Molluscs from underwater meadows - An Early Miocene seagrass mollusc community from Java, Indonesia

Molluscs from underwater meadows - An Early Miocene seagrass mollusc community from Java, Indonesia Reich, Sonja NCB Naturalis, P.O. box 9517, 2300 RA Leiden, the Netherlands sonja.reich@ncbnaturalis.nl A highly diverse and exceptional well preserved fossil molluscan assemblage has been found in a marine sandstone deposit near the village of Banjung-Ante (Yogyakarta, central south Java). The fauna is of Early Burdigalian (Early Miocene) age according to the associated larger benthic foraminifers. The molluscan assemblage contains predominantly small to minute gastropods. More than 3500 individuals assigned to 140 species were found. Bivalves are present with only 23 species and 325 individuals (with one valve counting for a half individual). For a further investigation of the ecological composition the gastropods and bivalves of the Banjung Ante fauna were assigned to six different feeding guilds. The percentage of each feeding guild is illustrated in terms of species numbers and abundance (number of individuals). In terms of abundance the fauna is dominated by grazers and detritivores that make up 69%. This group includes various cerithiid and rissoid species, as well as the genus Bothropoma. The abundance of small, herbivorous gastropods seems characteristic for seagrass associated assemblages with many of them grazing on the microalgae growing on seagrass blades. Also present is the snail Smaragdia that appears to feed exclusively on leafs of seagrasses. In terms of species numbers carnivores s.s. are the most common; they make up 36%. Another group of parasitic and browsing carnivores makes up an additional 24% of the species numbers. The herbivores/detritivores make up only about 25% of the fauna in species numbers. Although chemosymbiontic lucinids make up only 2% of the abundance, their presence indicates dysoxic settings in the seafloor. Currently we are assembling data for modern and fossil seagrass faunas in order to compare to the Banjung Ante fauna. The latter has a very similar abundance/diversity composition as modern seagrass faunas found so far. The Banjung Ante fauna has a very dissimilar composition of Holocene soft bottom and coral associated mollusc faunas from the region. We intend to find data about faunas related to macroalgae in order to test whether seagrass faunas can be characterized on gross diversity and abundance of guilds.