The evolution of Frasnian marine “community-types” in south-central New York represents a complex interaction of biological and physical processes. The two most important physical factors were shelf profile (bathymetric configuration) and regional differences in substratum type, and temporal changes in both. Offshore marine communities were more sensitive to changes in bathymetric configuration than were the near-shore communities; the latter retained an essentially unchanging ecological structure throughout Frasnian time. Three major biome phases can be discerned in the Frasnian strata of south-central New York. These reflect the progressive shallowing on the shelf, the increase in overall sand content with time, and the geographic expansion of sand-grade substrata in shallow marine habitats.