Moyra Wilson and Bryan Rosen (Natural History Museum)
The modern Indo-West Pacific centre of marine diversity, has the highest zooxanthellate coral diversity in the world, and is characterised by abundant coral reefs. Field and literature studies show that although corals do occur in carbonates of Eocene-Oligocene age in SE Asia, they are rare, and extensive coral reefs have not been reported. Corals and reefs of this age may have been better developed in the mid-Pacific, especially in the late Eocene, but the most diverse regions at this time were in Europe, and to a lesser degree, the Caribbean. The apparent scarcity of Paleogene corals in SE Asia is surprising in view of the long-standing theory that the Indo-West Pacific is a Centre of Origin. The question therefore arises whether this ‘Paleogene gap’ is a sampling problem, and if not what are its causes and implications?
A review of SE Asian Tertiary carbonates in their tectonic context shows that many shallow-water Eocene-Oligocene carbonates were dominated by larger benthic foraminifera and coralline algae. A new Eocene coral fauna (12 spp) from Sulawesi accords with earlier views that there was little endemicity in SE Asia during the Paleogene. The general paucity of corals in carbonate platform deposits confirms that the ‘Paleogene gap’ is real, and not an artefact. Since SE Asia, particularly the western part, has remained in, or close to, tropical latitudes throughout the Tertiary, climatic reasons cannot account for this. Although local ecological factors would have been important, the tectonics point to a degree of geographical isolation from other coral rich regions as a key factor. These patterns changed dramatically, however, in the earliest Neogene, as Australian fragments collided with SE Asia.
Collision led to decreased isolation of the region, generation of numerous shallow-water areas and similar to the present day diverse, abundant zooxanthellate corals occur in the Miocene of SE Asia. The geographical complexity of SE Asia appears to have favoured localized isolation and origination of new taxa, though the fauna also consists of older relicts and taxa which migrated into the region from elsewhere. All these processes contributed to making the Indo-West Pacific the richest region for corals from the Neogene onwards.
We compare these patterns with the three main models of high diversity in the Indo-West Pacific Centre (Centres of Origin, Accumulation and Survival). During the Paleogene the region was not a centre of any kind. From the Neogene onwards, no single model is applicable, but a combination of all three models is preferred. Comparison with Mesozoic coral data and the history of the region suggests that the patterns discussed here also apply to the Late Triassic and Late Jurassic. As with the Neogene, these were times when blocks rifted from Gondwana moved across the tropics to dock against Asia. We therefore emphasise the role of plate tectonics in controlling regional high diversity patterns of zooxanthellate corals. Since corals and coral reefs provide habitats for a myriad of marine organisms, the biogeographic history of Cenozoic reef corals has implications for the evolution of tropical reef ecosystems.
Wilson, M. E. J. & Rosen, B. R. 1998. Implications of paucity of corals in the Paleogene of SE Asia: plate tectonics or Centres of Origin. In: Hall, R. & Holloway, J. D. (Eds.) Biogeography and Geological Evolution of SE Asia Backhuys Publishers, Leiden, The Netherlands