Rhynchonelliform Brachiopods Research Articles

Late Ordovician brachiopod distribution and ecospace partitioning in the Tvären crater system, Sweden

Patterns of distribution and ecospace utilization of Late Ordovician brachiopods in a recently formed, contemporary meteorite crater are described and analyzed. Rhynchonelliformean brachiopod communities, dominated by a wide range of orthides and strophomenides, colonized the newly formed crater. At the crater rim communities were established early on, although the crater depression was not inhabited until deposition of the upper third of the remaining crater fill. The crater formed a protected but restricted microenvironment where sediments four times the thickness of the nearby basinal succession accumulated. Within this narrow space environments varied from shallow-water to deeper-waters, about 200m in depth, and from well oxygenated to hypoxic. Such varied environments generated a rough ecological landscape, facilitating niche partitioning across a relatively small geographic area. Analysis of the guild structure of the fauna permits explanation of a local biodiversity hotspot in otherwise low-diversity strata elsewhere in the Scandinavian region. The Tvären impact event had an important palaeobiologic effect upon the fossil record as it served as a local pump and reservoir for biodiversity. Moreover the development of new community types and narrowly-defined niches helped further drive both α and β biodiversity during a critical phase of the Great Ordovician Biodiversification Event.

New and revised occurrences of rhynchonelliformean brachiopods from the middle Cambrian of the Iberian Chains, NE Spain

New and revised occurrences of rhynchonelliformean brachiopods from the middle Cambrian of the Iberian Chains, NE Spain

Lower and middle Cambrian rhynchonelliform brachiopods from the Precordillera terrane of Argentina

The first Cambrian rhynchonelliformean brachiopods are described from the Precordillera mountain belt of west-central Argentina, including the new species Wimanella mollensis and Nisusia ancauchensis. Other forms are Diraphora cf. D. borealis (Walcott), Diraphora sp., and an indeterminate bohemiellid. Brachiopods come from the lowermost part of the Precordilleran Cambrian succession (El Estero and Soldano Members of the La Laja Formation, Cerro Totora Formation) and from the Ancaucha olistolith within the Los Sombreros olistostrome. The associated trilobites constrain the fossiliferous levels to the Bonnia-Olenellus, Poliella denticulata and Ehmaniella biozones. A re-assessment of paleobiogeographic affinities of lower and early middle Cambrian rhynchonelliform brachiopods using cluster analysis supports the existence of a widespread warm-water Tropical Realm—to which the described Precordilleran faunas belong—and a more restricted ‘Mediterranean’ Realm developed on the temperate, mixed clastic-carbonate platforms.

Micromorphology and differential preservation of Upper Permian brachiopod low-Mg calcite

Calcareous shells of Rhynchonelliform brachiopods from the Nesen Formation (Late Permian) of Iran were studied by scanning electron microscope, cathodoluminescence and trace element chemistry to define their microstructure and state of preservation. In the Strophomenata shell succession, the secondary layer consists of cross-bladed, laminar calcite, and in a few species there is a tertiary layer of prisms. In the Rhynchonellata shell succession, the secondary layer consists of elongated fibers and the tertiary layer of prisms. All specimens were classified into six micromorphological types based on the fabric of the layer and the morphology of their microstructural units (laminae, fibres and prisms).Shell micromorphology and enclosing whole rock were further analyzed by cathodoluminescence and trace chemistry to assess preservation of brachiopod low-Mg calcite and specifically of their layers and microstructural units. Relative color frequencies were determined for each morphological type and degree of luminescence based on the lithology of the host rocks to evaluate the main factors that affect shell preservation. Results suggest that diagenetic alteration may depend on shell fabric and taxonomy as well as lithology of the host rock, but it appears that the first and the second features are the most important factors in determining the degree of brachiopod low-Mg calcite preservation. In general, the fibrous secondary layer is likely more often and better preserved than its laminar counterpart irrespective of host rock lithology. Furthermore, tertiary layers tend to be better preserved than secondary ones in Rhynchonellata low-Mg calcite shells, whereas no preferential preservation of either layer over the other was observed in the shells of Strophomenata. This information is important when selecting fossil brachiopod low-Mg calcite shells as geochemical proxies for paleoclimatic, paleoecologic, paleoenvironmental and other studies.

Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen

We describe Early and Middle Ordovician brachiopod faunas from northeastern Spitsbergen and discuss their biostratigraphical and palaeoecological implications. We recognise 60 species and 41 genera representing both linguliform and rhynchonelliform brachiopods. These include one new genus, the obolid Lenticulella with the type species Lenticulella amphora (Krause & Rowell, 1975), and 23 new species: Ectenoglossa? oviforma, Elliptoglossa vulgaris, Rosobolus? elongatus, Broeggeria obscura, Mirilingula? svalbardensis, Schizotreta marginalis, Conotreta convexa, Cyrtonotreta profilbekkiensis, Cyrtonotreta spinosa, Eurytreta subtriangularis, Hisingerella maniformis, Semitreta basisslettaensis, Semitreta pustulosa, Semitreta spitsbergensis, Numericoma? proclina, Eoconulus subquadratus, Dictyonites mugilis, Pelonomia sulcata, Leptella (Leptella) inequicostellata, Protoskenidioides promontorium, Anomalorthis rossi, Nothorthis subpyramidalis and Phragmorthis noda. The Ordovician succession contains both intertidal and deep-sea deposits with brachiopod faunas from nearly the whole range of environments. The Tremadocian is represented by a succession of low-diversity, rhynchonelli-6 · Zootaxa 3076 © 2011 Magnolia Pressform-dominated assemblages occupying shallow-marine environments. With the abrupt and large-scale drowning during the Floian, these faunas were replaced by a sparse slope fauna of micromorphic linguliforms. The outer shelf environments established in the latest Floian were occupied by a highly diverse brachiopod fauna dominated by micromorphic linguliform taxa. As the environment shifted toward the mid-shelf during the Darriwilian, however, the composition of the fauna changed to an Orthidiella-dominated assemblage of mostly rhynchonelliform taxa. The very late occurrence of the Orthidiella assemblage suggests that the Dapingian Orthidiella Zone found in America is probably diachronic. Although it includes many endemic species, the Middle Ordovician fauna shows a strong resemblance to the brachiopod fauna of Nevada. This study is based on approximately 16 500 brachiopod specimens obtained from both crack-out samples and acetic-acid-treated bulk samples.

Did the amalgamation of continents drive the end Ordovician mass extinctions?

Global biodiversity has been punctuated throughout the Phanerozoic by extinction events that vary in their degree of intensity and devastation. The mass extinction event that occurred at the end of the Ordovician Period rapidly removed a wide range of species. Because taxonomic loss occurred during an ice age, this is believed to have initiated the extinctions and thus, these extinctions have often been viewed as a deep time analogue to the loss in species diversity during the present day glacial interval. The current study, however, indicates that temperature – though arguably being a trigger – was not the sole reason for the crisis. Based on a large, bibliographic database of rhynchonelliform brachiopods that specifically operates within very narrow time-slices where every locality has been precisely georeferenced for the Upper Ordovician–Lower Silurian interval, we show that the extinctions were not uniformly distributed, nor was the succeeding recovery. Here we argue that changing plate tectonic configurations during the Ordovician–Silurian interval may have exerted a primary control on biotic extinction and recovery. In particular the proximity and ultimate loss of microcontinents and associated smaller terranes around Laurentia may have restricted shelf and slope habitats during the latest Ordovician but, nevertheless, in a contracted Iapetus Ocean, provided migration routes to help drive a diachronous early Silurian recovery. The conclusion that plate tectonics was a primary factor controlling the extent of the extinctions and the subsequent diversity rebound, demonstrates that a reduction in γ-diversity was perhaps the most important manifestation of the end Ordovician crisis and further raises the question whether this could be applied to other large Phanerozoic perturbations in biodiversity levels.

