Aragonitic dendritic prismatic shell microstructure in Thracia (Bivalvia,Anomalodesmata)

The shells of most anomalodesmatan bivalves are composed of an outer aragonitic layer of either granular or columnar prismatic microstructure, and an inner layer of nacre. The Thraciidae is one of the few anomalodesmatan families whose members lack nacreous layers. In particular, shells of members of the genus Thracia are exceptional in their possession of a very distinctive but previously unreported microstructure, which we term herein “dendritic prisms.” Dendritic prisms consist of slender fibers of aragonite which radiate perpendicular to, and which stack along, the axis of the prism. Here we used scanning and transmission electron microscopical investigation of the periostracum, mantle, and shells of three species of Thracia to reconstruct the mode of shell calcification and to unravel the crystallography of the dendritic units. The periostracum is composed of an outer dark layer and an inner translucent layer. During the free periostracum phase the dark layer grows at the expense of the translucent layer, but at the position of the shell edge, the translucent layer mineralizes with the units typical of the dendritic prismatic layer. Within each unit, the c‐axis is oriented along the prismatic axis, whereas the a‐axis of aragonite runs parallel to the long axis of the fibers. The six‐rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its distinctive appearance, the dendritic prismatic layer of the shell of Thracia spp. is homologous to the outer granular prismatic or prismatic layer of other anomalodesmatans, while the nacreous layer present in most anomalodesmatans has been suppressed.     

First records of brachiopods of the family Discinidae (Class Lingulata) from the Upper Jurassic of West Siberia

The organophosphatic brachiopods of the superfamily Discinoidea, family Discinidae are described from the Upper Jurassic of West Siberia. The protegulum (embryonic shell) preserved in adult shells is for the first time discovered in Mesozoic discinids. The shell microstructure is studied. A new species, Discinisca undata Smirnova, sp. nov., is established.     

Shell structure and affinity of vermiform 'gastropods'

Vermiform ‘gastropods’ are reported from a variety of rocks ranging from Givetian to Lower Triassic age. Examples encrusting shells and plants have been identified in non-marine shales, in addition to previously recognized occurrences in shallow marine microbial bioherms and stromatolites. SEM studies of the planorbiform or trochiform protoeonch reveal a shell wall comprising three calcite layers: an outer, initial acicular layer: a blocky prismatic layer: and an inner irregular micro-lamellar layer. Minor irregularities and microstructural details suggest a high original organic content. allowing flexibility for attachment. The sinistrally coiled (or hyperstrophic dextral) calcitie teleoconch is composed of an outer simple prismatic layer and inner micro-lamellar layer comprising sheets of irregular, platy, sometimes fused calcite tablets. displaying ridges and grooves similar to those of cross-bladed fabric. Repetition of layers may occur. Regular closely-spaced punctae. passing through and disrupting the micro-lamellar layer. are unlike any mollusean tubulation. Punetation may he a shell-strengthening response to uncoiling. Septa bear a centrai, anteriorly-projecting, probebly perforate protrusion. reminiscent of the siphunele of cephalopods and similar structures in tentaculitoida. The micro-lamellar layer in the protoconch, the micro-lamellar layer with distinctive ridge and groove structure and punetation in the teleoconch, and the structure of the septa point to a close affinity between vermiform ‘gastropods’ and the Tentaculitoidea. The three-layered microstructure of the protoconch and the coiled nature of the shell distinguish vermiform ‘gastropods’ from tentaculitoids. A consideration of the shared characters indicates that the tentaculitoids and vermiform ‘gastropods’ should be regarded as a sister group to the molluses. ***Vermiform gastropods: microstructure, protoconch, teleoconch, acicular laver, prismatic laver, micro-lameller layer. cross-bladed structure. punetation, septation. Tentaculitoidea     

Endocochliate embryo model in the Mesozoic Ammonitida

An endocochliate embryo model for the Mesozoic Ammonitida is proposed based on scanning electron microscopy of the ammonitella (= embryonic shell) stage of well‐preserved Japanese Cretaceous specimens belonging to nine species of five superfamilies. As in other specimens described previously, the ammonitella wall succeeding from the initial chamber ("protoconch") in the species examined consists of the inner prismatic, middle subprismatic and outer prismatic layers, with minute tubercles resting on the outer. Developmental patterns of these structures and their comparison with primary shell formation in modern Nautilus and Spirula suggest that the outer thin prismatic layer with microtubercles was secreted by the exterior epithelium after the completion of the main ammonitella wall by the interior shell gland. Thus, the early ammonite embryo might have had an endocochliate structural plan as in coleoids, and at the time of hatching the overlying mantle epithelium had shifted anteriorly to become an ectocochliate condition.     

