Foraminifera are marine protozoa that are characterized by having tests (shells) that supports the cellular material. These single-celled organisms have inhabited the oceans for more than 500 millions years.
Both living and fossil foraminifera come in a wide variety of shapes and sizes, and occur in many different environments, from near-shore to the deep sea, and from near surface to the ocean floor. They are extremely abundant in most marine sediments and live in marine to brackish habitats. Foraminifera grow an elaborate, solid calcite skeleton made of a series of chambers. The complexity of their shell structures (and their evolution in time) is the basis of their geological usefulness.
Simple forms of foraminifera appeared in the Cambrian and are common in the early Palaeozoic. They had become abundant, with the evolutionary development of relatively large and complicated test architecture, by the late Palaeozoic, and provide a model example of evolutionary diversity throughout the Mesozoic and Cenozoic. This long and well-recorded evolutionary record makes foraminifera of outstanding value in zonal stratigraphy, palaeoenvironmental, palaeobiological and palaeoceanographic interpretation and analysis.
Foraminifera are separated into two groups following their life strategy, namely the planktonic and the benthic foraminifera. Many species of foraminifera are planktonic and of worldwide occurrence in broad latitudinal and temperature belts. They float in the surface or near-surface waters of the open ocean as part of the marine zooplankton. Their wide distribution and rapid evolution are a reflection of their successful colonisation of the pelagic realm. When this wide geographical range, achieved through the late Mesozoic and in the Cenozoic, is combined with a short vertical stratigraphic time range due to their rapid evolutionary characteristic, they make excellent index fossils at family, generic and species levels. There is, however, another group of foraminifera, which is as successful as the planktonic foraminifera group and that is even more abundant in modern seas, namely the benthic foraminifera. Benthic foraminifera live, attached or free, at all depths, and include a major group of foraminifera with complicated internal structures, the so called “larger benthic foraminifera”.
The larger benthic foraminifera are not necessarily morphologically bigger than small benthic foraminifera, although many are, but they are uniquely characterised by having internally complicated tests. While one can identify small benthic foraminifera from their external morphology, one must study thin sections to identify larger benthic foraminifera from their internal test architecture.
For a full discussion of Larger Benthic Foraminifera, see:
by M.K. BouDagher-Fadel