VARYING MODES OF SPECIATION IN CRETACEOUS AMMONOID CLADES

With their deep time perspective, paleontologists have particular insight into one of the great questions in the historical sciences: how do biological species form? Different modes of speciation have been proposed, including bifurcation (ancestral species splits into two new descendant species), budding (ancestral species gives rise to a descendant species while itself persisting), and anagenesis (ancestral species turns into a descendant species without splitting). The relative frequency of these three modes of speciation has long been debated and one mode or another has been proposed as characteristic of a particular group of organisms, e.g., anagenesis in marine microfossils. We used a Bayesian phylogenetic approach to assess speciation modes in three different clades of Cretaceous ammonoid cephalopods, including the planispiral Acanthoceratidae and Desmoceratoidea and the straight-shelled heteromorph Baculitidae. Separate Bayesian phylogenetic analyses were conducted on each ammonoid clade. Maximum clade credibility trees were produced for each clade and the number of occurrences of bifurcation, budding, and anagenesis associated with species origins were determined. The three ammonoid clades showed different dominant modes of speciation. Acanthoceratid speciation was dominated by bifurcation, with only minor budding and anagenetic events. In contrast, desmoceratoid speciation events were nearly evenly split among the three speciation modes. Baculitids showed an intermediate pattern, with relatively frequent bifurcating and budding speciation. These differences in speciation mode may relate to both biological (e.g., origination rates, morphological variation and constraint) and artefactual (e.g., taxonomic practices, biostratigraphic utility) differences among these three ammonoid clades. The pronounced differences in relative frequency of speciation modes demonstrates that not all ammonoid cephalopod clades speciated in the same way.