براکیوپد-Brachiopoda
Brachiopoda
A phylum of solitary,
exclusively marine, coelomate, bivalved animals, with both valves symmetrical
about a median longitudinal plane. Brachiopods are typically attached to the
substrate by a posteriorly located cuticle-covered stalk called a pedicle.
Anteriorly, a relatively large mantle cavity is always developed between the
valves, and the ciliated tentacular (filamentous) feeding organ, or lophophore,
is suspended within, projecting anteriorly from the anterior body wall (Fig. 1).
The name Brachiopoda means “arm foot” and refers to the morphology and location
of the lophophore.
Fig. 1 Principal organs of a brachiopod as
typified by Terebratulina. (After R. C. Moore, ed., Treatise on Invertebrate
Paleontology, pt. H, Geological Society of America, Inc., and University of
Kansas Press, 1965)
This brief description
serves to differentiate a brachiopod from any other animal. Within the
Brachiopoda, three groups currently regarded as subphyla are recognized, each
named after their most ancient living representative: Linguliformea,
Craniiformea, Rhynchonelliformea. Phoroniformea are recognized by some as a
fourth subphylum. The two classes (Inarticulata and Articulata), formerly
distinguished by the presence or absence of articulation between the two valves
of the shell, are no longer recognized as distinct taxa; rather, they appear to
represent the ends of a spectrum of articulatory types, some well delineated,
others less so. Rhynchonelliforms have impunctate, endopunctate, or
pseudopunctate calcitic shells with a fibrous (or laminar) shell structure; most
are articulated, with the valves typically hinged together by a pair of ventral
teeth with complementary sockets in the dorsal valve; most have pedicles with a
core of connective tissue. Craniiforms have punctate calcitic shells with a
laminar (or tabular) shell structure; all lack articulation, with valves that
are held together only by the soft tissue of the living animal, and all lack
pedicles. Linguliforms have canaliculate organophosphatic shells with a
stratiform shell structure; all lack articulation; and all have pedicles with a
coelomate core. See also:
Inarticulata; Lingulida; Rhynchonelliformea
Phylogeny and
classification
Molecular sequence
data from nuclear and mitochondrial genes in extant brachiopods provide strong
support for brachiopods as protostomous organisms, and not deuterostomes, as
some earlier morphological and developmental data seemed to suggest. These data
also indicate that the Phoronida is nested within the Brachiopoda, and is not
the brachiopod sister group, as previously thought. Lophophorates alone do not
appear to form a clade; the Bryozoa are more distantly related within the
Lophotrochozoa, a rather poorly resolved clade that also includes mollusks and
annelids, among other phyla (Fig. 2). Only approximately 5% of named brachiopod
genera (and most likely, species) are extant; most of our current understanding
of brachiopod phylogeny results from a composite of molecular data from a small
taxonomic sample, and morphological (and stratigraphic) data from a much larger
sample of extinct fossil taxa. See
also: Bryozoa; Phoronida; Rhynchonelliformea
Fig. 2 Phylogenetic relationships among
selected metazoans. (Simplified from K. M. Halanych and Y. Passamaneck, Amer.
Zool., 41:629–639, 2001; K. M. Halanych, Annu. Rev. Ecol. Evol. Sys.,
35:229–256, 2004; and B. L. Cohen and A. Weydmann, Organisms, Diversity and
Evolution, 5(4):253–273, 2005)
The phylum is
currently classified as follows:
Subphylum
Linguliformea
Class
Lingulata
Order:
Lingulida
Siphonotretida
Acrotretida
Class
Paterinata
Order
Paterinida
Subphylum Craniiformea
Class
Craniata
Order:
Craniida
Craniopsida
Trimerellida
Subphylum
Phoroniformea (?)
Subphylum
Rhynchonelliformea
Class
Chileata
Order:
Chileida
Dictyonellida
Class
Obolellata
Order:
Obolellida
Naukatida
Class
Kutorginata
Order:
Kutorginida
Class
Strophomenata
Order:
Strophomenida
Billingsellida
Orthotetida
Productida
Class
Rhynchonellata
Order:
Protorthida
Orthida
Pentamerida
Rhynchonellida
Atrypida
Spiriferida
Spiriferinida
Thecideida
Athyridida
Terebratulida
Orientation
The two brachiopod
valves are currently referred to as the dorsal and ventral valves, not brachial
and pedicle, because not all brachiopods possess pedicles. This body orientation
distinguishes them from bivalved mollusks, which have valves on the left and
right sides of the body. While topologically dorsal and ventral in adults, their
developmental orientation is not entirely clear; both valves of some extant
craniiforms may derive from the dorsal surface of the developing, modified
trochophore larva. The pedicle either protrudes between the valves (through the
delthyrium and notothyrium, which are small triangular apertures serving as
pedicle openings), or (Fig. 3) more commonly emerges from a variably modified
opening (pedicle foramen) in the ventral valve, which, in articulated forms, is
the valve bearing the hinge teeth. Whatever the form of the pedicle opening, it
is always at the posterior end of the animal and its enclosing shells, the
opposite end being regarded as anterior. Both valves have a characteristic
distribution of muscles, and any skeletal support for the lophophore is
invariably developed from the dorsal valve (Fig. 4).
