The conventional means of comparing the severity of the various mass extinction events has been to compute the percentage of taxonomic categories that went extinct. This monumental work has been carried out largely by paleontologists at the University of Chicago, through literature research initiated by
David Raup and Jack Sepkoski. The first tabulations were of families of marine animals. Several years later Sepkoski tabulated the number of genera going extinct, and he is now working on the number of species. It was through the use of such statistics that the "Big Five" (the Ordovician, Devonian, Permian, Triassic, and Cretaceous extinctions) were differentiated from other Phanerozoic extinctions. If the number of genera going extinct is used as a means of comparing the mass extinctions, then the Permo-Triassic event was the most catastrophic, followed by the Ordovician, Devonian, Triassic, and end Cretaceous. The Cambrian extinctions do not appear to be so "major." Sepkoski's most recent compilation (which he kindly sent to us as a personal communication in 1997) yields per-family extinction rates of 54% for the Permian, 25% for the Ordovician, 23% for the Triassic, 19% for the Devonian, and 17% for the famous K/T extinction. In two other analyses, however, performed separately by paleontologists Helen Tappan and Norman Newell, the Cambrian extinctions of marine families exceed those of the Permian. The Cambrian extinctions are the most consequential of all; about 60% of marine families went extinct in the Cambrian compared to about 55% in the Permian.
These results pose a dilemma. In the Permian period, the cause of the great mass extinction is quite clear. The continents coalesced into one large supercontinent, greatly affecting worldwide climate and temperature in the process, and near the end of the period an additional, sudden, and catastrophic event occurred as well: Enormous quantities of carbon dioxide were released into the seas and atmosphere, causing a sudden and deadly rise in global temperature. In the Cambrian, however, we have no overt cause. Our best guess is that the garden analogy holds true here. The garden of animals during Cambrian time had only just emerged, and although there were many different types or body plans (more so than now, in fact), there were very few species in each category. Even slight changes in environmental conditions were sufficient to wipe out entire categories, entire phyla. The Cambrian was the riskiest period of all time for animal life. Its extinctions, in our view, were the most important in the history of life on Earth, so the Cambrian should be judged as more important than any other such events, including the Permo-
Triassic extinction. There is more to comparing extinctions than simply adding up the number of species killed off.
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