THE PALEOENVIRONMENTAL DISTRIBUTION OF BRYOZOAN CLADES AND THEIR MORPHOLOGY: IMPLICATIONS FOR THE PERMIAN-TRIASSIC MASS EXTINCTION

The Permo-Triassic was a critical interval in the evolutionary history of marine bryozoans, characterized by large diversity drops and a taxonomic switch. Stenolaemate-dominated bryozoan communities of the Paleozoic were replaced by gymnolaemate-dominated bryozoans of the post-Triassic, a replacement driven by the end-Guadalupian and end-Permian extinctions but protracted through the Triassic. While the taxonomic changeover is well documented, the paleoenvironmental context has not been explored and little is known about the impact of Late Permian events on bryozoan ecology.

Using field data and primary sources, the onshore-offshore distribution of bryozoan orders, genera, and their morphology was examined throughout the Permian and Triassic, with particular emphasis on the Permian/Triassic boundary and the Early Triassic, a time well-known for its delayed biotic recovery and harsh environmental conditions.

Onshore-offshore paleoenvironments include nearshore, inner shelf and lagoon, middle shelf, reef, outer shelf, slope mound, and slope and deep basin settings. Results indicate that prior to the end-Guadalupian extinction and following the Early Triassic recovery, the paleoenvironmental distribution of bryozoans was largely driven by intrinsic biological processes. Bryozoan clades occupied all environments along the onshore-offshore transect but their frequency distribution across these environments varied according to clade membership and morphology. Extrinsic factors controlled the distribution of bryozoans through the Permian/Triassic boundary: following the end-Guadalupian crisis, bryozoans gradually retreated to nearshore settings, and in the earliest Triassic, bryozoans were restricted to offshore environments. These patterns support euxinia (anoxia and hydrogen sulfide poisoning) as the kill mechanism for the end-Permian extinction. Deep euxinic water propagating upwards would have gradually inundated shallower environments, restricting and killing deeper water bryozoans. The Early Triassic offshore sites, situated on the eastern edge of the Panthalassa Ocean, likely represented refugia where bryozoans could escape the vicissitudes of Early Triassic oceans.