1. An extinct marine microorganism of the order Fusulinida, characterized by a spindle-shaped or elongated chambered shell, abundant in Paleozoic seas.
Usage: paleontology; geology; technical

Examples

• Fusulinids were among the most common fossils found in Carboniferous limestone deposits.
• The shells of fusulinids are useful index fossils for dating rock layers.
• Paleontologists identified the age of the rock formation by examining the fusulinid assemblage.
• Fusulinids thrived in warm, shallow marine environments during the Paleozoic Era.
• The extinction of fusulinids at the end of the Permian period marked a major turning point in Earth's history.