CORALLINE ALGAE FROM THE MIOCENE MAHAKAM DELTA (EAST KALIMANTAN, SE ASIA)

Although the Miocene is the epoch of the onset of the biodiversity hotspot in South East Asia, crustose coralline algae (CCA) of this age are poorly known of this region. To fill this knowledge gap, crucial to understand the evolutionary history of reef building coralline algae, we studied CCA in Miocene reefs in the Kutai Basin, the largest sedimentary basin in Borneo that was dominated by siliciclastic sediments of the Proto-Mahakam delta. Local carbonate buildups comprising low relief patch reefs occur within the deltaic succession in shallow and turbid water, influenced by high siliciclastic input. CCA in the Kutai basin are mostly found in association with coral reefs, encrusting corals or fragments of them. In beds with very high siliciclastic content no coralline algae were observed in the outcrops, but corals were still present.

Two main CCA assemblages have been recognized in the studied Middle Miocene reefs: 1) a shallow-water assemblage, with two species of Neogoniolithon, thick crusts of Spongites, various Hydrolithon and also Sporolithon and Lithoporella. 2) In darker waters preferentilaly grew melobesioid assemblages, dominated by rhodoliths mainly consisting of Lithothamnion or by thin crusts of Mesophyllum and Sporolithon. This last one is the only assemblage found in mesophotic reefs.

CCA of East Kalimantan show occurrences of some extant species earlier than expected. These new first-occurrence dates are being used to produce a detailed time tree of the main reef building CCA species with the help of a new molecular phylogeny based on five genetic markers.