Coral-rudist bioconstructions in the Upper Cretaceous Haidach section (Gosau Group; Northern Calcareous Alps, Austria)

Summary In the area of Haidach (Northern Calcareous Alps, Austria), coral-rudist mounds, rudist biostromes, and bioclastic limestones and marls constitute an Upper Cretaceous shelf succession approximately 100 meters thick. The succession is part of the mixed siliciclasticcarbonate Gosau Group that was deposited at the northern margin of the Austroalpine microplate. In its lower part, the carbonate succession at Haidach comprises two stratal packages that each consists, from bottom to top, of a coral-rudist mound capped by a rudist biostrome which, in turn, is overlain by bioclastic limestones and, locally, marls. The coral-rudist mounds consist mainly of floatstones. The coral assemblage is dominated by Fungiina, Astreoina, Heterocoeniina andAgathelia asperella (stylinina). From the rudists, elevators (Vaccinites spp., radiolitids) and recumbents (Plagioptychus) are present. Calcareous sponges, sclerosponges, and octocorals are subordinate. The elevator rudists commonly are small; they settled on branched corals, coral heads, on rudists, and on biolastic debris. The rudists, in turn, provided settlement sites for corals. Predominantly plocoid and thamnasteroid coral growth forms indicate soft substrata and high sedimentation rates. The mounds were episodically smothered by carbonate mud. Many corals and rudists are coated by thick and diverse encrustations that indicate high nutrient level and/or turbid waters. The coral-rudist mounds are capped byVaccinites biostromes up to 5 m thick. The establishment of these biostromes may result from unfavourable environmental conditions for corals, coupled with the potential of the elevator rudists for effective substrate colonization. TheVaccinites biostromes are locally topped by a thin radiolitid biostrome. The biostromes, in turn, are overlain by bioclastic limestones; these are arranged in stratal packages that were deposited from carbonate sand bodies. Approximately midsection, an interval of marls with abundantPhelopteria is present. These marls were deposited in a quiet lagoonal area where meadows of sea grass or algae, coupled with an elevated nutrient level, triggered the mass occurrence ofPhelopteria. The upper part of the Haidach section consists of stratal packages that each is composed of a rudist biostrome overlain by bioclastic wackestones to packstones with diverse smaller benthic foraminifera and calcareous green algae. The biostromes are either built by radiolitids,Vaccinites, andPleurocora, or consist exclusively of radiolitids (mainlyRadiolites). Both the biostromes and the bioclastic limestones were deposited in a low-energy lagoonal environment that was punctuated by high-energy events.In situ-rudist fabrics typically have a matrix of mudstone to rudistclastic wackestone; other biogens (incl. smaller benthic foraminifera) are absent or very rare. The matrix of rudist fabrics that indicate episodic destruction by high-energy events contain a fossil assemblage similar to the vertically associated bioclastic limestones. Substrata colonized by rudists thus were unfavourable at least for smaller benthic foraminifera. The described succession was deposited on a gently inclined shelf segment, where coral-rudist mounds and hippuritid biostromes were separated by a belt of bioclastic sand bodies from a lagoon with radiolitid biostromes. The mounds document that corals and Late Cretaceous elevator rudists may co-occur in close association. On the scale of the entire succession, however, mainly as a result of the wide ecologic range of the rudists relative to corals, the coral-dominated mounds and the rudist biostromes are vertically separated.     

Upper Cretaceous rudist biostratigraphy of the Bey Da?lar? Carbonate Platform, Western Taurides, SW Turkey

