2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 2:45 PM


FALL, Leigh M., Department of Geology and Geophysics, Texas A&M University, MS 3115, College Station, TX 77843, lfall@geo.tamu.edu

Cohort survivorship curves are a method used to analyze rates of extinction over time of a group of taxa that originated at about the same time. A cohort analysis was performed at the resolution of geologic stages (ICS, 2004) to investigate the timing of rugose coral extinctions and recoveries associated with extinction events. The analysis is based upon 655 middle to late Paleozoic rugose coral genera obtained from Sepkoski's (2002) genus-level marine compendium. Initial results indicate that the Famennian cohort has a higher survivorship than most of the rugose taxa in the Devonian, Carboniferous, or the Permian. There are only three stages when rugose corals have lower rates of extinction; the Eifelian, Kungurian, and Emsian, respectively. Rugose corals originating during the stage immediately following the end-Frasnian extinction have increased longevity (i.e., decreased rates of extinction) relative to corals originating at other times just like recovery cohorts following post-Paleozoic mass extinctions (Miller and Foote, 2003). The results obtained in this analysis imply that Paleozoic faunas had a similar evolutionary response to extinction events.

Other cohorts with low survivorship (i.e., high rate of extinction) include the Kasimovian and Wordian. The Kasimovian cohort experiences the highest extinction of the Carboniferous, following a previously recognized small-magnitude North American extinction event at the Moscovian/Kasimovian boundary. Wordian taxa experience higher extinctions compared to other rugose taxa in the Permian. However, taxa originating in stages following the Kasimovian or the Wordian do not seem to show increased longevities as shown by the Famennian taxa. Lastly, there does not appear to be a Capitanian extinction event in rugose corals.

A cohort analysis of middle to late Paleozoic rugose corals indicates that the Famennian are a recovery cohort of the end-Frasnian extinction event. The dynamics of the extinction may be similar to other mass extinctions in the Phanerozoic, although triggering mechanisms may be different.