Impact of ocean acidification on plankton larvae as a cause of mass extinctions in ammonites and belemnites

Among the ten strong and catastrophic extinctions of ammonites and belemnites in geological history of Earth, at least eight of them (mainly for ammonites) occurred during the development of oceanic acidification events. Our analysis shows that the most vulnerable ontogenetic phase of these animals to oceanic acidification was their small and mainly planktonic paralarvae with either outer (ammonites) or inner (belemnites) calcareous shells. The thin walls of the paralarval shell with aragonite phragmocone serving to keep the neutral buoyancy might have prevented them to migrate deeper than the implosion depth (from several meters to first tens meters). Strong short-term ocean acidification events were the strongest in superficial water layers of the open ocean and shallow waters on the shelf, the very habitats of ammonitellas and paralarval belemnites, with catastrophic impact on integrity of their tiny calcareous shells. We suggest that the damage and corresponding malfunction of the fragile phragmocone to keep the neutral buoyancy was crucial during the development of ocean acidification. Disastrous short-term ocean acidification events contributed to extinctions of the calcareous plankton in Mesozoic, regardless whether they were planktonic during the entire life (e. g., coccolithophorids, foraminifers) or just during a relatively short ontogenetic phase as in ammonites and belemnites.