fusulinid, any of a large group of extinct foraminiferans (single-celled organisms related to the modern amoebas but having complex shells that are easily preserved as fossils). The fusulinids first appeared late in the Early Carboniferous Epoch, which ended 318 million years ago, and persisted until the end of the Permian Period, 251 million years ago. Where they occur, the fusulinids have proven to be extremely useful for correlating different rock units in widely separated regions and for dividing geologic time into smaller units. Petroleum geologists also use them as keys to the locations of economically important deposits of oil and natural gas. Many forms of fusulinids are known, from barely visible species to forms that are easily seen with the naked eye and may be as much as 5 cm (2 inches) long. Many fusulinids resemble grains of wheat; the internal structure, however, is very complex and distinctive. The shell consists of a series of chambers formed about a central longitudinal axis. Complex patterns in the number and arrangement of internal walls and deposits are present and aid in classification and the working out of evolutionary relationships. Most fusulinids lived in clear marine water far from the shore.