The Upper Cretaceous (Middle Cenomanian-Coniacian) successions of the Bey Da?lar? Carbonate Platform (Western Taurides, SW Turkey) are represented by rudist-bearing shallow-water limestones. Four rudist lithosomes are distinguished for the first time from the Eastern, Northern and Southern Areas of the Bey Da?lar? Autochthon. The oldest rudist assemblages dominated by caprinids are observed in the Eastern (Katran Da?) Area (caprinid lithosomes) and suggest a Middle-Late Cenomanian age. The uppermost part of the platform carbonates in the Northern Area is characterized by an association of hippuritid and radiolitid rudist bivalves dominated by Vaccinites praegiganteus (Toucas) (hippuritid lithosomes). The rudist fauna indicates the Late Turonian age, which is confirmed by the previously obtained ⁸⁷Sr/⁸⁶Sr values of well-preserved low-Mg calcite of Vaccinites praegiganteus (Toucas) shells. The rudist associations of the Southern (Susuzda?) Area are represented by two rudist formations. The lower lithosomes are mainly made up of hippuritids and radiolitids (hippuritid-radiolitid lithosomes). The stratigraphical distributions of the species of the assemblage indicate a Santonian-Early Campanian age. The rudist associations of the upper lithosomes are dominated by species of Joufia and Gorjanovicia (Joufia-Gorjanovicia lithosomes), which suggest a Late Campanian-Maastrichtian age. Identification of the rudist lithosomes yields information on the palaeobiogeographic distribution of the rudist species in the eastern Mediterranean region and also on the biostratigraphic frame of the Upper Cretaceous successions of the Bey Da?lar? Carbonate Platform.     

Taxonomical and biostratigraphical significance of the North African radiolitid rudist bivalve Praeradiolites biskraensis (Coquand, 1880)

Abstract: The revision of the radiolitid rudist bivalve Praeradiolites biskraensis (Coquand), including shell morphology and structure, taxonomical status, and palaeogeographical and stratigraphical distribution, was undertaken. We studied Coquand’s collection, other specimens from the type locality, Col de Sfa, and other Algerian fossil localities, as well as recently collected material from the Gafsa region in Tunisia. A neotype from Col de Sfa is proposed. The stratigraphical distribution of the species is bracketed in the Upper Cenomanian, using the distribution of co‐occurring ammonites in Tunisia and microfossils in Algeria. The palaeogeographical distribution is verified for Algeria and Tunisia based on all records in North Africa. Survey of the taxonomical status and problems of related radiolitid genera has resulted in revised diagnoses of Praeradiolites Douvillé, Eoradiolites Douvillé, Sphaerulites Lamarck and Radiolites Lamarck and revealed problems with Radiolites fleuriaui d’Orbigny, which is being the type species of Praeradiolites. Maghrebites gen. nov. is proposed for the North African radiolitid rudist Praeradiolites biskraensis (Coquand).     

Distribution of pigments in the shell of the gastropod <Emphasis Type="Italic">Littorina obtusata</Emphasis> (Linnaeus, 1758)

The patterns of pigment distribution in the shell of the gastropod Littorina obtusata were studied. It was determined that the background coloration of the periwinkle shell resulted from the coloration interaction of the external and internal parts of the ostracum. The coloration of the latter can be monochromatic or twolayer. The external and/or internal zones of the ostracum can be colored purple, white, orange, yellow and greenish. Only 11 of 25 possible color combinations were recorded. Four additional versions of shell background coloration resulted from partial depigmentation of external or internal parts of ostracum. The mottling pattern corresponded to white or depigmented lenticular sites in the external part of the ostracum. The bands on the shell may be subdivided into two groups according to their means of formation. The bands that appeared due to redistribution of pigments forming background coloration belonged to the first group, the bands arising as a layer of additional (white, yellow, orange) pigment belonged to the second group. Various band versions could occur simultaneously in the same individual.     

