THE APPEARANCE OF CHANGE: SEARCHING FOR AN ENVIRONMENTAL TRANSITION IN AN ACCUMULATING SUBFOSSIL ASSEMBLAGE

Organisms and their environments are invariably linked, and understanding their interrelationships is of great interest to paleontologists. If a habitat is altered, then a change in the associated community of organisms is expected, but, despite years of research on subfossil assemblages, the extent to which rapid environmental changes can be recorded in the fossil record remains uncertain. In this investigation, I assess the impact of a well-constrained, anthropogenic, environmental change on a marine molluscan death assemblage.

Previous experiments on Cross Bank, in southeastern Florida Bay, altered the local seagrass community through nutrient enrichment, prompting a temporal transition from the wide-bladed seagrass Thalassia, to the needle-bladed seagrass, Halodule. During the present study, two 1200 m transects were established along the crest of Cross Bank with stations at 200 m increments. The first transect was located on Halodule-covered, altered sites and the second was established as a control transect in a zone of unaltered Thalassia cover. A piston core with an average recovery of 80 cm was taken at each station and subsampled at 2 cm increments. Of interest here are the upper 20 cm of sediment from each core, because this zone is most likely to contain molluscan assemblages produced during and after the Thalassia-Halodule transition at altered sites.

Quantitative analyses of the subsamples demonstrate marked differences between altered and control sites in the rank orders and relative abundances of major molluscan taxa. These differences are tied to the physical characteristics of the two seagrasses and the suitability of these habitats to the fauna. Large-bodied grazing gastropods and infaunal bivalves suited to life in a Thalassia-habitat, such as Cerithium muscarum and Pitar fulminatus, decrease in relative abundance in the Halodule-rich habitat, while smaller bodied gastropods and epifaunal bivalves like Bittiolum varium and Brachiodontes exustus increase in abundance. Whether a similar differentiation can be recognized stratigraphically within cores from altered, Halodule-rich sites is currently being evaluated.