STANDARDIZED SAMPLING OF ORDOVICIAN-EARLY SILURIAN CRINOIDS: MACROEVOLUTION AND THE END-ORDOVICIAN EXTINCTIONS

Three crinoid evolutionary faunas existed during the Paleozoic. The middle Mississippian transition from the Middle to the Late Crinoid Evolutionary Fauna was previously demonstrated to be driven by increased rates of faunal turnover and not merely significant extinction. The transition from the Early to the Middle Crinoid Evolutionary Fauna coincided with the end-Ordovician mass extinction and subsequent recovery. Was this macroevolutionary transition due to rapid faunal turnover, or was it mediated by high rates of extinction at the end-Ordovician?

Occurrence- and biofacies-standardized resampling techniques are used to remove the effects of variable sampling in an Ordovician through Early Silurian (Llandovery) crinoid data set. The comprehensive, list-based data have been brought into compliance with current systematic practice, and the temporal distribution of faunas are resolved to the substage level. These standardized sampling procedures indicate that the highly volatile face-value genus richness trajectory is largely an artifact of variable sampling. Crinoid generic richness may not increase substantially after an initial Middle Ordovician (Sandbian, Harnagian) diversification. From the Rawtheyan (Katian) to the Hirnantian, a significant (~24%) decrease in richness occurred with recovery by the Aeronian (middle Llandovery). Thus, the faunal turnover among crinoids was mediated by significant extinction. Rates of extinction are positively correlated to rates of sedimentary package truncation, suggesting that physical environmental changes may have driven this macroevolutionary transition. Rates of origination do not resemble the record of sedimentary basin expansion, suggesting that other biological factors, such as evolutionary innovation and biotic interactions, played a more important role in determining origination.