Environmental change drove macroevolution in cupuladriid bryozoans

Abstract

Most macroevolutionary events are correlated with changes in the environment, but more rigorous evidence of cause and effect has been elusive. We compiled a 10 Myr record of origination and extinction, changes in mode of reproduction, morphologies and abundances of cupuladriid bryozoan species, spanning the time when primary productivity collapsed in the southwestern Caribbean as the Isthmus of Panama closed. The dominant mode of reproduction shifted dramatically from clonal to aclonal, due in part to a pulse of origination followed by extinction that was strongly selective in favour of aclonal species. Modern-day studies predict reduced clonality in increasingly oligotrophic conditions, thereby providing a mechanistic explanation supporting the hypothesis that the collapse in primary productivity was the cause of turnover. However, whereas originations were synchronous with changing environments, extinctions lagged 1â€"2 Myr. Extinct species failed to become more robust and reduce their rate of cloning when the new environmental conditions arose, and subsequently saw progressive reductions in abundance towards their delayed demise. Environmental change is therefore established as the root cause of macroevolutionary turnover despite the lag between origination and extinction.