On the origin and evolution of Superorder Flexibilia (Echinodermata): a Bayesian tip‐dated phylogenetic assessment of Ordovician–Silurian flexible crinoids

Abstract

The crinoid Superorder Flexibilia originated in the late Middle Ordovician and, following the Late Ordovician mass extinction (LOME), radiated into one of the major constituents of Palaeozoic crinoid communities. For over a century, the canonical origin of the flexibles has largely been posited to be direct descendance from the paraphyletic cladid genus Cupulocrinus , which went extinct during the LOME. However, this hypothesis has yet to be tested within a comprehensive, species‐level phylogenetic framework, especially one that capitalizes on the novel temporal and biogeographic context provided by new flexible species described in recent decades. Here, we conduct a Bayesian tip‐dated phylogenetic analysis using the fossilized birth–death process to investigate evolutionary relationships among 38 cupulocrinid and flexible crinoid species spanning the Ordovician to the middle Silurian. Results suggest that flexibles are sister to, but not direct descendants of, the genus Cupulocrinus , and that both lineages arose in the latest Darriwilian ( c . 459 Ma) during the Great Ordovician Biodiversification Event. Phylogenetic biogeographic analyses support the geographic point of origin for Flexibilia to be central Laurentia. By the middle Katian ( c . 448 Ma) flexibles had dispersed throughout Laurentia and into the neighbouring palaeocontinent of Baltica, demonstrating a widespread taxonomic and geographic radiation that was underway well before the onset of the LOME. These findings provide a spatiotemporally constrained understanding of the origin and early evolution of Superorder Flexibilia, and highlight the swift geographic spread and taxonomic diversification of this major crinoid clade during an important interval of faunal turnover in crinoid communities.