EXTINCTIONS / MILLION YEARS MAXIMUM EXTINCTION RATE (Epoch) A Series of (Random) Unfortunate Events? Distributions of Extinction Events through the Cenozoic. John Kloke, Pomona College, Claremont CA 91711 Jennifer Stempien, University of Colorado at Boulder, Boulder, CO 80309 K. Rebecca Thomas, Montgomery College, Rockville MD 20850 Introduction Results Estimates based on the fossil record suggest the majority of animals and plants that have ever existed on Earth are now extinct. Numerous studies have focused on determining the spacing and intensity of both mass and background extinction rates through the entire Phanerozoic (545 Million years ago to present) to suggest potential causes of extinction and the effects of extinction on evolutionary patterns. Some of these studies have suggested a cyclic nature of 26 million years for background extinction patterns for the past 250 million years (Raup and Sepkoski, 1984), however this pattern has been seen as controversial and no conclusive causative factors have been suggested for a cyclic pattern. Despite the controversy of a cyclic pattern for background extinctions, present focus of attention has been on investigating the intensity of extinctions to one another and not if the extinctions are distributed randomly across the temporal scale. Are extinction events for genera distributed randomly during the Cenozoic? Ho: Extinction events are distributed randomly with respect to time in the Cenozoic. Table 1: Extinction rates for all genera in dataset. EPOCH AGE (mya) EXTINCTIONS / MILLION YEARS Pleistocene 1.8 50.6 Pliocene 5.3 128.9 Miocene 23 69.3 Oligocene 33.9 43.3 Eocene 55.8 62.6 Paleocene 65.5 46.2 max = 128.9 p = 0 Materials and Methods Dataset: The number of genera that had their last occurrence in each epoch in the Cenozoic (65 million years to 10,000 years ago) were acquired from Sepkoski et al. 1981, available from the following website: http://strata.ummp.lsa.umich.edu/jack/. Time scale: The Geologic Time Scale, 1999, Palmer and Geissman, The Geological Society of America, Boulder CO. http://www.geosociety.org/ Method: Extinction rates per million years were calculated for each epoch by dividing the number of genera that went extinct by the length of the epoch (Table 1). The maximum extinction rate among the epochs was used as the test statistic. A distribution of maximum extinction rates was generated by randomly distributing 4059 extinction events between 0 and 65.5 million years for 10,000 repetitions. This distribution was used to test whether the maximum extinction rate found in the data (Pliocene) was significantly different than that expected by chance. This procedure was repeated for genera within five separate phyla (Table 2). Table 2: Maximum extinction rates for genera by phylum. PHYLUM MAXIMUM EXTINCTION RATE (Epoch) P-VALUE Arthropoda 11.7 (Pliocene) 0.0002 Brachiopoda 3.4 0.0042 Chordata 16.9 0.0074 Echinodermata 5.5 (Eocene) 0.7139 Mollusca 70.6 Conclusions We reject the null hypothesis that extinction events for all genera are randomly distributed in the Cenozoic. We reject the null hypothesis that extinction events for genera within Arthropoda, Brachiopoda, Chordata, and Mollusca are randomly distributed in the Cenozoic; however, we cannot reject the null for Phylum Echinodermata. For all phyla in which extinction events were nonrandomly distributed, maximum extinction rates were in the Pliocene. This is consistent with individual case studies exploring diversity patterns in the Pliocene. Future studies may further explore these patterns and their potential causes at a variety of temporal, spatial and faunal resolutions.