MOLLUSCAN TAPHONOMY AS A PROXY FOR RECOGNIZING FOSSIL SEAGRASS BEDS

It is important to establish proxies for identifying paleoenvironments beyond the recognition possible from taxonomic assemblages to avoid circular reasoning. These alternate proxies may increase in importance as the geographic and geologic ranges of an environment expand. Seagrass beds, important marine ecosystems both economically and environmentally, have a poor fossil record. As a result, little is known about the geographic distribution of seagrasses over historic and geologic time, or their abundance when unaltered by anthropogenic effects. We examined taphonomic characteristics of molluscan bioclasts within and adjacent to seagrass beds in order to develop and test a proxy for identifying the seagrass habitat in the fossil record. We collected sediment samples from Recent seagrass and non-seagrass environments; molluscan bioclasts from these samples were categorically ranked for the following taphonomic characteristics: encrustation, bioerosion, abrasion, dissolution, fragmentation, edge chipping, and edge rounding. Samples of faunal assemblages associated with deposits inferred to be from seagrass beds were collected from the Eocene Moodys Branch Formation (Copenhagen, LA) and an unnamed Pleistocene unit from Bocas del Toro, Panama. The molluscs from these samples were examined for the same characteristics as the Recent molluscs to test if taphonomic characteristics associated with seagrass habitats are preserved in fossil samples. Taphonomic data were analyzed using both canonical discriminant analysis and assignment tests. Differences between seagrass and non-seagrass environments are apparent, although these differences may be distorted by localized transport of shells between environments, and by differential response of various taxonomic groups to taphonomic processes. A combination of edge rounding, abrasion, dissolution, and fragmentation best discriminates between seagrass and non-seagrass environments. Assignment tests suggest that these taphonomic characteristics are preserved in the fossil record, making it possible to identify fossil seagrass beds based on their taphonomic signature. However, error rates associated with assignment tests suggest that increasing the number of individuals evaluated from some environments is necessary to better define the taphonomic signature of seagrass beds.