An introduction to planktonic foraminifera

Abstract

The planktonic foraminifera are a highly abundant and diverse group of marine pelagic protists that live in the surface or near-surface waters of the open ocean as part of the marine zooplankton. They evolved from benthic foraminiferal ancestors in the Late Triassic or Jurassic. These unicellular eukaryotes are encased in a hyaline test made of calcium carbonate, the morphology of which is of importance in their classification at the generic and specific levels. This classification normally depends on chamber arrangements, wall structures, external ornament, perforation, aperture type. These features are illustrated, and a variety of accessory structures, including perforation cones, pseudomuricae, muricae, costellae, favose reticulations, pustules, muricocarinae, or keels, are described. The planktonic mode of life for the foraminifera was enabled by the development of globular, buoyant chambers. Initially, simple forms occupied the surface waters, but later as ornamentation of the test developed, more massive forms could have achieved neutral buoyancy at greater depth beneath the ocean surface. We describe how death assemblages (thanatocenosis) of planktonic foraminifera on the sea floor (or in resulting sedimentary rocks) can be used to infer the living assemblage (biocenosis) and hence reconstruct the ecological structure of the overlying water column. We discuss how features of the tests of planktonic foraminifera were selectively amplified to provide them a competitive advantage to exploit different niches in the oceans. We show that the evolutionary histories of many of the morphological features of the planktonic foraminifera show adaptive convergent trends (i.e., identical shapes or structures appear again and again within the same lineage or in parallel lineages from different stocks). These trends are seen to develop independently in Mesozoic, Paleogene, and Neogene families. They occur because each specific ecological niche has an optimal shape or processes that are required for its most effective exploitation. Finally, we present a brief overview of the story of the evolution and periodic extinction of the planktonic foraminifera during their history that spans at least 170. million years. Because of their mode of life, fossil planktonic foraminifera are extremely abundant in most marine sediments and occur worldwide within broad latitudinal temperature belts. Combining their wide geographical range with their frequently short specific time ranges, they make excellent tools for biostratigraphy and are central to many regional geological studies and to the exploration of hydrocarbon-bearing basins. Their practical significance justifies, therefore, their further and detailed study. © 2012 Elsevier B.V.