TESTING FOR DEPRESSED RATES OF ORIGINATION AND EXTINCTION AMONG CRINOIDS DURING THE LATE PALEOZOIC ICE AGE

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), Stanley and Powell examined raw diversity counts and origination/extinction rates of several classes/orders of marine invertebrates throughout the Paleozoic. Stanley and Powell concluded that the LPIA created global environmental conditions that 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 used a similar approach as Stanley and Powell, with updated Devonian-Permian crinoid genera data assembled and analyzed to test the effect of the LPIA. The combined crinoid data supports Stanley and Powell’s hypothesis of depressed evolutionary rates during the LPIA. However, crinoid diversity did not decrease as was the case for other classes/orders of marine invertebrates during the LPIA. Additionally, evolutionary rates did vary between crinoid clades both outside of and during the LPIA. Most crinoid clades had reduced evolutionary rates during the LPIA, but crinoid clades with higher evolutionary rates outside the LPIA were less affected. During the Paleozoic, the order of clades by increasing evolutionary rates are: primitive cladids, disparids, camerates and flexibles, and advanced cladids. Evolutionary rates in flexibles were unaffected by the LPIA. For remaining clades the order of increasingly depressed LPIA evolutionary rates are: advanced cladids, camerates and disparids, and primitive cladids. Crinoid taxa with rapid evolutionary rates before the LPIA were less affected by global climate change than crinoid taxa with slower rates. Although evolutionary rates in flexibles were essentially unaffected by the LPIA, the most rapidly evolving clade, the advanced cladids, were the only survivors among crinoids of the Permian mass extinction, ultimately giving rise to the Mesozoic articulate crinoids.