Evolution of Erect Marine Bryozoan Faunas: Repeated Success of Unilaminate Species

Twice, over era-scaled lengths of time, the relative diversity of branched unilaminate species among the erect bryozoan fauna increased by a factor of 10. In this duplicate natural experiment, a colonial form that offers the least resistance to the outflow of filtered water apparently replaced all the others. The data suggest long-term progressive evolution by an increase in the number of species and lineages that are hypothesized to be more hydrodynamically efficient in eliminating filtered water. Except for the Late Permian extinction event, the increase in the proportion of unilaminate species continued through, and perhaps was enhanced by, periods of mass extinction.