Taxon age,origination, and extinction through geologic time

Changes in the taxon ages of fossil marine families that are alive and those that become extinct in each stage of the Phanerozoic reflect changes in the origination rate, differences in the extinction rate of families with different taxon ages, and mass extinction events. Extinct families are generally much younger than the population from which they were drawn. Periods dominated by higher numbers of younger families are more susceptible to larger size extinctions and greater variation in extinction size. As a result the relative size of extinction peaks must be viewed with regard to the taxon age structure of the population. Mass extinctions cause little change in the taxon age of the fauna. However, adaptive radiations cause a large drop in the average age of the families that are alive at any given time. Families must be treated as dynamic entities in macroevolutionary studies because their probabilities of extinction change over time.