Fig. 3 Diagrammatic representation of the
external features of a generalized brachiopod seen in (a) posterior, (b) left
lateral, (c) dorsal, and (d) dorsolateral views. (After R. C. Moore, ed.,
Treatise on Invertebrate Paleontology, pt. H, Geological Society of America,
Inc., and University of Kansas Press, 1965)
Fig. 4 Generalized representation of
distribution of epithelium in relation to other tissues and organs in (a)
lingulides and (b) terebratulides. (After R. C. Moore, ed., Treatise on
Invertebrate Paleontology, pt. H, Geological Society of America, Inc., and
University of Kansas Press, 1965)
Anatomy
The pedicle is the
only organ protruding outside the valves, for the remainder of the animal is
enclosed in the space between them. This space is divided into two unequal
parts, a smaller posteriorly located body cavity and an anterior mantle cavity.
The ectodermal outer epithelium underlying the shell bounding the body cavity is
in a single layer; but anteriorly, laterally, and even posteriorly in most
inarticulated brachiopods, it is prolonged as a pair of folds forming the
ventral and dorsal mantles, from which the shells are mineralized (at the
generative zones). In extant species, the two mantles approach each other and
ultimately fuse along the posterior margin of rhynchonelliform brachiopods; in
contrast, the mantles are invariably discrete in craniiforms and linguliforms,
and are separated by a strip of body-wall inner epithelium (Fig. 4).
The body cavity
contains the musculature; the alimentary canal; the mixonephridia, which are
paired excretory organs also functioning as gonoducts; the reproductive organs;
and the rather poorly understood circulatory and nervous systems. Except for the
openings through the mixonephridia, the body cavity is enclosed, but the mantle
cavity communicates freely with the sea when the valves are opened. The
lophophore is a feeding and respiratory organ, typically suspended from the
anterior body wall within the mantle cavity, and is always symmetrically
disposed about the median plane. The lophophore consists of a ciliated,
filament-bearing tube with two arms in varying configurations. The ciliary beat
produces a laminar flow of water into and then out of the mantle cavity, flowing
across the filaments while inside. The latter trap food particles which are
carried along a groove in the lophophore to the mouth situated medially between
the two arms. See also: Lophophore
The alimentary canal
of all brachiopods is broadly similar in structure, but differs in orientation
in the body cavity. The mouth opens into a muscular tube, the esophagus, which
continues to a stomach and intestine. In addition, there are a variable number
of digestive diverticula that communicate with the stomach through narrow ducts
(Fig. 1). In rhynchonelliform brachiopods, the intestine curves into a C-shape
and ends blindly (with no anus) in a posteroventral location; in linguliforms,
it curves into a U-shape, leaving the anus right-lateral or ventrolateral; in
phoronids, it is also U-shaped, but the anus is anterodorsal; in craniiforms,
the intestine is straight and does not curve or fold, leaving the anus
medioposterior.
The diductor and
adductor muscles that control the opening and closing of the valves are
contained within the body cavity (Fig. 1), but their distribution varies among
the three brachiopod subphyla. In most rhynchonelliforms, they are disposed to
effect a rotation of the valves about a hinge axis located on the valves; in
craniiforms and linguliforms, the hinge axis is located in the viscera between
the valves; in selected linguliforms, the two valves can “scissor” past one
another, allowing burrowing in a soft substrate. Adjustor muscles effect
movement of the shell relative to the pedicle, and other muscles may be present
that can move the lophophore slightly relative to the valves. Because of a
differential rate of secretion of shell material by the epithelium at the bases
of the muscles, the site of muscle attachment is commonly impressed in the
valves, producing muscle scars. Rarely, the muscle scars may be elevated above
the adjacent shell.
Although some
small-bodied species are herma-phroditic and brood their larvae, the sexes are
separate in the majority of brachiopods. The gonads, or reproductive organs, are
located either within the body cavity (as in the lingulids and discinids) or
more typically in slender tubelike extensions of the body cavity which project
into the mantle, called the mantle canals (in craniids and rhynchonelliforms).
