genus of extinct fusulinid foraminiferans (single-celled animals with a hard, complexly constructed shell) found as fossils in Permian marine rocks (the Permian Period began 286 million years ago and ended 245 million years ago). Parafusulina is more specifically restricted to the Leonardian and Guadalupian stages, smaller divisions of Permian rocks and time, and is thus an excellent index, or guide, fossil. The shell is characterized by distinct flutings, and the details of internal structure are best studied in thin sections.