Shell Structure And Inferred Growth, Functions And Affinities Of The Sclerites Of The Problematic Micrina

The stratiform laminae of Micrina sclerites originally consisted of rheomorphic successions of monolayers of micrometric–sized, apatitic tablets, presumably interleaved with chitin and glycosaminoglycans (GAGs). Paired laminae enclose slot–like chambers swelling into lobes distally that originally contained GAGs and deposits of spherulitic and prismatic apatite. The laminae are pervaded by apatitic tubes, apparently secreted by microvillous setoblasts and containing, at the surface, chitinous setae. Internal markings suggest that the triangular (sellate) sclerite supported a pair of muscles and the planospiral (mitral) sclerite, a medial muscle and gonadal sacs flanked by a pair of crescentic muscle bases. Both sclerites were secreted by a mantle with a circumferential fold. The sellate and mitral sclerites are homologized with the anterior and posterior shells of Halkieria and could have become the dorsal and ventral valves of the ancestral brachiopod by a sequence of transformations. These include: the folding of the halkieriid body axis; accelerated mixoperipheral growth of the anterior (dorsal) shell to enclose, with the posterior (ventral) shell, a mantle cavity lined with modified ciliated epithelium of the foot; reduction of sclerite–secreting epithelium to the locus of the brachiopod pedicle epithelium; and the anterior (dorsal) spread of gonadal lamellae.     

AN EARLY CAMBRIAN ORGANOPHOSPHATIC BRACHIOPOD WITH CALCITIC GRANULES

Abstract:  The linguliform brachiopod Eoobolus from the Early Cambrian Mural Formation (Jasper National Park, Canadian Rocky Mountains) exhibits various calcitic features in its otherwise apatitic shell. It is argued here that the decomposition of the organic matter within the shell led to a microenvironment similar to those resulting in the phosphatization of soft tissues. This diagenetic regime encouraged the initial precipitation of apatite cements followed by calcite cements. By fully coating primary structures early apatite cements separate primary structures from the later precipitation of calcite cement. Round calcareous grains, about 3  µ m in size, that occur in the centre of apatite botryoids must therefore represent original components of the shell. The equivalent pits of such calcareous granules are seen in the larval shells of many Palaeozoic linguliform brachiopods. This suggests that mixed organophosphatic-calcareous shells were relatively common at that time but that they have been overlooked owing to the obliteration of original calcareous structures by traditional acid preparation methods for the extraction of phosphatic fossils. The Eoobolus shell structure is intermediate between purely organophosphatic and calcitic shells. Although one such genus is not sufficient to reconstruct the ancestral composition of the brachiopod shell, it provides a means of recognizing other transitional forms that are needed to understand fully the shift in shell mineralogy.     

A Stem Group Brachiopod From The Lower Cambrian: Support For A Micrina (Halkieriid) Ancestry

The shell structure of the Lower Cambrian Mickwitzia , a bilaterally symmetrical bivalve hitherto doubtfully assigned to the Brachiopoda, confirms that the genus shares characters with linguliform brachiopods. The columnar lamination of its organophosphatic shell is homologous with that characterizing acrotretides. The shell, however, is also pervaded by striated apatitic tubes indistinguishable from those permeating the sclerites of the problematic organophosphatic, laminar–shelled Micrina which is close to Halkieria . No crown group brachiopods have such tubes that are presumed to have contained setae. The presence of both these features in the Mickwitzia shell suggests that the stock is a stem group brachiopod with a halkieriid ancestry.     

Chemico-structure of the organophosphatic shells of siphonotretide brachiopods

The organophosphatic shell of siphonotretide brachiopods is stratiform with orthodoxly secreted primary and secondary layers. The dominant apatitic constituents of the secondary layer are prismatic laths and rods arranged in monolayers (occasionally in cross-bladed successions), normally recrystallized as platy laminae. Sporadically distributed, interlaminar, lenticular chambers, containing apatitic meshes of laths and aggregates of plates and spherulites, probably represent degraded, localized exudations of glycosaminoglycans (GAGs) with dispersed apatite.
The shells of Helmersenia and Gorchakovia are perforated by canals with external depressions (antechambers) that possibly contained chitinous tubercles in vivo . The immature shell of Siphonotreta and most other siphonotretids is similarly perforated and pitted; but the mature part bears recumbent, rheomorphic, hollow spines that grew forward out of pits. Internally, spines pierce the shell as independent structures to terminate as pillars in GAGs chambers. Spines and pillars were probably secreted by collectives of specialized cells (acanthoblasts) within the mantle.
The shell of the oldest siphonotretide, Schizambon , is imperforate but the ventral valve has a pedicle foramen that lies forward of the posterior margin of the juvenile valve. This relationship characterizes all siphonotretides, suggesting that the pedicle, in vivo , originated within the ventral outer epithelium and not from the posterior body wall as in lingulides.     

