BIOGEOGRAPHIC ORIGINS OF MIDDLE SILURIAN CRINOIDS

In 1980, the largest gap in our understanding of Paleozoic crinoid history was the latest Ordovician (Hirnantian) and earliest Silurian (Llandovery). This missing interval was pivotal in the evolutionary history of crinoids, because it followed end-Ordovician extinctions and included the interval of faunal recovery following that extinction. Also, the Llandovery spanned the transition between lower Paleozoic and middle Paleozoic crinoid faunas, which are fundamentally different. The earliest Silurian was particular poorly known because Llandovery rocks are relatively rare. In 1980 only 19% of presently recognized Llandovery crinoid genera were known, and, therefore, a concerted effort was made to fill this gap by Ausich, Brett, Donovan, Eckert, and Witzke with significant work in Iowa, New York, Ohio, Ontario, Quebec, Scotland, and elsewhere. Included in these efforts was a description of the diverse fauna of the Brassfield Limestone in the greater Dayton area. Today, more 110 Llandovery genera are known, and even more likely existed if range-through taxa are included. Acknowledging the bias for Laurentian faunas, evaluation of the temporal and biogeographic distribution of currently documented Llandovery faunas indicates that the well-known middle Silurian crinoid faunas of the world were derived largely from the Llandovery fauna of Laurentia. Laurentian Llandovery faunas provide a link between Ordovician crinoids on Laurentia and the more cosmopolitan middle Silurian faunas.