Which extinct group of marine invertebrates had a three-lobed exoskeleton? Trilobites are the canonical answer.
Trilobites were extinct marine arthropods that formed the class Trilobita. They appeared in the fossil record during the Cambrian Period and survived for nearly 270 million years before disappearing in the end-Permian extinction about 252 million years ago. Their name refers to the three lengthwise lobes of the body: a central axial lobe and two lateral pleural lobes.
A trilobite’s body was also divided from front to back into three major sections: the cephalon, or head; the thorax; and the pygidium, or tail shield. Its mineralized dorsal exoskeleton was rich in calcite, which helped trilobites leave an abundant fossil record. Many fossils are incomplete because the animal shed its exoskeleton during molting.
Trilobites were not dinosaurs, mollusks, or modern insects, although they were arthropods. They occupied many marine roles, acting as predators, scavengers, or filter feeders. More than 22,000 trilobite species have been described, ranging from tiny forms to giants over 70 centimeters long.