RECALIBRATING PHANEROZOIC DIVERSITY USING TAXONOMIC RATES

The canonical view of global diversity in marine animals is that diversity increased in the early Paleozoic, held roughly steady during the rest of the Paleozoic, declined drastically in the Late Permian, and gradually increased over the Mesozoic and Cenozoic, with some setbacks, to a present-day maximum. This view has been reconsidered in at least two ways over the past few years. (1) Variation in preserved sediment has been shown once again to correlate with number of sampled taxa; thus some aspects of the global pattern, especially short-term fluctuations, may be artificial. (2) Sampling standardization suggests that diversification in the Mesozoic and Early Cenozoic may have been less pronounced than in the canonical view. Here I consider a new approach to this problem and apply it to Sepkoski's Compendium of Fossil Marine Animal Genera. The rationale is simple: If we knew the true rates of origination and extinction over time, this would tell us the diversity pattern, since diversity reflects the net balance of origination over extinction. Of course, taxonomic rates are also biased by sampling, but the bias can be circumvented by a numerical optimization method that finds the maximum-likelihood estimates of origination, extinction, and sampling most consistent with the observed data on first and last appearances. Given the true rates, the true diversity at any time is estimated simply as the cumulative excess of origination over extinction up to that time. The picture that emerges from this analysis has a few prominent features: (1) True diversity was more volatile from stage to stage than the raw data would imply. (2) Some events, like the Triassic rebound, were more rapid than suggested by the raw data. (3) True diversity peaked sometime in the Early to Middle Cenozoic, and has been in decline since the Middle Miocene. The Cenozoic peak is roughly twice the height of the Paleozoic maximum, much less than the four-fold difference seen in the raw data.