Trilobites rank among the most important of early animals. Our knowledge of them has been gained from the study of their fossils, usually impressions left of their shells after burial in sediment which subsequently hardened into rock. They appeared abruptly in the early part of the Cambrian Period, and came to dominate the Cambrian 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 belonged to the phylum Arthropoda—like crustaceans, spiders and insects. They aredivided into three parts from head to tail (head, thorax and tail) and from side-to-side (the central axis and two lateral lobes). The word ‘trilobite’ refers to the side-to-side partition, not the head, thorax and tail.