Miocene Coral Assemblages in Anguilla, B.W.I., and Their Implications for the Interpretation of Vertical Succession on Fossil Reefs

Three common coral assemblages have been quantitatively identified in the middle Miocene Anguilla Formation of Anguilla (Lesser Antilles) using cluster analysis on three-dimensional coordinates derived from nonmetric multidimensional scaling. The data consisted of tallies of coral species identified at 25 points within 178 1-meter-square quadrats placed haphazardly across vertical exposures of coral-rich lenses at five localities. Eighteen species were recorded. The resulting clusters intergrade and are characterized by moderately high diversities which are not significantly different. They include: 1) a thick-branched assemblage dominated by Porites portoricensis, 2) a mound-shaped assemblage including Porites waylandi, Siderastrea conferta, Goniopora hilli, and G. imperatoris, and 3) a platy/thin-branched assemblage composed of Porites macdonaldi and Porites baracoaensis. The results of chi-square tests show that the assemblages are randomly distributed, both in space and in stratigraphic sequence. The patchy, intergradational distribution of the assemblages, their equally variable but high diversities, and their association with calcareous sand lenses suggest that they probably all developed as small patch reefs or thickets of varying composition on a shallow, gently sloping backreef platform. The three assemblages developed under similar environmental conditions in one major reef zone on a platform subjected to frequent abiotic disturbance by shifting sand. The mere existence of distinctive assemblages of reef-building corals having different shapes, therefore, needs not in itself imply ecological succession or large-scale physical environmental gradients, as is commonly interpreted in many similar fossil reef sequences.