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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.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.The trilobites were among the most successful of all early animals, existing in oceans for almost 300 million years.By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed.Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician.Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida.

include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups.

Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues).

All Olenellina lack facial sutures (see below), and this is thought to represent the original state.

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.

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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.