THE EFFECT OF LARVAL BROODING ON DIVERSIFICATION RATES IN CHEILOSTOME BRYOZOA FROM THE CRETACEOUS TO RECENT

Life-history traits such as fecundity, dispersal strategy, and parental investment determine the demographic structure of populations. As a result, many have hypothesized that life-history traits influence genetic drift and natural selection through population size and gene flow. In Bryozoa, we have the rare opportunity to test whether lineages’ life-history strategies have by extension affected macroevolutionary rates of origination and extinction. Bryozoa are colonial organisms with a rich fossil record, and the Cheilostome order exhibits life-history variation reflected in fossil morphology. Since the Cretaceous, the Cheilostome order convergently evolved larval brooding several times, while non-brooding lineages also persisted. Larval brooding is a high parental investment, low fecundity, low dispersal life-history strategy compared to non-brooding. Previous studies have suggested low gene flow within brooding populations may have driven the late Cretaceous Cheilostome radiation. Alternatively, we hypothesize larval brooding may through a variety of mechanisms increase lineages’ fitness, decreasing extinction rates relative to non-brooders.

We compile a novel database of first and last occurrences and the presence or absence of larval brood chambers for 642 Cheilostome bryozoan genera from 480 publications. We estimate origination, extinction, and net diversification rates using Foote’s method and find brooding strategy is in the best-supported model by AIC comparison for all three rates. This pattern is driven in part by a spike in non-brooders’ extinction rate in the Miocene, which warrants further investigation. With the subset of genera for which we have occurrence data, we plan to additionally apply a mark-recapture approach. Larval brooding’s effect on Cheilostome diversity suggests life-history evolution played a role in the Cheilostome order overtaking the Cyclostome order in abundance and diversity, demonstrating life-history traits can affect macroevolutionary turnover.