Brachiopods are sedentary marine invertebrates that possess a hard, mineralized shell consisting of two hinged halves (valves) that enclose the delicate soft body of the animal. Although to the untrained eye brachiopods might resemble bivalves (e.g., clams), the similarities are superficial, and they are not closely related, each belonging to a distinct phylum. Brachiopods are entirely marine organisms, living on the sea bottom where they filter small particles of food from the water using a ciliated structure known as a lophophore. They have a rich and well-documented fossil record that extends back to the Cambrian Period and were among the most diverse and successful marine invertebrates of the Paleozoic Era, with over 12,000 fossil species described. Brachiopods were especially hard-hit during the mass extinction at the end of the Permian Period (~250 mya) that marked the end of the Paleozoic Era. Although they survived this extinction, brachiopods never again attained the levels of diversity, abundance, and ecological importance that they had during the Paleozoic. Today, brachiopods can still be found in marine environments, but their distribution and abundance is much reduced, with only a few hundred living species known.
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