Fusulinid

Fusulinid is a superfamily of extinct single-celled organisms belonging to the foraminifera. These marine protozoa are characterized by their large, elongate, and often coiled shells, known as tests. Fusulinids lived during the Carboniferous and Permian periods, roughly 360 to 252 million years ago. Their fossils are found predominantly in shallow marine sedimentary rocks worldwide.

The shell of a fusulinid is typically composed of calcite and exhibits a complex internal structure. This structure includes multiple chambers that are added sequentially as the organism grows. The coiling pattern and the shape of these chambers are key diagnostic features used to identify different fusulinid genera and species. Their size can vary considerably, with some reaching up to several centimeters in length, making them among the largest foraminifera known.

Fusulinids were widespread and abundant in Paleozoic seas, often forming a significant component of the fossil record in their respective environments. Their rapid evolution and wide geographic distribution have made them exceptionally useful index fossils for biostratigraphy, particularly in correlating rock layers from the Carboniferous and Permian periods. Their presence or absence, along with the specific species found, can help geologists pinpoint the age of sedimentary rocks. The extinction of fusulinids is linked to the end-Permian mass extinction event, the largest extinction in Earth's history.