THE RELATIONSHIP BETWEEN SEXUAL DIMORPHISM AND TAXON LONGEVITY IN FOSSIL AMMONOIDS: A ROLE FOR SEXUAL SELECTION IN EXTINCTION

Recent studies have suggested that the strength of sexual selection in modern birds can be linked to increased species extinction risk. If this is the case, this relationship may represent an example of species level selection, i.e., selection occurring at or above the species level. The fossil record provides an ideal opportunity to test this possibility over long timescales. The goal of this study is to investigate the relationship between sexual dimorphism (a proxy for the strength of sexual selection) and taxon longevity (a proxy for extinction risk) in ammonoids.

We compiled global data on the presence and absence of genus-level sexual dimorphism in Paleozoic and Mesozoic ammonoids from a recent literature compilation. We also compiled first and last global occurrence data from the Paleobiology Database and the biostratigraphic literature to calculate genus-level taxon durations. Additionally, we assessed survivorship across larger events, including the Cenomanian-Turonian extinction. The possible relationship between dimorphism and extinction risk was quantified by comparing taxon durations between 124 monomorphic and 122 dimorphic genera, ranging from Triassic to Cretaceous in age. Taxa that went extinct at the K/T boundary were eliminated from this analysis to avoid artificially truncating their durations. Preliminary results indicate no significant effect of sexual dimorphism on taxon survivorship, when the data are pooled across all genera. However, when we account for phylogenetic inertia, the presence of sexual dimorphism does appear to be positively correlated with increased extinction risk, at least in some superfamilies.

To explore this possibility in more detail, we are currently focusing on a single superfamily (Acanthocerataceae) and compiling data on microconch versus macroconch diameter in genera exhibiting sexual size dimorphism. This will allow us to assess the relationship between the degree of sexual dimorphism and taxon longevity. The resulting database is under construction and currently contains 65 macroconch and 78 microconch measurements from 31 species with 20 genera within this superfamily.