Gradient analysis of Middle Ordovician brachiopod biofacies: biostratigraphic, biogeographic, and macroevolutionary implications

The distribution and abundance of Middle Ordovician brachiopod genera along an onshore-offshore gradient in eastern North America is investigated. Six biofacies are recognized that occur in shore-parallel environmental zones ranging from intertidal to deep basin environments. The distribution of brachiopod biofacies through time and space was governed by changes in bathymetry and lateral shifts in lithofacies associated with basin evolution. Recognizing these patterns clarifies previous biostratigraphic and biogeographic interpretations of this fauna. By the Middle Ordovician, articulates had diversified to become important components of all marine environments except the deep basin; inarticulates were most important in offshore and basinal settings. This pattern is consistent with hypotheses of displacement of the Cambrian fauna (inarticulate brachiopods) by the offshore expansion of the Paleozoic fauna (articulate brachiopods). -from Author