Persistent Form

an organism that has maintained a constant external appearance in the course of evolution, passing from one epoch to another without substantial changes. Persistent forms are often called living fossils, or phylogenetic relicts. Examples include a brachiopod of the genus Lingula and king crabs of the genus Limulus, both of which have existed for about 400 million years; a marine mollusk of the genus Neopilina, which has existed for 350 million years; and a lungfish of the genus Latimeria, which has existed for about 250 million years. The occurrence of persistent forms is always due to the stability of the principal features of the organisms’ environment. A closely related term is “bradytelic forms” (seeBRADYTELY).