Members of Kingdom Protista consist of single eukaryotic cells (cells having a nuclear wall and organelles). Some members are photosynthetic and others (protozoa) must consume other protists. Four major types are discussed here:
foraminifera, radiolaria, diatoms, and coccoliths (see Fig. 4.15).
Foraminifera are a marine protista group in which some members are planktonic (zooplankton) and others are benthic. Foraminifera are protozoans that secrete tests of calcite or create tests of cemented silt grains (called agglutinated). The structure of the test is a single chamber or a series of chambers, and the size is about that of a grain of sand. Some fossil forms, such as Nummulites, were considerably larger. The protoplasm of the living cell extends out from the main opening, or aperture, and also from pores in the test (see Fig. 4.15). This external net of pseudopodia traps food particles for digestion inside the cell. Foraminifera reproduce by alternation of sexual and asexual phases.
In the Paleozoic era agglutinated foraminifera were most common. Wheat grain-sized calcareous forms with complex chamber structure, the fusulinids, are good index fossils for the late Paleozoic. Foraminifera with calcite tests were more common in the Mesozoic and Cenozoic. Foraminifera are extremely useful in biostratigraphy and can be used to subdivide geologic time into finer intervals called zones.
Radiolaria are marine zooplankton that secrete a test of opaline silica in spherical, helmet-shaped, and spiny forms commonly with open pores. Radiolaria became abundant in the Mesozoic era. As in the foraminifera, the cell is protozoan, and pseudopodia extend from openings in the lattice of the test to trap food particles. Some radiolaria contain algae within their tissue, which supply them with oxygen. Radiolaria are smaller than foraminifera, closer to silt size, and are abundant in modern seas.
Diatoms are a form of algae and are therefore photosynthetic organisms. They first appeared in the early Mesozoic and became abundant later in that era. These algae secrete minute silt-sized tests of opaline silica that are usually round or oval-shaped. The two valves of the test fit together like a box and its lid. They are found in fresh water as well as marine waters. Diatoms and radiolaria are the primary components of deep-sea siliceous oozes, and after lithification these deposits form one variety of the sedi-
mentary rock chert.
Coccoliths are extremely small calcareous platelets secreted by single cells of photosynthetic yellow-green algae that are abundant today as phytoplankton in the sea. These algae first appeared in the Triassic. Their shell fragments, along with foraminifera, are an abundant component of pelagic calcareous oozes. Coccolith platelets are the primary constituent of the sedimentary rock chalk. The chalk cliffs of Dover along the English Channel are composed mostly of coccoliths.