 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 before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetids died out.The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago.Trilobites were among the most successful of all early animals, existing in oceans for almost 300 million years.