TWO POPULATIONS OF PHANEROZOIC EXTINCTION EVENTS

The Phanerozoic fossil record suggests on the order of 10 to 20 stages of substantial taxonomic turnover in the marine realm. Data on marine animal genera yield a frequency distribution of extinction events with a distinct mode at intermediate event sizes; prior work has been equivocal as to whether there is a separate statistical population of mass extinctions. These conclusions tacitly assume that first and last appearances are good proxies for times of origination and extinction. Incomplete sampling has the potential to take a single rate anomaly and smear it out or to create spurious anomalies that flank intervals of poor sampling.

The history of taxonomic turnover underlying the observed first and last appearances was assessed with a maximum likelihood method that jointly estimates rates of origination, extinction, and sampling. The results suggest many fewer intervals of major taxonomic turnover than would be implied by a literal reading of the raw data on first and last appearances. The frequency distribution of event sizes is highly skewed with near-zero extinction rates most common. There seem to be two populations of events after all. Rather than background versus mass extinction events, however, these two populations represent extreme quiescence on the one hand and detectable turnover on the other hand. The history of animal life is better characterized by even longer periods of boredom and shorter intervals of terror than previously thought.