Homeobox gene expression in Brachiopoda: The role of Not and Cdx in bodyplan patterning, neurogenesis, and germ layer specification

The molecular control that underlies brachiopod ontogeny is largely unknown. In order to contribute to this issue we analyzed the expression pattern of two homeobox containing genes, Not and Cdx, during development of the rhynchonelliform (i.e., articulate) brachiopod Terebratalia transversa. Not is a homeobox containing gene that regulates the formation of the notochord in chordates, while Cdx ( caudal) is a ParaHox gene involved in the formation of posterior tissues of various animal phyla. The T. transversa homolog, TtrNot, is expressed in the ectoderm from the beginning of gastrulation until completion of larval development, which is marked by a three-lobed body with larval setae. Expression starts at gastrulation in two areas lateral to the blastopore and subsequently extends over the animal pole of the gastrula. With elongation of the gastrula, expression at the animal pole narrows to a small band, whereas the areas lateral to the blastopore shift slightly towards the future anterior region of the larva. Upon formation of the three larval body lobes, TtrNot expressing cells are present only in the posterior part of the apical lobe. Expression ceases entirely at the onset of larval setae formation. TtrNot expression is absent in unfertilized eggs, in embryos prior to gastrulation, and in settled individuals during and after metamorphosis. Comparison with the expression patterns of Not genes in other metazoan phyla suggests an ancestral role for this gene in gastrulation and germ layer (ectoderm) specification with co-opted functions in notochord formation in chordates and left/right determination in ambulacrarians and vertebrates. The caudal ortholog, TtrCdx, is first expressed in the ectoderm of the gastrulating embryo in the posterior region of the blastopore. Its expression stays stable in that domain until the blastopore is closed. Thereafter, the expression is confined to the ventral portion of the mantle lobe in the fully developed larva. No TtrCdx expression is detectable in the juvenile after metamorphosis. This expression of TtrCdx is congruent with findings in other metazoans, where genes belonging to the Cdx/caudal family are predominantly localized in posterior domains during gastrulation. Later in development this gene will play a fundamental role in the formation of posterior tissues.

Brachiopod taxa and shell portions reliably recording past ocean environments: Toward establishing a robust paleoceanographic proxy

[1] Fossils of rhynchonelliform brachiopods (marine invertebrates) constitute ∼70% of the samples used for delineating a well-known Phanerozoic trend in oxygen isotope composition (δ18O) of low-Mg calcite shells. The trend represents secular variations in temperature and/or δ18O of ancient seawater. However, the use of brachiopods as a paleoceanographic proxy is based on the presupposition that the shell calcite is precipitated in isotopic equilibrium with ambient seawater. Here, we show that high-resolution time series of the shell δ18O values along the maximum growth axes of two long-lived cool-water brachiopods are identical to, greater than, or less than those of calcite precipitated in isotopic equilibrium with ambient seawater, depending on the difference in shell growth rates. Coupled with δ18O data from subtropical and warm-temperate brachiopod species examined in our previous studies, we provide a sound framework illustrating which taxa and shell portions reliably recorded past ocean environments.

Interrogation of distributional data for the End Ordovician crisis interval: where did disaster strike?

The uppermost Ordovician–lowermost Silurian (Upper Katian–Rhuddanian) is surveyed with respect to the α‐ and β‐diversity of rhynchonelliformean brachiopods. The survey is based on new collections as well as existing literature, compiled in a large, georeferenced database. The brachiopod faunas are plotted using a Geographical Information System (GIS) and subjected to density analysis to display fluctuations within the different faunas through time. In addition, an analysis is performed on the preferred relative depth ranges of the brachiopod communities through the latest Ordovician–earliest Silurian crisis interval, following the concept of Benthic Assemblage zones. Both analyses support the view of a two‐phased late Ordovician (Hirnantian) decline in diversity followed by a radiation in the early Silurian (Rhuddanian). These data show that the main taxonomic loss was geographically located on the peri‐Laurentian terranes, in the Laurentian epicratonic seas and on the margins of the Ægir Ocean. Refuges during the survival interval were probably located in the shallow‐water zones of especially Baltica, but also Gondwana, the peri‐Laurentian terranes and the Kazakh Terranes. Except for Baltica, these refuges may to a large extent be obscured by the success of the Hirnantia fauna and, thus, may not contain many true survivor taxa. South China apparently operated as a refuge for the mid‐shelf to deeper‐water faunas. Avalonia, Baltica, Laurentia and South China experienced increased diversity in shallow‐water settings in the recovery interval. Most regions experienced increased diversity among deeper‐water faunas, although this appears to have been delayed until the late Rhuddanian on Laurentia. At this time the deeper‐water faunas disappeared from the peri‐Laurentian terranes (as did the terranes themselves) probably as a consequence of the progressively narrowing Iapetus Ocean. Copyright © 2011 John Wiley & Sons, Ltd.

