Enhanced dominance by cheilostomes relative to cyclostomes in low-latitude bryozoan assemblages through 100 million years

Abstract Clade-clade interactions are thought to be a major force in the changing representation of organisms with similar ecological niches over time and space. It has long been known that the global number of cheilostome bryozoan species have increasingly outnumbered cyclostome bryozoan species, due to higher rates of both origination and extinction in cheilostomes. However, the latitudinal differences in this evolutionary turnover, and their ecological mechanisms, have been underexplored. Here we test the hypothesis that the numerical dominance of cheilostome species is enhanced at low latitudes today, and quantify this dominance in the geological past. Our data comes from 288 extant and 1257 fossil bryozoan assemblages reported in the published literature. We show using generalized linear mixed models that assemblages contain a greater proportion of cheilostome species in lower latitudes through the last 100 million years. We discuss why higher latitudes may have represented a stronghold for cyclostomes, even as cheilostomes have continued to outnumber cyclostomes in species richness over the Cenozoic. We hypothesize that a combination of lower average temperatures and greater seasonality could give cyclostome bryozoans with relatively slower metabolism an edge in higher latitudes, even as cheilostome bryozoan species numbers dominate globally.