OLIGOCENE-MIOCENE BIOGEOGRAPHY OF THE WESTERN INDIAN OCEAN AND THE MYTH OF FAUNISTIC EXCHANGE WITH THE MEDITERRANEAN SEA (Invited Presentation)

The Indian Ocean is among the largest oceans of earth covering an area of more than 70 million square kilometers. Its marine biodiversity constitutes an important part of the Indo-West-Pacific Region. Gastropods represent one of the best resolved taxonomic groups in this region in respect to taxonomic inventories and biogeographic patterns comprising >2000 species. The origin of this diversity, however, is still poorly known. Despite the huge area of coastal strips that had fringed the Indian Ocean during Oligocene and Miocene times, the preserved sediments are restricted to rather small areas in northern India and Pakistan, the Sultanate of Oman, coastal Tanzania, Pemba Island, the Indian Kerala province, the Garo Hills in Assam and the Burmesian Irrawaddy region.

Biogeographically, these faunas witness a pronounced provinciality during Oligocene and Early Miocene times, differing markedly from the much more homogeneous modern faunas. Oligocene faunas are known from Arabia, Pakistan and N-India. Throughout the Oligocene, several of the species are also found in the Western Tethys, documenting a still passable seaway for nearshore molluscs. Already at that time a clear biogeographic separation from the Western Tethys is evident. Moreover, the similarities between Indo-Pakistani faunas and Arabian ones are surprisingly low suggesting a further provincialism. During the Aquitanian the West-East interrelation drops to zero despite the passage having been open during this interval. Only the Burdigalian assemblages of N-India witness a very minor re-appearance of Western Tethys taxa, due to the re-establishment of rather ineffective migration pathways prior to the final closure of the Tethyan Seaway. During the Early Miocene, a distinct Central East African Province (CEAP) is distinguished from an Eastern-African-Arabian Province (EAAP) and a Western Indian Province (WIP) ranging along the western margin of India. All have only limited similarities with coeval faunas of the Proto-Indo-Polynesian Province (PIPP), usually ranging below 10% of shared species. No faunistic relation with the proto-Mediterranean Sea was established for eastern Africa and southern India during Miocene times pointing against any immigration of “Indo-Pacific” elements into the circum-Mediterranean area.