Complexity and Scale of the Term ‘event’ in Palaeozoic Stratigraphy

The Palaeozoic Era, spanning approximately 290 million years from the Cambrian to the Permian, represents one of the most dynamic intervals in Earth’s history. It witnessed profound evolutionary innovations, major biotic turnovers, and significant environmental changes that shaped the trajectory of life and the structure of sedimentary basins. Understanding these events and bioevents is crucial for refining stratigraphic correlations, interpreting the interplay between life and Earth systems, and reconstructing global palaeogeography. The term ‘event’ has been applied inconsistently over the Palaeozoic Period, with no clear definition. It currently encompasses a wide range of intervals across a broad spectrum of scales, including more substantial radiations, such as the Cambrian ‘explosion’ or the Great Ordovician Biodiversification ‘Event’ (GOBE), that clearly spanned millions of years, but also much shorter population bursts, such as the ‘Biotic Immigration Events’ (BIMEs), that are sometimes only recorded locally. On the other hand, the term is also widely used for intervals with extinction ‘events’ of variable duration, including the Late Ordovician Mass Extinction (LOME) or the relatively minor extinction events in the Silurian (the Ireviken, Mulde and Lau events), and the Kellwasser and Hangenberg events in the Devonian, among many others. The Palaeozoic Era closes with the severest mass extinction ‘event’ of the Phanerozoic – the end Permian Nemesis - which duration and pattern are still debated. With the aim to allow a chronostratigraphical definition of the term and its correlation of the chronostratigraphical charts, we attempt to provide an overview of the numerous terminologies of ‘events’ applied in the Palaeozoic.