WHY ARE GENUS EXTINCTIONS AGE-BIASED IN THE MARINE FOSSIL RECORD?

Although the Red Queen model predicts that species extinction risk is independent of prior duration, extinction rates of marine invertebrate genera are inversely associated with genus age throughout the Phanerozoic. Three explanations have been offered to reconcile these observations with the Red Queen model: 1) Older genera tend to contain more species than younger genera. 2) Older genera tend to be more geographically widespread than younger genera. 3) The apparent age-selectivity of extinction in the global marine invertebrate fauna is a statistical artifact resulting from the mixing of higher taxa with different, but internally age-independent, extinction rates. We used data from the Paleobiology Database to evaluate these explanations throughout the Phanerozoic. Multiple logistic regression shows that association between genus age and extinction risk persists even after controlling for species richness and geographic range. Although a substantial fraction of the apparent age-selectivity of extinction in the global fauna can be attributed to mixing of higher taxa, extinction risk is usually inversely associated with genus age even within classes and orders. Thus, the association between extinction risk and genus age appears to require further explanation; some additional variable(s) must be associated both with extinction risk and genus age. Because many extinction events in the fossil record appear to reflect similar forcing mechanisms, one additional component in the age bias of genus extinctions may be the gradual accumulation, via a winnowing process, of genera resistant to these rare but recurring physical perturbations. Whereas interspecific Red Queen dynamics predict that biologically caused extinctions will be independent of prior duration, it is possible that physically-driven extinctions are age-selective because long-ranging genera are more likely to have previously faced –and survived- a given type of physical stress.