Reconstruction of a paleo-seagrass ecosystem using molluscan paleoecology and stable isotope geochemistry: A case study from the Quilon Limestone (Miocene), India.

Seagrasses are a vital part of the marine ecosystem, owing to their contribution to oceanic primary productivity and supporting highly diverse marine ecosystems. The fossil record of seagrasses is rare because of their poor preservation potential. Sedimentological and taphonomic indicators are generally used to identify the paleo seagrass habitat. The fossil record of seagrass-associated taxa such as foraminifera, corals, bryozoans, molluscs, and sirenians can also provide indirect evidence of this specific habitat. These constitute Indirect Paleo-Seagrass Indicators (IPSIs). The early Miocene (Burdigalian) fossil assemblage of Quilon limestone of Kerala, India, has been interpreted as a seagrass habitat based on the species association of gastropods, bryozoans, and foraminifera. This is the only reported seagrass ecosystem from West Indian Province (WIP). In our study, we attempt to develop a new multi-proxy approach using morphology, ecology, and geochemical signatures of seagrass-associated molluscs from the Quilon Formation and evaluate its potential to reconstruct a paleo-seagrass ecosystem.

Our sample consisted of ~2000 specimens of microbivalves representing nine families of bivalves. We also included previously reported 16 families of macrobivalves in our ecological analysis. The molluscan community of the Quilon Formation is dominated by families that are mobile (67%), infaunal (65%), and suspension feeders (78%) in comparison to other ecological guilds. The seagrass ecosystem, characterized by soft substratum and high suspension load, favors these ecological strategies. The high proportional abundance of Lucinidae bivalves (8.9%) in the assemblage agrees with the expectation of dominance of chemosymbionts in the seagrass meadows. The small body size (<10mm ) and low predation intensity (drilling frequency 0.06, repair scar frequency 0.04) reported from this community also match the expected pattern of a seagrass ecosystem serving as a nursery.

We analyzed molluscan shells from Quilon limestone for stable isotope ratios. We also supplemented this data with published data of present and past seagrass-associated molluscs. Using this data, we evaluated the influence of ecological variables in shaping the stable isotope signature of molluscs in the seagrass ecosystem. The 𝛅13C values of bivalves ranged from -3 to 3 ‰  and of gastropods ranged from -1 to 4 ‰. Our results show that deposit feeders and grazers have 𝛅13C values in comparison to chemosymbionts. The herbivores and carnivores have comparable 𝛅13C and 𝛅18O isotopic values implying a limited role of diet in shaping the isotopic signature of seagrass molluscs. 

The multi-proxy results support the previous interpretation of Quilon limestone as a seagrass ecosystem. This study provides insight into using a multi-proxy approach of combining molluscan taxonomy, morphology, ecology, and geochemistry in developing a reliable IPSI for identifying paleo-seagrass ecosystems.