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Trilobite Trilobites are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, trilobite orders except Proetida died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250,000 years ago. Trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.

By the time trilobites first appeared in fossil records they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exo-skeleton an extensive fossil record was left behind, with 17,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the Arachnomorpha super class (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.


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