Trilobites are a group of extinct marine arthropods that first appeared around 521 million years ago, shortly after the beginning of the Cambrian period. They are one of the earliest known groups of arthropods and are known from more than 10,000 fossil species. Trilobites had a lobed structure of the exoskeleton, which has a raised central lobe (or axis) and a pair of side lobes, called pleurae. The trilobite body is also divided lengthwise into three regions or tagmata: a head or cephalon, a middle region (thorax) composed of several to many articulated segments, and a tail plate called a pygidium, which consists of fused segments. The name "trilobite" comes from the distinctive three-fold longitudinal division of the dorsal exoskeleton into a central axis, flanked on either side by lateral (pleural) areas.
Trilobites had compound eyes, consisting of a number of separate lenses. The number of lenses and the complexity of the eye structure varied enormously. Some trilobites had large, convex compound eyes (like a fly’s) with a large number of lenses, giving them a wide field of view forwards, backwards, sideways, upwards and even downwards, depending on the actual curvature of the eye. Other trilobites had much smaller eyes, with fewer lenses, giving them a more restricted view.
Trilobites were highly diversified and geographically dispersed by the time they first appeared in the fossil record. They had a wide diversity of modes of life, such as burrowing in sediment, crawling over the sea floor, swimming in open water, detritus feeding, scavenging, and predation. The study of their fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics.
