Euryhalinity of Palaeozoic articulate brachiopods

OMonotypic and very low diversity virgianid shell beds from the Upper Ordovician to Lower Silurian dolomites of North Greenland were formed in marginal marine quiet-water hypersaline environments. In the light of this evidence the salinity tolerances of other Palaeozoic articulate brachiopods is evaluated. There are only a small number of species apparently invading hypersaline or brackish environments, but it is significant considering that previously all articulate brachiopods were thought to be fully marine. Two types of occurrence are noted, those species specifically related to marginally marine environments, disappearing with the introduction of fully marine faunas, and the majority of species which extend their normal marine range into marginal conditions. No brachiopod species appears to have invaded very hypersaline or truly brackish conditions. No single group of articulate brachiopods specifically specialised in colonising marginal marine environments, apart from possibly the virgianid pentamerids. Palaeozoic, Upper Ordovician, Lower Silurian, Brachiopoda. Pentamerida, Virgianidae, Greenland, palaeoecology. hypersaline environments, brackish environments .     

Shell microstructure in mesozoic articulate brachiopods

The shell microstructure was studied based on extensive material of Triassic, Jurassic, and Cretaceous brachiopods from the orders Rhynchonellida, Terebratulida, and Thecideida. Evolutionary changes in relationships between the shell layers of thecideids were revealed; the narrowing of fibers in historical development of rhynchonellids was established; the importance of the prismatic layer for taxonomy was recognized; and differences in width and shape of fibers within the superfamilies of rhynchonellids and terebratulids were determined.     

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.     

Body volumes and internal space constraints in articulate brachiopods

Brachiopods were once dominant in all the oceans of the world. but their distributions are non more restricted. There are few species which are found in shallow warm habitats and these are predominantly small. They have exceptionally low metabolic rates and exhibit low energy lifestyles. The majority of living articulate brachiopods are punctate (possessing mantle extensions. or caeca. which traverse the shell). Evidence produced hei-e suggests that the evolution of these phenomena may have been strongly affected by architectural constraints placed on articulate brachiopods by the use of the lophophore for feeding and respiration. They are essentially space limited because of the large volume needed for this organ. In some punctate brachiopods over 75% of their total body volume may be occupied by the lophophore and mantle cavity. This figure is only 60% in an impunctate (no caeca) species and may be only 20% in bivalve molluses. The implications are that caeca evolved to reduce pressure on space requirements, that maximum sizes may be set by the scaling patterns of space allocation and metabolic efficiency is a consequence of space constraints. Current distribution patterns may be strongly affected by the low metabolism and low energy lifestyles. The relative success of small brachiopods in warm shallow seas may have been facilitated by the scaling patterns of space allocations which show small specimens to have similar mantle cavity volumes to bivalve molluscs.     

Geochemical investigation of growth in selected Recent articulate brachiopods

Buening, N. & Carlson. S. J. 1992 07 15: Geochemical investigation of growth in selected Recent articulate brachiopods.
Growth increments have traditionally been used to determine age and growth rates of shelled organisms, particularly bivalves. Brachiopod growth increments and the time span they represent are rather poorly understood, however. Geochemical analyses of trace element concentrations preserved in skeletal calcite may provide an alternative method to determine the age of Recent brachiopods and to interpret their patterns of growth. Magnesium is a particularly important trace element in skeletal calcite because it is thought to vary with temperature, growth rate. and taxonomic affiliation. Electron microprobe analyses of Mg concentrations in the Recent articulate, temperate water brachiopods Terebratulina unguicula and Terebrotalia transversa , have revealed two distinct ontogenetic patterns of Mg concentration. The primary ontogenetic pattern is characterized by elevated and more variable levels of Mg concentration during early growth. followed by lower, more stable Mg concentrations during growth after sexual maturity. This pattern appears to be related to a predictable decrease in growth rate through ontopeny. Secondary peaks of Mg superimposed on this primary pattern may represent growth spurts related to annual cycles of productivity. Preliminary Fourier analyses of patterns of Mg concentration provide additional support for this hypothesis. Thus, a record of productivity characterized by annual peaks of Mg concentration may well allow us to age individual brachiopods by means other than size-frequency histograms. growth lines, and other less precise and accurate methods. Brachiopods, growth. biomineralization, calcification, Terebratali transversa, Terebratulina Unogicui.a, magnesium, Calcium, geochemical. ontogeny .     

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.     