An obolellate brachiopod with soft-part preservation from the Early Cambrian Chengjiang fauna of China

The obolellates represent a poorly understood group of the oldest known cosmopolitan calcareous rhynchonelliform brachiopods. They made their first appearance in the early Atdabanian and became extinct at the end of the Middle Cambrian. Consequently, any information concerning the soft anatomy of this ephemeral lineage of brachiopods has great phylogenetic significance. This is the first report on two specimens of an obolellate with fine preservation of soft parts including the pedicle, marginal setae and possible imprints of a spiral lophophore, recovered from the early Cambrian Chengjiang Konservat Lagerstätte of Kunming, southern China. The setae are thin and densely fringed along the shell margin. The stout pedicle is distinctly composed of densely stacked tabular bodies, lacking a central coelomic lumen; it emerges through a possible foramen, and slightly tapers posteriorly with the distal end attached to exoskeletons of other organisms.

The exceptionally preserved Early Cambrian stem rhynchonelliform brachiopod Longtancunella and its implications

Zhang, Z., Holmer, L. E., Ou, Q., Han, J. & Shu, D. 2011: The exceptionally preserved Early Cambrian stem rhynchonelliform brachiopod Longtancunella and its implications. Lethaia, Vol. 44, pp. 490–495. The extraordinary Longtancunella chengjiangensis is one of the rarest and most problematic brachiopods from the Early Cambrian Chengjiang Lagerstätte, where it occurs in unique gregarious clusters with up to ten individuals attached by their stout pedicles to a single hard exoskeleton. Although the general morphology and soft anatomy of Longtancunella is available, its taxonomy and systematic placement within the Brachiopoda remains problematic. New exceptionally preserved specimens show that the pedicle of Longtancunella cannot be considered homologous with the pedicles in crown Rhynchonelliformea or Linguliformea. The pedicle of Longtancunella emerges from a ventral umbonal foramen with a rounded plate that must have been secreted by specialized ventral mantle epithelium. The rounded plate is proposed to be a homologue to the colleplax – a plate covering the umbonal foramen, which is otherwise known from the extinct rhynchonelliform chileates. Longtancunella is proposed to represent a soft-shelled stem rhynchonelliform brachiopod with chileate features, thus demonstrating for the first time that the chileate-like umbonal perforation functions as a pedicle opening. The unusual preservation of the annulated chileate-like holdfast supports the view that it may have been secreted as stacks of chitinous or even mineralized ‘attachment pads’. Together with having a spirolophous lophophore and U-shaped gut, Longtancunella displays a unique mixture of linguliform and rhynchonelliform characters that throw new light on an early stage of rhynchonelliform diversification. □Brachiopods, Cambrian, Chengjiang Lagerstätte, Chileata, colleplax, pedicle.

Crystallographic contribution to the vital effect in biogenic carbonates Mg/Ca thermometry

ABSTRACTThe processes involved in vital effects, defined as biological processes overriding environmental signals, are not well understood and this hampers the interpretation of environmental parameters such as seawater temperature. Insufficient knowledge is available about changes in physico-chemical parameters, in particular those related to crystallography, associated with biomineral formation and emplacement. This paper assesses the influence of crystallography on Mg2+ concentration and distribution in calcite biominerals of bivalved marine organisms, mussels and rhynchonelliform brachiopods, and considers the implications for Mg/Ca thermometry. In the mussel Mytilus edulis, changes in Mg2+ are not associated with crystallography; but in the brachiopod Terebratulina retusa, increases in Mg2+ concentrations (∼0·5–0·6 wt. ) are associated with the {0001 planes of calcite biominerals. A comparison between mussels and brachiopods with avian eggshells, which form at constant ambient temperature, also reveals that there is at least a common 0·1 wt. variation in magnesium concentration in these calcite biomineral systems unrelated to temperature or crystallography. Results demonstrate that the integration of contextual crystallographic, biological and chemical information may be important to extract accurate environmental information from biominerals.

