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The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out.

Trilobites disappeared in the mass extinction at the end of the Permian about 252 million years ago.

By the Ordovician mass extinction, vigorous trilobite radiation has stopped and gradual decline beckons.

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meaning "three lobes") are a group of extinct marine arachnomorph arthropods that form the class Trilobita.

Trilobites form one of the earliest-known groups of arthropods.

Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.

Trilobites had many lifestyles; some moved over the sea bed as predators, scavengers, or filter feeders, and some swam, feeding on plankton.

The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician.

Later evolution in trilobites was largely a matter of variations upon the Ordovician themes.

Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend.

Notable examples of this were the orders Agnostida and Asaphida, and the suborder Illaenina of the Corynexochida.

Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period.

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