The relationship between major lamellae and epithelial regressions in some articulate brachiopods

Brunton, C. Howard C. & Alvarez, Fernando 1989 07 15: The relationship between major lamellae and epithelial regressions in some articulate brachiopods. Lethaia , Vol. 22, pp. 247–250. Oslo. ISSN 0024–1164.
Hiller (1988, Lethaia , Vol. 212) proposed three relationships between the secretory epithelium of articulate brachiopods and the shell surface ornamentations of growth lines, lamellae and spines. None of his models satisfy the growth of strongly lamellose athyrid shells and we propose a fourth involving strong regressions effecting both primary and secondary shell layers. In Recent Tegulorhynchia we suggest a growth function for the 'frayed' shell of Hiller occurring immediately in front of the spines.     

Electron microscopic studies of growth margins of articulate brachiopods

Summary The brachiopod shell is secreted by the mantle epithelium lining the internal surfaces of its two valves. Growth lines, seen on their external surfaces, have been interpreted in terms of mantle regression and transgression from the valve margins. This scanning electron microscope study of the shell microstructure in recent brachiopods confirms these views and shows the skeletal evidence upon which such interpretations can be made. Electron micrographs reveal that from a growth line a plane dips posteriorly into the shell substance along which normal skeletal secretion was interrupted. Commonly a mosaic of secondary fibres, similar to that seen on the inside surface of the valve, is preserved upon this regression plane, most of the inside surface of which is covered by primary shell, usually extending posteriorly well into the secondary shell layer. The regression plane marks the area from which the mantle withdrew and the area over which shell secretion was interrupted. During mantle transgression primary shell was deposited over much of this surface, before the redevelopment of secondary fibres, so that the old internal surface of the valve was preserved as a false mosaic within the shell. In this way it is possible to recognise the extent of mantle regression and to note the position of the primary — secondary shell secreting junction of the mantle at the time when shell secretion was resumed.     

The origin of the spiriferidine brachiopods

Wright, Anthony D. 197901 15: The origin of the spiriferidine brachiopods. Lethaia . Vol. 12. pp. 29–33. Oslo. ISSN 0024–1164.
In recent years doubts have been expressed as to whether the strophic spiriferidines should continue to be grouped with the non-strophic spiriferids in the order Spiriferida or not. The solution to this problem will only be found by establishing the origin of the spiriferidines. A characteristic feature of the spiriferidines is their well developed micro-ornament; this is shared also by the orthid Platystrophiinae, and together with the striking similarity in overall morphology (well recognized in the early nineteenth century) it suggests that here, in the Platystrophia plexus, is the ancestral stock of the strophic spirebearers.
The Platystrophünae share with the non-strophic Rhynchonellida the potential to develop a spire. It is here considered that the former gave rise to the spiriferidines and the latter to the non-strophic spire-bearers. These quite separate developments justify the separation of the spire-bearers into the two orders Spiriferida and Atrypida.     

Large extinctions of articulate brachiopods in the paleozoic and their ecological and evolutionary consequences

The largest Paleozoic extinctions of articulate brachiopods occurred at the Frasnian—Famennian boundary in the Late Devonian and at the Permian—Triassic boundary. Both extinctions affected taxa of all levels, including orders, but differed in scale, course, and ecological and evolutionary consequences. The Frasnian—Famennian extinction event was selective and evolutionary activity after the crisis varied in different orders. However, in the Early Carboniferous, the brachiopod diversity was mostly restored in comparison with the Devonian maximum. In particular groups, preadaptation played a role in changes in diversity and reconstruction of communities. The brachiopod composition at this boundary changed sharply. The extinction event at the end of Permian was global and accompanied by changes in the biota. Later, in the Meso-Cenozoic, the brachiopod diversity was not restored, and bivalves acquired primary importance in various bottom communities of different sea zones where Paleozoic brachiopods previously dominated. Extinction of brachiopods at this boundary was long and gradual. The symptoms of the ecological crisis in the development of Permian brachiopods are recognized beginning from the Roadian Age, which was probably the onset of this crisis.     

Generic longevity of articulate brachiopods in relation to the mode of stabilization on the substrate

The geologic ranges of the articulate brachiopod genera which appear in the Treatise are converted into numerical values with the aid of radiometric dates on the Phanerozoic time scale. Longevity—frequency distributions are plotted for the genera segregated according to their mode of stabilization on the substrate. The categories and their subdivision for the mode of stabilization include: (1) anchorage by spines, (2) cementation (early in and throughout life), (3) unattached, commissure horizontal (questionably mobile and sessile), (4) unattached, commissure vertical (interarea stabilization and umbonal weighting), (5) functional pedicle (tethered and combined with interarea stabilization).Statistical tests (Median and Mann-Whitney U) reveal that the unattached, free-lying, questionably mobile genera and the cemented genera among the endemic brachiopods display a significantly greater generic longevity distribution than the unattached, free-lying, stationary and pedunculate genera. Genera cemented throughout their life show a significantly greater longevity distribution than spine-anchored genera. Brachiopod genera which combined interarea stabilization with a pedicle show a significantly greater longevity distribution than genera tethered on a pedicle. Among the cosmopolitan genera, pedunculate and cemented brachiopods display a significantly greater longevity distribution than the unattached, interarea-stabilized genera.     

