Post-Ordovician trilobite diversity and evolutionary faunas

Abstract

Trilobites were the most successful clade of marine invertebrates during the Cambrian, as highlighted by Sepkoski’s Cambrian Evolutionary Fauna. After the Cambrian they were still abundant, widespread and highly diversified. Previous analyses show that trilobite diversity dynamics during the Ordovician can be summarized by several trilobite evolutionary faunas. Trilobites, however, were severely affected by the end-Ordovician extinction event, and never again reached such a high diversity. Here we investigate the evolutionary history of trilobites after this crisis by evaluating how their diversity dynamics are structured in evolutionary faunas and what could be the environments influencing these faunas. Using occurrences from the Paleobiology Database, we estimate the post-Ordovician palaeobiodiversity of trilobites by coverage rarefaction (SQS) and identify trilobite evolutionary faunas by means of factor analysis. During the Silurian, trilobite taxonomic richness was relatively high and the Silurian Fauna was dominated by the families that appeared in the Ordovician and survived the endOrdovician extinction, such as Odontopleuridae and Encrinuridae. A Devonian Fauna established progressively since the end of the Silurian as other families diversified, such as Acastidae and Tropidocoryphidae. The Early Devonian records the highest post-Ordovician diversity, probably favoured by the spreading of epicontinental shallow seas. Abrupt sea-level changes and anoxic events during the Middle Devonian strongly reduced trilobite diversity and brought both Silurian and Devonian Faunas close to their disappearance. During the Late Devonian, trilobite diversity was low and dominated by phacopids and proetids, which characterize a Kellwasser Fauna. Finally, the Hangenberg event (end-Devonian) affected all existing trilobite clades and the following Tournaisian diversification led to the Late Palaeozoic Fauna, composed of phillipsiids and brachymetopids. Despite an important diversity decrease in the Visean–Serpukhovian, this fauna was the only one to persist until the extinction of trilobites at the end of the Permian.