Trilobites showed strong resilience capacity through the Late Devonian events despite an inexorable decline

Abstract

The Kellwasser events constituted a major biocrisis at the Frasnian–Famennian boundary, that killed many organisms. The subsequent Hangenberg event also had important effects on marine fauna, although it is not traditionally considered a mass extinction. This study aims to understand the tempo and mode of the Late Devonian extinctions using trilobites as a case of study. To estimate the trilobite dynamics both taxonomic richness and morphological disparity were assessed. The diversity was calculated using conodont biozones, which provide a high-resolution timeline, allowing tracking changes in taxonomic richness and the environmental events implied. The morphological disparity was analysed using space occupancy measures and visualized in morphospaces. The Frasnian began with a diversification of trilobites at generic levels. The number of orders remained constant and relatively high with the presence of Corynexochida, Harpetida, Odontopleurida, Phacopida and Proetida. Environmental conditions were favourable for the proliferation of trilobites until the late Frasnian. However, the Kellwasser events disrupted trilobite evolution, leading to a double extinction event. A first random extinction affected low taxonomic levels, while the second extinction was selective, causing losses at both the generic and order levels, and a strong decrease in morphological disparity. Nonetheless, in response to the end-Frasnian environmental changes, blind and reduced-eye trilobites became more common. Following a dip in generic richness at the beginning of the Famennian, the two surviving orders, Phacopida and Proetida, recovered. Although no novelties characterized them, they partially refilled the morphospace. A slight decline occurred in the middle Famennian before a strong morphological and taxonomical diversification was identified in the expansa and preasulcata zones. The final Hangenberg event at the end of the Famennian affected trilobites at all taxonomic ranks as the Kellwasser events. Only Proetida survived.