Novel adaptation increases resilience in Planktic Foraminifera

Abstract

What controls the macroevolutionary history of a group: biological interactions (the \say{Red Queen}) or abiotic circumstances like climate, volcanism, or bolide impacts (the \say{Court Jester})? Planktic foraminifera are a small group of single celled marine eukaryotes. They have one of the best fossil records of any group, resolvable to the species level and deposited within relatively continuous deep-sea sections composed sometimes primarily of their skeletal material. It is likely that their morphology corresponds to their ecology, and we use the former as a proxy for the latter. We measured all recognized morphospecies and used a unique metric to examine their morphological complexity and its relationship with diversity. The connection between the subdivision of morphospace/ecospace (complexity) and diversity appears to correspond to the previously observed diversity dependence governing their evolution. At several points in the last 150 Myr lower mean test complexity-per-diversity corresponds to suppressed origination rates, indicating that ecospace is subdivided so much that it could not be further occupied. We find that the occupation of ecospace in planktic foraminifera is unlinked from climate, except for events large enough to cause a complete reorganization (e.g., the Eocene-Oligocene Transition). Similar reorganizations of ecospace occupancy are driven by key adaptations, like the evolution of photosymbiosis. The relationship of ecospace occupation and diversity is more volatile prior to the evolution of photosymbiosis than after, suggesting that this novel adaptation provides added resilience for the group.