IS THERE A SEPARATE CLASS OF "MASS" EXTINCTIONS?

As Raup noted in his work on kill curves, when all time intervals from the start of the Cambrian through the Pleistocene are considered in rank order of extinction intensity, the intensities are distributed as a continuous exponential series with no marked discontinuity separating "mass extinctions" from the range of other events. But when the 107 substages and stages tabulated are viewed in time order extinction intensities not only decrease over time but vary widely from one time to another. Extinction intensity is persistently high in the Cambrian and Early Ordovician, with a mean value of 45% genus extinction per interval (n=19), compared to a mean of 24% per interval for the rest of the Paleozoic (n=43) and 16% per interval for the entire Mesozoic and Cenozoic (n=45). High Cambrian and Early Ordovician extinction rates may reflect community instability associated with low faunal diversity, a predominance of extinction-prone taxa in Cambrian and Early Ordovician communities, high levels of physical disturbance, or biological and physical factors in combination. In any case, the low diversity and distinct taxonomic makeup of Cambrian and Early Ordovician faunas indicate that they may not be comparable to later faunas. When the Cambrian and Early Ordovician are omitted, the 82 lowest extinction intensities of the remaining 88 intervals still form a continuum when arrayed in rank order, but the six highest values [the end Ordovician, end Permian (and its flanking stages), end Triassic, and end Cretaceous] now fall on a different distribution and arguably form a distinct set. These six time periods are the only post Arenigian intervals that match or exceed the mean extinction intensity of the Cambrian and Early Ordovician. Given this distribution, we can recognize four mass extinction events that differ from the exponential continuum of extinction intensities characterizing the last 470 million years. No stage of the Devonian has an extinction intensity that departs from the main trend, although seven of eight Middle and Late Devonian intervals, plus the remaining four Triassic intervals, comprise 11 of the 15 highest extinction intensities in the continuum of 82 intervals.