Drill hole analysis reveals evidence of targeted predation on modern brachiopods

If drill holes are made in prey either by mistaken or opportunistic predators or parasites rather than those which do so on a more regular basis we might predict that such drill holes will commonly be incomplete or repaired and show no evidence of behavioural familiarity such as size selection or constancy of attack site. These predictions were used to examine the patterns of drill holes found in bulk samples of death assemblages of modern rhynchonelliform brachiopods from around New Zealand. Four samples, yielding data for five different taxa (four terebratulides and one rhynchonellide) were examined. In each the proportion of incomplete or repaired holes was low. Samples of Terebratella sanguinea (Fiordland) and Gyrothyris mawsoni (Antipodes Islands) show statistically significant evidence of both size selection and preferential drill hole siting. The sample of Notosaria nigricans (Waikawa) shows only evidence of size selection. Examination of drill hole morphology and the co-occurring gastropod fauna suggests that the distinctive countersunk holes in T. sanguinea and G. mawsoni were likely made by the trophonid gastropod Xymene ambiguus. Aquarium experiments demonstrate that the ranellid gastropod Argobuccinum tumidum is also capable of feeding on brachiopods but that the resulting hole is not likely to be readily recognisable in death assemblages.

Latest Changhsingian orthotetid brachiopods in the Dolomites (Southern Alps, Italy): ecological opportunists at the peak of the end-Permian mass extinction

The majority of Changhsingian orthotetid brachiopod species from the Southern Alps were described by the end of the nineteenth century, but were then neglected by subsequent authors who proposed further new species, giving rise to great taxonomical confusion. An examination of type-specimens and newly collected material permits the consideration ofOrthothetina ladina(Stache, 1878) andOmbonia tirolensis(Stache, 1878), type-species ofOmbonia, as the only valid species and describeTeserina neriias a new genus and species. The former two species are mostly located within a short stratigraphic interval, which is a few centimetres thick, limited by the trigger and peak of the end-Permian mass extinction.Teserina neriin. gen. n. sp. occurs 2–3 m above the extinction peak and represents one of the last Permian rhynchonelliform brachiopod holdovers in the Southern Alps.

Palaeoenvironmental aspects of Late OrdovicianSericoideashell concentrations in an impact crater, Tvären, Sweden

Frisk, Å.M. & Harper, D.A.T. 2010: Palaeoenvironmental aspects of Late Ordovician Sericoidea shell concentrations in an impact crater, Tvären, Sweden. Lethaia, Vol. 44, pp. 383–396. Numerous studies have reported the presence of the small, thin-shelled cosmopolitan rhynchonelliformean Sericoidea as being environmentally controlled and, together with its close relatives, characteristic of deep-water, distal, clastic Ordovician and Silurian settings. Assemblages of Sericoidea have been analysed from post-impact strata in a newly formed Late Ordovician impact crater. In the crater succession, colonization of benthic faunas can be monitored through the post-impact limestone, demonstrating a number of environmental preferences. Consequently, the crater, as a result of its restricted area, provides an experimental arena for faunal distributions to be correlated with specific environments. The continuous infilling of the crater following its formation reveals a transition from argillaceous mudstones to carbonates deposited in deeper-water environments to shallower regimes. Rhynchonelliformean brachiopods inhabited the crater depression very late after the impact and are entirely represented by the genus Sericoidea, occurring abundantly in the upper third of the existing crater infill. The deep-water regime that existed in the depression during the initial interval of crater formation had been substantially reduced. Clearly Sericoidea-bearing associations associated with shaly substrates did not merely favour and occupy deep-water environments as previously suggested. The unfavourable conditions triggered by the impact and the inhospitable aftermath allowed Sericoidea to exploit a less-crowded ecospace. This reorganization, following the catastrophe, from a deep-water related ecological niche to considerable shallower settings suggests that Sericoidea was a pioneer colonist displaying an opportunist r-strategy. The shell beds analysed are related to shallower water and this may, moreover, help unravel the dilemma of the general absence of Sericoidea in the deeper-water Foliomena fauna. □Dalby Limestone, impact crater, Late Ordovician, opportunists, Sericoidea, Tvären.

