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 Phoroniformea (?)
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)
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.
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
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Tree of Life Web Project
University of California, Berkeley, Museum of Paleontology