EVOLUTIONARY-ECOLOGICAL SUBUNIT TURNOVER EVENTS: DIFFERING MODELS FOR COORDINATED FAUNAL TURNOVER EVENTS WITHIN THE DEVONIAN OF THE APPALACHIAN BASIN

Long-ranging intervals of relative biotic stability, primarily at the compositional level, have long been recognized within the Silurian and Devonian intervals of the Appalachian Basin. Detailed study of these intervals by Brett and Baird 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. Subsequent studies focused initially upon examining the relative nature of faunal stability within EE-SUs along with documentation of similar patterns of faunal stability and turnover within correlative strata in adjacent sedimentary basins. More recent work has focused upon the intervals of rapid turnover between EE-SUs, particularly matching regional turnover events to global bioevents within the Devonian interval, as well as examining the nature of turnover events, i.e., near wholesale extinction and replacement with immigrant taxa within the basin vs. concurrent turnover within multiple endemic evolutionary lineages.

Examples of extinction-incursion 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. It is also possible to see a mixing of these two types of turnover, as in the Recurrent Hamilton-Geneseo EE-SU transition.