Geological Range

Foraminifera range from the Cambrian to Recent. Most of the Lower and Middle Palaeozoic forms had an agglutinated test whereas calcareous forms were most common in late Palaeozoic and later times. Planktonic foraminifera did not appear until the Mesozoic but are important constituents of Cretaceous and younger deep-sea deposits. The large-sized, lensshaped nummulitid foraminifera (up to about 15 cm) are widely distributed in Early Tertiary carbonate ramp and platform deposits.

They are particularly useful as an index fossil in Cenozoic marine deposits of the Mediterranean Sea region. Foraminifera provide the basis for much of the stratigraphic subdivision of the Upper Palaeozoic, Mesozoic and Cenozoic. Foraminifera commonly have a low reservoir potential due to their small particle size and stable primary mineralogy or neomorphic replacement. Some primary intragranular porosity may be of local significance, but generally connectivity is poor, giving low permeability.

In its simplest form, the body is sac-shaped with a large internal cavity which opens upwards, the closed end being attached to the substrate. The wall is perforated by numerous small pores leading to the central cavity. The skeleton is internal and may be spongin (an iodine-bearing protein which is rarely preserved in the geological record), siliceous or calcareous. Some sponges may incorporate foreign particles like sand grains in their skeleton. However, the skeleton typically consists of soft tissue supported by a complex network of spicules (Fig. 5.16).

The soft tissue decays on death and spicule-bearing forms are therefore seldom preserved intact. The spicules may occur in two different sizes within a single individual. The large spicules (megascleres) vary from 0.1 to 1 mm and constitute the skeleton proper. The small spicules (microscleres) vary from 0.01 to 0.1 mm and are found isolated in the sponges, especially around the pores. The megascleres may occur as isolated spicules, be in close contact with each other, or be firmly intergrown to form a skeleton.

Ecology

Nearly all sponges are marine although a few freshwater forms are known. Sponges live mostly in relatively clear water, from the littoral zone down to abyssal depths. Siliceous sponges are generally found in deep water, whereas calcareous forms are more common in shallow-water environments. Most sponges lived attached to the bottom. Many sponges are important boring organisms which help to break down calcareous rocks and carbonate skeletons produced by larger organisms into finer-grained material. The holes are 0.2–1 mm in diameter, forming thin holes up to 0.5 cm long.