On the origin systematic position of the calcareous-shelled inarticulate brachiopods

Available data on the anatomy, ontogeny embryology of Brachiopoda ( sensu lato ) suggest that this Phylum in the traditional view is in fact a clade of organization includes two stocks of lophophorate organisms of quite different origins. Their rank does not correspond to existing Class divisions. The phosphatic-shelled in articulates are regarded as a separate Class Lingulata. The Phylum Brachiopoda ( sensu stricto ) is restricted to the calcareous-shelled inarticulate articulate lineages. Ancestors of the calcareous-shelled brachiopods probably differentiated from the protolophophorates before the radiation of the other lophophorate stocks took place. The articulation of the valves appeared independently in several lineages during the early stages of brachiopod evolution.     

The origin and geometry of radial ribbing patterns in articulate brachiopods

Ackerly, S. C. 1992 07 15: The origin and geometry of radial ribbing patterns in articulate brachiopods.
Geometric models for simple. radial ribbing in articulate brachiopods include (1) ribs radiating isometrically from the shell umbo. (2) divergence of thc ribs from some 'point' within the shell, and (3) reorientation of the ribs at right angles to the shell margin. Analyses of the Orthida, the ancestral taxon of articulate brachiopods, indicate that rib geometries are isometric in Early Cambrian taxa (model 1). but that by the Early Ordovician rib orientations are generally perpendicular to the shell margin (model 3). A combination of functional and morphogenetic Factors explains the ribbing geometries observed in orthide brachiopods.     

Biogeography of the recent brachiopods

The vertical, latitudinal, and circumcontinental zonality of the distribution of the species, genera, and families of recent brachiopods is considered. The distortions of the latitudinal and meridional symmetry of the biogeographic structure of the ocean are analyzed in view of the patterns of the global circulation of the surface and intermediate waters. Thus ancient faunas may be reconstructed based on data on the structural characteristics of the taxocene of recent brachiopods. The features of the paedomorphic evolution of brachiopods from the different families in extreme habitats (interstitial, underwater caverns, submarine rises, abyssal depths, hydrothermal areas, and margins of habitats) are discussed. The biogeographic structure of bottom dwellers is shown to simplify with depth as well as with simplification of the hydrological structure of the ocean. The important role of the bathyal oceanic zone (slopes of continents, islands, submarine mountains, ridges, and rises) in the preservation of faunal relicts is shown. The historical change from brachiopods to bivalves that occurred from the Paleozoic to the Mesozoic and Cenozoic is shown to have resulted not from competitive exclusion, but from complex and global changes in the plankton composition, which were unfavorable for articulate brachiopods, which had already developed specialized feeding habits, feeding on food that led to the production of almost no metabolic waste products; they had even partly lost their alimentary canal. The development of shelly plankton and, especially, of diatoms hampered the post-Paleozoic revival of large assemblages of articulate brachiopods in shallow-water habitats. The unfilled ecological niches were colonized by bivalves, which were widely adapted to feeding on live phyto-and zooplankton. Recent articulate brachiopods, which are adapted to feeding on the products of decay of dead plankton, form a belt of densely populated settlements of the organic biofilter outside the photic zone on the seaward edge of shelves and on the upper parts of the slopes of continents, islands, and submarine rises throughout the world.     

The function of the brachial valve septa in plectambonitacean brachiopods

Eoplectodonta transversalis (Wahlcnberg) and other sowcrbyellids have a complex arrangement of septa in the brachial valve. It is argued that the function of these is primarily to support the lophophore and to strengthen the shell, and that the septa mirror is almost exactly the shape of the lophophore. In Eoplectodonta transversalis the ontogenetic develop ment of the lophophore varies from a simple to a complcx ptycholophe. Other plectam-bonitaceans have trocholophous and schizolophous lophophores.     

