MORPHOLOGICAL RESPONSE OF BIVALVE TAXA TO PALEOCOMMUNITY COMPOSITION

In assessing the systematic relationships among paleoecological communities, the role played by the biotic interactions of various organisms is one that is often overlooked. In order to assess the effects of biotic interactions within communities, we analyzed two Neogene bivalve communities from the Miocene Cercado Formation (Rio Gurabo section) of the Dominican Republic. Both samples were collected from shell beds with well-preserved fossil material and the bivalves were extracted from the bulk fossil samples. The sampling areas are nine meters apart stratigraphically. Due to the close proximity of the two samples, the similarities and differences among organisms that are shared between the two samples are more likely to be a result of ecological conditions, and less a product of evolutionary change over time. The two samples are comparable in levels of taxonomic diversity, with each having approximately twenty-five bivalve genera. The two samples also share several taxonomic groups, such as pectinids, Arca, and Divaricella. However, there are some groups restricted to one sample, such as Glycymeris and various venerid species. The common genera differ in abundance within their respective samples. Pectinids are the absolute most abundant taxonomic group in one sample, whereas there are significantly fewer pectinids than the relatively abundant Arca and Divaricella in the other sample.

Groups that are shared by the two communities will be compared morphologically to determine whether biotic interactions such as competition or predation played a role in the properties possessed by that organism across the community. For example, an organism that displays a significant difference in size between the two communities may be a product of one sample having a greater or more diverse set of interactors. Biotic interactions can explain differences in community composition as well as account for differences in size and shell morphology. Thus, the members of a community are a key factor in understanding morphological differences between taxa across different paleocommunities.