The mantle canal pattern may be retained on the inner surface of the valves,
even in fossils, by processes of differential secretion comparable with those
producing the muscle scars.
Given the phylogenetic
nesting of phoronids within brachiopods, the anatomy of Phoronida can be
interpreted as having evolved from a brachiopod body plan; phoronid anatomy is
modified largely as a reflection of the loss of the two mineralized valves. See also: Phoronida
Embryology and
ontogeny
Linguliforms,
craniiforms, phoronids, and rhynchonelliforms share numerous aspects of
embryogenesis and larval development, yet each has distinct characteristics not
shared with any others (Fig. 5). Extant linguliforms and phoronids are
planktotrophic and have likely remained so from their origin in the Cambrian (or
earlier); extant craniiforms and rhynchonelliforms are lecithotrophic, and have
each evolved independently from a planktotrophic ancestral state. Planktotrophic
larvae remain in the water column for weeks to months, while lecithotrophic
larvae remain in the water column only briefly, usually less than a week; these
differences have important implications for the timing of differentiation of
adult structures, and for the ability of the larvae to disperse
biogeographically. Planktotrophic brachiopods develop more structures as larvae,
prior to metamorphosis, and lecithotrophs develop more structures following
metamorphosis, as young juveniles. Shell deposition begins at metamorphosis in
all three subphyla, although mantle formation can begin during embryogenesis or,
more commonly, during the larval growth period, just prior to metamorphosis.
Fig. 5 Comparison of fate maps and selected
developmental stages of the three brachiopod subphyla and Phoroniformea,
considered by some to be a fourth brachiopod subphylum. (a) Fate maps of
uncleaved eggs. (b) 16-cell embryos. (c) Late gastrula stage. (d) Lateral views
of the early larva. (Reprinted from G. Freeman, Developmental Biology,
261:263–287, 2003, with permission from
Elsevier)
Larvae in all three
subphyla share a three-part body organization: an apical lobe, from which the
lophophore differentiates in the larval stage of linguliforms and immediately
following metamorphosis in craniiforms and rhynchonelliforms; a mantle lobe,
from which the mantle (on which the periostracum and shells later mineralize),
alimentary canal, and most of the viscera develop; and a pedicle lobe, from
which the pedicle develops in linguliforms during the larval stage, and in
rhynchonelliforms following settlement and metamorphosis. In rhynchonelliforms
alone, the mantles are reversed after metamorphosis so that they are oriented
anteriorly, partially covering what was the apical lobe of the larva, and with
the original inner surface of the mantle forming the outer surface of the
organism. The mantles subsequently secrete the earliest, first-formed shell, the
protegulum, which enlarges by terminal accretion during later shell growth. Only
after metamorphosis does the rudimentary lophophore, alimentary canal, and adult
musculature develop. Rhynchonelliforms and phoronids share many aspects of
embryogenesis (Fig. 5a–c), providing further support for genetic data suggesting
they evolved among brachiopods. Craniiforms do not develop pedicles or a larval
pedicle lobe, but the larvae settle at their posteriormost ends, comparable to
where a pedicle lobe would be located. Craniiforms do not undergo mantle
reversal.
Linguliform larvae are
much like miniature adults that undergo minimal morphological change at
metamorphosis, having developed most of their adult features while larvae in the
plankton. The two mantle rudiments are separated from each other early in larval
life, and are not fused along the posterior margin in the manner characteristic
of rhynchonelliform brachiopods.
Shell morphology
The two valves are
commonly of unequal size, with the ventral valve typically larger. Because the
shell increases in size by increments laid down at the mantle margin,
ontogenetic changes in shape are faithfully recorded by growth lines. The valves
may be further ornamented by concentric folds (rugae), growth lamellae, or
radially disposed ribs of various amplitude and wavelength. Numerous extinct
genera are characterized by extravagant development of spines. In
rhynchonelliforms, the posterior region of one or both valves is commonly
differentiated from the remainder of the valve as a somewhat flattened cardinal
area. The ventral area is typically further modified by the pedicle opening;
among articulated brachiopods this consists of a triangular delthyrium which may
be partially closed by a pseudodeltidium or a pair of deltidial plates. A
corresponding triangular opening, the notothyrium, may be developed on the
dorsal cardinal area. Internally, muscle scars and mantle canal impressions may
be apparent, together with structures of varying complexity associated with
articulation (for example, dental and socket plates) and support of the
lophophore (crura, spiralia, loops). Calcite spicules are present in the mantle
tissues of some extant terebratulides. Linguliform valves are less heavily
mineralized and more organic-rich than rhynchonelliforms. Extant craniiforms,
cemented to a hard substrate, have ventral valves that are flat and extremely
thin, with gently cap-shaped dorsal valves.
