauna is reasonably full and extensive, although new species are still being described and there are significant gaps in sampling off Wilkes Land and in the Bellingshausen and Amundsen Seas. Species richness was highest in those areas that have been subject to the most intense research activity and this pattern remained even after correction for sampling intensity. The low species richness of the Southern Ocean molluscan fauna compared with many tropical sites is confirmed, and is related principally to the absence of the large number of rare taxa that characterize some tropical assemblages. There is as yet no convincing evidence for a latitudinal cline in molluscan diversity within the Southern Ocean. Multivariate analyses defined biogeographical provinces very similar to those established previously, though they also identified a number of finer‐scale sub‐provinces including a small area of high diversity off Enderby Land. Most Southern Ocean gastropods and bivalves are rare, with limited distributions; relatively few taxa have circumpolar distributions.