23-1 Species level diversity dynamics during the Great Ordovician Biodiversification Event: a phylogenetic case study of Laurentian brachiopod lineages

Session: Phylogenetic and Computational Approaches in Paleobiology and Paleoecology, Part I

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Abstract:

Marine life underwent a dramatic global biodiversity increase and ecological restructuring during the Ordovician Period with global diversification rates peaking during the main pulse of the Great Ordovician Biodiversification Event (GOBE) in the Middle Ordovician. Because diversification patterns represent the summation of discrete speciation events within individual lineages, a detailed understanding of diversification patterns and rates is required to constrain factors and mechanisms that may have promoted speciation during the GOBE.  Explicit consideration of speciation processes requires species-level phylogenetic hypotheses. Herein, we present a case study in which we analyze diversification dynamics using phylogenetically-constrained analyses of speciation and extinction rates across the GOBE interval for three clades of rhynchonelliformean brachiopods: Hesperorthis Schuchert and Cooper, 1931, Mimella Cooper, 1930, and Oepikina Salmon, 1942. Each clade was widely distributed across Laurentia, radiated during the Great Ordovician Biodiversification Event (GOBE), and flourished into the Sandbian and Katian. Together, they provide the opportunity to reconstruct a detailed snapshot of diversification across the GOBE in Laurentia within a critical benthic clade.  

Evolutionary relationships and diversity rates within each clade were assessed through Bayesian methods. Bayesian phylogenetic analysis was conducted using Markov Chain Monte Carlo (MCMC) sampling in MrBayes based on a combination of internal and external morphological characters for each clade. These topologies were then subjected to a Bayesian ‘fossil tip-dating’ analysis by implementing the fossilized birth–death process to constrain speciation and extinction rates during each clade’s history. We then evaluated speciation dynamics within each clade across the entire Ordovician Period.

Results indicate that all the three clades originated in the Early Ordovician, experienced a dramatic diversification of lineages during the Middle Ordovician (Darriwilian), and suffered a reduction in diversity in the Late Ordovician. Maximum net diversification occurred centrally within each lineage’s history which indicates slow initial diversification followed by more explosive phase in which speciation rate outpaced extinction rate during the main pulse of the GOBE. Notably, the highest speciation rates often occurred in the Late Ordovician, but speciation were outpaced by elevated extinction which produced biodiversity decline. The symmetrical diversification trajectories among clades indicate that environmental factors, such as sea level oscillations, that impacted all lineages likely provided a common speciation mechanism.  However, offset timing suggest that speciation triggers and dynamics also differed in detail among clades.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8337

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