Fecundity in fossil Bryozoa: Accounting for colony fragmentation and the spatial division of reproductive labor

Our ability to measure evolution by natural selection in the fossil record is limited by the near impossibility of estimating the fecundity and thus relative fitness of most fossil organisms. Neocheilostome bryozoans are an important exception, because they have calcified larval brood chambers known as ovicells that provide an approximate estimate of the colony’s sexual fecundity. This clade has a rich fossil record dating back ~100 million years, providing potential opportunities to observe changes in relative fitness and natural selection through many past intervals of environmental change. However, neocheilostome fossil specimens are often highly fragmented, and fragments are not necessarily randomized subsets of a colony. To make use of the majority of the neocheilostome fossil record, we need to test the effect colony organization has on our methods of inferring colony fecundity from fragmented specimens.

In this study, we measure colony fecundity in a population of Recent neocheilostome bryozoan specimens of the species Parasmittina eccentrica Winston & Jackson, 2021, and quantify the nonrandom spatial arrangement of ovicells due to colony organization. We then simulate fragmenting these specimens and test the statistical robustness of standard methods one might use to reconstruct fecundity from fossil specimens. We find that ovicells are clustered and concentrated at mid-distances from the ancestrula (the oldest part of the colony). As a result, estimates of a colony’s fecundity from a single fragment have higher variance than would be expected if ovicells were randomly distributed. When estimating average population fecundity, observed variance among fossil fragments is a better estimator of sample variance than methods that assume spatial independence (such as a binomial distribution), especially for fragment sizes of 8 mm or less. While there is much to be learned about neocheilostome ovicell arrangement across taxa and environments, we can robustly estimate fecundity from small fossil fragments even in extinct neocheilostome species.