The development of growth lines on articulate brachiopods

Three types of growth lines are recognised on articulate brachiopod shells: (1) very fine diurnal growth lines formed by calcite increments at the shell margin, (2) seasonal growth lines, formed by inward reflection (doubling back) of the mantle edge, seen as concentric steps on the shell surface and marked by re-orientation of growth vectors evidenced by secondary shell fibres, (3) disturbance lines, formed by abrupt regression of the mantle edge, also seen as concentric steps on the shell surface, but indicated by a dislocation in the shell fabric. Lamellose and spinose ornaments of the sort seen in Tegulorhynchia are essentially genetically controlled. Periodic outgrowths from the outer mantle lobe secrete frills of primary shell that project from the shell surface and form short hollow spines where they cross the radial ornament. In longitudinal section spine formation is seen to involve gradual increase in the rate of secretion of primary shell followed by retraction, and often collapse, of the mantle outgrowth, accompanied by regression. Reflection of the mantle edge usually follows spine formation.     

Ultrastructure of the loop of terebratulide brachiopods

The folded and twisted calcareous ribbon, forming both the ascending and descending lamellae of the loop of Waltonia inconspicua (Sowerby), is a two-layered structure consisting of a wedge of regularly stacked secondary layer fibres that overlie a thin layer of non-fibrous calcite (herein termed brachiotest). On one surface, that facing into the mantle cavity, secondary fibrous mosaic predominates, but smooth, finely banded brachiotest occurs as a narrow marginal lip upon which secondary layer fibres proliferate and progressively overlap. This growing edge of the ribbon is secreted by long, folded epithelial cells with digitate extensions to their apical plasmalemmas, which are distinguishable from the cuboidal epithelium-secreting fibres and their membranous sheaths. The other surface, facing the body cavity and the brachial coelom, consists entirely of roughened brachiotest exhibiting prominent banding that is aligned parallel to the growing loop edge. This surface is overlain by microfilamentar epithelium acting as a holdfast for the connective tissue frame of the lophophore. The other edge of the ribbon consists of truncated sections of both secondary-layer fibres and brachiotest which bear signs of resorption consistent with the degenerated state of the associated epithelium. Growth of the Waltonia loop is controlled by these localized processes of secretion and resorption of the fibrous and brachiotest layers and is typical of all terebratulides so far studied. The brachiotest is not homologous with the non-fibrous primary shell secreted at the valve margin. □ Brachiopoda, Articulata, Terebratulida, ultrastructure, lophophore, loop.     

'Flapping valves' in brachiopods

M. J. S. Rudwick and others postulate 'rhythmic-flow' feeding for the Permian richtho-feniacean bi-achiopods, whereas R. E. Grant claims that they fed by normal ciliary action. Suspension-feeding has two components, current generation and food capture; normal brachiopod lophophores do both, but this is neither universal nor compulsory among animals. Opening and closing the richthofeniid shell generated a 'tidal-flow' current precisely analogous to respiratory currents in mammals; this is neither inefficient nor 'self-defeating', as Grant claims. Grant's analysis fails because he chose the wrong mechanical analogy (a pump). Morphological features of richthofeniids are better explained on a tidal-flow hypothesis than on a ciliary-flow model, and all the data adduced by Grant in rejecting the former is compatible with it or favorable to it. It explains morphological features that are bizarre mysteries on the ciliaiy-current model, and is therefore superior even though it implies that these Permian brachiopods were radically innovative.     

Commissural asymmetry in brachiopods

Consistent asymmetric folding of the commissure is a characteristic feature of a small but significant number of brachiopod species. The feature may be obligate or facultative and is almost entirely confined to rhynchonellids, most of which are Mesozoic. The detailed nature of the asymmetry is very variable, but does not extend to internal hard parts such as crura. Taken as a whole, asymmetric brachiopod species show no preference for any particular environment or geographic region, and in no circumstances seem to have been markedly more or less successful than symmetric species. We are thus led to suggest that asymmetry was a genetically based condition which cropped up periodically in brachiopod evolution, and which possibly was selected neither particularly for nor against.     