The locomotion system of Mesozoic Coleoidea (Cephalopoda) and its phylogenetic significance

A morphological comparison of shell‐muscle contacts in coleoid cephalopods mainly from the Early Jurassic (Toarcian) Posidonia Shales of Holzmaden (Germany), the Middle Jurassic (Callovian) Oxford Clay of Christian Malford (UK), Late Jurassic (Kimmeridgian‐Tithonian) plattenkalks of Solnhofen (Germany), and the Late Cretaceous (Cenomanian) of Hâdjoula and Hâkel (Lebanon) provides new and meaningful insights into their locomotion systems. The study shows that both pro‐ostracum‐ and gladius‐bearing coleoids are typified by a marginal mantle attachment and by distinctly separated fins, which usually insert (indirectly via the shell sac and basal fin cartilages) to posterior shell parts. While absent in gladius‐bearing forms, mantle‐locking cartilages might have existed already in pro‐ostracum‐bearing belemnoids. Similar to ectocochleate ancestors, funnel‐ and cephalic retractors are generally attached to the internal (ventral) shell surface. A comparison of Mesozoic and Recent gladius‐bearing coleoids shows that the locomotion system (most significantly the dorsal mantle configuration, and the presence of nuchal‐ and funnel‐locking cartilages) is fundamentally different. This does not support the concept of ‘fossil teuthids’, but suggests, owing to similarities with Recent Vampyroteuthis, placement of Mesozoic gladius‐bearing coleoids within the Octobrachia (Octopoda + Vampyromorpha). Classification of Mesozoic gladius‐bearing coleoids as octobrachians implies that: (1) unambiguous teuthids are still unknown in the fossil record and (2) the similarity between Recent and some fossil gladiuses represents a matter of homoplasy.     

Taxonomy and paleobiogeographic implication of Glabrobournonia Morris and Skelton (Hippuritida,Radiolitidae) from the Late Cretaceous Yigeziya Formation,southwestern Tarim Basin

A re-examination of the specimens that were identified as Biradiolites minor Pojarkova from the late Campanian to early Maastrichtian middle member of the Yigeziya Formation of southwestern Tarim Basin reveals that they should be assigned to the genus Glabrobournonia Morris and Skelton. Glabrobournonia is a genus of radiolitids characterized by indented radial bands, salient ridges on the shell margins and absence of fine ribs on the surface of the right valve. Apart from southwestern Tarim Basin, Glabrobournonia minor (Pojarkova) has also been recorded from the late Campanian of Fergana and Alai basins. The central Asian, late Campanian to early Maastrichtian G. minor differs from the late Campanian to Maastrichtian, eastern Arabian type species Glabrobournonia arabica Morris and Skelton in the flat left valve and an additional fourth ridge on the junction of the dorsal and posterior sides of the right valve. Biradiolites ingens (Des Moulins) could be the direct ancestor of Glabrobournonia. The paleogeographic distribution of Glabrobournonia suggests that this genus dispersed to central Asia from the late Campanian time, becoming widely distributed in the eastern Tethyan region rather than endemic to eastern Arabia. Correspondingly, specimens belonging to Gyropleura yielded from the same bed as G. minor in southwestern Tarim Basin, are similar to the specimens which were attributed to the eastern Arabian Gyropleura sp.; Campanian to early Maastrichtian Osculigera specimens described from the Yigeziya Formation are comparable with those known from the Campanian–Maastrichtian of Iran, Afghanistan and eastern Arabia. The similarity of the rudist assemblages between central Asia and eastern Arabia suggests a faunal connection and affinity between the north and south margins of the eastern Tethyan realm during Campanian to early Maastrichtian times.     

Evolutionary and Diagenetic Changes in the Chemico-structure of the Shell of Cranioid Brachiopods

The laminar dorsal valve of living Neocrania consists of: a primary layer of rhombohedral tablets, composed of granular calcite and commonly forming slats orthogonal to the margin, associated with polysaccharides and a fibrous 60 kDa protein; and a secondary layer of spirally growing (10.4) rhombohedra, doped with the 60 kDa protein and interleaved with membranes of a fibrous 44 kDa protein. The ventral valve consists exclusively of a primary layer with the same composition and basic structure as that of the dorsal valve. Investigation of selected antecedents shows that the chemico-structure of the Neocrania shell has been virtually unchanged since the first appearance of the stock in the Early Ordovician (Arenig). The greatest phylogenetic change affected the ventral valve that varied, even in Ordovician genera, from a film of calcitic blades to a complete succession of primary and secondary laminae. The most profound diagenetic changes occurred before the Late Cretaceous with proteins degrading into peptides that were dispersed, with the loss of less stable amino acids, during laminar recrystallization. Palaeozoic shells suffered further recrystallization but, even after pressure solution, the original laminar fabric was replicated long after it had lost its constraining organic membranes.     

