THE MID-PHANEROZOIC BRYOZOAN CRISIS: ECOLOGIC AND EVOLUTIONARY IMPLICATIONS OF THE END-PERMIAN AND END-TRIASSIC MASS EXTINCTIONS

Diversity and spatio-temporal analyses of Permian to Jurassic marine stenolaemate bryozoans provide new evidence linking the effects of mass extinction-related environmental stress to the evolutionary trajectories of marine invertebrates. Environmental stress associated with the end-Permian mass extinction was initiated in the Middle Permian, persisted through the Early Triassic, and was followed by a second episode during the Norian and Rhaetian ahead of the end-Triassic crisis. The ecological and evolutionary impacts of these two intervals of sustained environmental instability were protracted through the Permian to Jurassic interval and permanently altered the ecological structure of post-Triassic bryozoan communities.

Our results show that elevated bryozoan extinction rates during the Late Permian and Late Triassic were coupled with major changes in the habitats they occupied. Bryozoans gradually declined in offshore deep-water settings during the Late Permian and in nearshore and offshore settings during the Late Triassic. Re-colonization of these environments in the wake of each crisis was delayed but coupled with increases in global generic diversity. The taxonomic effects of the end-Guadalupian extinction were milder than previously interpreted, even though ecologically, bryozoans were becoming restricted to onshore settings. The end-Permian mass extinction was the largest for bryozoans, drastically reducing global and assemblage generic diversity and triggering a permanent change in their paleoenvironmental preferences from onshore to mid-shelf settings. The 285 million year dominance of stenolaemate bryozoans ended during the Late Triassic, by which time all but one order (Cyclostomata) had become extinct, initiating a taxonomic switch between stenolaemate and gymnolaemate bryozoans. These results further emphasize the important ecologic and taxonomic implications of the mid-Phanerozoic mass extinctions and their associated environmental degradation for the evolutionary history of lophophorates.