Fusulinids

Fusulinids are an extinct group of large foraminiferans (foraminifera) known for their long, spindle-shaped shells. They belong to the order Fusulinida and lived in the world's oceans from the Late Silurian to the end of the Permian. They were predominantly benthic, inhabiting warm, shallow marine environments such as continental shelves and carbonate platforms, often in carbonate-rich limestones.

The shells, or tests, are calcareous and can reach several centimeters in length. They are composed of many chambers added in a linear or curved series, giving a fusiform or loaf-like shape. The walls are robust and their microstructure and sutures allowed for complex internal partitioning. The interior housed the protoplasm and the usual foraminiferal features of pore-rich walls and irregular chamber connections.

Fusulinids are among the best-known Paleozoic foraminifera and are widely used in biostratigraphy. Their rapid evolutionary succession and abundance in Carboniferous to Permian strata make them valuable index fossils for correlating rock units and calibrating geological timescales. Genera such as Fusulina, and related forms, illustrate the group's diversity.

Extinction occurred at the end of the Permian during the Permian-Triassic mass extinction, which eliminated many marine organisms, including fusulinids. Today, fusulinid fossils are important tools in paleontology and petroleum geology, helping to reconstruct paleoenvironmental conditions and dating Paleozoic rocks.