EPICONTINENTAL SEAS VERSUS OCEAN-FACING SETTINGS: THE KINETICS OF ORIGINATION AND EXTINCTION DURING THE PALEOZOIC ERA

During the Phanerozoic, there was a decline in epicontinental seas (ES) as central loci of marine biodiversity and a concomitant increase in open-ocean-facing settings (OFS), primarily in association with continental margins. Previously, we compared the kinetics of genus origination and extinction in both settings during the Permian through Cretaceous and found that origination rates exhibited no consistent differences between the two, but extinction rates were significantly higher among genera that preferred OFS during three major mass extinctions. Here, we extend our analysis back into the Paleozoic, for an interval encompassing the Ordovician through Permian.

Genus-level data were acquired from The Paleobiology Database (http://paleodb.org/), and paleogeographic maps were used to assign occurrences to the two regimes based on proximity to the open ocean. Genera were calibrated as preferring one setting over the other when a statistically-significant preponderance of their occurrences were located in that setting.

As in our previous analyses, origination rates exhibited little noteworthy difference for genera preferring either setting. Extinction rates, however, revealed several intriguing patterns. First, during the Permian, for which the present study incorporates more extensive data and a more highly resolved timescale than our earlier investigation, extinction rates were higher for genera preferring ES than those preferring OFS for a protracted interval encompassing nearly the entire period, except for the Changhsingian, when the opposite was the case. Second, the Givetian through Famennian “mass-extinction” stages of the Devonian all exhibited higher extinction rates for genera preferring OFS, but as part of an interval for which this was the case from the late Early Devonian through the earliest Carboniferous. Finally, extinction rates were apparently higher for genera preferring ES during the Late Ordovician, but we are still working to accommodate our Ordovician data in a more highly resolved Ordovician timescale. Coupled with our earlier analyses, the present study provides additional evidence of significant differences between ES and OFS in the kinetics of mass extinction, likely reflecting the responses of the two regimes to causal mechanisms.