DIFFERING MODELS FOR COORDINATED FAUNAL TURNOVER EVENTS WITHIN THE DEVONIAN OF THE APPALACHIAN BASIN

Detailed study of long-ranging intervals of relative biotic stability at the compositional level within the Silurian and Devonian interval of the Appalachian Basin led to the development of the hypothesis of Coordinated Stasis and the identification of a number of evolutionary-ecological subunits (EE-SUs) bounded by relatively rapid faunal turnover. Further investigation of the hypothesis of Coordinated Stasis has focused upon quantitative analysis of patterns of faunal stability within and between EE-SUs, documenting similar patterns in adjacent sedimentary basins, and matching regional turnover events to global bioevents within the Devonian interval. Recent examination of the processes of faunal change occurring between EE-SUs has yielded two unique patterns: 1) near wholesale extinction and replacement with immigrant taxa within the basin; 2) concurrent turnover within multiple endemic evolutionary lineages.

Examples of extinction-replacement turnover events include the boundaries between the Onondaga-Stony Hollow, Stony Hollow-Hamilton, Hamilton-Lower Tully, and Lower Tully-Recurrent Hamilton EE-SUs. In the Onondaga-Stony Hollow and Hamilton- Lower Tully cases, nearly the entire seafloor gradient consisting of a diverse, cooler-water, Eastern Americas Realm fauna was replaced by a more depauperate, warmer-water Old World Realm fauna due to watermass changes associated with rising global seawater temperatures within the basin. Conversely, the Stony Hollow-Hamilton and Lower Tully-Recurrent Hamilton cases reflect water mass changes associated with global cooling events and reassembly of the previous more diverse cool-water endemic faunas along with new additions. Examples of the concurrent evolutionary lineage turnover events include the boundaries between the Helderberg-Oriskany, Oriskany-Esopus, Esopus-Schoharie, and Schoharie-Onondaga EE-SUs. While there seems to be little evidence of extra-basinal controls on this type of turnover event, they may be caused by restriction of habitat during major lowstands within the basin.