MASS EXTINCTIONS AND GREATEST SPECIATIONS: THE EMERGENCE OF CALCAREOUS PHYTOPLANKTON IN THE EARLIEST MESOZOIC (Invited Presentation)

The end-Permian mass extinction was a most catastophic event in Life history on Earth. It was the time of the “Strangelove ocean” with prompt and large-scale perturbations of the carbon cycle, associated with global anoxia and warming along with some ocean acidification. The construction of the Siberian Traps is regarded as the trigger of complex feedbacks culminating into a biotic crisis: the ocean became habitale only for a few opportunistic “disaster species”, but harsh on most organisms.

Lower Triassic samples from Southern China revealed the occurrence of calcareous nannofossils in Olenekian strata: coccospheres of small-sized coccoliths in Smithian samples unequivocally indicate that coccolithogenesis was already operating some 249 Ma. Thus, the Permian/Triassic boundary extreme conditions had “mors tua vita mea” effects. Preference for specific trace metals, low trophic levels, and suppressed CO₂ in the ocean-atmopshere system favored/pushed some Prymnesiophytes to evolve calcification: this was the beginning of the long and successful evolutionary history of coccolithophores.

The origin of phytoplankton biocalcification changed, forever, the oceanic ecosystem dynamics and biogeochemical cycles at short- and long-term. Possibly biomineralization was key for coccolithophores to invade the open ocean and progressively contribute to sustain the marine biosphere, increase the export and burial of organic matter and shift/extend carbonate deposition from neritic to pelagic areas. The coccolithophore emergence soon after the end-Permian mass extinction is a spectacular example of intimate coupling between geological processes and Life evolution. It fixes the birth of modern ocean at the very beginning of the Mesozoic, when coccolithogenesis started to direct the carbonate system, regulating CO₂ exchange with the atmosphere, and transferring the results of complex biogeochemical interactions within the Earth system to the geological record.