The extinct marine arthropod with a three-lobed body that is a common fossil is the trilobite. Its name refers to the three longitudinal lobes of its mineralized exoskeleton: a central axial lobe and two pleural lobes on either side.
Trilobites belonged to the class Trilobita and lived in marine environments from the Cambrian Period, about 521 million years ago, until the end-Permian mass extinction roughly 252 million years ago. More than 22,000 species have been described, ranging from less than a millimeter to more than 70 centimeters long.
Their calcite-rich exoskeleton fossilized readily, while repeated molting also left behind numerous discarded shells. This helps explain why trilobites are so common in the fossil record and why geologists use them as index fossils for dating Paleozoic rocks. They were not dinosaurs, mollusks, or modern insects, although they were arthropods. Some species crawled across the seafloor as scavengers, predators, or deposit feeders, while others swam or filtered food from the water.