Brachiopod
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{{Taxobox | color = pink | name = Brachiopods | image = Usgs_boulder_brachiopods.jpg | image_width = 250px | image_caption = Living brachiopods. | regnum = Animalia | phylum = Brachiopoda | phylum_authority = Duméril, 1806 | subdivision_ranks = Subphyla and classes | subdivision = See Classification }}
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, and therefore unrelated. 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. Whereas bivalve shells open by means of an external ligament, brachiopods use internal diductor and adjustor muscles. Brachiopod shells may be either phosphatic or calcitic (very rarely possibly aragonitic). Some fossil forms also had elaborate flanges and spines.
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General description
Image:Lingula anatina 2.jpg Brachiopods come in two easily distinguished varieties. Inarticulate brachiopods are held together entirely by musculature, whereas articulate brachiopods have a hingelike articulation between the shells. Brachiopods are always marine and are found either attached to substrates by a structure called a pedicle or else resting on muddy bottoms. Brachiopods are filter feeders with a distinctive feeding organ called a lophophore. 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 (1/4 of an inch) to just over 8 cm (3 inches). Fossil 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 38.5 cm (15 inches) in width.
Evolutionary history
The earliest known brachiopods are found in the late Neoproterozoic (although these are only questionably assigned to Brachiopoda). The first brachiopods were inarticulate (hingeless), but articulate (hinged) brachiopods appeared soon thereafter, in the Early Cambrian. 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.
Brachiopods -- both articulate and inarticulate -- are still present in modern oceans. The most abundant are the terebratulides (class Terebratulida). The perceived resemblance of terebratulide 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/Kamptozoa, these phyla constitute the informal superphylum Lophophorata.
Image:Brachiopoda-morphology.png
The inarticulate brachiopod genus Lingula has the distinction of being the oldest, relatively evolutionarily unchanged animal known. The oldest Lingula occur in the very early Cambrian, 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.
Image:Brachiopod Neospirifer.jpg
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
| Brachiopod Taxonomy Extant taxa in green, extinct taxa in grey | |||
| Subphyla | Classes | Orders | Extinct |
|---|---|---|---|
| Linguiliformea | Lingulata | Linguilida | no |
| Siphonotretida | Ordovician | ||
| Acrotretida | Devonian | ||
| Paterinata | Paterinida | Ordovician | |
| Craniformea | Craniforma | Craniida | no |
| Craniopsida | Carboniferous | ||
| Trimerellida | Silurian | ||
| Rhychonelliformea | Chileata | Chileida | Cambrian |
| Dictyonellidina | Permian | ||
| Obolellata | Obolellida | Cambrian | |
| Kutorginata | Kutorginida | Cambrian | |
| Strophomenata | Orthotetidina | Permian | |
| Triplesiidina | Silurian | ||
| Billingselloidea | Ordovician | ||
| Clitambonitidina | Ordovician | ||
| Strophomenida | Carboniferous | ||
| Productida | Permian | ||
| Rhynchonellata | Protorthida | Cambrian | |
| Orthida | Carboniferous | ||
| Pentamerida | Devonian | ||
| Rhynchonellida | no | ||
| Atrypida | Devonian | ||
| Spiriferida | Jurassic | ||
| Thecideida | no | ||
| Athyridida | Cretaceous | ||
| Terebratulida | no | ||
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.
Of note
On July 16, 1986, the Kentucky State Legislature designated the brachiopod to be the Kentucky state fossil.
External links
- UC-Berkeley Museum of Paleontology
- Palaeos Brachiopoda
- BrachNet
- Information from the Kansas Geological Survey
- site of R.Filippi
Reference
- Williams, A., Carlson, S.J., and Brunton, C.H.C. (2000). "Brachiopod classification" in v. 2 of Williams, A. et al. Brachiopoda (revised), part H of Treatise on Invertebrate Paleontology (Kaesler, R.L., ed.). Boulder, Colorado: Geological Society of America and Lawrence, Kansas: The University of Kansas (ISBN 0-8137-3108-9)de:Armfüßer
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