DEVONIAN MARINE EXTINCTIONS WERE CAUSED BY TROPHIC COLLAPSE DUE TO DECLINING PCO2

The evolution of the land plants, beginning with bryophyte-grade cover in the Middle Cambrian and continuing with the Wenlockian origin of primary wood and the Givetian origin of secondary wood and trees, contributed to atmospheric CO₂ drawdown via increased weathering, biomass sequestration and subsequent carbon burial. Thus, evolutionary novelty, in the form of woody tissue, ultimately lead to profound change in the physical global environment. The species and genus richness distribution of the acritarchs follows very closely the pCO₂ GEOCARB III model. Speciation of phytoplankton during the Great Ordovician Biodiversification Event took place under relatively high dissolved CO₂(aq) in algae that possessed low Rubisco efficiency. Thus, the early Paleozoic phytoplankton were not limited by CO₂ availability. Devonian phytoplankton, however, were subjected to a progressive decline in dissolved CO₂(aq) which caused extinction in phytoplankton lineages with inefficient Rubisco and no Carbon Concentration Mechanisms (CCMs). The gradual loss of phytoplankton during the Devonian should have caused a progressive trophic collapse in marine ecosystems whose severity would have been greater in the zooplankton and nekton, than in the benthos, which is trophically buffered. This trophic collapse model predicts that the Devonian extinction was ecologically progressive, was fundamentally gradual in character, and was somewhat decoupled from benthonic invertebrate standing diversity. Intriguingly, this gradual acritarch “phytoplankton blackout” fits more recent diversity estimates which also document gradual Devonian extinction of marine invertebrates over the older catastrophic extinction models.