Shell morphology
varies considerably externally in overall shape and size, relative biconvexity,
and degree of ornamentation. Shell morphology also varies internally in
structures involved in articulation, lophophore support, and muscle position,
from order to order, among the 26 orders recognized. This extensive variation
makes it difficult to succinctly characterize overall trends. See also: Rhynchonellida; Terebratulida
Ecology and
biogeography
All modern brachiopods
are marine, and there is little doubt from the fossil record that brachiopods
have always been confined to the sea. A few genera, however, notably the closely
related linguliform brachiopods Lingula and Glottidia, can tolerate reduced
salinities and may survive in environments that would be lethal to the majority
of forms. Recent brachiopods occur commonly beneath the relatively shallow
waters of the continental shelves, which seem to have been the most favored
environment in terms of diversity and abundance, but the bathymetric range of
the phylum is very large. Some modern species live intertidally and, at the
other extreme, some have been dredged from depths of over 16,500 ft (5000 m).
The majority of
brachiopods form part of the sessile benthos and are attached by their pedicle
during postlarval life. Paleozoic brachiopods not uncommonly exhibit boreholes
penetrating their shells, suggesting death by boring predation; interestingly,
modern brachiopods have very few known predators, boring or otherwise, even
though most are epifaunal, typically living on hard substrates. Glottidia and
Lingula are exceptional in being infaunal and making burrows with the help of
complex musculature connecting the two valves that lack articulation. The loss
of the pedicle has occurred multiple times over the course of brachiopod
evolution, by either complete suppression or atrophy early in the life of the
individual. Such forms lie free on the sea floor, are attached by cementation of
part or all of the ventral valve, or are anchored by spines. The geographic
distribution and geological setting of some fossil species suggest that they may
have been epiplanktonic, attached to floating weed, but such a mode of life is
unknown in modern faunas.
Biogeographically,
brachiopods today exhibit an antitropical distribution, with their highest
diversity closer to the Poles than to the tropics, and higher in the Southern
Hemisphere than the Northern Hemisphere. Paleobiogeographical data from the
distribution of fossil species indicate that Paleozoic brachiopods exhibited a
more typical latitudinal diversity gradient, with high tropical diversity,
decreasing toward the polar regions, such as is observed today in the majority
of marine invertebrates.
Taxonomic diversity and stratigraphic
distribution
Brachiopods formed a
major part of skeletonized marine faunas throughout the Paleozoic; linguliforms
follow a pattern of diversity through time more or less typical for the Cambrian
Evolutionary Fauna described by J. J. Sepkoski, while rhynchonelliforms typify
the Paleozoic Evolutionary Fauna. The oldest undoubted brachiopods, the
paterinates, occur in the Lower Cambrian Tommotian stage. Representatives of
seven of the eight classes are known from the Lower Cambrian, indicating that
morphological diversity was high even very early in the history of the phylum.
Throughout the Cambrian, linguliforms were more abundant than rhynchonelliforms;
but during the Early Ordovician radiation, rhynchonelliform diversity and
abundance both increased dramatically and remained high until the end-Permian.
In the late Paleozoic, a number of major brachiopod groups became extinct, and
the end-Permian extinction event caused diversity to plummet. Four of the five
extant orders survived this extinction event, and rediversified, although much
more modestly through the Mesozoic and Cenozoic than in the Paleozoic. In the
present-day oceans, approximately 120 genera are recognized, dominated by
rhynchonelliforms (terebratulides and rhynchonellides in particular). See also: Extinction (biology);
Paterinida; Rhombifera
S. J. Carlson
A. J. Rowell
Bibliography
S. J. Carlson and M. R.
Sandy (eds.), Brachiopods Ancient and Modern: A Tribute to G. Arthur Cooper,
Paleontol. Soc. Pap. 7, 2001
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Treatise on Invertebrate Paleontology, pt. H: Brachiopoda (revised), 6 vols.,
Geological Society of America and
M. J. S. Rudwick, Living
and Fossil Brachiopods,
Additional
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long before the Early Cambrian, Organisms, Diversity & Evolution,
5(4):253–273, 2005
P. Copper and J. Jin
(eds.), Brachiopods: Proceedings of the Third International Brachiopod Congress,
Sudbury, Ontario, Canada, 2-5 September 1995, A. A. Balkema, Rotterdam, 1996
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specification during embryogenesis in Rhynchonelliform brachiopods, Dev. Biol.,
261:268–287, 2003
Paleopolis
Tree of Life Web Project
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