Biotic recovery of conodonts following the end-Ordovician mass extinction

Abstract

The end-Ordovician mass extinction dramatically altered the course of conodont evolution. This extinction event is probably unique in that it can be strongly correlated with a glacial climatic control. This study has identified, through the application of high-resolution stratigraphy, events within the extinction and recovery intervals. Elements of the uppermost Ordovician Shelf-edge Biofacies were severely affected by the oceanic cooling and introduction of cold-water currents associated with the initiation of the glacial maximum. In contrast, elements of the Shelf Biofacies were more severely affected by the intense cooling, shallowing and overcrowding during the glacial maximum. A number of the Shelf-edge taxa that had survived the glacial maximum suffered extinction at the hands of increasing water temperatures, rising anoxia and/or the cessation of oceanic circulation during the post-glacial transgression. Recovery was initiated by the appearance of Crisis Progenitor Taxa within the glacial maximum in the Shelf Biofacies and during the post-glacial transgression in the Shelf- edge- Slope biofacies. The Shelf-edge Biofacies identified within the uppermost Ordovician is not recognised in the Lower Silurian. Two main biofacies occurred on the Shelf and Slope, which had directly evolved from their Upper Ordovician equivalents. The long-term recovery involved the evolution of Crisis Progenitor Taxa and Ecological Generalists within the Shelf and Slope Biofacies (autochthonous taxa). Punctuated equilibrium likely predominated in the Shelf Biofacies as a consequence of widely fluctuating physical conditions. In contrast, the more stable environments of the slope encouraged gradualistic evolution within the Slope Biofacies (Plus ça change Model). Transgressive episodes within the Llandovery, possibly linked to eccentricity cycles, caused the iterative appearance of Long-term Refugia Taxa (allochthonous taxa), sourced from a Pterospathodontid Biofacies. The transgressive episodes also drove elements of the Slope Biofacies onto the shelf. It has been observed that the mechanisms driving extinction, namely environmental disruption and temperature changes, were also responsible for fuelling the subsequent recovery.