Trilobite

Trilobite, common name for a class of extinct marine arthropods. Trilobites ranged in length from a few millimetres up to about 65 cm (26 in), although most species were between 3 and 7 cm (1 and 3 in) long. Trilobites lived during the Palaeozoic era (570 million to 245 million years ago) and were common in the early part of that era. The trilobites were named after the arrangement of their exoskeleton, or outer shell, into three lobes. The exoskeleton, the part of the organism that is most commonly preserved, was made of calcium carbonate and covered the back of the animal.

Trilobites had a pair of compound eyes. In some trilobites, the eyes had densely packed lenses and may have served merely as a light-sensitive warning device to detect movement. In other trilobites, the eyes had fewer and more complex lenses and may have been capable of forming images and perceiving depth.

Trilobites lived in shelf and slope environments around continental margins and in the shallow continental seas that covered areas of the Earth that today are land masses. Most trilobites were bottom-dwellers, although some may have been swimmers or floaters. Some that possessed exceptionally large eyes and a large field of vision, such as Carolinites, are thought to have been swimmers that inhabited surface waters. Others, with reduced eyes or no eyes at all, preferred deeper, darker waters. Many trilobites burrowed into the sea bottom for protection and to seek food.

Trilobites employed a variety of feeding strategies. Many ploughed through mud at the bottom of oceans and seas, ingesting the sediment to sift out organic matter. Others were scavengers or predators. Most trilobites could roll themselves up into a defensive position so that only the exoskeleton was exposed.

The fossilized remains of trilobites are useful because they help scientists develop relative timescales for the ancient marine environment. Because trilobites evolved quickly and were widely distributed, comparing the trilobite fossils found in rock layers in different regions of the Earth can indicate which rock layer is older than the other. Trilobite fossils are particularly helpful in developing timescales for the Early Palaeozoic era.

Scientific classification: Trilobites belong to the phylum Arthropoda and the subphylum Trilobita.