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Brachiopods (from Latin bracchium, arm + New Latin -poda, foot) make up one of the major animal phyla, Brachiopoda. Also known as lamp shells, 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 also had elaborate spines.

1 General description

Brachiopods come in two easily distinguished 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. Modern brachiopods generally live in areas of cold water, either near the poles or in deep parts of the ocean.

Modern brachiopods range in the size of their shell from less than 5 mm (1/4 of an inch) to just over 8 centimetres (3 inches). Fossils brachiopods generally fall within this size range, but some adult species have a shell of less than 1 millimetre across, and a few gigantic forms have been found measuring up to 37.5 cm (15 inches) in width.

2 Evolutionary history

The earliest known brachiopods are found in the late Neoproterozoic. The first brachiopods were inarticulate (hingeless), but articulate (hinged) 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 most abundant are the terebratulids (class Terebratulida). The perceived resemblance of terebratulid shells to ancient oil lamps gave the brachiopods their common name "lamp shell". The phylum most closely related to Brachiopoda is probably the small phylum Phoronida (known as "horseshoe worms").

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" known as halkeria that has recently been found to have had small brachiopod-like shields on its head and tail.

During the OrdovicianThe Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. The Ordovician follows the Cambrian period and is followed by the Silurian period. The Ordovician named after the Welsh tribe of the Ordovices was define and SilurianAlternate use: The Silurians, a reptillian race from the science fiction series " Doctor Who . The Silurian is a major division of the geologic timescale that extends from about 408. 5 million years before the present (BP) with the end of the Ordovician p periods brachiopods became adapted to life in most marine environments and became particularly numerous in shallow water habitats, in some cases forming whole banks in much the same way as bivalves (such as mussels) do today.

Throughout their long geological history the brachiopods have gone through several major proliferations and diversifications, and have also suffered from major extinctions as well.

It has been suggested that the slow decline of the brachiopods over the last 100 million years or so is a direct result or the rise in diversity of filter feeding bivalves which have ousted the brachiopods from their former habitats. However, it should be noted that the greatest successes for the bivalves have been in habitats which have never been adopted by the brachiopods, such as burrowing and free swimming.

The abundance, diversity, and rapid evolution of brachiopods during the Paleozoic make them useful as index fossilIndex fossils (or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that although different sediments may look different depending on the conditions under which they were laid down, they mas when correlating strata across large areas.

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