Ecophenotypic strategies of dalmanellid brachiopods

General ecophenotypic patterns, of particular interest when they apply to all, or most, taxa of the group concerned, can never be demonstrated until after monophyletic taxa have been recognized, that is, until after the initial stages of phylogeny construction have been carried out. In criticizing certain dalmanellid phylogenies, and based in large part on a study of five 'species subgroups'. Hurst & Watkins (1978; Geologica et Palaeontologica 12 ) postulate ecophenotypic patterns for Isorthis , and Hurst (1978; Palaeontology 21 ) postulates general patterns of ecophenotypic variation for dalmanellid brachiopods. These patterns may be invalid for four reasons: (1) Univariate and 'bivariate' statistical analysis of the samples used to define the five subgroups reveals no significant differences between subgroups, or vertical trends, for the very morphological characters claimed to exhibit the ecophenotypic patterns; (2) Hurst & Watkins' discriminant function analysis contains procedural errors and its results are ambiguous; (3) several of the five subgroups represent mixtures of unrelated taxa; (4) in recognizing the alleged patterns, Hurst & Watkins ignored contrary evidence from many taxa (and from many dalmanellid studies). □ Brachiopoda, Dalmanellidae, Silurian, ecology, evolution, systematics.     

Allometric studies of fossil brachiopods

In different groups of fossil brachiopods new adaptations were formed by changes in size, shape, and proportions of morphological structures in ontogenetic and phylogenetic development.     

Origin and dispersal of the earliest brachiopods

Brachiopods first appeared at the very beginning of the Phanerozoic together with the first skeletal organisms. Most brachiopod taxa that arose in the first half of the Cambrian had a short temporal range and became completely extinct by the middle of the Middle Cambrian. Rigid articulation of the valves of brachiopods was provided by various structures, which also appeared in the Early Cambrian. This fact points to the importance of this feature for the formation of the whole group and at the same time testifies to the high variability of rigid articulation at the early stages of brachiopod evolution. This is a typical manifestation of archaic diversity in this animal phylum, which appeared very early in the Phanerozoic. Another important property of the archaic diversity of the early brachiopods was the large number of centers of diversification. As for the majority of groups, climatic zonality was the main factor determining the distribution of brachiopods at the beginning of the Phanerozoic. The main ecological types of brachiopods also appeared in the Early Cambrian.     

The role of the coelom as a hydrostatic skeleton in lingulid brachiopods

Anatomical and experimental studies of the perivisceral coelom and pedicel of Lingula ana lina indicate that the coelomic fluid functions as a hydrostatic skeleton in respect of valve and pedicel movements, valve opening always being associated with positive pressures. The perivisceral coelom is surrounded by a body wall containing circumferential muscle fibres, whilst all muscles passing between the valves (principally adductor and oblique fibres) are located within the body wall. These two sets of muscles function similarly to the circular and longitudinal muscles of a classical hydrostatic skeleton.
Pressure recordings from the lumen of the pedicel and perivisceral coelom, during opening or rotary movements of the valves, were similar and showed pressure pulses of up to 0.8 kPa. During the initial stages of burrowing, pulses of up to 2.5 kPa were observed when the valves were being pressed into the sand. These values are well within the capability of the circumferential muscles of the body wall.     

Molecular data indicate the protostome affinity of brachiopods

Although the phylogenetic position of brachiopods has always been subject to debate, many authors place them as a sister group to deuterostomes on the basis of morphological and developmental characters. However, molecular phylogeny consistently places them among protostomes. More precisely, brachiopods are predicted to branch inside the lophotrochozoan assemblage, together with annelids, molluscs, nemerteans, flatworms, and others. That result has been criticized on the basis of (1) prior knowledge of brachiopod morphology and (2) the known limitations of molecular phylogenies. Here I review recent data of molecular origin, particularly those displaying qualitative properties close to those of morphological characters. The complement of Hox genes present in all metazoa tested to date has proved to be a powerful tool for broad phylogenetic reconstruction. The mitochondrial genome also provides qualitative characters, showing discrete events of gene rearrangements. After discussing the data and the way they should be interpreted in the perspective of several hypotheses for metazoan phylogeny, I conclude that they argue strongly in favor of the protostome (and lophotrochozoan) affinity of the brachiopods. There is therefore a need for a reinterpretation of brachiopod morphological and developmental characters. I also identify some research axes on brachiopod morphology.     

