CHANGING THE MENU IN THE LATE DEVONIAN APPALACHIAN SEA: STATISTICAL ANALYSES OF THE FRASNIAN/FAMENNIAN EXTINCTION IN VIRGINIA

Elevated levels of extinction at or near the Frasnian/Famennian boundary are evident in a new database of fossil occurrences from Virginia, USA. Rhynchonelliform (articulated) brachiopods and bivalves dominate the data set; the brachiopods lost about half of their diversity at or near the Frasnian/Famennian boundary, while the bivalves were largely unaffected. The last occurrences of species are spread through the latest Frasnian, an effect that could represent either a non-simultaneous extinction or the Signor-Lipps effect. By placing confidence intervals on stratigraphic ranges, we tested the hypothesis that the victims of the extinction died out simultaneously. With generalized confidence intervals, the probability of recovering a species may change through a stratigraphic section to reflect changes in paleoenvironment and/or collecting intensity. Using generalized confidence intervals, a single, sudden extinction event can be rejected; the data are statistically consistent with either a gradual extinction or more than one extinction pulse. The distribution of last occurrences suggests two pulses of extinction; because two pulses have been described elsewhere in the world (the Kellwasser events), the possibility of a two-pulse extinction in Virginia deserves further study.

Since the Virginia data set includes local abundance data, we examined general trends in paleocommunity structure. From a latest Frasnian high, standardized within-community (alpha) diversity and taxonomic evenness drop modestly into the Famennian. However, Famennian alpha diversity and evenness are similar to levels that were common earlier in the Frasnian. Thus, while the extinction appears to have affected local communities in this level-bottom, siliciclastic setting, community structure in the Famennian did not stray from the range of variation seen during much of the Frasnian. At single locations on the Virginia seafloor, the seafood menu was longer in the latest Frasnian than before or after.