Analyzing niche stability and biogeography of Late Ordovician brachiopod species using ecological niche modeling

Gauging the potential impacts of environmental change on the geographic distributions of species is a central area of modern biogeographic analysis, often involving complex models of species–environment interactions. The geographic distribution of fossil species can also provide a framework to test the impact of environmental change on biogeography and ecological niches of species, yet few paleontological analyses have attacked this question in deep time. Herein we present a quantitative biogeographic analysis to examine the stability of ecological niches and geographic ranges of rhynchonelliform brachiopods during an interval of sea level change preserved in Upper Ordovician strata of the Cincinnati Arch.The intensive sampling, excellent preservation, and numerous prior paleoecological and sedimentological analyses within the tri-state region of Kentucky, Indiana, and Ohio provide a robust framework for detailed paleobiogeographic study. Quantitative biogeographic modeling methods incorporating GIS (Geographic Information Systems) are utilized in order to spatially analyze the geographic ranges of brachiopod species of the Corryville and Mt. Auburn Formations of the C3 (uppermost Maysvillian) depositional sequence.This study employs the ecological niche modeling program GARP (Genetic Algorithm using Rule-set Prediction) to predict the geographic distribution of eight brachiopod species during three time slices within the C3 sequence. This method estimates a species’ geographic range by modeling the ecological niche of the species based on a set of known species occurrence data coupled with environmental data inferred from sedimentologic proxies. Once environmental tolerances for a species are modeled; the species is predicted to occur wherever its preferred set of environmental conditions occurs within the study region.Distributional patterns were reconstructed for three time slices during the C3 sequence. Recovered range predictions were quantitatively analyzed for evidence of temporal range changes. Results indicate that average species range within the study area decreased and species tracked their preferred niche with high fidelity during the transition from the early to middle portions of the C3 depositional sequence, an interval of rapid relative sea level change. However, during the transition from the middle to late portions of the sequence, gradual shallowing within the basin and development of discontinuous habitat patches correlates with niche evolution of five of the eight species modeled. The average area a species occupied within the basin increased during this interval, but there is a mixed response including both increases and decreases in range size within the study group. In general, the species that exhibit niche evolution increased their geographic range size while those that continue to track their niche with high fidelity experience a decrease in geographic range size. During the latter half of the C3 sequence, previously continuous habitats become fragmented, thereby isolating individual populations and providing a mechanism for niche evolution. The rate of sea level change and the corresponding fragmentation of previously continuous habitats into isolated patches appear to be the primary controls on both mean geographic range size and relative degree of niche evolution.

Using ecological niche modeling to assess biogeographic and niche response of brachiopod species to the Richmondian Invasion (Late Ordovician) in the Cincinnati Arch

