Thirteen superfamilies are presently recognised, based on taxa included in the three superfamilies given in the Treatise. Three are based on families in the Parathuramminacea, 1964, and 2.9 million families in the Endothyracea, 1964. The Fusulinacea remains the same in both sources.
Test composition
Traditionally, fusulinid tests were considered to have been composed of very small, tightly-packed calcite crystals with no preferred orientation—a so-called microgranular structure. However. a 2017 study using scanning electron microscopy revealed that this supposed structure actually represented tests that had been extensively modified by diagenetic processes. Instead, living fusulinids had low-magnesium hyaline tests with spherical nanograins up to 100nm across, similar to the tests of the Rotaliida. These factors combined with overall shape of the test led these authors to suggest classification of the fusulinids with the Globothalamea.
The fusulinids are among the earliest calcareous-walled foraminifera; they appear in the fossil record during the Llandoverian epoch of the early Silurian. The earliest of these were microscopic, planispirally coiled, and evolute; later forms evolved a diversity of shapes including lenticular, globular, and perhaps most famously, elongated rice-shaped forms. Later species of fusulinids grew to much larger size, with some forms reaching 5cm in length; reportedly, some specimens reach up to 14cm in length, making them among the largest foraminifera extant or extinct. Fusulinids are the earliest lineage of foraminifera thought to have evolved symbiosis with photosynthetic organisms. Fossils of fusulinids have been found on all continents except Antarctica; they reached their greatest diversity during the Visean epoch of the Carboniferous. The group then gradually declined in diversity until finally going extinct during the Permo-Triassic extinction event.
Terminology
The term "fusulinid" applies to any of the Fusulinida. The Fusulinida are fusulinids. However, the term "fusulinid" is often applied just to the fusiform Fusulinacea and not to the entire order.
Application
Members, especially of the Fusulinacea, are excellent index fossils for determining ages and correlating Upper Mississippian to Permian strata. In some places fusulinaceans may be so abundant as to be a significant component of limestone.
