THE FATE OF TRIASSIC BRYOZOANS IN THE WAKE OF THE PERMIAN-TRIASSIC 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 diversification of Mesozoic marine faunas were delayed until the middle Triassic (approximately 6-10 my) and pre-extinction diversity levels were not reestablished.

Patterns of diversity change have been studied extensively in bryozoans from older (Ordovician & Silurian) and younger (post-Triassic) rocks. We currently have few details about bryozoan diversity changes associated with the Permian extinction and subsequent Triassic recovery. We do know that two bryozoan orders became extinct at the end of the Permian (Fenestrida and Rhabdomesida) and that the surviving 5 orders appear to have recovered slowly during the Triassic.

We have examined the occurrences of Triassic bryozoan species worldwide. Preliminary results indicate a generally depauperate, geographically restricted, fauna in the early Triassic followed by a rapid rise in bryozoan diversity from the Ladinian (Middle Triassic) through the Carnian Stage (early Late Triassic). During this interval, diversity increased by 433% (12 Ladinian, 52 Carnian species) as bryozoans began to occupy a variety of habitats and exhibited a more cosmopolitan distribution. This increase was followed by a rapid decline through the Rhaetian Stage (Late Triassic). Representatives of two orders, the Trepostomida and Tubuliporida, were largely responsible for the Triassic diversification. Conversely, representatives of Orders Cryptostomida and Cystoporida are rare throughout the entire Triassic. Cryptostome species numbers reached a peak in the Ladinian whereas cystoporate species are most abundant in the Carnian.

One goal of this study is to determine the factors that may have affected the survival and recovery of bryozoans following an extinction. Preliminary work on species-richness (no. of species per genera) and survivorship across the end-Carnian extinction phase reveals that survival was unrelated to species-richness.