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 oceans and are often found as fossils in sedimentary rock. These creatures were among the first animals to develop complex eyes, allowing them to see in their underwater environment. Trilobites played a significant role in the marine ecosystem, feeding on small organisms and detritus. Their extinction at the end of the Permian Period marked a significant event in Earth's history.