Palaeobiology of a Biradiolites mooretownensis rudist lithosome: seasonality,reproductive cyclicity and population dynamics

Hennhöfer, D., Götz, S. & Mitchell, S.F. 2012: Palaeobiology of a Biradiolites mooretownensis rudist lithosome: seasonality, reproductive cyclicity and population dynamics. Lethaia, Vol. 45, pp. 450–461. During the Cretaceous, rudist bivalves were among the most important benthic carbonate producers on tropical to sub‐tropical carbonate platforms. Yet questions concerning the biology of rudist bouquets remain unanswered to a great extent. In this study a monospecific bouquet of the small radiolitid rudist Biradiolites mooretownensis has been evaluated from a palaeobiological angle. Three‐dimensional, high‐resolution, quantitative analysis provides a detailed evaluation of growth and reproduction in an in situ rudist association. A total of 1237 consecutive tomograms with a vertical spacing of 0.1 mm were produced of which 1150 have been digitally measured for total area, number of specimens, packing density, spat density, recruitment, survival time, mortality and accommodation space. The results show constant coverage of about 60%, a stable packing density of 3.2 specimens per cm² and constant reproduction throughout the bouquet. Time series analysis (spectral analysis) using PAST statistical software shows cyclic spat density every 14.9 mm of vertical growth. Combined with the results of the δ¹⁸O isotope analysis (showing cyclicities of 14 mm) one reproduction cycle appears to be annual. 46.4% of all counted specimens died before 3 mm of vertical growth. More than 93% of the initial spat does not exceed 15 mm shell height or 1 year respectively. Two mortality peaks in the juvenile’s life at 4 and 10–15 mm shell height either represent important obstacles in the ontogenetic development of the species or reflect external influences. □ Biradiolites, grinding tomography, palaeobiology, population dynamics, reproduction, rudists, sclerochronology, seasonality.     

Endosymbiotic algae in rudists?

The supposition that rudists may have lived in association with endosymbiotic zooxanthellae has been tested by comparing the shell wall structure of the radiolitid genus Osculigera with the recent bivalve Corculum. It is concluded that the wall structure of the upper valve could have been able to provide endosymbiotic algae in the mantle with light.     

Shell structure of the Early Cretaceous (Berriasian-Barremian) Rhynchonellids from Dagestan

The shell structure of five rhynchonellid species from the Lower Cretaceous of Dagestan was studied for the first time. The primary layer is rarely preserved in fossil rhynchonellids. Most of the species studied have a primary layer composed of prismatic calcite, one species has a finely granulated primary layer. The secondary fibrous layer consists of crossing bundles of fibers the (throughout shell thickness) and parallel bundles of fibers on the valve bottom.     

Succession of rudistid lithosomes along the western coastal margin of the Iberian Basin (Coniacian,Castrojimeno Section,central Spain)

Rudistid lithosomes cropping out near Castrojimeno, at the northern margin of the Central System in north-central Spain, provide detailed information on their composition and structure, on their development and succession, and about their relationship with the Coniacian sequence stratigraphy framework of the Iberian Basin. Most rudist assemblages are oligospecific, with a dominant species, or monospecific. The radiolitids Biradiolites angulosus, Praeradiolites requieni, and Radiolites sauvagesi and the hippuritids Hippurites incisus and Vaccinites giganteus were identified. Radiolitids demonstrate wide intraspecific morphological variability. The following Riding’s structural categories of organic reefs are represented: segment reefs, spaced and close cluster reefs, and close cluster/frame reefs. Bioclastic beds of reworked rudist fragments occur below or in between the rudist reefs. The vertical succession of all five types of rudistid lithosomes distinguished evidences a shallowing-upward trend. Rudistid lithosomes developed on the coastal margin during the superposition of the highstand sea-level stage of third- and fourth-order depositional sequences.     

