PALEOECOLOGY OF THE DYSAEROBIC ZONE PRESERVED IN DEVONIAN BLACK SHALES

The distribution of body fossils within Devonian aged black to gray shales of the Appalachian basin varies in a predictable manner in association with variable relative bottom water oxygen levels. Throughout the range of dysaerobic conditions, rhynchonelliform brachiopods of the super-family Camarotoechioidea are overwhelmingly dominant, but species within this clade are ecologically variable. Dysaerobic taxa can be divided into two groups: those that are adapted to the lowest bottom water oxygen conditions, and those that are adapted to fully oxygenated conditions. Within the latter group, taxa have distinct ranges of tolerance to low oxygen conditions. As a result, the successive loss of species in a predictable order, not specific taxonomic assemblages, serve to delineate the dysaerobic zone. Valve size in species with a broad range of oxygen tolerance varies in association with relative oxygen levels. Further, variation in life habit distribution is recognized through the dysaerobic zone with epifaunal filter feeders dominating under extremely reduced oxygen levels, and specific oxygen thresholds are recognized by the addition and/or dominance of specific life habits.