Within the Cincinnati, Ohio region, the beginning stage of a large-scale biotic invasion is recorded in the C4 depositional sequence (Late Ordovician, Richmondian Stage), which includes the Arnheim Formation and its correlates. The fauna and strata of this region are well studied and the ecological changes present provide a unique opportunity to study biogeographic changes associated with the onset of a biotic invasion. In this study, GIS-based ecological niche modeling (ENM) is used to examine the biogeographic and paleoecological impacts of the biotic invasion during the C4 sequence. In particular, this study assesses whether species respond to invasion pressure through niche conservatism (exhibiting stasis and maintaining the parameters of their ecological niche through time) or niche evolution (altering the ecological parameters of their niche through adaptation). Ranges of ten rhynchonelliform brachiopod species were modeled for three temporal intervals within the C4 sequence. Modeled ranges provide the basis for quantitatively assessing the shifting roles of native and invasive species throughout the C4 sequence. GARP (Genetic Algorithm for Rule-set Prediction), a computer-learning based ENM program, was used to model species' ecological niches from environmental data as estimated from sedimentological variables associated with known species occurrence data. Once environmental parameters of the niche were modeled, the resulting rule set was used to project the geographic boundaries of potential habitat and estimate the spatial extent occupied by each species. Three prominent trends are apparent within the fauna. (1) Generalist species tended to track their preferred ecological niches closely from the early into the middle time slice and maintained or expanded the size of their geographic range during this transition. (2) Ecologically specialized species exhibited decreasing geographic range size through the sequence and were more likely to shift geographic areas inhabited within the study area. (3) The amount of niche evolution increased as the invasion progressed; species that adapted to the new biotic regime though niche evolution persisted through the invasion interval while species with high levels of niche conservatism did not. During early C4 sequence, species respond individualistically to changes in abiotic conditions (shallowing upward), but do so primarily by tracking their previously preferred environmental conditions laterally and maintain high fidelity of niche dimensions through time. This matches the null hypothesis that species will exhibit niche conservatism during intervals of slow or gradual environmental change. Following the geologically sudden introduction of the intrabasinal invaders in the second half of the C4 sequence, native species responded to the combined abiotic (shallowing upward) and biotic (invader introduction) changes in the Cincinnatian environmental by altering the parameters of their ecological niches and through fundamental changes in geographic distribution. Extrapolating these results to modern ecosystems, suggests that most species should exhibit niche conservatism and habitat tracking in response to gradual changes in environmental conditions, while species should exhibit niche evolution and adaptive response following rapid environmental changes. Furthermore, modern species with reduced potential for adaptive response may also be the most susceptible to extinction.

Neuromuscular development in Novocrania anomala: evidence for the presence of serotonin and a spiralian‐like apical organ in lecithotrophic brachiopod larvae

The phylogenetic position of Brachiopoda remains unsettled, and only few recent data on brachiopod organogenesis are currently available. In order to contribute data to questions concerning brachiopod ontogeny and evolution we investigated nervous and muscle system development in the craniiform (inarticulate) brachiopod Novocrania anomala. Larvae of this species are lecithotrophic and have a bilobed body with three pairs of dorsal setal bundles that emerge from the posterior lobe. Fully developed larvae exhibit a network of setae pouch muscles as well as medioventral longitudinal and transversal muscles. After settlement, the anterior and posterior adductor muscles and delicate mantle retractor muscles begin to form. Comparison of the larval muscular system of Novocrania anomala with that of rhynchonelliform (articulate) brachiopod larvae shows that the former has a much simpler muscular organization. The first signal of serotonin-like immunoreactivity appears in fully developed Novocrania anomala larvae, which have an apical organ that consists of four flask-shaped cells and two ventral neurites. These ventral neurites do not stain positively for the axonal marker alpha-tubulin in the larval stages. In the juveniles, the nervous system stained by alpha-tubulin is characterized by two ventral neurite bundles with three commissures. Our data are the first direct proof for the presence of an immunoreactive neurotransmitter in lecithotrophic brachiopod larvae and demonstrate the existence of flask-shaped serotonergic cells in the brachiopod larval apical organ, thus significantly increasing the probability that this cell type was part of the bauplan of the larvae of the last common lophotrochozoan ancestor.

Mechanical properties of modern calcite- (Mergerlia truncata) and phosphate-shelled brachiopods (Discradisca stella and Lingula anatina) determined by nanoindentation

