Antarctica and the evolution of taxonomic diversity gradients in the marine realm
Although it will probably never be as complete as that known from any of the other continents, the Antarctic fossil record comprises a surprising amount of information. Originally gathered primarily for stratigraphical purposes, much of this is now amenable to biogeographical synthesis. In theory there are a number of ways in which this can be achieved, but one of the most important, and exciting, lies in the study of taxonomic diversity gradients. Polar-equatorial gradients, in particular, hold one of the keys to elucidating the origin and maintenance of large scale biodiversity patterns. Latitudinal gradients in species richness in the Southern Hemisphere are not necessarily a mirror-image of those seen in the north. There is growing evidence to suggest that the Southern Ocean is a repository of considerable taxonomic diversity, especially for epifaunal marine invertebrate taxa which have adapted to coarse-grained, glacial substrates. Living Antarctic gastropods, for example, comprise a surprisingly diverse group and there is evidence from the fossil record that they are the product of a substantial historical legacy. We still have a lot to learn about the nature of taxonomic diversity gradients in the fossil record. Nevertheless, a picture is beginning to emerge of comparatively steep latitudinal gradients for both bivalves and gastropods in the latest Jurassic (Tithonian). If this is substantiated through the Cretaceous, it would seem to provide striking evidence of steep gradients on a pre-glacial Earth. To some this will be taken as confirmation of the fact that gradients are a time-invariant feature of the Earth' s surface; a direct consequence of the tropics comprising by far the largest natural region (or biome). However, care must always be exercised in attributing simple patterns to simple processes, and other factors may well be involved. Studies based on the fossil record have not yet been able to show that there have been either enhanced rates of speciation in the tropics or extinction towards the poles. This fact alone suggests that something else besides area must be involved in the generation of large scale biodiversity patterns. Time is an obvious candidate here, but it should be stressed that this is very much on a geological timescale of tens or even hundreds of millions of years. Rather than concentrate on intrinsic properties of organisms which may have enabled them to proliferate in the tropics or limited their success at the poles, it may be that more attention should be paid to the role of extrinsic factors. Of prime importance among these may well have been serial range expansions and contractions in concert with repeated climatic change.