Palaeoecological patterns within the dysaerobic biofacies: Examples from Devonian black shales of New York state

Two distinct body fossil distribution patterns are recognized within Devonian dysaerobic strata of the Appalachian Basin: 1) taxa that are adapted to the lowest bottom water oxygen conditions, and 2) taxa that are adapted to fully oxygenated conditions with distinct ranges of tolerance to low oxygen conditions. The latter group, taxa demonstrate a nested pattern of distribution in association with relative bottom water oxygen levels. As a result, rather than discrete, unique taxonomic assemblages within the dysaerobic biofacies, the successive loss of species in a predictable order in association with decreasing oxygen, serves to delineate 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. Valve size of Eumetabolotoechia multicostata 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. Epifaunal filter feeders dominate under extremely reduced oxygen levels, and specific oxygen thresholds are recognized by the addition and/or dominance of specific life habits.This nested pattern of taxonomic distribution through the dysaerobic zone is unexpected based on observations in modern and other post-Palaeozoic low oxygen communities and is likely associated with frequent and pervasive periods of dysoxia in shallow epeiric seaways.