EVOLUTION AND EXTINCTION IN THE PELAGIC REALM OF EARTH’S OCEANS

The pelagic realm, the largest of all realms of Earth’s life, covers nearly 70% of Earth’s surface and extends from the surface to 4000+m deep. It is a 3-demensional realm unlike any other. Every major group of animals, algae and protists have occupied countless horizontal and vertical habitats. Its fossil record is limited due to the paucity of fossil species and pelagic sedimentary deposits on land or in the ocean basins prior to the mid-Mesozoic. Skeletonized forms are commonly abundant however, and reliably show periods of evolutionary radiation and of extinction. Ever since the Precambrian, pelagic organisms radiated multiple times and underwent the five major and another 25 or so mass extinctions. These are distinguished by the number of taxa that were extinguished in relatively short geological intervals. The extinctions eliminated organisms in different systematic, functional, and ecologic groups thus indicating a widespread cause with impacts on many different biotas. Among a variety of causes, ecologic factors, of course, have been suggested; for example, that the “niches” of organisms were eliminated, which means any or all attributes of species, and then reoccupied. Relatively sudden impulses of greenhouse gas (CO₂ in particular) warming marine hydroclimates resulted in the extinctions. But how? Sudden greenhouse warming has its greatest effect through the elimination of vertical and horizontal habitats by homogenizing the oceans ecologically by warming it. When these habitats are destroyed, most organisms die. Radiations, which may take many millions of years, have been interpreted as the reoccupation of “empty niches” but that is wrong. No old niches existed after an extinction. Instead, new ecologic opportunities developed where survivors in refuges or habitats less impacted could adapt and evolve through physical and biological processes, and these allowed the evolutionary radiations that followed extinction events. The time of these radiations is chiefly a measure of the increasing ecologic opportunities and complexity of new horizontal and vertical habitats.