BETWEEN-HABITAT DISSIMILARITY (BETA-DIVERSITY) RESPONSE OF TRILOBITES TO THE END-ORDOVICIAN MASS EXTINCTION: A MECHANISM TO MAINTAIN WITHIN-HABITAT RICHNESS (ALPHA-DIVERSITY)

The end-Ordovician mass extinction is documented to have resulted in at least a 50% reduction of trilobite global taxonomic richness at genus and family level. Despite this loss, within-habitat species richness (alpha-diversity) did not significantly change after the event. Therefore, it is likely that dissimilarity, either along environmental gradients (beta-diversity) or across geography (provincialism), was reduced in the early Silurian (Llandovery–Wenlock) to compensate for the loss of overall species richness. While there have been qualitative studies of Silurian trilobite distributions, particularly involving reduced provincialism associated with closure of the Iapetus Ocean, there have been no quantitative investigations of distribution changes across the extinction event.

This study investigates whether within-habitat taxonomic richness was maintained following the extinction at least partly by increasing the environmental range of trilobite taxa, resulting in a reduced number of less discrete biofacies. In order to assess this, all taxonomic data on trilobite fossil assemblages were compiled from the literature on Laurentian trilobites. Each collection was assigned to one of five major marine habitats based on sedimentological criteria. Taxonomic assignments of species were assessed and updated as necessary.

Trilobite biofacies were identified via Q- vs. R-mode two-way cluster analysis of abundance or occurrence data. Dendrograms of collections clustered by taxonomic content and of taxa clustered by occurrence in collections were compared on the two-way plots. Biofacies were defined as the intersections of major clusters of collections and taxa. Separate analyses were carried out for the Late Ordovician and the early Silurian, and the results compared to assess biofacies differentiation along a first-order ecological gradient (in marine assemblages, generally depth).

Preliminary results indicate substantial changes to the characteristics and distribution of biofacies after the extinction event, strongly suggesting that taxa may have been more broadly distributed along environmental gradients than during the Ordovician. Since between-habitat dissimilarity is only one component of overall dissimilarity, future work will focus on changes to geographic distributions.