Structure and composition of the boundary zone between aragonitic crossed lamellar and calcitic prism layers in the shell of Concholepas concholepas (Mollusca,Gastropoda)

Mollusc shells are composed of two or three layers. The main layers are well‐studied, but the structural and chemical changes at their boundaries are usually neglected. A microstructural, mineralogical, and biochemical study of the boundary between the inner crossed lamellar and outer prismatic layers of the shell of Concholepas concholepas showed that this boundary is not an abrupt transition. Localized structural and chemical analyses showed that patches of the inner aragonitic crossed lamellar layer persist within the outer calcitic prismatic layer. Moreover, a thin aragonitic layer with a fibrous structure is visible between the two main layers. A three‐step biomineralization process is proposed that involves changes in the chemical and biochemical composition of the last growth increments of the calcite prisms. The changes in the secretory process in the mantle cells responsible for the shell layer succession are irregular and discontinuous.     

A possible mechanism for the formation of annual growth lines in bivalve shells

We report a unique shell margin that differed from the usual shell structure of Pinctada fucata. We observed empty organic envelopes in the prismatic layer and the formation of the nacreous layer in the shell margin. All the characteristics of the growing margin indicated that the shell was growing rapidly. To explain this anomaly, we propose the concept of “jumping development”. During jumping development, the center of growth in the bivalve shell jumps forward over a short time interval when the position of the mantle changes. Jumping development explains the unusual structure of the anomalous shell and the development of annual growth lines in typical shells. Annual growth lines are the result of a discontinuity in the shell microstructure induced by jumping development.     

Microstructural details in shells of the gastropod genera Carychiella and Carychium of the Middle Miocene

Microstructural details are revealed via scanning electron microscopy (SEM) in two carychiid species from the early Middle Miocene of Styria, SE Austria. The protoconchs of the shells of Carychiella eumicrum (Bourguignat 1857) and Carychium gibbum (Sandberger 1875) show different types of microstructure on the embryonic shell during ontogeny. Total, superficial punctate structure on the shell of Carychiella eumicrum contrasts with the protoconch–teleoconch demarcation (p/t boundary) observed on the protoconch of Carychium gibbum. Both species exhibit aragonitic microstructure. Diagenetic effects, prismatic, homogeneous and crossed lamellar microstructures are detectible in both species. Rheomorphic folding and dense pitting within the columella of Carychiella eumicrum suggest a structure–function relationship for tensile strength and bulk weight reduction in carychiid snails. We hypothesize that total superficial pitting on the shell of C. eumicum, seen here for the first time in the Carychiidae, suggests paedomorphosis as a life‐history strategy to palaeoecological conditions of the Rein Basin during the early Middle Miocene.     

New Studies of Enamel Microstructure in Mesozoic Mammals: A Review of Enamel Prisms as a Mammalian Synapomorphy

Characters from enamel microstructure have not been used in recent phylogenetic analyses of Mesozoic Mammalia. Reasons are that enamel characters have been perceived as (A) variable without regard to systematic position of taxa, (B) inconsistently reported within the literature, and (C) simply scored as either prismatic or not prismatic in earlier mammals. Our work on Mesozoic mammals such as Sinoconodon, Gobiconodon, Triconodontidae, Docodon, Laolestes, and others suggests that synapsid columnar enamel (SCE) structure was easily transformed into plesiomorphic prismatic enamel (PPE) and that PPE may be described with at least five independent character states. Two PPE characters—a flat, open prism sheath and a planar prism seam—were present in the cynodont Pachygenelus and in several Jurassic and Cretaceous mammals. We propose that appearance of a prism sheath transforms SCE into PPE and that reduction and loss of a prism sheath reverse PPE into SCE, in both phylogeny and ontogeny. We further propose that no amniote vertebrates other than the trithelodontid cynodont, Pachygenelus, plus Mammalia have ever evolved an ameloblastic Tomes process capable of secreting PPE and that the genetic potential to secrete PPE is a synapomorphy of Pachygenelus plus Mammalia, whether or not all lineages of the clade have expressed that potential.     

