DEPRESSED RATES OF ORIGINATION AND EXTINCTION AMONG CRINOIDS DURING THE LATE PALEOZOIC ICE AGE: ADVANCED CLADIDS WERE EXCEPTIONAL

Daniel C. Segessenman and Thomas W. Kammer

Stanley and Powell (2003) hypothesized that marine invertebrates experienced depressed rates of origination and extinction during the Late Paleozoic Ice Age (LPIA; Serpukhovian-Sakmarian) as well as reduced overall diversity. Using data from Sepkoski’s Compendium of Marine Genera (2002), they examined raw diversity counts and origination/extinction rates of several classes of marine invertebrates throughout the Paleozoic. Stanley and Powell concluded that the LPIA created global environmental conditions that reduced environmental diversity leading to suppressed rates of origination and extinction in marine invertebrates. Subsequent studies by other researchers on Paleozoic brachiopods, foraminifera, and trilobites have supported Stanley and Powell’s hypothesis.

We chose to test Stanley and Powell’s results with an updated Devonian-Permian crinoid genera data set. The overall crinoid data supports Stanley and Powell’s hypothesis of depressed evolutionary rates during the LPIA. All crinoid clades have reduced evolutionary rates during the LPIA with respect to background rates. However, crinoid diversity did not decrease as it did in other classes/orders of marine invertebrates during the LPIA. In particular, the advanced cladids crinoid clade had their peak diversity during the LPIA (Asselian). All other clades of crinoids experienced reduced diversity as predicted by Stanley and Powell (2003). The increased diversity of the advanced cladids during the LPIA is likely due to the evolutionary innovation of muscular arm articulations leading to the expansion of this clade relative to other clades lacking this innovation as environmental conditions fluctuated within the time frame of the LPIA. Despite the disparity between the advanced cladids diversity and the other crinoid clades, the results of the analyses performed on the updated crinoid database in general support Stanley and Powell’s (2003) hypothesis of depressed evolutionary rates and reduced overall diversity in marine invertebrates during the LPIA.