COMMUNITY-LEVEL PALEOECOLOGICAL CHANGE ASSOCIATED WITH REGIONAL EXTINCTION IN THE LATE ORDOVICIAN (MOHAWKIAN) APPALACHIAN BASIN OF THE EASTERN UNITED STATES

During the Late Ordovician (Mohawkian), a regional extinction occurred among marine taxa in the Appalachian Basin of eastern North America. To examine community-level paleoecological change across this extinction boundary, field censusing of macroinvertebrate genera was completed for shallow and deep subtidal settings from three regions of the Ordovician Appalachian Basin including the Nashville Dome of Tennessee, the Jessamine Dome of central Kentucky, and southwestern Virginia. In each region, both environments were sampled (when present) for each of three stratigraphic sequences representing pre-extinction (sequence M4) and post-extinction (sequences M5 and M6) intervals.

Diversity and evenness metrics calculated from field samples decline across the extinction boundary in all regions and environments. 28% of the taxa present in the M4 sequence are not found in the following sequences, and the majority of those taxa (71%) are articulate brachiopods. Over half of the taxa present within the M4 are also found through both the M5 and M6, including ramose bryozoans and common brachiopods (Dalmanella, Strophomena, and Zygospira). This suggests the extinction did not lead to major reorganization of marine paleocommunities.

Multivariate analyses indicate strong regional patterns in the data. During the M4, Virginia communities were dominated by many brachiopods not seen in subsequent sequences. In Tennessee, shallow and deep subtidal communities were dominated by ramose bryozoans and articulate brachiopods. Kentucky shallow subtidal communities were similar to those of Tennessee, with locally abundant mollusks and corals. During the M5, taxonomic variability among the regions declined and biofacies emerge based on differing proportions of common articulate brachiopods, including Dalmanella, Rafinesquina, Sowerbyella, and Zygospira. However, in Tennessee and Kentucky, shallow subtidal communities diverged, and within Tennessee, deep and shallow communities diverged, reflecting the physical changes occurring in the basin. During the M6, communities in all regions are similar to those of the M5. Tennessee and Kentucky shallow subtidal communities are compositionally more similar, as in the M4, perhaps indicating some degree of faunal recovery from the extinction.