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.     

Development of rudist lithosomes in the Coniacian–Lower Campanian carbonate shelves of central-southern Italy: high-energy vs low-energy settings

The Late Cretaceous shallow-water depositional areas of southern Tethys were complexes of unprotected shelves occupied by foramol assemblages that produced loose, diagenetically stable bioclastic debris not involved in significant in situ cementation processes. Both storm- and wind-induced currents and waves exercised a strong control on the distribution of the shifting biogenic sediments which covered the open sea-floor, constituting large coalescing sheets of winnowed fine to coarse skeletal sands. Rudists spread over all shelf sectors, from more open and external areas to more internal ones, occupying different substrata and furnishing the bulk of the skeletal component by means of bioerosion processes. They colonised mobile sediments giving rise to complex bodies with peculiar characteristics related to environmental parameters of the different sectors of the shelf. On the basis of detailed sedimentological, taphonomic and palaeontological data, we recognised two main rudist-rich depositional settings (‘end members’) in the southern Italy Senonian rudist-bearing successions. In successions pertaining to hypothesised marginal shelf sectors, characterised by high-energy regime deposits, rudist lithosomes are metric in thickness and lateral extent and lens-like in morphology, rich in bioerosion-derived skeletal sand and silt. Rudists are highly diversified. Large elongated cylindro-conical hippuritids (mostly pertaining to the genera Hippurites and Vaccinites), thick-shelled radiolitids and plagioptychids largely dominate. Rudists clustered in life position are subordinate; they often form small bouquets. More commonly these organisms appear fallen but only barely reworked. The rudist-rich bodies laterally pass into clean bioclastic grainstone in which sedimentary structures, related to current and/or storm erosional action, are common. No evidence of significant original relief of the rudist bodies in respect of the neighbouring sediment can be recognised. The submarine erosion and/or the high-energy processes operating presumably inhibited the aggradation of the tidal sediments above the marginal ones. As a consequence the vertical facies organisation shows widespread subtidal cycles, as commonly recognised in open shelves with ramp-like morphologies. In successions pertaining to more internal and/or low-energy sectors, rudist-rich beds rhythmically alternate with finer-grained foraminiferal limestones. Small elevator radiolitids with oligospecific diversity are dominant, mostly concentrated in clumps. Rudists in growth position are abundant, although a large quantity of shells appear toppled with little reworking. They may form laterally continuous biostromal shell beds. Sedimentary structures such as cross-lamination and gradation are only occasionally present. The resulting facies are commonly arranged in peritidal/shallow subtidal cycles in which evidence of subaerial (up to pedogenic modifications on a large and small scale) and, less frequently, submarine exposure is common. Intermediate successions have been recognised, characterised by deposits of silty-sand plains, which present intercalations of graded, bioclastic, storm-related beds. Sedimentological characteristics seem to document more open conditions in which submarine erosion was intermittently prevalent. In these successions rudist species that are commonly found both in high-energy and low-energy assemblages coexist.     

The original composition of the pro‐ostracum of an early Sinemurian belemnite from Belgium deduced from mode of fossilization and ultrastructure

Abstract: The pro‐ostracum of the early Sinemurian belemnite Nannobelus from the Belgian Province of Luxembourg is preserved as a thin, irregularly mineralized (phosphatized and pyritized), finely laminated structure, which is situated dorsally between the calcified rostrum and phragmocone. It has a median field with a criss‐cross pattern of bluntly pointed, curved growth lines and fine longitudinal ridges, as well as two lateral fields characterized by a fine ornament of closely spaced, longitudinal striae, each lateral field showing a narrow anterior belt‐like portion, the width of which equals about one‐third of that of the median field. Their posterior portion is remarkably asymmetrical, because its free margin (which does not about the median field) curves ventrally and the interspace between striae gradually increases here. The striation of the lateral field is formed by the longitudinally exposed narrow portions of succeeding, overlapping sublayers of the pro‐ostracum. Additionally, an internal sublayer with a silicified, honeycomb‐like structure is demonstrated in the pro‐ostracum. Based on microlamination that is comparable to that of the chitinous gladius in extant squids and on the irregular mineralization (unlike the rest of the shell), the pro‐ostracum is considered to have been originally mainly organic, containing an intermediate cartilaginous sublayer with a typical honeycomb‐like structure. The cartilaginous sublayer supposedly provided protection of the pro‐ostracum against fractures which might have resulted from regular contractions together with the muscular mantle during jet‐propulsion. Ultrastructural and chemical data on Nannobelus favour the interpretation of the pro‐ostracum as a novelty of the skeleton in coleoids rather than as a dorsal projection of the phragmocone wall.     

