MIDDLE PALEOZOIC DYSOXIC FAUNAS AND ECOLOGICAL-EVOLUTIONARY TURNOVER EVENTS: A MIDDLE DEVONIAN CASE STUDY

Biofacies of Paleozoic dysoxic black to dark gray marine mudrocks are characterized by low diversity, high dominance assemblages of small brachiopods, mollusks, and trilobites. As with other biofacies, dysoxic benthic faunas in the middle Paleozoic of the Appalachian basin exhibit extended intervals (1-7 million years) of species- and community-level stability ("coordinated stasis"). These stable intervals end abruptly with brief (~10-100 Kyr) periods of faunal extinction and species turnover. Faunal turnovers are commonly associated with transgression and expansion of dysoxic facies. Predictably, dysoxic biofacies appear be highly sensitive to these crises, though less strongly affected in terms of overall structure than diverse oxic biotas. Research comparing the paleoecological dynamics of dysoxic and oxic assemblages is clearly critical to understand the possible mechanisms for the evolutionary patterns of stability/turnover. Unfortunately, the "unspectacular" nature of many dysoxic assemblages has resulted in a paucity of studies of these biofacies. To this end, we have begun the process of collecting bulk samples of dysoxic facies through several cycles within the Middle Devonian upper Hamilton-Tully succession in central New York. A unique feature of this strategy is the inclusion of primary and secondary school students and teachers in field sampling, data collection, and interpretation. Low-diversity dysoxic faunas are ideal for student involvement. Statistical tests of data accuracy (e.g., % accuracy) are being developed to insure the scientific quality of the data. This project will allow us to generate a large database of high stratigraphic resolution, while also providing students with a meaningful introduction to scientific research. Preliminary results suggest that turnover in black to gray shales involves the substitution of indigenous forms by analogous taxa derived from other basins, as well as diminutive (paedomorphic?) ecomorphs of species typical of low-diversity, shallow marine environments (e.g. small Allanella, Tropidoleptus). Moreover, episodes of turnover also appear to begin considerably earlier in Devonian dysoxic shale faunas than in diverse oxic biotas.