1. Extinct marine protozoans of the order Fusulinida, characterized by a spindle-shaped or elongated test, abundant in Paleozoic seas and commonly used as index fossils for dating rock formations.
Usage: paleontology; geology; plural form; singular is fusulinid

Examples

• Fusulinids were among the most successful microfossils of the Carboniferous and Permian periods.
• Geologists use fusulinids to correlate and date sedimentary rock layers across different regions.
• The abundance of fusulinids in limestone deposits makes them valuable for stratigraphic analysis.
• Fusulinids disappeared during the Permian-Triassic extinction event.
• Thin sections of limestone often reveal numerous fusulinid shells under the microscope.