REDUCED BIOFACIES DIFFERENTIATION IN SILURIAN TRILOBITE FAUNAS OF LAURENTIA

Over the past 30 years, several major studies have concluded that Cambrian and Ordovician trilobite assemblages on the Laurentian paleocontinent were arranged in discrete biofacies along environmental gradients, with water depth the chief control. Biofacies were originally determined qualitatively, but more recent work has focused on q- and r-mode hierarchical cluster analyses using taxon-abundance data. Studies have typically identified 4-6 coeval biofacies occurring on a proximal to distal transect at any one time.

The end-Ordovician mass extinction saw the reduction of global trilobite taxic diversity by about half. Recent work has demonstrated that Silurian trilobite within-habitat (alpha) diversity was largely unaffected, suggesting that there must have been serious reductions in the beta (between habitat) and gamma (geographic) components of Silurian diversity.

While the Silurian has long been known as a time of increased cosmopolitanism, the potential effects of the end-Ordovician extinctions on ecological partitioning have not been quantitatively examined. We compiled all available literature and field based taxon-abundance information on Llandovery, Wenlock, and lower Ludlow trilobite faunas of Laurentia and carried out q- and r-mode cluster analyses. The results demonstrate a greatly reduced range of biofacies development in Silurian trilobite faunas. Nearshore and basinal biofacies remain distinct, but there is little environmental differentiation of assemblages occuring in normal marine settings between these extremes.

Trilobite taxa which survived the end-Ordovician extinction typically had relatively distinct biofacies occurrence during the Ordovician, but their ecological range was greatly expanded following the extinction. Although loss of gamma diversity likely played a significant, if thus far unquantified, role in the fall in global clade diversity, our results indicate that breakdown in between habitat diversity was also a major factor.