A SEA CHANGE IN SMUGGLER’S COVE: COMPOSITIONAL DYNAMICS OF SUBFOSSIL MOLLUSCAN ASSEMBLAGES

To better understand the processes that produce time-averaged fossil assemblages, paleontologists have long investigated the formation of subfossil assemblages on modern seafloors. Generally, it has been demonstrated that these assemblages faithfully reflect the living assemblages and benthic environments from which they were derived. Nevertheless, because they may typically contain skeletal material that is hundreds to thousands of years old, questions remain concerning the compositional stability of these assemblages beyond the one to two year timeframes of most previous investigations.

Here, we evaluate the degree of temporal stability among subfossil molluscan assemblages over a span of more than 20 years in Smuggler’s Cove, St. Croix, US Virgin Islands. A transect sampled initially by Miller in 1979-1980 was reestablished approximately and resampled during 2002. The transect paralleled a gradient of decreasing seagrass cover and density and increasing bioturbation by the decapod crustacean Callianassa. Subfossil molluscan samples and benthic vegetation counts were taken at 30 m lateral intervals for direct comparison with the earlier suite of data.

Results demonstrate that, after 22 years, species richness and composition of subfossil molluscan communities continue to reflect the fundamental environmental gradient, characterized by a transition from herbivorous gastropods and lucinid bivalves in seagrass beds to actively burrowing bivalves and predatory gastropods in bioturbated zones containing less seagrass. However, the abundance of key taxa changed significantly along the transect during the intervening period; these differences can be recognized in both the subfossil and living assemblages. Most notably, there was a significant decline throughout the study area of the previously ubiquitous herbivorous gastropod, Cerithium litteratum, and an increase in the abundance of another gastropod, Tricolia affinis. This transition suggests that there was a change in the physical or ecological dynamics of Smuggler’s Cove during the period between the two studies. By conducting similar, comparative assessments in other modern settings that were sampled intensely in the past, it can be determined whether dynamism is typical during the formation of time-averaged fossil assemblages.