Brachiopods (from Latin brachium, arm + New Latin -poda, foot) are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling pelecypods (for instance, clams) of phylum Mollusca to which they are not closely related. It is estimated by paleobiologists that 99 percent of all documented lamp-shell species are both fossils and extinct.
Despite superficial similarities, bivalves and brachiopods differ markedly: Bivalves usually have a plane of symmetry between the valves of the shell, which are mirror images of each other; most brachiopods have a plane of bilateral symmetry through the valves and perpendicular to the hinge. The two brachiopod valves differ in shape and size from one another. Bivalves use adductor muscles to hold their two valves closed, and they open them by means of an external or internal ligament once the adductor muscles are relaxed. Brachiopods use internal diductor muscles to pull their two valves apart; they close the two with adductor muscles.
A second major difference is that most brachiopods are attached to the substrate by means of a fleshy "stalk" or pedicle. In contrast, although some bivalves (pelecypods such as oysters, mussels and the extinct rudists) are fixed to the substrate, most are free-moving, usually by means of a muscular "foot".
Furthermore, brachiopod shells may be made of either Calcium Phosphate or -- much more commonly -- Calcium Carbonate (CaCO3), as mollusks generally are. Lastly, in contrast to most bivalves, some extinct brachiopods exhibit elaborate flanges and spines.
Brachiopods may be divided into two types: inarticulate brachiopods are held together entirely by musculature, whereas articulate brachiopods have a hinge-like articulation between the shells. All brachiopods are marine and are found either attached to substrates by a structure called a pedicle or resting on muddy bottoms. Brachiopods are suspension feeders with a distinctive feeding organ called a lophophore, which is found in two other animal phyla (Bryozoa and Phoronida). Modern brachiopods generally live in areas of cold water, either near the poles or in deep parts of the ocean.
Modern brachiopods range in shell size from less than 5 mm (¼ in) to just over 8 cm (3 in). Fossil brachiopods generally fall within this size range, but some adult species have a shell of less than 1 mm across, and a few gigantic forms have been found measuring up to 38½ cm (15 in) in width.
The earliest unequivocal brachiopods in the fossil record occur in the early Cambrian, with the hingeless, inarticulate forms appearing first, followed soon thereafter by the hinged, articulate forms. Possible brachiopods have also been found in much older upper Neoproterozoic strata, although their assignment remains uncertain. Brachiopods are extremely common fossils throughout the Paleozoic. The major shift came with the Permian extinction. Before this extinction event, brachiopods were more numerous and diverse than bivalve mollusks. Afterwards, in the Mesozoic, their diversity and numbers were drastically reduced, and they were largely replaced by bivalve mollusks. Mollusks continue to dominate today, and the remaining orders of brachiopods survive largely in fringe environments of more extreme cold and depth.
The most abundant modern brachiopods are the 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"). Along with the Bryozoa and possibly the Entoprocta, these phyla constitute the informal superphylum Lophophorata.
The inarticulate brachiopod genus Lingula is the oldest, relatively evolutionarily unchanged animal known. The oldest Lingula fossils are found in Lower Cambrian rocks dating to roughly 550 million years ago. The origin of brachiopods is unknown. A possible ancestor is a sort of ancient "armored slug" known as Halkieria that was recently been found to have had small brachiopod-like shields on its head and tail.
During the Ordovician and Silurian 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. In some places, large sections of limestone strata and reef deposits are composed largely of their shells.
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 of (1) the rise in diversity of filter feeding bivalves, which have ousted the brachiopods from their former habitats; (2) the increasing disturbance of sediments by roving deposit feeders (including many burrowing bivalves); and/or (3) the increased intensity and variety of shell-crushing predation. 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.
The abundance, diversity, and rapid evolution of brachiopods during the Paleozoic make them useful as index fossils when correlating strata across large areas.
The taxonomy is still unstable, however, so different authors have made different groupings. In their 2000 article as part of the Treatise on Invertebrate Paleontology, Alwyn Williams, Sandra J. Carlson, and C. Howard C. Brunton present current ideas on brachiopod classification; their grouping is followed here. They subdivide Brachiopoda into three subphyla, eight classes, and 26 orders. These categories are believed to be approximately phylogenetic. Brachiopod diversity declined significantly at the end of the Paleozoic. Only five orders in three classes include forms which survive today, a total of between 300 and 500 extant species. Compare this to the mid-Silurian Period, when 16 orders of brachiopods coexisted.
|Brachiopod Taxonomy |
Extant taxa in green, extinct taxa in grey
after Williams, Carlson, and Brunton, 2000