Origination and Extinction through the Phanerozoic: A New Approach

Temporal patterns of origination and extinction are essential components of many paleontological studies, but it has been difficult to obtain accurate rate estimates because the observed record of first and last appearances is distorted by the incompleteness of the fossil record. Here I analyze observed first and last appearances of marine animal and microfossil genera in a way that explicitly takes incompleteness and its variation into consideration. This approach allows estimates of true rates of origination and extinction throughout the Phanerozoic. Substantial support is provided for the proposition that most rate peaks in the raw data are real in the sense that they do not arise as a consequence of temporal variability in the overall quality of the fossil record. Even though the existence of rate anomalies is supported, their timing is nevertheless open to question in many cases. If one assumes that rates of origination and extinction are constant through a given stratigraphic interval, then peaks in revised origination rates tend to be displaced backward and extinction peaks forward relative to the peaks in the raw data. If, however, one assumes a model of pulsed turnover, with true originations concentrated at lower interval boundaries and true extinctions concentrated at upper interval boundaries, the apparent timing of extinction peaks is largely reliable at face value. Thus, whereas rate anomalies may well be real, precisely when they occurred is a question that cannot be answered definitively without independent support for a model of smooth versus pulsed rate variation. The pattern of extinction, particularly the major events, is more faithfully represented in the fossil record than that of origination. There is a tendency for the major extinction events to occur during stages in which the quality of the record is relatively high and for recoveries from extinctions to occur when the record is less complete. These results imply that interpretations of origination and extinction history that depend only on the existence of rate anomalies are fairly robust, whereas interpretations of the timing of events and the temporal covariation between origination and extinction may require substantial revision. Origination and Extinction in Earth History Quantitative analysis of the stratigraphic ranges of fossil taxa has revealed striking patterns in the history of life on Earth, particularly during the Phanerozoic Eon. At the most overarching scale, there has been a secular decline in the average rate of appearance of new species, genera, and families and in the disappearance of existing taxa since the Early Cambrian (Raup and Sepkoski 1982; Van Valen 1984; Gilinsky and Bambach 1987). With respect to the overall statistical relationship between origination and extinction, peak rates of first appearance tend to lag peak rates of last appearance by several million years (Kirchner and Weil 2000b). Manuscript received March 28, 2002; accepted July 1, 2002. There is also evidence that changes in observed biological diversity are more strongly linked to extinction than origination during the Paleozoic era, whereas origination plays a stronger role in diversity variation during the Mesozoic and Cenozoic eras (Foote 2000a). Numerous other examples could be cited. What these cases have in common is, first, that they reflect some of the largest-scale features that one sees when studying the fossil record and, second, that their explanations, which are yet to be found, are potentially of great geological and biological importance. A third factor that these and many other studies have in common is that, faute de mieux, they take observed times of first and last appearance as rea-