CHANGES IN BIVALVE FUNCTIONAL ECOLOGY IN RESPONSE TO ENVIRONMENTAL CHANGE IN THE CARIBBEAN NEOGENE

We compared changes in the relative abundance of bivalve genera and functional groups in the southwest Caribbean over the past 11 Ma to determine the response of bivalve community composition to environmental changes associated with the final closure of the Central American Seaway approximately 3.5 Ma. Quantitative samples from 29 faunules produced 106,000 specimens in 145 genera. All genera were assigned to functional groups based on diet, relationship to the substrate, and mobility. Valves were then counted and weighed to determine the abundance of different taxa and functional groups. Separate Principal Components Analyses (PCA) of faunules were performed based on generic and functional groups using three kinds of data: presence/absence, counts, and weights. Ordinations based on quantitative data for functional groups demonstrated strong shifts in community structure with a stark contrast between faunules older than 4.25 Ma and those younger than 2.5 Ma, with an intermediate group between these ages. PCA axis 1 explains 50% of the variability, and is strongly correlated with the percent carbonate content of sediments and mean annual range in temperature (MART), both of which are proxies for paleoproductivity. However, PCA 1 is not correlated with age, which further supports the hypothesis that changes in productivity were major drivers of ecological changes. In contrast, PCA based on genera did not result in clear groupings of faunules, and PCA axis 1 explained only 30% of the overall variation with no clear relationship to environmental change or age. In addition, ordinations performed on both functional groups and genera using presence/absence data were driven by sporadic occurrences of rare groups or taxa and showed no clear patterns, emphasizing the need for quantitative data in paleoecological analysis. Ironically, detailed taxonomic analysis obscured the major evolutionary responses to environmental change that are clearly apparent for functional groups.