Community Ecology of the Middle Ordovician Black River Group of New York State

Abstract

The faunas of the Black River Group were studied within an inferred environmental framework in eight stratigraphic sections in northwestern New York State. The group was deposited in seven nearshore carbonate environments, and the lithologic record of each environment contains a distinctive fossil assemblage. These assemblages represent seven organic communities which are given the names of their environments of occurrence. The supratidal flat community was dominated by blue-green algal mats. Burrowing suspension feeders now represented only by trace fossils were dominant in the intertidal flat community. Blue-green algal mats also dominated the subtidal marine pond community. The most important elements in the subtidal channel community were two burrowing deposit feeders, and blue-green algae coating bioclastic grains. The wave-baffle community was dominated by colonies of Tetradium (coral) which formed a current and wave baffle. The baffle margin community was very diverse, but was dominated by byssate molluscan bivalves (Cyrtodonta) and browsing snails (Loxoplocus). The codiacean green alga Hedstroemia was the most abundant element of the level-bottom marine community, but the assemblage is a diverse one. When these reconstructed communities are analyzed with respect to relations with physical environmental variables and trophic (feeding) structure, they are found to be analogous to Holocene marine communities. For example: (1) community diversity increased offshore with decreasing environmental stresses, (2) the infaunal habit was particularly adaptive for life in nearshore, high-stress environments, (3) the proportion of deposit feeders in a community depended on sediment organic content, (4) the proportion of suspension feeders depended on water exchange rate, (5) the communities show a distribution of dominant members among several feeding categories, and (6) each feeding category is dominated by one species. Although the analogy between Holocene and ancient communities is not unexpected, it is demonstrated quantitatively here.