THE RE-DIVERSIFICATION OF BRYOZOANS AFTER THE PERMO-TRIASSIC MASS EXTINCTION

The end-Permian mass extinction was the most severe in Earth history eliminating up to 96% of existing marine species. The causes of the extinction remain elusive and the recovery of life following it is even less well understood. The reappearance and re-diversification of Mesozoic marine faunas was delayed approximately 6-10 Ma until the middle Triassic and pre-extinction diversity levels were not reestablished.

Patterns of diversity change have been studied extensively in bryozoans from older and younger rocks, but we currently have few details concerning changes in bryozoan diversity associated with the Permian extinction and subsequent Triassic recovery. It is known that 2 bryozoan orders became extinct at the end of the Permian and that the surviving 5 orders appear to have recovered slowly during the Triassic.

An examination of the occurrences of Triassic bryozoan species, worldwide, indicates generally depauperate and geographically restricted faunas during the early Triassic. Diversity rose rapidly during the Ladinian and the Carnian Stages increasing bryozoan diversity by 433% (12 Ladinian, 52 Carnian species) as species began to exhibit more cosmopolitan distributions. This increase was followed by a rapid decline through the Rhaetian Stage. Representatives of Orders Trepostomida and Tubuliporida were largely responsible for Triassic re-diversification. Conversely, species of Orders Cryptostomida and Cystoporida are rare throughout the entire Triassic. The Tethys Ocean was a center of bryozoan origination and diversification with species eventually extending their geographic ranges into higher latitudes.

Carbonate rock volume and outcrop surface area are correlated positively with bryozoan diversity throughout the Triassic. Similar correlations with other (including non-marine) lithologies suggest that a global event was responsible for increasing rock volumes. Carbonate substrates appear to have been increasingly available throughout the Triassic whereas bryozoan diversity increased at a slower rate until peaking in the Carnian. Therefore, the Carnian peak in bryozoan diversity does not appear to be a preservational artifact produced by the increasing availability of carbonate rock.