UNIFORM EXTINCTION WITHIN CLADES OF THE MIDDLE PALEOZOIC CRINOID MACROEVOLUTIONARY FAUNA IN THE NORTH AMERICAN MIDCONTINENT

Global crinoid generic diversity reached its Paleozoic maximum during the lower Visean (Lower Mississippian). Diversity subsequently declined 75 percent by the upper Serpukhovian (Upper Mississippian). This decline was accompanied by a change in clade dominance patterns as the camerate-dominated Middle Paleozoic crinoid macroevolutionary fauna transitioned to the advanced cladid-dominated Late Paleozoic crinoid macroevolutionary fauna. Various hypotheses, relating to rapid faunal turnover or increased predation, have been proposed to explain this evolutionary event that took place during a time when the biosphere was experiencing only background extinction.

In the North American Midcontinent this transition took place by the early Chesterian. What happened during the Meramecian? The Middle Paleozoic fauna was well established in the preceding Osagean with camerates near their peak Paleozoic diversity, although advanced cladids had already started their radiation. Data for the Midcontinent indicates that all the major crinoid clades (camerates, advanced cladids, primitive cladids, disparids, and flexibles) declined in diversity by about half without a significant change in clade dominance patterns between the Osagean and Meramecian. This suggests uniform extinction across clades as taxa were exposed to extinction stresses such as habitat reduction or increased predation. All clades were free to radiate once extinction stresses were reduced in the Chesterian. The fact that the advanced cladids were the only clade to radiate suggests they were better adapted than other crinoid clades. Presumably this reflects the unique muscular arm articulations of this clade, which was also the ancestral clade of the articulates, the only post-Paleozoic crinoids.

The existing global database (Sepkoski, 2002) does not strongly support the pattern of uniform extinction as seen in North America. Currently we are collecting global data to better understand why the North American record is unique. We will attempt to test whether the persistence of the preferred habitat of camerates during the Meramecian may have extended the Middle Paleozoic crinoid fauna longer in this region while simultaneously restraining the radiation of the advanced cladids.