ESTIMATION OF TRUE TAXONOMIC LONGEVITIES FROM OBSERVED FOSSIL RANGES: A COMPARISON OF TWO METHODS

Many problems in evolutionary paleobiology require adequate knowledge of the times of origination and extinction of taxa. Because of the incomplete and biased nature of the fossil record, however, observed temporal ranges may underestimate true taxonomic durations or longevities. Several methods estimate the extent of truncation of lower and upper range boundaries, and thus provide a measure of the completeness of the fossil record. The classical approach uses the number of fossil horizons and the length of the observed range to establish confidence intervals on the range endpoints. Its central assumptions are 1) a random distribution of fossil horizons and 2) continuous and uniform sampling of the stratigraphic range, which may preclude analysis of discretely sampled deep-sea drilling cores (e.g., a wide variety of micropaleontological data). A new method incorporates estimates of extinction and preservation rate in a branching model of cladogenesis. This approach has the advantage of requiring only first and last appearance data (making it applicable to commonly used compendia of fossil ranges), and the frequency of singleton taxa. Unlike classical confidence intervals, it also requires an explicit estimate of the number of subtaxa existing during a taxon’s first appearance datum, as well as a formal model of taxonomic diversification. Sufficient sample size is critical; available data may allow the method to estimate the time of origin of Aves but not Archaeopteryx, for example. This method is thus perhaps best suited for estimating the true longevities of higher taxa. Both methods are vulnerable to variable sedimentological regimes; if the disappearance of a taxon merely reflects the closing of a taphonomic window, neither technique can properly estimate its true time of extinction. I compare these two methods using case studies from the fossil records of trilobites, scleractinian corals, inoceramid bivalve mollusks, and titanotheres.