COMPARATIVE PATTERNS OF RECURRENT PALEOECOLOGICAL STABILITY AND TURNOVER IN THE MIDDLE DEVONIAN OF CENTRAL OHIO AND NEW YORK: IMPLICATIONS FOR BASIN-SCALE COMMUNITY CHANGES

Ecological Evolutionary Subunits (“EESUs”) are periods of faunal and paleoecological stability on the order of 10⁵ – 10⁶ years, bounded by intervals of rapid faunal turnover on scales smaller than are geologically recorded. As defined by Brett and Baird (1995) based on Silurian-Devonian fossils of New York, detailed stratigraphic evidence shows that these faunal turnovers are not associated with major unconformities. They appear abrupt even in thick, relatively conformable successions, indicating a true biotic change and not an artefact of stratigraphic bias. Further, there are multiple instances of typical faunal assemblages disappearing for one EESU interval, only to return in the next, with varying degrees of modification. However, questions remain about the degree to which the stability and synchronous turnovers of the EESUs are expressed on a broader basin to province scale, as well as the possible refugia of recurrent faunas.

To this end, we analyzed faunal occurrence data from Stauffer’s classic 1909 work The Middle Devonian of Ohio in the context of newly updated stratigraphic correlations between central Ohio and New York. Presence/absence data on species/genera of brachiopods, bryozoans, corals, mollusks, and others were tallied by “zones” as defined by Stauffer, approximately equivalent to member to sub-member scale stratigraphic units of NY. A preliminarily updated stratigraphic correlation was constructed, based on field surveys and lithologic analysis of core samples, conducted in cooperation with the Ohio Geological Survey.

The faunal compositions of the Columbus and Delaware limestones of central Ohio show strong similarities and differences to the known patterns of EESUs in the equivalent Onondaga Limestone and lower Hamilton Group of New York. We observed a broadly similar pattern of faunal turnover, with parallel changes in both taxonomic and functional diversity at likely equivalent bioevents. Most genera and species within EESUs are common to the two regions, however, there are differences in the total diversity and some of the specific taxa present. These patterns of recurrence of entire faunas on long time scales of up to a few million years and approximately simultaneous biotic changes within different sub-basins of a biogeographic province implies a regional-global scale process.