Shifting speciation mode and biogeographic patterns during the Late Ordovician (Sandbian-Katian) in Laurentian brachiopods (Atrypida, Anazygidae)

The Late Ordovician was a period of substantial environmental change. The co-evolution of Earth systems changes involving tectonic uplift, eustasy, and biotic changes are recorded in the Late Ordovician in Laurentia. In this study, species of the brachiopod family Anazygidae within the genera Zygospira and Catazyga were analyzed to assess dispersal pathways among eight sedimentary basins. Stochastic analysis applied to a Bayesian phylogeny through BioGeoBEARS facilitated estimation of speciation events including episodes of vicariance and basin-to-basin dispersal events within the clade. Three algorithms were implemented and compared. Dispersal-extinction-cladogenesis (DEC) + jump dispersal parameter was the best supported model. Results indicate that Anazygidae originated from an ancestral vicariance event, and most subsequent speciation events occurred via dispersal (total of 14 distinct episodes). Vicariance events are constrained to Mohawkian (Sandbian 2 to Katian 1) time and correlate to tectonic events. Dispersal events dominated during Cincinnatian (Katian 1–4) time and promoted increased diversification of anazygid species. Dispersal events are related to epicontinental circulation patterns and regional sea level changes and can be linked to connectivity and isolation pulses between the mid-continent and southern basins. Dispersal events exhibit a directional trend from the mid-continent toward the southern-most basins in Laurentia. Long-distance dispersal can be explained by jump dispersal events, with marginal-Laurentian islands acting as “stepping-stones” for intracratonic species. Anazygid speciation declined in the Richmondian (Katian 4). Zygospira and Catazyga became extinct in Laurentia as the epicontinental seas drained and climate cooled during the late Katian.