We measured distribution patterns of hardness and elastic modulus by nanoindentation on shells of the rhynchonelliform brachiopod Mergerlia truncata and the linguliform brachiopods Discradisca stella and Lingula anatina. The rhynchonelliformea produce calcitic shells while the linguliformea produce chitinophosphatic shells. Dorsal and ventral valves, commissure and hinge of the calcitic shell of M. truncata show different nanohardness values (from 2.3 to 4.6 GPa) and E-modulus (from 52 to 76 GPa). The hardness of the biocalcite is always increased compared to inorganic calcite. We attribute the effects to different amounts of inter- and intracrystalline organic matrix. Profiles parallel to the radius of curvature of the valves cutting through the different layers of shell material surprisingly show quite uniform values of nanohardness and modulus of elasticity. Nanoindentation tests on the chitinophosphatic brachiopods D. stella and L. anatina reflect the hierarchical structure composed of laminae with varying degree of mineralization. As a result of the two-phase composite of biopolymer nanofibrils reinforced with Ca-phosphate nanoparticles, nanohardness, and E-modulus correlate almost linearly from (H=0.25 GPa, E=2.5 GPa) to (H=2.5 GPa, E=50 GPa). The mineral provides stiffness and hardness, the biopolymer provides flexibility; and the composite provides fracture toughness. Gradients in the degree of mineralization reduce potential stress concentrations at the interface between stiff mineralized and soft non-mineralized laminae. For the epibenthic chitinophosphatic D. stella the lamination is also present but less pronounced than for the infaunal L. anatina, and the overall distribution of material strength in the cross-sectional profile shows a maximum in the center and a decrease towards the inner and outer shell margins (modulus of elasticity from 30 to 12 GPa, hardness from 1.7 to 0.5 GPa). Accordingly, the two epibenthic forms, calcitic M. truncata and chitinophosphatic D. stella display fairly bulky (homogeneous) nanomechanical properties of their shell materials, while the burrowing infaunal L. anatina is distinctively laminated. The strongly mineralized laminae, which provide the strength to the shell, are also brittle, but keeping them as thin as possible, allows some bending flexibility. This flexibility is not required for the epibenthic life style.

Survival patterns of macrobenthic marine assemblages during the end-Permian mass extinction in the western Tethys (Dolomites, Italy)

The ecological competition between brachiopods and bivalves is analysed by means of a quantitative palaeoecologic method applied on four assemblages located within a short stratigraphic interval, approximately 2 m thick, in the lower Tesero Member of the Werfen Formation (in the Southern Alps). The assemblages originate from the Tesero, Bulla and Sass de Putia sections. The analysed stratigraphic interval, uppermost Changhsingian in age, is located between the early and heaviest phase of the end-Permian mass extinction, which occurred across the Bellerophon/Werfen formational boundary (Event Boundary), and the Permian/Triassic boundary (Chronological Boundary), when nearly all the Permian stenotopic holdovers disappeared. These assemblages are characterised by small sized skeletons (“Lilliput effect”), which represent an adaptive survival strategy in stressed and harsh habitats resulting from the climatic and palaeoceanographic changes connected with the mass extinction. The Tesero assemblages are dominated by rhynchonelliform brachiopod Orbicoelia (bed CNT10) or Streptorhynchus (bed CNT11A), which were mostly attached at the top of shallow microbialitic mounds. These assemblages are again dominated by Permian stenotopic taxa and show a Palaeozoic structure. The Tesero habitat, which again permitted the survival of brachiopods, represented one of the last refuges in the western Tethys. On the contrary, the Bulla (BU9-10) and Sass de Putia (wPK13A) assemblages are bivalve-dominated, and thus show an ecologic structure typical of Early Triassic post-extinction marine benthic communities or Palaeozoic stressed marine communities. The bivalve-dominated assemblages proliferated in prevailing muddy siliciclastic substrates, with brief episodes of microbial algal growth. The most important environmental limiting factors and leading causes of end-Permian mass extinction are discussed in terms of palaeoautecologic and palaeosynecologic analysis. The different taxonomic composition and ecologic structure of the assemblages is related to palaeogeography, including water depth and connections with the open sea. The brachiopod-dominated assemblage, exclusive of the Tesero section, proliferated in microbial carbonate habitats in near-shore environments. The bivalve-dominated assemblages, which were more widespread than the brachiopod assemblages in the Dolomites and also occurred in other western Tethys localities, occur in more open and deeper marine environments. In the western Tethys margins, the local distribution of mixed faunas suggests that the extinction of Permian stenotopic taxa was caused by the onset of poisonous water on the shelves originating from deep marine environments. This extinction pattern appears to be a regional phenomenon and does not seem be applicable on a global scale. The extinction events were controlled by a complex network of interactive factors and the survival of faunal elements was probably stochastic.