Oligo-Miocene faunal dispersal in the Indian Ocean region

At present-day, the center of maximum marine biodiversity is located in the tropical waters of the Indo-Australian Archipelago in the central Indo-Pacific. However, the origin of this marine species richness is largely unknown. Recent molecular genetic and palaeontological data indicate that the role of Pleistocene sea level changes in the formation of the biodiversity center may be less important than previously thought and that the enormous richness of underwater life has evolved through millions of years by immigration, displacement and local evolution of species controlled by the displacement of major tectonic plates and global climate fluctuations. Some modern day typical Indo-Pacific faunal elements (Tridacna giant clams, Acropora corals) originated in the area of the present Mediterranean Sea during the Eocene Epoch (~5634 Ma) and spread over the Indian Ocean region not until the Oligocene and Miocene (~345 Ma). Due to the often incorrect taxonomic identification and unprecise age-estimates of the sparse fossil record only few information exist concerning the biogeography of the early Indian Ocean and its relationships with the Mediterranean region. In particular the Gulf of Bengal must be classed as palaeontological Terra incognita. The FWF-project Closing the gap OligoceneMiocene patterns of faunal dispersal and biodiversity in the Indian Ocean aims at a better understanding of the origin and changing patterns of Indo-Pacific marine biodiversity in the context of global tectonic and climatic changes. To close the knowledge gap in the NE Indian Ocean it is intended to create a modern taxonomic dataset, which includes different groups of marine organisms (gastropods, bivalves, echinoids, decapods, corals, benthic foraminifers) and offer a high level of temporal resolution (at least chronostratigraphic stages). For this purpose it is planned to sample key localities in Sri Lanka, NE India and Myanmar, which document the critical late Oligocene to middle Miocene time interval, when large-scale plate movements interrupted the marine gateways to the Mediterranean basins and the emergence of the Indo-Australian Archipelago. The integration of various palaeontological, sedimentological and stratigraphic informations will allow identifying local processes possibly overprinting the regional or global signals. The superordinate objective of the project is a comprehensive study that will integrate the palaeontological, sedimentological and stratigraphic results with comparable data sets from the central and eastern Mediterranean, Iran, Oman, western India and Tanzania, which were generated in previous FWF-projects by the working group. The high temporal and resolution and spatially precise biogeographical division intended will not only allow correlating changes in species distribution and richness to palaeoclimatic, palaeoceanographic and tectonic events in the studied late Oligocene to middle Miocene time interval but may also provide valuable information to evaluate the differential responses of recent shallow marine ecosystem in the Indo-Westpacific region to current global change.

The Indian Ocean region is an area of high biodiversity comprising both marine and terrestrial biodiversity hotspots. This project attempted to contribute to a broader understanding of the modern biodiversity distribution in the Indian Ocean from a deep-time (Oligocene to Miocene) perspective. For this purpose, it was intended to collect new fossil material of different groups of shallow marine organisms from different areas in the Indian Ocean region, reconstructing the respective palaeoenvironments and refining the stratigraphic resolution of the fossil-bearing deposits. The combined data was used to put palaeobiogeographic changes into relation with geodynamic, climatic and oceanographic changes. The data on Miocene mollusks and reef corals document a strong early Miocene bioprovincialism until the middle Miocene, which differs fundamentally from the modern biogeography of the Indian Ocean. This provincialism ceased when Miocene Indian Ocean Equatorial Jet, a precursor of the present-day South Equatorial Current, became established 14 to 9 million years ago. As one consequence, East Africa became faunistically connected with the Malay Archipelago and the present hotspot of coral reef biodiversity in the Mozambique Channel evolved as an offshoot of the Coral Triangle. The latter is the global center of shallow-marine biodiversity at present-day. It formed during the early-middle Miocene due to an eastward shift of tropical shallow marine biodiversity, which was primarily driven by the collision of the Afro-Arabian and Indian plates with Eurasia and the resultant successive closure of the Tethyan Seaway. In support of this Hopping Hotspot Hypothesis, a new Burdigalian ostracod assemblage from SW-India shows biogeographic affinities with Eocene-Miocene faunas from the Western Tethys as well as with Miocene-recent faunas from the Indo-Pacific. The time-equivalent Jaffna Limestone in northern Sri Lanka records an extensive shallow water carbonate platform in the Palk Strait area that provided an important stepping stone for the dispersal of coral reef faunas at the junction between the Western (Arabian Sea) and Eastern (Bay of Bengal) Indian Ocean during the eastward movement of global marine biodiversity. The reconstruction of environmental changes within a sequence stratigraphic framework, provides closer insight into the palaeogeographic evolution of the Palk Strait and its terrestrial and marine faunal permeability during the Miocene. This helps to better understand the sequence of origination and dispersal events in the molecular genetic records of Sri Lankan and south Indian terrestrial species within the Western Ghats-Sri Lanka biodiversity hotspot. As an additional research subject, we analyzed skeletal density and high-resolution stable isotope and trace element time series from middle Miocene Porites corals. These combined sclerochronological-sclerochemical records provide valuable archives for constraining seasonal environment and climate variability during the middle Miocene warming event and document for the first time the calcification responses of reef corals to natural conditions of ocean warming and acidification.