Provenance Of Atlantic Lingulid Brachiopods

Living lingulid brachiopods are ubiquitous in low-latitude, marine infaunas. Lingula occurs throughout the Pacific and Indian oceans with the only Atlantic species, L. parva, confined to West Africa. Glottidia is restricted to offshore America from Virginia to California and Peru, and is assumed to have descended from a Pacific Lingula during the early Tertiary. Lingulid organophosphatic shells differ structurally. That of Glottidia is characterizedby trellised rods (baculate); that of Indo-Pacific species of Lingula by spheroidal and rod-like microstructures (virgose); and that of L. parva by apatitic rods arranged as spherulites. A spherulitic fabric is unknown in fossil lingulids, but the distinction between GlottidiaLingula can be traced back to the Carboniferous, which accords with the deep molecular divergence between the two genera. The common occurrence of lingulids with baculate shells in European post-Palaeozoic sediments suggests that ancestral Glottidia entered the Atlantic by the Tethyan Current during the Late Cretaceous/early Cenozoic, and migrated into the Pacific before the formation of the Panama Isthmus. Penecontemporaneously, antecedents of L. parva possibly migrated from east Tethys along the trans-Saharan seaway.     

Chemico-structural phylogeny of the discinoid brachiopod shell

Stratiform shells of living discinids are composed of membranous laminae and variously aggregated, protein-coated granules of apatitic francolite supported by proteinaceous and chitinous nets in glycosaminoglycans (GAGs) to form laminae in rhythmic sets. The succession is like that of living lingulids but differs significantly in the structure of the periostracum, the nature of baculate sets and in its organic composition. In particular, discinids have a higher level of amino acids although with relatively lower acidic and higher basic concentrations; and their overall lower organic content is owing to lower levels of hydrophilic components, like GAGs and chitin. The organic constituents are not completely degraded during fossilization; but data are presently too meagre to distinguish between linguloid and discinoid ancestries. Many differences among three of the four described extant genera emanate from transformations with a long geological history. Pelagodiscus is characterized by regular, concentric rheomorphic folding (fila) of the flexible periostracum and the plastic primary layer and by sporadically developed hemispherical imprints of periostracal vesicles. Both features are more strikingly developed in Palaeozoic discinids. In the oldest discinid, the Ordovician Schizotreta, and the younger Orbiculoidea and related genera, vesicles were persistent, hexagonal close-packed arrays fading out over fila. They must have differed in composition, however, as the larger vesicles of Schizotreta were simple (possibly mucinous), whereas the smaller vesicles of Orbiculoidea and younger genera were composites of thickly coated spheroids, possibly of lipoproteins, which survive as disaggregated relicts in Pelagodiscus. Baculate sets within the secondary layer are also less well developed in living discinids, being incipient in Pelagodiscus and restricted to the dorsal valve of Discinisca. The trellised rods (baculi) with proteinaceous cores are composed of pinacoids or prisms of apatite, depending on whether they are supported by chitinous nets or proteinaceous strands in GAGs. This differentiation occurred in Schizotreta but in that stock (and Trematis) the baculate set is symmetrical with baculi subtended between compact laminae, whereas in younger and post-Palaeozoic species the outer bounding lamina(e) of the set is normally membranous and/or stratified. The most striking synapomorphy of living discinids is the intravesicular secretion of organsiliceous tablets with a crystalline habit within the larval outer epithelium and their exocytosis as a close- or open-packed, transient, biomineral cover for larvae. Canals, on the other hand, are homologous with those pervading lingulid shells. Both systems interconnect with chitinous and proteinaceous sets and have probably always served as vertical struts in an organic scaffolding supporting the stratiform successions. A phylogenetic analysis based mainly on shell structure confirms the discinoids as the sister group of the linguloids but, contrary to current taxonomic practice, also supports the inclusion of acrotretoids within a ''discinoid'' clade as a sister group to the discinids.     

