101-2 Abiotic Drivers of Paleozoic Echinoderm Diversification: Continental Fragmentation Fuels Origination, Temperature Triggers Extinction

Abstract:

The echinoderm fossil record spans the entire Phanerozoic. Major echinoderm classes were established in the Cambrian, and order-level diversity expanded during the Great Ordovician Biodiversification Event; many of the clades were short-lived, morphologically experimental, and regionally restricted, making the group a sensitive recorder of Paleozoic environmental change. Identifying what drove this diversification is representative of a broader, foundational question in paleobiology: do origination and extinction respond to the same or different abiotic forces throughout deep time? 

We extracted 29,943 Paleozoic Echinodermata fossil occurrences from the Global Biodiversity Information Facility, assigned to 1,011 genera, and dated using formation-level ages from Macrostrat. Genus-level evolutionary rates were estimated using two complementary Bayesian birth-death approaches implemented in PyRate: a birth-death model with constrained rate shifts and a reversible-jump MCMC model, both of which infer the number, timing, and magnitude of rate shifts. A Multivariate Birth-Death (MBD) model was used to test six candidate abiotic drivers: continental fragmentation, global temperature, sea level, atmospheric carbon dioxide, ice volume, and volcanism, plus within-clade diversity dependence, against origination and extinction.

Diversity increased from ~0 at the base of the Cambrian (~552 Ma) to a sustained plateau of ~150–175 genera through the Ordovician to Devonian (~470–360 Ma). Maximum diversity was reached during the early Carboniferous (~340–320 Ma) before declining to ~0 at the end- Permian mass extinction. Origination and extinction rates are variable throughout the dataset, with the largest uncertainties in the Cambrian. The Late Ordovician Mass Extinction exhibits elevated extinction rates and stable origination rates, whereas the end-Devonian extinctions exhibit stable extinction rates but depressed origination rates, providing additional support that this extinction event was a biodiversity crisis. Rates at the end-Permian mass extinction exhibit highly elevated extinction relative to stable origination, but with greater error than in prior time periods. 

Results of the MBD model indicate speciation of Paleozoic echinoderms was driven by continental fragmentation. This follows similar studies which have identified founder-event speciation, or jump dispersal, and dispersal over vicariance as important processes to echinoderm diversification. Extinction of echinoderms was driven by temperature, a finding also in line with prior studies of marine invertebrates. Although resilient, echinoderms are sensitive to changes in ocean temperature and warrant careful consideration under future warming scenarios.

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