'Brachiopods' (from
Latin ''bracchium'', arm + New Latin ''-poda'', foot) are a nearly
extinct, small
phylum of
benthic,
invertebrates . Also known as 'lamp shells' (or 'lampshells'), "'brachs'" or 'Brachiopoda', they are
sessile, two-
shelled,
marine animals with an external
morphology superficially resembling
pelecypods (for instance,
clams) of phylum
Mollusca to which they are not even closely related.
On July 16, 1986, the Kentucky State Legislature designated the brachiopod to be the
Kentucky state fossil. It is estimated by
paleobiologists that 99 percent of all documented lamp-shell species are both
fossils and
extinct.
[1]
Despite superficial similarities, bivalves and brachiopods differ markedly:
Bivalves usually have a plane of symmetry between the shells, whereas most brachiopods have a plane of
bilateral symmetry ''through'' the shells and perpendicular to the
hinge. Both brachiopod shells are symmetrical as individual shells, but the shells differ in shape from one another. Whereas bivalves use
adductor muscles to hold their two shells closed, and open them by means of an external or internal
ligament once the adductor muscles are relaxed, brachiopods use muscle power (by internal
diductor and
adjustor muscles) to pull their two shells apart, and to close the two (by 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 either
phosphatic or -- much more commonly --
calcitic, as mollusks generally are. Only rarely do brachiopods may produce
aragonitic shells, which are composed of a less-permanent form of
calcium carbonate. Lastly, in contrast to most bivalves, some extinct lamp-shells exhibit elaborate
flanges and
spines.
General description
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.
Evolutionary history
Brachiopod fossils are often found in dense assemblages, such as these specimens of the
Ordovician species ''Onniella meeki''.
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.
A
Carboniferous brachiopod ''Neospirifer condor'', from Bolivia. The specimen is 7 cm across.
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.
Classification
In older classification schemes, Phylum Brachiopoda was divided into two classes: Articulata and Inarticulata. Since most orders of brachiopods have been extinct since the end of the Paleozoic Era, classifications have always relied extensively on the morphology (that is, the shape) of
fossils. In the last 40 years further analysis of the fossil record and of living brachiopods, including
genetic study, has led to changes in
taxonomy.
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.
Footnotes
1. See, for instance, data provided by paleontologist W. H. Easton (1960) in ''Invertebrate Paleontology'' (New York: Harper and Brothers).
External links
★
UC-Berkeley Museum of Paleontology
★
Palaeos Brachiopoda
★
BrachNet
★
Information from the Kansas Geological Survey
★
site of R.Filippi
★
Brachiopoda World Database
References
★
Brachiopoda (revised), , A, Williams, , 2000, Part H of
Treatise on Invertebrate Paleontology, , , , Geological Society of America and The University of Kansas, , ISBN 0-8137-3108-9
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