Evolution of a Rhythmic Lamination in the Organophosphatic Shells of Brachiopods

The secondary layer of organophosphatic-shelled brachiopods consists of stratiform laminae in glycosaminoglycans (GAGs), pervaded by canals with chitinous walls. The laminae are composed of various aggregates of protein-coated granules of apatite, 4–8 nm in size, 10 or so soluble proteins, β-chitinous meshes and sheets, and a possibly proteinaceous actin-like network. The succession is disposed in rhythmic sets grading between predominantly apatitic and wholly membranous laminae. The most conspicuous set consists of apatitic rods (baculi) in trellised arrays associated with laminae of compacted mosaics and spherules of apatite. The baculi are composed of mosaics accreting around axial fibrous proteins; apatitic aggregates are also scattered throughout the GAGs and accrete on proteinaceous networks and chitinous meshes. Baculi, subtended between two compact laminae, first appeared in Early Cambrian times and are a synapomorphy of lingulides. Subsequently, the secretion of one or another of the compact laminae was suppressed in the two surviving clades, with sets of the lingulid Glottidia and the discinids, respectively, grading inwardly from, and to, compact laminae. Suppression of baculate secretion also occurred in the dorsal valve of living Discinisca while, in Lingula, baculi have been replaced by botryoidal aggregates of mosaics at least since Carboniferous times.     

Comparison of aragonitic molluscan shell proteins

Acidic macromolecules, as a nucleation factor for mollusc shell formation, are a major focus of research. It remains unclear, however, whether acidic macromolecules are present only in calcified shell organic matrices, and which acidic macromolecules are crucial for the nucleation process by binding to chitin as structural components. To clarify these questions, we applied 2D gel electrophoresis and amino acid analysis to soluble shell organic matrices from nacre shell, non-nacre aragonitic shell and non-calcified squid shells. The 2D gel electrophoresis results showed that the acidity of soluble proteins differs even between nacre shells, and some nacre (Haliotis gigantea) showed a basic protein migration pattern. Non-calcified shells also contained some moderately acidic proteins. The results did not support the correlation between the acidity of soluble shell proteins and shell structure.     

Distribution of pedicle boring traces and the life habit of Late Paleozoic leiorhynchid brachiopods fiom dysoxic habitats

Alexander, R.R. 1994 10 15: Distribution of pedicle boring traces and the life habit of Late Paleozoic leiorhynchid brachiopods from dysoxic habitats.
Pedicle boring traces, Podichnus isp., are concentrated on the anterolateral commissure of both valves of Leiorhynchoidea carboniferum and L. weeksi from the black ferruginous micrites of the Chainman Formation (Upper Carboniferous; west-central Utah, USA) and black phosphatic ('False Cap') limestone of the Phosphoria Formation (southeastern Idaho, USA), respectively. The absence of Podichnus isp. from the posterior of shells of both species indicates that the beak of the shell was buried in the mud, inaccessible to colonization by conspecific larvae. The concentration of pedicle boring traces near the anterolateral incurrent regions of leiorhynchid shells with a well-developed central fold further suggests that settling conspecific larvae behaved rheotaxically. Larvae were induced to metamorphose near the commissure of the host shell, where suspended food was drawn to the incurrents of the host. The piggybacked mode of life on the anterior of conspecific hosts provided a refuge for juveniles above the dysoxic black sediments. In contrast, modem brachiopods that live as epibionts on skeletal substrates display either a random or posteriorly concentrated distribution of pedicle boring traces. Permian, Carboniferous, brachiopods, leiorhynchids, Podichnus, dysoxic, rheotaxis .     

The first report of epipunctae in non-plaesiomyid brachiopods from the lowest Silurian of southeastern China

Epipunctae are microscopic perforations that do not penetrate the shell but are confined to the outer layers of the shell. They have been known previously only in the orthidine family Plaesiomyidae. The striated walls of epipunctae are considered unique to brachiopods and thus of significance for the study of stem-group brachiopod. In this study, we report the first occurrence of epipunctae in a non-plaesiomyid brachiopod based on a well-preserved external mould of Cathaysiorthis yushanensis from the lower Silurian Shiyang Formation of Jiangxi, southeastern China. The species belongs to the Family Cathaysiorthidae which differs from the Family Plaesiomyidae in many aspects especially in dorsal internal structures. The new data imply that epipunctae may be more widespread in orthide brachiopods than previously thought, strengthening the notion that this is a plesiomorphic character of the brachiopod shell. The discovery of epipunctae, however, depends on excellent preservation of the outer surface of the shells as well as careful cleaning and observation.     

An intracrystalline chromoprotein from red brachiopod shells: implications for the process of biomineralization.

