Origins: Antarctica formed at the centre of an ancient supercontinent called
Gondwana, comprising Africa, South America, India, Australia and New Zealand. Scientists have now reconstructed a realistic chain of events documenting the passage on Gondwana through geological time. The final break in Gondwana occurred 25-35 million years ago between South America and Antarctica, at which point Antarctica reached its final polar position with widespread consequences (Livermore et al., in press). This led to the opening of the Drake Passage and the
surrounding of the continent in its entirety by water. The subsequent development of the Antarctic Circumpolar Current completed the climatic isolation of the continent and at this time the region began to correspondingly cool. Low temperatures encouraged the expansion of mountain glaciation to lower altitudes eventually producing the continental ice sheet, ice shelves and the full scale East Antarctic ice cap we recognise today. This occurrence also led to the inception of the Polar Front, an important (but not absolute) boundary to the physical passage of water from the
Antarctic into the three connecting ocean basins to the north and the migration of species and genetic material between hemispheres. The progressive isolation of the continent had important implications for Antarctic flora and fauna. In some instances extinction was inevitable as ice sheets ploughed across the continent. However, many marine species were able to adapt and accommodate change and soon became established, which resulted in the current high incidence of endemism (Arntz et al.,
1994; Clarke and Johnston, 2003). White (1984) and Picken (1980) developed the concept that high levels of endemism resulted from the isolation of the Antarctic fauna after the circumpolar current system became properly established, and formed a hydrological barrier to easy dispersal.
Hypotheses discussing the origins of Antarctica’s biology have been diverse suggesting colonization from the deep-sea and/or South America along the Scotia arc, and evolution in situ from a relict fauna to be likely (Lipps and Hickman, 1982).
Clarke and Crame (1989) reviewed the origins of the Southern Ocean marine fauna and commented on the continuity with which shallow water habitats have been present over geological time. Geophysical evidence suggests that shallow waters have been accessible since the late Mesozoic possibly even longer, and on occasions were considerably more extensive than present. Fossil evidence dating from the Cretaceous/Tertiary suggests the existence of a diverse shallow water marine fauna with affinities with both the Tethys and Pacific basins, with the notable absence of two main groups: the decapods and teleost fish. Explanations for the absence of some groups are not yet forthcoming and it is unlikely that low temperatures have had any real part to play, as the teleost fish and decapods are well represented in the Arctic fauna. The exclusion of groups by geographical constraints is by no means an exhaustive explanation either, as all fauna would have been subject to the same barriers. Possibly habitat reduction from past intermittent ice events holds the key, encouraging the diversification of some groups into vacant ecological niches. Therefore, the apparent absence of some species and the success of others is more likely the effect of historical contingency (Clarke & Crame, 1992).
The extant fauna, is in many cases, highly diverse, reminiscent of numbers and diversities noted in any lower latitude location. However, the poor representation of some groups has given support to the theory of a latitudinal cline in species richness
(Clarke and Johnson, 2003). The reason for a cline in some species is not yet clear. It is unlikely that low temperature or the age of the Antarctic has played a pivotal role as many high diversity groups exist at the same temperature and unlike the lower diversity Arctic, Antarctica has evolved over longer time scales. However, this cline does appear striking in some animals, for example in molluscs. The occurrence of small size, poor calcification and the incidence of low ornamentation has been widely documented in cold water molluscs (Nicol, 1967). Clarke (1983) has suggested that this may reflect a high metabolic penalty of precipitating calcium carbonate from solution in seawater at low temperature and may explain the low numbers observed. However, other taxa with calcareous skeletons are diverse, such as echinoderms, brachiopods and pycnogonids, suggesting this explanation to not be exhaustive. Clarke and Crame (1989) suggested that reduction in temperature during the Cenozoic was an unlikely evolutionary barrier for most fauna. Such a barrier is now believed to have evolved in situ, and to have been present since at least the Late
Cretaceous and possibly before. Therefore, the biological signatures we see today in the Antarctic are more likely to be a response to the intense seasonality, which characterises the continent, and not the low temperatures per se (Clarke, 1990).
Several modern day groups comprise a large number of closely related species, which are almost certainly the product of adaptive radiations in situ over long time
periods e.g. pycnogonids, gastropods, echinoderms, ascidians and notothenioid fish (Dell, 1972). The isolation of the Southern Ocean and the limited means for dispersal across the Polar Front lend more weight to the argument for the existence of a long established fauna with Cretaceous ancestry, and has eradicated the perception of the Antarctic marine habitat as an evolutionary backwater.