Trilobites

Trilobites are extinct marine arthropods whose bodies were divided into a head shield, a segmented middle and a tail, and into three lobes lengthwise. The earliest appear about 521 million years ago, in rocks of Siberia, Morocco and elsewhere, and within a few million years they were everywhere. Over about 270 million years they gave rise to some 20,000 named species, from specks barely a millimetre long to Isotelus rex of Canada, about 70 centimetres.

They were among the first animals to grow a hard exoskeleton of calcite-reinforced chitin, probably made possible by the arms race that began with skeletons and predators in the Ediacaran. They moulted as they grew, so each animal left many fossils, one reason they are so numerous. Their compound eyes had lenses of clear calcite crystals, a rare case of mineral lenses, as Euan Clarkson showed. Trilobites crawled, swam, burrowed and filter-fed, some rolled up like a woodlouse for defence, and some had long spines or huge eyes.

Trilobites diversified in the Cambrian and Ordovician, declined through the Devonian and fell to a few families after the Late Devonian extinction. The last, a few lineages, vanished in the Great Dying about 252 million years ago. Because species changed quickly and had hard shells, trilobites became the main tool for dating Paleozoic rocks, and they remain among the world's most collected fossils.