1. The red colour of some terebratulid brachiopod shells is caused by a small chromoprotein that occurs within the calcium carbonate matrix of the shell. 2. This carotenoid-protein complex was isolated from within the calcite shell of three different brachiopod genera and may therefore be involved in the process of biomineralization. 3. The apparent molecular weight of this protein, as judged by SDS-PAGE, is 6.5 kDa. 4. The partial N-terminal amino acid sequence of the protein is virtually identical in three different brachiopod genera, indicating homology. 5. Two carotenoids are present in Terebratella sanguinea: canthaxanthin and the tentatively identified monoacetylinic analogue of astaxanthin.     

Study of fossil and recent brachiopods,using a skyscan 1172 X-ray microtomograph

Fossil and recent brachiopods were studied with the aid of a Skyscan 1172 microtomograph. The capabilities of this method at different stages of studying, X-ray scanning and producing slices and 3D models are described. The method enables the study of punctuation, microornamentation, and inner structures of the brachiopod shells and soft tissues. The contrast of shell structures of fossil brachiopods is discussed; it depends on differences in the mineral composition of the shell and surrounding matter. This method allows studying the inner structure of the holotypes of brachiopod species without damaging their shells. The data on the efficiency of the method are provided.     

Bioerosion of oyster shells in brackish modern mangrove swamps,Nigeria

Studies on shells of the intertidal oyster Crassostrea tulipa cemented to aerial prop roots and stems of the mangroves Rhizo‐phora racemosa, R. harrisonii, and R. mangle from the Cross River and Qua Iboe River estuaries show that photosynthetic en‐dolithic cyanobacteria (blue‐greens) are the major bioerosional agent, affecting about 94% of the shells examined. Pblychaetes attacked less than 10% of these materials and thus have low bioerosional impact.

Herbivorous gastropods play a secondary role because, by grazing, they clear the shell surface of encrusting cyanobacteria and thus enhance the activity of boring forms. Where epilithic cyanobacteria have been removed, the gastropods sometimes leave faint grazing traces of low fossilization potential.

Microfloral boring activity is high at all stations, but the assemblage is of markedly low diversity compared with those of littoral and shallow sublittoral marine communities and may thus be useful as a paleoenvironmental tool. Bioerosional agents are directly or indirectly responsible for the disintegration of oyster shells, whose fragments are incorporated in muddy intertidal sediments.     

Magnesium and sulphur in the calcite shells of two brachiopods, Terebratulina retusa and Novocrania anomala

This study determines the distribution of magnesium and sulphur in the shells of two species of brachiopod from the same environment to highlight environmental and biological influences on shell composition. In Terebratulina retusa there are differences in magnesium concentration between the primary layer and the outer and inner regions of the secondary layer. In contrast, Novocrania anomala has a shell composed of high magnesium calcite and there is no significant difference in magnesium concentration between the primary and the secondary shell layers. Sulphur provides an indication of the distribution of sulphated organic matrix within the shells of T. retusa and N. anomala . In T. retusa the distribution of magnesium and sulphur correlates across the shell; however, there is no evidence for a relationship between magnesium and sulphur distribution in N. anomala . The relationship between magnesium and sulphur in T. retusa indicates that a proportion of the magnesium content of the shell is associated with the sulphated fraction of the organic matrix. In these two species of brachiopod, from the same environment, magnesium and organic concentration and distribution are very different, emphasizing the importance of fully understanding the factors that control biomineral composition before the application of these biominerals to environmental studies.     

Macromolecules in brachiopod shells: characterization and diagenesis

An immunological investigation was conducted of soluble intra-crystalline macromolecules isolated from living and fossil brachiopod shells, which had previously been used for an immunologically based study of phylogeny (serotaxonomy). The soluble intra-crystalline macromolecules comprised 0.03% by weight of the extant shell material. Bulk analysis and gel electrophoresis indicated that the organic material is predominantly glycoprotein, and contains up to 30% by weight carbohydrate. Treatment of the macromolecules with periodate and proteinase K revealed that antibodies were raised predominantly against the carbohydrate moieties. Using a specially adapted dot blot immunobinding assay (DIBA) the decay in immunological signal over geological time was determined. Pleistocene shells have lost between 99 and 99.9% of immunological reactivity, and original antigenic determinants form a declining proportion of total organic matter. It is suggested that condensation reactions between amino acids and sugars account for the rapid destruction of determinants; this has important implications for the direction of future studies on fossil macromolecules. □ Serotaxonomy, biomolecular palaeontology, glycoproteins, melanoidins, brachiopods.     