Taphonomic and paleoecologic implications of flow-induced forces on concavo-convex articulate brachiopods: an experimental approach

The Orientation of benthic marine organisms may be disturbed by flow-induced forces (i.e. drag and lift) caused by wave and current activity. Drag and lift are partly a function of organism size and shape. Consequently, morphology may affect stability (defined as resistance to reorientation, flipping, or entrainment) both during the life of an organism and after its death. An understanding of drag-and-lift effects is therefore essential to the interpretation of paleoecology and biostratinomic processes. An experimental method for quantifying the relative effects of flow-induced forces is described. These forces are measured during flume experiments using transducers and plaster replicas of fossils. As an illustration of the method's potential for taphonomic research, results from experiments investigating the effects of concavo-convex morphologies of articulate brachiopods are presented. Concave-up and convex-up orientations are commonly used to infer paleohydraulic conditions. Two geniculate brachiopods (Rafinesquina alternata and Leptaena richrnondensis) and three flattened forms (a second morphotype of Rafinesquina altemata, Strophodonta demissa , and Tropidoleptus carinatus) were tested in convex-up and concave-up postures and in three azimuthal orientations (hingeline oriented upstream, hingeline downstream, and hingeline parallel to flow). Concave-up orientations consistently exhibit higher drag than convex-up orientations, and this supports the common observation that valved fossils are typically found convex up in paleoenvironments dominated by traction transport. The presence of geniculation significantly increases drag. Lift is relatively insignificant for all models in most orientations. □ Taphonomy, paleoecology, brachiopods, flow-induced forces, transport.     

Mechanical strength of shells of selected extant articulate brachiopods: Implications for paleozoic morphologic trends

Dried shells of Terebratalia transversa, Laqueus californianus, Hemithyris psittacea, and T. unguicula and alcohol‐soaked, tissue‐lined shells of Terebratulina retusa, Dallina septigera, Cryphus vitreus, and Liothyrella uva were crushed in an apparatus that facilitated measurement of the force (newtons) against the valves at the instant of fracture. The results revealed that the costate shells of T. transversa and T. retusa were the strongest. Force is correlated with valve thickness, but not with size (length). When normalized for valve thickness, the force required to fracture shells is correlated with shell biconvexity (height/length) among pooled species of dried specimens. Geniculate specimens of T. retusa were not stronger than the intraspecific variants with a constant radius of curvature to their valves.

The percent‐frequency of plicate, spinose, lamellose and rugate genera increase significantly in the successive stages, Caradocian (Late Ordovician) through Famennian (Late Devonian) at the expense of smooth to costellate genera. The percent‐frequency of rectimarginate (central fold lacking) genera also decreases appreciably in this time frame. These morphologic trends, in combination with the experimental crushing data, support the hypothesis that selection favored species with such anti‐predatory adaptations during a time of escalation of shell‐crushing predators.     

Divaricate patterns in Cambro-Ordovician obolid brachiopods from Gondwana

A new brachiopod species, Westonia mardini, from the Furongian of Turkey and a new occurrence of Westonia urbiona from the Cambrian Series 3 of Iberian Peninsula are reported. These new finds of ‘westoniids’ collected in Gondwana allow the discussion of the functional morphology of their terrace ridges. This structure enabled an effective and rapid reburrowing and allowed the occupation of the shallow marine unstable sands and silts in the shoreface environments. This burrowing mechanism, with their pedicle directed downwards, was unknown before the Cambrian Series 2 and was abandoned chiefly in the Early Ordovician. Here we also review the distribution of ‘westoniids’ in space and time to analyse the diversification, decline and replacement of this important group of brachiopods. They became a significant part of the semi-infaunal marine associations beginning with the Cambrian Series 2 and proliferated in shallow arenaceous shelves during the Cambrian Series 3 and during the Furongian in most palaeocontinents. The acme of ‘Westoniid’ obolids associations was in the Furongian, but locally relict associations were present in high-latitude Gondwana until the Tremadocian. Decline of westoniid associations began during the Tremadocian, being replaced by very diverse smooth-shelled obolid associations with a novel burrowing mechanism and deeper lifestyle.

