BRYOZOAN DIVERSITY IN THE MIOCENE OF EAST KALIMANTAN (INDONESIA)

Cenozoic bryozoans from the Indonesian Archipelago have been poorly investigated. Previous publications have reported a total of 21 genera and only 11 identified species. This paucity contrasts with the high diversity of reef-associated bryozoans living in the same area at the present day and is probably related to difficulties in locating well-exposed sections with adequately preserved fossils rather than due to a genuine rarity of bryozoans during Cenozoic times in this region. Under the umbrella of the Throughflow Project, a pioneering study has been undertaken to narrow this gap in our knowledge and to revise bryozoan diversity in the Miocene of East Kalimantan (Indonesian Borneo). A total of 14 sections, ranging in age from Burdigalian to Tortonian, have been sampled for bryozoans in the vicinities of Samarinda, Bontang and Sangkulirang. All studied outcrops were located in the Kutai Basin, the largest sedimentary basin in Borneo, characterized by high rates of siliciclastic sedimentation and the development of Oligocene to Miocene patch reef complexes associated with turbid waters. Six of the sections have yielded a total of 123 bryozoan species. This fauna includes 15 cyclostome and 108 cheilostome species, the latter comprising 23 anascans and 85 ascophorans, far surpassing the previously known bryozoan diversity for the entire Cenozoic of the Indonesian Archipelago. The majority of species are encrusting (76%), usually sheet- or spot-like; 23% of species are erect, mostly jointed and attached to the substrate by rhizoids; only one species is free-living (1%). The undersides of thin platy corals provide the dominant substrates. However, despite the high diversity of bryozoans, they do not contribute significantly to carbonate productivity in this coral reef environment, which is mainly dominated by calcareous coralline algae and larger benthic foraminifera. The number of bryozoan colonies is low and most are small, as is typical for oligotrophic tropical environments where bryozoans are diverse but subordinate in biomass with respect to the other biota.