Calymene was able to roll into a ball for protection.
Trilobites rank among the most important of early animals.
Our knowledge of them hasn 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 became to dominate the Cambrian and early Ordovician sea. A prolonged decline then set in before they finally became extinct in the Permian Period, about 250 million years ago. For information regarding the copyright of BGS material for this page , click here .