Trilobite

Trilobites are extinct arthropods that form the class Trilobita. They appeared at the start of the Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Late Devonian extinction, all trilobite orders, with the sole exception of Proetida, died out. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago (m.y.a.).

Trilobites are very well-known, and possibly the second-most famous fossil group after the dinosaurs. When trilobites appear in the fossil record of the Lower Cambrian they are already highly diverse and geographically dispersed. Because of their diversity and an easily fossilized exoskeleton, they left an extensive fossil record with some 15,000 known species spanning Paleozoic time. Trilobites have been important in both paleontology and plate tectonics research. For example, trilobites have been important in estimating the rate of speciation during the period known as the Cambrian Explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian (Lieberman, 1999), and are readily distinguishable because of complex and well preserved morphologies. The trilobites are often placed either within subphylum Chelicerata or grouped as its sister group to form the group Arachnomorpha, although several alternative taxonomies are found in the literature.

Different trilobites made their living in different ways. Some led a benthic life as predators, scavengers or filter feeders. Some swam in order to feed on plankton. Still others are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.