Morphogenetic regulation in the shells of bivalves and brachiopods: evidence from the geometry of the spiral

Ackerly. S. C. 1992 07 15: Morphogenetic regulation in the shells of bivalves and brachiopods: evidence from the geometry of the spiral.
Analyses of the spiral geometry in shells of the mollusc Pecten maximus and the brachiopod Terebratulina retusa indicate a relative reduction in morphological variability within the population during growth. The spiral, as measured by a model of exponential radial expansion, tends to converge on a particular adult form, irrespective of irregularities during early growth phases. The recurrence of this pattern of variability in populations from two separate phyla (molluscs and brachiopods) suggests a common mechanism controlling shell form. Brachiopoda. Mollusca, coiling, spiral. morphogenetic regulation, growth .     

The brachiopod <Emphasis Type="Italic">Eoobolus</Emphasis> from the Early Cambrian Mural Formation (Canadian Rocky Mountains)

The Early Cambrian brachiopod, Eoobolus, is one of the first representatives of the superfamily, Linguloidea, the defining characteristics of which include the classical morphology of oval shells and a pedicle that emerges from between the two valves. The material described here from the Mural Formation (Jasper National Park, Canadian Rocky Mountains) provides well-preserved muscle scars and larval shells that allow a discussion of the muscle system and the larval morphology of Eoobolus. The dorsal larval shell exhibits a morphology similar to other Cambrian linguloids, but also to paterinids, Mickwitzia muralensis, and some rhynchonelliforms. This suggests that there was a lesser degree of disparity among brachiopod larvae in the Cambrian than there is today. The muscle system of Eoobolus is similar to other linguloids, but differs from that of Recent lingulids and discinids by having one or two more pairs of oblique muscles. New data on the distribution of features characteristic of the family Eoobolidae question the validity of this family.     

Chemico-structural evolution of linguloid brachiopod shells

Chemico-structures of shells representing all families presently assigned to the Linguloidea have undergone significant transformations since the Early Cambrian. Superficial hemispherical to hemi-ellipsoidal pits on the larval and/or mature shells are interpreted as casts of deformable, membrane-bound vesicles of mucus or rigid vesicles of glycoproteins or GAGs with thickened coats. Flat-bottomed, sub-circular imprints characterize acrotheloids and many acrotretides, and could be impressions of biconvex tablets of apatite like those exocytosed within the primary layer of the obolid ‘Lingulella’? antiquissima, whilst the rhomboidal imprints of the Paterula shell could have held tablets of proteinaceous silica like those of living discinid larvae. The ancestral fabric of the linguloid secondary layer was probably composed of rubbly and virgose sets, but trellised rods of apatite (baculation) are characteristic of most linguloids and also acrotheloids. This condition was suppressed in shells identified as ‘Lingula’ from at least the Early Carboniferous to the present day. In early Palaeozoic acrotretides and lingulellotretids, columnar and camerate fabrics evolved in place of baculation. Baculation in Discinisca tenuis and Glottidia pyramidata is associated with the amino acids glutamic acid, glycine, alanine, arginine and proline which may be components of an organic polymer axial to baculate accretion.     

Cocoa shells for heavy metal removal from acidic solutions

The development of economic and efficient processes for the removal of heavy metals present in acidic effluents from industrial sources or decontamination technologies has become a priority. The purpose of this work was to study the efficiency with which cocoa shells remove heavy metals from acidic solutions (pH 2) and to investigate how the composition of these solutions influences heavy metal uptake efficiency. Adsorption tests were conducted in agitated flasks with single-metal solutions (0.25 mM Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn), multi-metal solution (comprised of 0.25 mM of each of the cations above) and an effluent obtained from chemical leaching of metal-contaminated soil, in the presence of different cocoa shell concentrations (5–40 g/l). Results from the single-metal solution assays indicated that the fixation capacity of heavy metals by cocoa shells followed a specific order: Pb > Cr > Cd=Cu=Fe > Zn=Co > Mn=Ni=Al. Cocoa shells are particularly efficient in the removal of lead from very acidic solutions (q_max=6.2 mg Pb/g, pH_i=2.0 and T=22 °C). The presence of other metals and cations in solution did not seem to affect the recovery of lead. It was also observed that the maximum metal uptake was reached in less than 2 h. This research has also demonstrated that the removal of metals caused a decline in solution proton concentration (pH increase) and release of calcium, magnesium, potassium and sodium from the cocoa shells.