(pl. foraminifera), foraminiferan, foraminiferid; superclass Sarcodina, class Rhizopoda)
An order (or in some classifications a subclass, Foraminifera) of amoeboid (see amoeba) Protozoa in which the cell is protected by a test, consisting of one to many chambers, whose composition is of great importance in classification. The three main types are: (a) most primitively, a test wall composed of tectin (which also forms an underlying layer in the other two types); (b) a test formed from agglutinated sedimentary particles; and (c) a fully mineralized test composed of secreted calcareous or siliceous minerals of which aragonite and calcite are the most common. The arrangement of the chambers may be linear, spiral, cone-like, etc. Numerous fossil foraminifera are known, usually less than 1 mm across; though some, like the fusilinids (Carboniferous to Permian) and nummulitids (Eocene to Oligocene) were appreciably larger: some measured up to 100 mm in diameter. Most species live in marine environments. Agglutinated forms predominated in the Cambrian and Ordovician, presumably derived from a tectinous ancestor, while forms with fully mineralized tests appeared in the Ordovician and diversified greatly in the Devonian. The Foraminiferida are important zonal fossils, and some planktonic varieties can be used for stratigraphic correlation on virtually a world-wide scale. Accumulations of their tests make up a substantial part of certain geological formations (e.g. chalk deposits of the Cretaceous and Globigerina ooze).
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.