Fusulinida

Fusulinida is an extinct order of foraminiferans (single-celled protists) known for their large, calcareous tests and complex architecture. These organisms are among the largest foraminifers, with tests ranging from a few millimeters to several centimeters in length.

Typically, the tests are multi-chambered and elongated, often fusiform or plump, and built of microgranular calcite. The chambers are added sequentially and arranged in a spiral or uncoiled pattern, giving a complex internal morphology that is highly diagnostic in the fossil record. The outer wall is usually thick and multi-layered, sometimes with a perforated structure and lamellae that reflect rapid, layered growth.

Fusulinids were benthic marine organisms living in warm, shallow to shelf environments, though some occupied deeper settings. They are most abundant in limestone sequences deposited during the Carboniferous and Permian periods (roughly Mississippian to Permian) and are widely distributed in many paleo-ocean basins, especially the Tethyan realm.

Fossils of Fusulinida are important index fossils for Paleozoic strata, helping to date rocks and correlate strata across continents due to their rapid evolution and geographic spread. The group includes the type genus Fusulina and other genera such as Fusulinella and Parafusulina.

Fusulinida became extinct by the end of the Permian period, in the Permian-Triassic extinction event, with no surviving descendants. Etymology derives from Latin fusus, meaning “spindle,” referring to the spindle-shaped tests.