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The extinctions that brought the Paleozoic era to a close about 245 million years ago constituted the most severe biotic crisis in the history of animal life. About 67% of all genera of marine animals known from the uppermost (Tatarian) stage of the Permian System appear not to have survived into the Mesozoic era (1-3). The Guadalupian Stage of the Permian, which precedes the Tatarian, also exhibits a high incidence of apparent extinction; 58% of marine animal genera known from the Guadalupian are unknown from any later interval. This is the highest recorded incidence of extinction for any stage of the past 430 million years except the Tatarian (2,4). This pattern has led some investigators to recognize a protracted interval of heavy extinction at the end of the Paleozoic era (5). Indeed, recorded percentages of extinction for major tax decline more-or-less monotonically backward from the Tatarian to the earliest Permian (Fig. 1). (Figure 1 omitted.) The fossil record is imperfect, however, and since the Signor-Lipps effect was elucidated more than a decade ago, it has seemed possible that the high percentages of extinction measured for pre-Tatarian intervals of the Permian are artifacts of an incomplete fossil record (6). The Signor-Lipps effect is the erroneous assignment of extinctions that occurred during a crisis interval to earlier intervals; this misassignment results from failure to discover the extinct taxa in rocks that represent the crisis interval. Even if severe extinction in the Late Permian was actually confined to the Tatarian Age, a strong Signor-Lipps effect could give a false appearance that rates of extinction began to increase earlier.
Because the Tatarian record cannot be perfect, there must be some Signor-Lipps effect for the latter part of the Permian. Nonetheless, three tests that we have devised all indicate that the very high rates of extinction recorded for Guadalupian marine faunas are not artifacts of the Signor-Lipps effect but instead reflect actual extinction. Two of the tests reveal that heavy extinction was concentrated at or near the end of the Guadalupian Age. We conclude that one of the largest mass extinctions of the past half billion years occurred about 5 million years before the even larger crisis that ended the Paleozoic era. This Guadalupian crisis appears to have been as...