Seagrass mollusks as a model group for paleoecological and paleodiversity studies

Abstract

Today’s global marine diversity hotspot is located in the Indo-Malayan region of the Indo-West Pacific. Numerous groups of marine organisms including mollusks contribute to this high taxonomic richness that is thought to have originated in the Early Neogene. Because the available fossil data to document patterns of diversification in the Cenozoic of SE-Asia are comparatively sparse, the collection of new data is needed. To perform this task, the Marie-Curie Initial Training Network Throughflow was formed in 2010, focusing on the Miocene of East Kalimantan, Indonesia. This thesis represents one of eleven projects of the Throughflow program. The initial aim of the presented research was the reconstruction of marine mollusk biodiversity in the Miocene of Indonesia. To make useful comparisons of species diversity over time it is necessary to compare faunas from the same paleohabitat, because species richness varies considerably among different environments. For the following reasons seagrass meadows and associated mollusk assemblages were selected as a model habitat and model group for this research: significance of seagrass meadows as ecologically important marine habitats; abundance and species richness of mollusks in seagrass meadows; high biodiversity of seagrasses and associated mollusks in the study region; excellent preservation of fossil material; availability of fossil seagrass-associated mollusk faunas in the study area that represent different stratigraphic intervals. The focus on a specific paleohabitat also comes with challenges; in particular the addressed paleohabitat has to be reliably identified. Seagrasses have a poor fossil record and their former presence has to be inferred using other approaches. This is why the focus of this research shifted towards investigating possibilities of the successful identification of paleo-seagrass meadows as a base for future biodiversity studies. Therefore, a suite of indirect paleo-seagrass indicators has been established in order to differentiate paleo-seagrass meadows from other shallow marine habitats in the geological record. This also includes the detailed characterization of two mollusk assemblages from a seagrass meadow and a coral carpet environment in the study area as well as studies on the potential of mollusk assemblages as paleoenvironmental indicators. Especially, assessments of feeding ecology, δ13C signals of shells and the presence of the obligate seagrass feeder Smaragdia enable the identifications of paleo-seagrass habitats.Using these indicators paleo-seagrass meadows can be discriminated and the study of timing and context of diversification of associated mollusk assemblages becomes possible. High species richness and high numbers of undescribed taxa make such studies challenging. In order to investigate the timing and context of diversification standardized analyses are required, and material from existing museum collections appears to be of limited use as unknown collecting biases may apply.