EXTINCTION AND RADIATON IN PELAGIC AND BENTHIC PROTISTS

The five major mass extinctions, a number of intermediate level extinctions and many other minor ones all impacted pelagic and benthic protists. For pelagic (~65% of Earth’s surface) and benthic (~70%) environments, the extinctions are characterized by the number of taxa that were extinguished in relatively short geological intervals, including many protists. All of these extinctions affected protists in different systematic, functional, skeletal, and ecologic groups, thus suggesting a widespread cause with impacts on many different ecosystems. The extinctions were followed by completely new major taxa, not simply by a renewal or radiation of old types. A host of organism-related (like physiologic changes) and ecologic (like cooling) factors have been suggested as causes. A common inadequate explanation is that “niches” of organisms were eliminated, which means any or all attributes of species, and then reoccupied. Recent discoveries and analyses indicate that relatively sudden pulses of greenhouse gases (CO₂ in particular) warmed marine climates resulting in the extinctions. But how? The fossil record of protists provides indications. Certainly organism-specific mechanisms may be involved but not as general causes. At a first order level, sudden global warming events account for most observations, specifically the elimination of major habitats and ecosystems in pelagoc and benthic marine ecosystems. Post-extinction radiations may take many millions of years and have been interpreted as the reoccupation of “empty niches” but that is wrong. Since the ecosystems changed or were eliminated, no old niches existed after extinction. Instead, new ecologic opportunities developed where survivors could adapt and evolve through physical processes often interacting with biological ones, and these allowed the evolutionary radiations of new biotas that followed extinction events. The time of these radiations is chiefly a measure of increasing ecologic opportunities as the ecology becomes more complex with time.