http://zoobank.org/urn:lsid:zoobank.org:act:4D74A740-34FD-4E3A-89BA-F80128800C0E     

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 protegulum and juvenile shell of a Recent articulate brachiopod: patterns of growth and chemical composition

Patterns of shell formation and the chemical composition of the shell deposited during early post-larval life were investigated in laboratory-reared cultures of the Recent articulate brachiopod Terebraralia transversa (Sowerby). A non-hinged protegulum averaging 148 pm in length is secreted by the mantle within a day after larval metamorphosis. The inner surface of the protegulum exhibits finely granular, non-fibrous material. A rudimentary periostracum constitutes the outer layer of the primordial shell. and concentrically arranged growth lines are lacking. By four days post-metamorphosis, a brephic type of juvenile shell develops from periodic additions of shell material to the anterior and lateral edges of the protegulum. Imbricated secondary fibers occur throughout the inner layer of the newly formed juvenile shell, and a rudimentary hinge apparatus is present posteriorly. The external surface of the shell exhibits concentric growth lines anterior to the caudally situated protegulum, and unbranched punctae begin to form in the subperiostracal region of the shell. At 23 days post-metamorphosis, the shell weighs an average of 1.7 μg and measures 318 μm in length. Electron microprobe analyses reveal that the protegulum is calcified. Minor amounts of sulfur, magnesium, iron, chlorine, aluminum, and silicon are also present in protegula and juvenile shells. Based on electron diffraction data, the mineral phase of juvenile shells consists of calcite, and protegula also appear to contain calcite.     

Influence of valve geometry, ornamentation, and microstructure on fractures in Late Ordovician brachiopods

Among Late Ordovician brachiopods from southeastern Indiana. strophomenids display a ratio of 4:1 parabolic to linear repaired fractures in contrast to the 1:2 ratio found for orthids and rhynchonellids. Additionally, only strophomenids display repaired elliptical fractures. The weakest parts of strophomenid and orthid-rhynchonellid shells are the regions of the adductor muscle scars and the sulcus, respectively. Fractured biplanate shells of strophomenids are commonly cleaved anteriorly to posteriorly, whereas fractures are localized on the anterior of strongly curved to geniculate conspecific specimens. Rugae on leptaenids, thickened anterior margins of the brachial valve of rafinesquinids, and the dense distribution of pseudopunctate in all strophomenids, functioned to localize anteriorly the (un)repaired linear and parabolic fractures. No sublethal fracture occurs on any biconvex shell where the height is greater than 14 mm, despite the fact that numerous specimens of certain species attained shell heights of 20 mm or more, an observation suggesting the upper limit in the gape of the crushing elements of the predator. Crushing experiments on valve 'models' reveal that the inflated equibiconvex, plicate shape of the shells of Plarysfrophia is the strongest. However, the architecture of the concave strophomenid valves is relatively stronger than the corresponding valves of many orthids and rhynchonellids when normalized for valve thickness.     

Comparative larval myogenesis and adult myoanatomy of the rhynchonelliform (articulate) brachiopods Argyrotheca cordata, A. cistellula, and Terebratalia transversa

Background

Primary agametic-asexual reproduction mechanisms such as budding and fission are present in all non-bilaterian and many bilaterian animal taxa and are likely to be metazoan ground pattern characters. Cnidarians display highly organized and regulated budding processes. In contrast, budding in poriferans was thought to be less specific and related to the general ability of this group to reorganize their tissues. Here we test the hypothesis of morphological pattern formation during sponge budding.

Results

We investigated the budding process in Tethya wilhelma (Demospongiae) by applying 3D morphometrics to high resolution synchrotron radiation-based x-ray microtomography (SR-μCT) image data. We followed the morphogenesis of characteristic body structures and identified distinct morphological states which indeed reveal characteristic spatiotemporal morphological patterns in sponge bud development. We discovered the distribution of skeletal elements, canal system and sponge tissue to be based on a sequential series of distinct morphological states. Based on morphometric data we defined four typical bud stages. Once they have reached the final stage buds are released as fully functional juvenile sponges which are morphologically and functionally equivalent to adult specimens.

Conclusion

Our results demonstrate that budding in demosponges is considerably more highly organized and regulated than previously assumed. Morphological pattern formation in asexual reproduction with underlying genetic regulation seems to have evolved early in metazoans and was likely part of the developmental program of the last common ancestor of all Metazoa (LCAM).