A novel extrapallial fluid protein controls the morphology of nacre lamellae in the pearl oyster, Pinctada fucata

Mollusk shell nacre is known for its superior mechanical properties and precisely controlled biomineralization process. However, the question of how mollusks control the morphology of nacre lamellae remains unresolved. Here, a novel 38-kDa extrapallial fluid (EPF) protein, named amorphous calcium carbonate-binding protein (ACCBP), may partially answer this question. Although sequence analysis indicated ACCBP is a member of the acetylcholine-binding protein family, it is actively involved in the shell mineralization process. In vitro, ACCBP can inhibit the growth of calcite and induce the formation of amorphous calcium carbonate. When ACCBP functions were restrained in vivo, the nacre lamellae grew in a screw-dislocation pattern, and low crystallinity CaCO(3) precipitated from the EPF. Crystal binding experiments further revealed that ACCBP could recognize different CaCO(3) crystal phases and crystal faces. With this capacity, ACCBP could modify the morphology of nacre lamellae by inhibiting the growth of undesired aragonite crystal faces and meanwhile maintain the stability of CaCO(3)-supersaturated body fluid by ceasing the nucleation and growth of calcite. Furthermore, the crystal growth inhibition capacity of ACCBP was proved to be directly related to its acetylcholine-binding site. Our results suggest that a "safeguard mechanism" of undesired crystal growth is necessary for shell microstructure formation.     

Upper cretaceous rudist and stromatoporid associations of Central Oman (Arabian Peninsula)

Summary Rudist and stromatoporid associations of the Campanian from Central Oman are nearly monospecific. They are dominated byDurania aff.nicholasi, Vaccinites vesiculosus, Torreites milanovici or phaceloid and massive stromatoporids. Several other rudist genera play a secondary role. The thickness of the associations is rarely more than one metre. Solitary corals do not occur in the associations. Colonial corals are less common, although they are up to 1 m high and show considerable diversity. There are no binders. The reef structure indicates variable hydrodynamic conditions. They are always associated with very shallow water. The pureDurania aff.nicholasi patches with large colonial corals andTorreites milanovici are presumably the most rigid structures. The near monospecific associations ofVaccinites vesiculosus are widely distributed. Although mostly preserved in situ, strong currents, presumably caused by tropical storms, have repeatedly impaired and interrupted growth. The specific growth characteristics of the shell of some rudists, especially the radiolitids, enable an estimation of the individual lifespan. Frameworks of approximately 1 metre thickness probably developed in ±100 years. The sediments of the complete sections are predominantly bioclastic.     

Cenomanian carbonate facies and rudists along shallow intraplatform basin margin-the island of Cres (Adriatic Sea, Croatia)

