Trilobite diversity in Avalonia prior to the end Ordovician extinction — the peak before the trough
Abstract The trilobite biodiversity curve for Avalonia differs markedly from the global and other regional patterns in having the peak of diversity immediately prior to the Hirnantian extinction event rather than in the Caradoc or even earlier. This reflects both the very wide spectrum of biofacies represented in this part of the succession in Avalonia and the immigration of taxa resulting from the closure of the Iapetus and Tornquist oceans. Detailed analysis of the faunas demonstrates that Avalonia exemplifies the many factors involved in provincial breakdown and their influence on biodiversity. Furthermore, it emphasises the need to understand the palaeoenvironmental context within which such changes took place. Over 90 trilobite genera are known from the mid-Ashgill (Cautleyan and Rawtheyan stages) of Avalonia with half of these making their first appearance on this microcontinent in that interval. The wide environmental spectrum reflected in the rock record not only includes the return of the deepest water (cyclopygid–atheloptic) biofacies but also marks the first recorded development in Avalonia of the pure carbonate illaenid–cheirurid association which was first established in low latitude Laurentia much earlier in the Ordovician. Its appearance in Avalonia may reflect a combination of the drift of the microcontinent towards tropical latitudes and a possible warming event. Recruitment into this and other environments was overwhelmingly from low latitudes with the largest cohort having its earlier history in Laurentia. Setting aside the ‘new’ pure carbonate environment, the marked rise in diversity and concomitant provincial breakdown resulted from the insinuation of immigrant taxa into existing communities across the shelf and upper slope rather than from any substantial replacement of the established incumbents. The peak of diversity in the mid-Ashgill made the climatically triggered Hirnantian extinction all the more dramatic in Avalonia.