The Upper Triassic spumellarian genus Ferresium Blome ranges from upper Norian to Rhaetian, a relatively short period of geologic time equivalent to three ammonoid zones. During this time Ferresium undergoes rapid evolutionary change on a scale far exceeding the gradual change observed among other radiolarian genera of this age. Evolutionary progression is based on the changing shape of the ferresid cortical shell, a morphological feature easily traced from species to species. By the middle to upper Rhaetian, shell shape changes sufficiently to warrant the establishment of a new genus. In the uppermost Rhaetian, Ferresium is less common and the new genus dominates all assemblages. At the end of the Triassic all members of this lineage are extinct.