Ontophylogenetic studies of the brachiopods of the order terebratulida

The patterns of ontophylogenetic change of the brachidium in Late Paleozoic, Mesozoic, and Cenozoic terebratulids are analyzed. The work of Russian specialists in establishing new evolutionary trends and working out terebratulid taxonomy on the basis of ontophylogenetic studies is discussed.     

Morphobiological study of the brachiopods of the order Chonetida

A comparative morphological study of the brachiopods of the order Chonetida revealed a key part of the development of the shell structures connected with the feeding and respiration organs, such as the lophophore, musculature, and mantle, in the morphological evolution of the group. The general trends revealed in the development are adaptive and were restored based on morphofunctional analysis. Against the background of these trends, the correlative changes of the shell shape and its external ornamentation led to the repeated appearance of homeomorphs, whose similarity cannot be explained by adaptation. The phylogeny of the superfamily Anoplioidea is described as an example.     

Predatory asteroids and the decline of the articulate brachiopods

Various causes, such as increased predation pressure, the lack of planktotrophic larvae, a 'resetting' of diversity, increased competition from benthic molluscs and the decline of the Palaeozoic fauna, have been suggested to explain the failure of the brachiopods to reradiate following the Permo-Triassic mass extinction. Increased predation pressure has hitherto appeared improbable, because typical predators of brachiopods, such as teleostean fish, brachyuran crabs and predatory gastropods, did not undergo major radiation until the late Mesozoic and early Cenozoic. However, new evidence strongly suggests that one important group of predators of shelly benthic organisms, the asteroids, underwent a major radiation at the beginning of the Mesozoic. Although asteroids appeared in the early Ordovician, they remained a minor element of the marine benthos during the Palaeozoic acme of the brachiopods. However, these early asteroids lacked four important requirements for active predation on a bivalved epifauna: muscular arms (evolved in the early Carboniferous); suckered tube feet, a flexible mouth frame and an eversible stomach (all evolved in the early Triassic). Thus radiation of the Subclass Neoasteroidea coincided with both their improved feeding capability and the decline of the articulates. The asteroids were the only group of predators of brachiopods that underwent a major adaptive radiation in the earliest Mesozoic. The asteroids may therefore have contributed to inhibiting a Mesozoic reradiation of the brachiopods. Epifaunal species lacking a muscular pedicle may have been particularly vulnerable. Unlike bivalve molluscs, modern brachiopods show only a limited range of adaptations to discourage asteroid predation. □ Asteroidea, Brachiopoda, evolution, predation, functional morphology.     

Climatic control of brachiopods in the lower Namurian

The fauna and lithology of a marine band in the lower Edale Shales (Namurian E₂b) at Edale, Derbyshire, England, are described quantitatively. Two brachiopods, Rugosochonetes sp. and Productus hibernicus were the dominant elements of the fauna. The average age at death was about 3 years, although the maximum age of Rugosochonetes (ca. 7 years) was probably a little greater than that of Productus (ca. 5 years). Both sediment deposition and fauna were climatically controlled. The climate was seasonal from wet to dry. Quartz was deposited and Rugosochonetes sp. flourished during the wet periods. Carbonate deposition and Productus hibernicus were dominant during the dry periods.     

Independent similarity in the morphological evolution of brachiopods

The close similarity of the shell exterior of articulate brachiopods from different orders, which must be taken into account in taxonomic identifications and phylogenetic reconstructions, is analyzed. A possible mechanism of the appearance of such brachiopods in connection with the morphogenetic generality of the structurally similar organisms is evaluated.     

Lines of phylogenetic studies of brachiopods

The main techniques used in phylogenetic studies of brachiopods are reviewed, onto-phylogenetic and morphobiological. The advantages and shortcomings of each method are discussed.