CHANGING PROPORTIONS OF BIVALVE GUILDS REFLECT ENVIRONMENTAL CHANGE IN THE SOUTHWEST CARIBBEAN DURING THE PAST 5 MILLION YEARS

Caribbean macrobenthos experienced major evolutionary turnover in the late Pliocene, in response to oceanographic changes associated with the formation of the Isthmus of Panama. We examined the ecologic signature of this event for bivalve mollusks from the southwest Caribbean (SWC) based on changes in the relative abundance of bivalve guilds defined by life habits and diet. All clams were identified to genus and assigned to one of four life habits (infaunal, semi-infaunal, epifaunal free-living, epifaunal attached) and four diets (suspension feeder, deposit feeder, chemosynthetic, carnivore). Because the functional morphology of bivalves is well understood, we can be confident in these classifications. Data are counts and weights of all bivalves from 85 quantitative bulk samples representing 13 faunules ranging in age from 4.25 to 1.25 MA. Three strong trends are apparent. The proportion of bivalves compared to other mollusks (mostly gastropods) declined markedly from about 85 to 30% towards the present. At the same time, suspension feeders declined from nearly 100% of total bivalves to 65-70%, whereas the proportion of infaunal bivalves declined from nearly 100 to 30%. The corresponding increase in epifaunal bivalves was due primarily to a growing number of attached species associated with increasing reef development. All these patterns are highly statistically significant for analyses based on numbers of specimens or shell weights, but are undetectable based only on numbers of taxa. This strongly supports earlier results that quantitative data are essential for paleoecological analysis of faunal turnover. Our results are consistent with ecological predictions based on decreased productivity and increased reef development previously documented from the SWC over the past 5 MA.