Brachiopods (Brachiopoda) are one of the major animal phyla. They are sessile, two-shelled, marine animals that somewhat resemble pelecypod mollusks (i.e. "clams") externally but are quite different internally. Unlike bivalves, which have a left shell and a right shell, brachiopods are always bilaterally symmetric, although the top and bottom shells usually differ in shape. The shells may be either phosphatic or calcaerous. Some fossil forms had elaborate spines.
Brachiopods come in two varieties. Inarticulate brachiopods are held together entirely by musculature whereas articulate brachiopods have hinges. Brachiopods are always marine and are found either attached to hard substrates by a structure called a pedicle or resting on muddy bottoms. Brachiopods are filter feeders with a distinctive feeding organ called a lophophore, found among the various different groups of lophophorates.
The earliest known brachiopods are found in the late Neoproterozoic. The first brachiopods were inarticulate, but articulate brachiopods appeared soon thereafter, in the Lower Cambrian. Brachiopods are extremely common fossils throughout the Paleozoic. They were largely replaced by mollusks in the Mesozoic. Brachiopods -- both articulate and inarticulate -- are still present in modern oceans.
The inarticulate brachiopod genus Lingula has the distinction of being the oldest more or less unchanged animal known. The oldest Lingula occur in the very early Cambrian and are roughly 550 million years old. The origin of brachiopods is unknown. A possible ancestor is a sort of ancient "armored slug" called a halkeriid that has recently been found to have had small brachiopod-like shields on its head and tail.