Trilobite

Trilobite, common name for a class of extinct marine arthropods. Trilobites lived from more than 500 million to about 250 million years ago, during the Paleozoic Era. They were most prevalent in the early part of that era, but members of the class existed for the extraordinary time period of nearly 300 million years. Trilobites ranged in length from a few millimeters up to about 65 cm (26 in), although most contemporary species were between 3 and 7 cm (1 and 3 in) long. The trilobites were named for 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 a hard substance; it covered the back of the animal.

Trilobites had two 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 Earth’s surface 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, such as Olenellus, burrowed into the sea bottom for protection and to seek food.

Trilobites employed a variety of feeding strategies. Many plowed through mud at the bottom of oceans and seas, ingesting the sediment to sort 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 time scales for the ancient marine environment. Because trilobites evolved quickly and were widely distributed, comparing the trilobite fossils found in rock layers in different regions can indicate which rock layer is older than the other. Trilobite fossils are particularly helpful in developing time scales for the early Paleozoic Era.