The most primitive arthropod class, known from more than 3900 fossil species. Inhabitants of Palaeozoic seas, the trilobites appeared first in the early Cambrian, had their widest distribution and greatest diversity in the Cambrian and Ordovician periods, and became extinct in the Permian. The body was divided into three regions: an anterior cephalon, comprising at least five, fused segments; a mid-body or thorax, with a varying number of segments; and a hind region or pygidium. All three regions were divided by a pair of furrows running the length of the body, giving a trilobite appearance (i.e. a median or axial lobe, flanked on either side by a lateral lobe). The mouth was situated in the middle of the central surface of the cephalon. Paired gill-bearing limbs were attached to the membranaceous, pleural skeleton. X-ray studies show the eyes to have resembled the compound eyes of living arthropods (see trilobite eye). Trilobites ranged in size from 0.5 mm long planktonic (see plankton) forms to those nearly 1 m in length; most species were 3–10 cm long. There were nine orders: Redlichiida; Agnostida; Naraoiidae; Corynexochida; Lichida; Phacopida; Ptychopariida; Asaphida; and Proetida.

Trilobita