Updating and Recoding Enamel Microstructure in Mesozoic Mammals: In Search of Discrete Characters for Phylogenetic Reconstruction

The previously unknown enamel microstructure of a variety of Mesozoic and Paleogene mammals ranging from monotremes and docodonts to therians is described and characterized here. The novel information is used to explore the structural diversity of enamel in early mammals and to explore the impact of the new information for systematics. It is presently unclear whether enamel prisms arose several times during mammalian evolution or arose only once with several reversals to prismless structure. At least two undisputed reversions or simplifications are known—in the monotreme clade from Obdurodon to Ornithorhynchus (via Monotrematum?), and (perhaps more than once) within the clade from archaeocete to a variety of odontocete whales. Similarly, both prismatic and nonprismatic enamel is present among docodonts. Seven discrete characters showing enough morphological diversity to be of potential importance in phylogenetic reconstructions may be identified as a more appropriate summary of enamel microstructural diversity among mammaliaforms than the single character “prismatic enamel-present/absent” employed in recent matrices. Inclusion of five of these characters in the matrix of Luo et al. (2002) modifies the original topology by collapsing several nodes involving triconodonts and other nontribosphenic taxa. There is considerable support for prismatic enamel as a synapomorphy of trithelodonts plus Mammaliamorpha, and multituberculates appear to have small or “normal” sized prisms as the ancestral condition, with some (as yet) enigmatic changes to nonprismatic structure in some basal members of the group and the appearance of “gigantoprismatic” structure as an autapomorphic state of less inclusive clades. Other potential qualitative characters and the need for attaining appropriate methods to incorporate quantitative features may be important for future analyses.     

Influence of shell morphometry,microstructure, and thermal conductivity on thermoregulation in two tropical intertidal snails

Tropical intertidal organisms tolerate large fluctuations in temperature and high desiccation rates when exposed during low tide. In order to withstand the short‐term heat stress, intertidal organisms adopt behavioral responses to maximize their survival. Our previous research showed that tropical littorinids found at the upper and lower intertidal shores in Singapore exhibited different behavioral adaptations during low tide. Most of the upper‐shore Echinolittorina malaccana kept a flat orientation, with the aperture against the substrate and the long axis of the shell towards the sun, whereas a majority of the lower‐shore individuals of Echinolittorina vidua stood with the edge of the aperture perpendicular to the substrate on the rocky shore during low tide. This prompted analyses of the shells of these two species to determine whether the differences in the shell morphometry, microstructure, and thermal conductivity of shells of E. malaccana and E. vidua were associated with their respective behavioral responses to thermal stress. Analyses of shell morphometry and thermal conductivity showed that shells of E. malaccana were more likely to minimize heat gain, despite having a higher thermal conductivity on the outer surface, due to their light‐gray, elongated shell. By contrast, the dark‐colored, globose shells of E. vidua probably gain heat more readily through solar radiation. Scanning electron microscopy images of the shells of both littorinid species further revealed that they have cross‐lamellar structure; however, only individuals of E. vidua showed the presence of disjointed rod layers and a pigmented inner shell surface. Individuals of E. malaccana had a rough outer shell surface with holes that inter‐connect to form water‐trapping channels that probably aid cooling. Individuals of E. vidua, however, had a smooth outer surface with rows of kidney‐shaped depressions as microsculptures which probably help to stabilize shell shape. In both Echinolittorina species, behavioral responses were used to overcome thermal stress during low tide that was associated with shell morphometry and shell thermal conductivity. Such combined adaptations increase survivability of the littorinids at their respective tidal levels.     

In situ chemical speciation of sulfur in calcitic biominerals and the simple prism concept

The microstructure and composition of two mollusc shells were investigated using a combination of light microscopy, SEM, EPMA, and XANES. The shells of Pinna and Pinctada are composed of calcite prisms separated by organic walls. The prismatic units of Pinna are monocrystalline, and those of Pinctada are polycrystalline with internal organic radial membranes. High-spatial-resolution XANES maps for the different S species across adjacent prisms show that sulfate is the principal component in both the intraprismatic organic matrices and the outer membranes. Additionally, these maps confirm that the inner structures of the prismatic units are different for both genera. In many ways, the prisms of Pinna and Pinctada are different and invalidate the "simple prism" concept.     

