ECOLOGICAL RESTRUCTURING AFTER EXTINCTION: THE LATE ORDOVICIAN (MOHAWKIAN) OF THE EASTERN UNITED STATES

Abstract

Most research regarding ecological response to extinction in the fossil record focuses on qualitative, taxonomically based analyses. Shifting to quantitative analyses that incorporate paleocommunity-level data offers a richer ecological perspective on how paleocommunities respond to extinction. 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 the Nashville Dome of Tennessee, the Jessamine Dome of central Kentucky, and the Valley and Ridge area of western Virginia. In each region, shallow and deep subtidal environments were sampled in four stratigraphic sequences representing preextinction and postextinction intervals (M3 and M4 sequences and M5 and M6 sequences, respectively). Diversity metrics calculated from subsampled field data generally decrease across the extinction boundary and into the M6 sequence, with more significant declines in the deep subtidal facies and Virginia samples. These facies and regional differences in diversity reflect environmental and geographic variability not only in the effects of the extinction but also in the extent of immigration of new taxa to different areas of the basin. Despite diversity declines, evenness varies little among sequences, facies, and regions across the extinction boundary, suggesting no major change to the relative abundance structure of paleocommunities. Analyses of shifts in ecospace use (particularly tiering and motility of taxa) and multivariate ordination, however, also reveal strong environmental and geographic differences in ecological response due to changes in rank abundance of common taxa.