Trilobites were marine arthropods that lived during the Paleozoic Era, approximately 521 to 252 million years ago. They are known for their distinctive three-lobed body structure, which includes a head, a segmented body, and a tail. Trilobites varied in size and shape, with some species reaching lengths of over a foot. They inhabited various environments, from shallow seas to deep ocean floors. These creatures were among the first animals to develop complex eyes, allowing them to see in their underwater habitats. Trilobites played a significant role in the marine ecosystem, serving as both predators and prey. They became extinct during the Permian-Triassic extinction event, but their fossils are commonly found and studied by paleontologists today.