Post-larval and larval shells ofJuranomia Fürsich & Werner 1989, andAnomia Linnaeus 1758 (Anomüdae, Bivalvia)

Post-larval and larval shells ofJuranomia calcibyssata from the Bathonian to Callovian of Poland and RecentAnomia membranacea from the Mediterranean are described and compared to other fossil and Recent members of the family Anomüdae. The stratigraphic range of the monospecific genusJuranomia, which, up to now, was only known from the Kimmeridgian, can be extended to include the Lower Bathonian. The state of preservation of the fossil species allows recognition of an internal aragonitic, branching and complex cross-lamellar shell layer in the post-larval left valve, which previous studies could only assume to be present.Anomia membranacea is a member of theA. ephippium lineage as proved, among other characters, by the presence of an outer calcitic prismatic layer in its right valve. It possesses an anterior pedal retractor in the left valve, which, in the original discussion of the phylogenetic affinities ofJuranomia, was thought to be lacking in species ofAnomia. Consequently, the generaJuranomia andAnomia only differ in two important shell characters: closeness or distance between the three central muscles and thickness of the inner aragonitic shell layer of the left valve. Larval shells ofJuranomia are similar to those of Recent anomiids in shape, size, the presence of a byssal notch in the right valve, and an external sinus and internal shelly process in the left valve. The last three features are parts of a single character which is considered as an autapomorphy of the stem species of the Anomiiudae. The small P I size ofJuranomia calcibyssata suggests a purely planktic-planktotrophic development and thus, high potential of dispersal, just as its modern counterparts. Irrespective of the general similarity in shell size, the mean dimensions of the P II are likely species-specific.     

Imbricate radial sculpture: a convergent feature within externally shelled cephalopods

Radial sculptural elements (e.g. ribs, lirae), formed by imbrication of two succeeding shell lamellae are found in members of both the Nautiloidea (Cymatoceras) and Ammonoidea (Phylloceratinae and Aspidoceratinae). Their formation involves periodic cessation of shell growth due to weak to moderate withdrawal of the shell secreting mantle. The radial lirae (0.5–1.5 mm in width) of Phylloceratinae and Aspidoceratinae (Aspidoceras and Pseudowaagenia) are created by the succession of sigmoid lamellae of the organic periostracum or of the outer prismatic layer, respectively. Each lira has a characteristic adorally‐projecting, scythe‐like appendage, arising from its crest. The prismatic lirae of Aspidoceras and Pseudowaagenia are analogous to the larger scaled pseudoribs of Cymatoceras. Garland‐like lamellae of the outer prismatic layer form the radial lirae of Mirosphinctes and Epaspidoceras (Aspidoceratinae), but these lack a conspicuous, projecting scythe‐like appendage. Additional prismatic cement is formed within adoral, oval hollow spaces of scythe‐appendage‐bearing lirae, either through diagenetic crystal growth, remote biomineralization or as a component of the dorsal shell. In Aspidoceratinae these prismatic infillings are replaced by a continuous herringbone layer, accompanied by a reduction of the lirae.     

How did the carrier shell Xenophora crispa (König, 1825) build its shell? Evidence from the Recent and fossil record

The genus Xenophora comprises species of marine gastropods (Cretaceous-Recent) able to add fragments of various origins to their shell surface. Agglutination potentials vary, from species lacking attachments to species completely covered by agglutinated materials, as in the Mediterranean species Xenophora crispa. Here, we analyse Recent and fossil specimens of Xenophora crispa from the Mediterranean area using SEM and XRD, to better understand their biomineralization patterns and the mechanisms leading to the agglutination of shells, bioclasts and lithoclasts, and their evolution in time. We also provide new data on poorly studied gastropod shell microstructures. We conclude that: (1) most of the Xenophora crispa shell consists of an aragonitic crossed lamellar fabric, but fibrous to spherulitic prismatic fabrics, seemingly of calcite, have been found in the columella and peripheral edge (the thickest parts of the shell); (2) attachment of objects is mediated by a prismatic microstructure, indicating that this may be the most functional fabric in attachment areas in molluscs; and (3) the functional purpose of the agglutination in Xenophora crispa may be related to a snowshoe strategy to successfully colonize muddy substrates, coupled with tactile and olfactory camouflage. Indeed, this species secretes in the columella and peripheral edge a less dense and a more organic rich calcitic fabric, possibly to lighten the shell thickest parts in order not to sink in soft sediments and to facilitate the shell raising from the substrate to create a protected feeding area. This behaviour seems to have been maintained by X. crispa over 2 My time span.     

