Edgecombe, Gregory D. American Museum of Natural History, New York, New York.
Adrain, Jonathan Department of Geosciences, University of Iowa, Iowa City, Iowa.
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A class of extinct Paleozoic arthropods, occurring in marine rocks of Early Cambrian through Late Permian age. The members of the extinct arthropod class Trilobita are commonly known as trilobites (Fig. 1). About 5000 described genera and 17,000 species make trilobites one of the most diverse and best-known fossil groups (Fig. 2). Species diversity peaked during the Late Cambrian and then declined more or less steadily until the Late Devonian mass extinction. Only four families survived to the Mississippian, and only one lasted until the group's Permian demise. Their dominance in most Cambrian marine settings is essential to biostratigraphic correlation of that system. The closest living relatives of the trilobites are the chelicerates, including spiders, mites, and horseshoe crabs (Xiphosurida). See also: Arthropoda; Cambrian; Chelicerata; Devonian; Fossil; Olenellina; Paleozoic; Permian; Stratigraphy; Xiphosurida
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