MONOSPECIFIC ASSEMBLAGES OF PHYLOGENETICALLY DISPARATE TAXA IN DEVONIAN BLACK SHALES

Low diversity assemblages are common in the rock record, and exceptionally they are composed of a single species. Monospecific assemblages, when independent of mechanical processes, are interpreted to represent extremely reduced bottom water oxygen conditions. The exaerobic biozone is the unique setting between anaerobic and dysaerobic conditions defined in the rock record by the presence of monospecific assemblages in association with laminated sediments (Savrda and Bottjer, 1987). The implication is that exaerobic taxa harbor chemosymbionts. An unusual diversity of clades including 3 rhynchonelliform, 2 lingulaform, and 1 bivalve species are present in monospecific assemblages within Middle and Upper Devonian black shales of New York State. These groups are associated with laminated sediments and are interpreted to be equally well adapted the lowest bottom water oxygen levels, however none are clearly established as exaerobic species. Several of these groups are unusual because they are not restricted to reduced oxygen settings, and are common and even dominant members of communities in fully oxygenated depositional settings. The recognition of these groups as inhabiting the most reduced oxygen conditions is significant in that these shelly macrofauna provide a useful tool for environmental interpretation in strata without a well developed trace fossil signal, typically used to interpret dysaerobic settings.