AMMONOIDS ARE TAXA TOO: DIVERSITY DYNAMICS IN JURASSIC-CRETACEOUS AMMONOIDEA AND WHY SCALE MATTERS

Recently developed quantitative methods have provided new insights into the large-scale diversity dynamics of marine animal life. Most of these studies use the ~5-million-year stratigraphic stage as the level of temporal resolution, which introduces the “singleton” problem—taxa known from only a single stage introduce unacceptable bias and must thus be ignored. This omission is not significant for most animal groups, but for the short-lived ammonoid cephalopods it results in the exclusion of the majority of genera. With better temporal resolution, however, the same techniques used for large-scale studies can be applied to the ammonoid record. What diversity dynamics do the ammonoids show, and how do they compare to the more global pattern established for other marine animals?

I have compiled a dataset of 1057 Jurassic-Cretaceous ammonoid genus originations and extinctions (including the suborders Ammonitina, Ancyloceratina, Lytoceratina, and Phylloceratina) at the substage (~2.5-million-year) level. The shift from stage to substage resolution reduces the number of singletons considerably (e.g., from 85% to 12% of Ammonitina genera). Standing diversity and per-capita rates of origination and extinction were calculated for the four suborders. Correlations among changes in origination, extinction, and diversity were determined, and Fourier and lacunarity analyses were performed to investigate possible multifractal behavior in the time series.

Evolutionary volatility in ammonoids was highest in the Early Jurassic, with origination and extinction rates declining through the Jurassic and Cretaceous. The strength of the correlations among changes in diversity, origination, and extinction also dropped from the Jurassic to the Cretaceous. The temporal and taxonomic scale at which these correlations are calculated affects the relative importance of origination and extinction. Spectral and lacunarity analyses of origination and extinction time series reveal some multifractal behavior, as has been found in global studies of marine animals. However, while origination and extinction show different scaling regimes in most marine animals, ammonoid originations and extinctions appear to involve similar forcing mechanisms.