Summary The island of Cres is located in the northern part of the Adriatic Sea. The island is built up of predominantly Cretaceous carbonates deposited in north-western part of extensive and long-lasting Adriatic Carbonate Platform. Owing to the influence of synsedimentary tectonics supported by eustatic changes during the latest Albian/Early Cenomanian, different sedimentary environments were established: from shallow intraplatform basin and related slope, across basin margin to protected shallow-platform. During the Early to Middle Cenomanian rudist communities (ichthyosarcolitid/caprinid/radiolitid)flourished along a relatively high-energy intraplatform basin margin. Fair amounts of coarse-grained bioclasts, derived almost exclusively from broken rudist shells, were deposited over a marginal depocenter. Contemporaneously, pithonellid wackestone-packstones containing microbioclasts and planktonic foraminifera were deposited basinward while marginal bioclastic sediments and limestone blocks of the basin margin origin were sporadically deposited within the basin. The opening of the Cres intraplatform basin was aborted and the basin was finally filled up during the Late Cenomanian. Since the Cres intraplatform basin was established at the beginning of the Cenomanian it probably represented the initiation phase in the north-western extension of the later Adriatic Trough development.     

Carbonate facies evolution from the late albian to Middle Cenomanian in Southern Istria (Croatia): influence of synsedimentary tectonics and extensive organic carbonate production

Summary During the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (presentday Istria) specific depositional systems characterised by frequent lateral and vertical facies variations were established within a formerly homogeneous area, ranging from peritidal and barrier bars to the offshore-transition zone. In southern Istria this period is represented by the following succession: thin-bedded peritidal peloidal and stromatolitic limestones (Upper Albian); well-bedded foreshore to shoreface packstones/grainstones with synsedimentary dliding and slumping (Vraconian-lowermost Cenomanian); shoreface to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shoreface carbonate sand bodies (Lower Cenomanian); prograding rudist bioclastic subaqueous dunes (Lower to Middle Cenomanian); rudist biostromes (Lower to Middle Cenomanian), and high-energy rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian). Rapid changes of depositional systems near the Albian/Cenomanian transition in Istria are mainly the result of synsedimentary tectonics and the establishment of extensive rudist colonies producing enormous quantities of bioclastic material rather than the influence of eustatic changes. Tectonism is evidenced by the occurrence of sliding scars, slumps, small-scale synsedimentary faults and conspicuous bathymetric changes in formerly corresponding environments. Consequently, during the Early Cenomanian in the region of southern Istria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were characterised by high-energy environments with extensive rudist colonies, and high organic production leading to the progradation of bioclastic subaqueous dunes. This resulted in numerous shallowing- and coarsening-upwards clinostratified sequences completely infilling formerly deeper environments, and the final re-establishment of the shallow-water environments over the entire area during the Middle Cenomanian.     

Ultrastructure of the loop of terebratulide brachiopods

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

Structure,crystallography, and morphogenesis of the cryptic shell of the terrestrial slug Limax maximus (Mollusca,gastropoda)

The structure and crystallography of the internal shell of the pulmonate gastropod slug Limax maximus were studied at the levels of light and scanning electron microscopy, revealing patterns of shell ontogeny and morphogenesis. The calcified portion of the slightly convex ovoid shell is composed of a single palisade layer of calcitic crystals. Numerous projections, 100 μm in width at the dorsal tip, are found on the dorsal surface of the shell and coincide with local nucleation sites of primordial calcium salt deposition onto the periostracum. With continued calcification these projections coalesce ventrally, forming the single crystalline shell layer. The organic portion of the shell includes the periostracum and an extensive PAS-staining conchiolin. In EDTA-etched preparations, conchiolin appears as a spongy network of fibers throughout the shell. Both horizontal and vertical components of the conchiolin are present, the former of variable thickness and occurring in an intercrystalline manner, the latter always occurring normal to the horizontal set. Macromorphogenic growth is characterized by three distinct temporal stages. Primary growth occurs radially from the umbonal region. Secondary growth is synonymous with shell thickening. Tertiary growth is characterized by both a lateral component, in which the shell extends beyond the primary growth boundaries, and a ventral component, in which the shell continues to grow in thickness. SEM of the ventral shell surface reveals a pattern of growth at the crystalmatrix interface. Proteinaceous fibers of the conchiolin occur unidirectionally in horizontal rows. Zones of incipient calcitic crystallization onto these hypostracal fiber bundles are contrasted by zones of increasing crystallization until the fibrous template (reduced hypostracum) is completely covered by crystals.