Evolutionary tempo in Jurassic and Cretaceous ammonites

Stage-level range data for 983 Jurassic and Cretaceous ammonite genera, distributed within 83 families, were analyzed by assigning absolute ages to stages or substages. We found ammonite genera to have a mean generic range of 7.3 Myr/ammonite genus. Using a similar methodology, mean generic range per family was also computed. The distribution of long-ranging genera (arbitrarily chosen as those ammonite genera ranging for 12 Myr or more) among families was found to be nonrandom. Instead, long-ranging genera were found to be concentrated in a few families, resulting in significant heterogeneity in the distribution of generic longevities within families (taxotely sensu Raup and Marshall 1980). Although some of the long-ranging genera were found to be morphologically simpler than shorter-ranging genera, others were equally or even more complex, indicating that longevity among ammonite genera is not merely a taxonomic artifact, dependent on degree of differentiable conch morphology. Those Cretaceous families composed of a large number of long-ranging genera were also among the leaders in mean species longevity per family, based on species-level range data for Cretaceous ammonites of the Great Valley Sequence of California. Many of the long-ranging genera and species possess a similar morphologic attribute: siphuncular tubes (connecting rings) of small diameter but high wall thickness.