ONSHORE-OFFSHORE DISTRIBUTION OF PERMIAN TO JURASSIC BRYOZOANS AND THE ENVIRONMENTAL SIGNATURE OF THE PERMIAN-TRIASSIC AND TRIASSIC-JURASSIC MASS EXTINCTIONS

We use the onshore-offshore distribution of marine bryozoans to evaluate the environmental impact(s) and extinction scenarios for the Permian-Triassic (P/T) and Triassic-Jurassic (T/J) biotic events. A range of mechanisms, from oceanographic and climatic to extraterrestrial causes, has been proposed for these extinctions; however, the fundamental trigger for each remains unknown. Bryozoans, highly sensitive indicators of environmental disturbances, enable us to test the severity of these mass extinctions in a range of environments.

Bryozoan clades were widespread in the Early Permian, but became increasingly restricted to onshore and reefal environments during the Middle and Late Permian, and largely disappeared from offshore settings by the Triassic. Despite having a dramatically lower diversity in the Triassic, bryozoans exhibited a similar onshore restriction prior to the Jurassic. Early Triassic and Early Jurassic bryozoan faunas were depauperate and environmentally and geographically restricted. Euxinia, a favored kill mechanism for the P/T crisis, is linked to anomalous oceanic conditions that persisted through the Early Triassic. Euxinic deep water propagating upwards would have gradually inundated shallower environments, restricting and killing deeper water bryozoans. However, there is currently no evidence for widespread euxinia at the T/J boundary; geochemical and paleobotanical studies instead support a climate-based mechanism related to volcanogenic emissions from the Central Atlantic magmatic province. Despite potential differences in their proposed causal mechanisms, the onshore-offshore distributions of Late Permian and Late Triassic bryozoans point to similarities in the environmental signature of both mass extinctions.