The Paleozoic evolution of the gastropod larval shell: larval armor and tight coiling as a result of predation‐driven heterochronic character displacement

Early and middle Paleozoic gastropod protoconchs generally differ strongly from their corresponding adult morphologies, that is, most known protoconchs are smooth and openly coiled, whereas the majority of adult shells are ornamented and tightly coiled. In contrast, larval and adult shells of late Paleozoic gastropods with planktotrophic larval development (Caenogastropoda, Neritimorpha) commonly resemble each other in shape and principle ornamentation. This is surprising because habitat and mode of life of planktonic larvae and benthic adults differ strongly from each other. Generally, late Paleozoic to Recent protoconchs are tightly coiled. This modern type of larval shell resembles the adult shell morphology and was obviously predisplaced onto the larval stage during the middle Paleozoic. The oldest known planktonic‐armored (strongly ornamented) larval shells are known from the late Paleozoic. However, smooth larval shells are also common among the studied late Paleozoic gastropods. The appearance of larval armor at the beginning of the late Paleozoic could reflect an increase of predation pressure in the plankton. Although there are counter examples in which larval and adult shell morphology differ strongly from each other, there is statistical evidence for a heterochronic predisplacement of adult characters onto the larval stage. Larval and adult shells are built in the same way, by accretionary secretion at the mantle edge. It is likely that the same underlying gene expression is responsible for that. If so, similarities of larval and adult shell may be explained by gene sharing, whereas differences may be due to different (planktic vs. benthic life) epigenetic patterns.     

Evolutionary relationships among American mud crabs (Crustacea: Decapoda: Brachyura: Xanthoidea) inferred from nuclear and mitochondrial markers,with comments on adult morphology

Members of the brachyuran crab superfamily Xanthoidea sensu Ng, Guinot & Davie (2008) are a morphologically and ecologically diverse assemblage encompassing more than 780 nominal species. On the basis of morphology, Xanthoidea is presently regarded to represent three families: Xanthidae, Pseudorhombilidae, and Panopeidae. However, few studies have examined this superfamily using modern phylogenetic methods, despite the ecological and economic importance of this large, poorly understood group. In this study we examine phylogenetic relationships within the superfamily Xanthoidea using three mitochondrial markers, 12S rRNA, 16S rRNA, and cytochrome oxidase I (COI), and three nuclear markers, 18S rRNA, enolase (ENO) and histone H3 (H3). Bayesian and maximum‐likelihood analyses indicate that the superfamily Xanthoidea is monophyletic; however, the families Xanthidae, Panopeidae, and Pseudorhombilidae, as defined by Ng et al., are not, and their representative memberships must be redefined. To this end, some relevant morphological characters are discussed. © 2013 The Linnean Society of London     

CONVERGENT SHELL MORPHOLOGY IN INTERTIDAL GASTROPODS

The functional morphology of shell infrastructure in 2 speciesof intertidal trochid was compared with that in 2 species ofnerite. The shell of Monodonta constrictais typical of the majorityof trochids. The shell is composed of 4 layers: a distal layer(calcite), anouter prismatic layer (aragonite), a nacreous layer(aragonite), and an oblique prismatic layer (aragonite). Monodontalabio lacks a distal layer and an oblique prismatic layer. Theoblique prismatic layer is replaced by an inner prismatic layerwhich forms an apertural ridge as a result of deposition andresorption. The shells of Nerita versicolor and N. tessellataconsistof 3 layers: an outer prismatic layer (calcite), a crossedlamellar layer (aragonite), and a complex crossed lamellar layer(aragonite). The complex crossed lamellar layer is covered withinclined platelets which superficially resemble the surfaceof the ique prismatic layer of trochids. In addition, the complexcrossed lamellar layer forms an apertural ridge which is similarin appearance to that of Monodonta labio. The outer surfaceof the mantle of Nerita versicolor and N. tessellata is throwninto a series of large folds which lie in contact with the inclinedplatelets of the omplex crossed lamellar layer. The interactionof the mantle folds with the inclined platelets is thought toserve as a rachet mechanism to aid in extension of themantle;a similar function has previously been proposed for trochids.The apertural ridges of Monodonta labio and Nerita are thoughtto prevent excessive desiccation when these gastropodsare exposedat low tide. ¹Contribution No. 56 of the Tallahassee, Sopchoppy & GulfCoast Marine Biological Association (Received 6 July 1979;