Since about 1925, fusulinids have been studied extensively for purposes of zoning Pennsylvanian and Permian strata. The value of these fossils in stratigraphic zonation is enhanced by their rapid and divergent evolution into many short-lived lineages which are recognizable both at specific and generic levels. Rapid evolution of this type in groups having recent representatives, such as mammals, insects, and Foraminifera, commonly show a specialization into specific habitats or modes of life.