Trilobites are one of the oldest animals that first lived on the Earth. They lived in the Cambrian Period and early Ordovician seas. A prolonged decline then set in before they finally became extinct in the Permian Period, about 250 million years ago.
Trilobites were arthropods—like crustaceans, spiders and insects. They are divided into three parts from head to tail (head, thorax and tail) and from side-to-side (the central axis and two lateral lobes). Hence their name 'trilobite'.