Skeletal Organisms Research Articles

Origin and role of non-skeletal carbonate in coralligenous build-ups: new geobiological perspectives in biomineralization processes

Abstract. The coralligenous build-ups located on the Mediterranean shelf in front of Marzamemi (SE Sicily, Italy) represent useful natural examples to use in studying the relationship between skeletal organisms and non-skeletal components in marine bioconstructions. Coralligenous build-ups are formed in open marine systems, and their comparison with coeval bioconstructions (biostalactites) of confined environments, like submarine caves, allows depicting the complex interactions between metazoans and microbial communities in the formations of recent bioconstructions in different Mediterranean settings. In this study, two coralligenous build-ups were characterized in terms of organisms and sediments involved in their formation. The framework mainly consists of coralline algae and subordinate bryozoans and serpulids. Sponges affect the general morphology of the bioconstructions both interacting with skeletonized organisms and through bioerosion activity. The micrite or microcrystalline calcite is present in minor amounts compared to other components that form the build-ups and consists of two types: autochthonous (in situ) and allochthonous (detrital). Fine autochthonous micrite mineralized directly inside the framework cavities and shows aphanitic or peloidal fabric, produced by organomineralization processes of soft sponge tissues and microbial metabolic activity, respectively. The detrital micrite occurring inside cavities derives from external sources or erosion processes of the bioconstructions themselves. This component has been classified as organic or inorganic based on the organic matter contents deduced by UV epifluorescence. A great quantity of sponges live in cavities of the coralligenous build-ups and compete with carbonatogenic bacteria for the same cryptic spaces, limiting the production of microbialites. The sharing of a similar relationship between sponges and microbial communities by coralligenous concretion and biotic crusts of particular submarine caves suggests that this competition is not habitat-specific. On the contrary, it may develop in a range of environmental settings, from open to cryptic systems, and could be used to clarify the role of metazoans vs. microbialites in palaeoecological reconstructions.

Skeletal abundance of Upper Ordovician coral reefs, Lourdes Formation, western Newfoundland

Reefs experience profound evolutionary and ecological changes through the Early Paleozoic, with the rise and fall of the lower Cambrian archaeocyathan reef and the later advent of other sponge and coral reefs. The Upper Ordovician Lourdes Formation of the Long Point Group, western Newfoundland, preserves small coral bioherms in a series of fault blocks along Long Point on the Port au Port Peninsula. These bioherms, preserved in the Black Duck Member 3, contain tabulate corals, bryozoans, and stromatoporoids adding to their framework. The overlying Beach Point Member preserves deeper water, storm-dominated deposition, and bioherms are absent from this setting. An analysis of skeletal abundance through the point counting of thin sections from samples within and outside the reefs reveal a mosaic of skeletal abundance across these settings. The bioherms and associated channel deposits preserve the highest skeletal abundance whereas the settings just outside of the reefs and in facies overlying the reefs preserve notably lower skeletal abundance. This suggests that reef settings harbored a higher diversity and abundance of skeletonizing organisms, more so than environments adjacent to but outside of the reefs. Though variations in depositional processes can affect the abundance of skeletal material through physical processes, the storm-dominated facies of the Beach Point Member generally contain more abundant oncoids, peloids, and intraclasts than skeletal material. Even when examining broadly similar facies within and outside of reefs, this difference in animal abundance remains. Examining Upper Ordovician coral reefs of the Lourdes Formation reveals that the Great Ordovician Biodiversification Event resulted in an increase in skeletal abundance from their earlier reef counterparts, but that the abundance of skeletal animals is driven by particular environmental conditions in reef settings, such as supersaturation of CaCO3, wave activity fostering local increases in oxygen, and warm temperatures. In this way, reefs provided some of the most suitable places for skeletal organisms to thrive in the Early Paleozoic.

DNA repair-related genes are the key for stony coral ancestors to survive under elevated levels of UVR

DNA damage response (DDR) is a complicated network to defend against physical or chemical changes in DNA in all animals. Elevated levels of ultraviolet radiation (UVR) caused DNA damage, which was a reason for the mass extinction that occurred at the Devonian/Carboniferous (D/C) boundary approximately 359 million years ago (Ma). However, the molecular adaptation of the stony coral ancestors that strangely survived the D/C boundary mass extinction is not well understood. In the present study, the molecular clock analysis using fourfold degenerate sites of 1,463 homologous genes of different stony coral species (a representative group of marine organisms with calcareous skeletons) suggested that their common ancestors originated 384.24 Ma, i.e., slightly earlier than the D/C transition. We identified 21 rapidly evolving genes (REGs) and 49 positive selection genes (PSGs) that were significantly enriched in diverse pathways, including the mitotic cell cycle process, intracellular protein transport, and DNA synthesis involved in DNA repair. Interestingly, four REGs and 21 PSGs were significantly enriched in DDR pathways, including the mitotic cell cycle process, DNA synthesis involved in DNA repair, and cellular response to DNA damage stimulus pathways. We hypothesize that enriched DDR genes are likely involved in the enhanced ability of ancient stony corals to detect and repair DNA damage. For example, the DNA polymerase epsilon catalytic subunit (POLE) gene, which encodes the DNA polymerase epsilon catalytic subunit A, mediates interaction with the other three catalytic subunits through its nonenzymatic carboxy-terminal domain. POLE may potentially enhance the binding ability to other subunits to strengthen its function because eight positive selection sites were distributed in the C-terminal of POLE and on the surface of the simulated 3D protein model. Therefore, our results demonstrated for the first time that the precise transfer of DNA information may help stony coral ancestors survive in elevated levels of ultraviolet radiation, suggesting that DDR levels may be critical to the environmental adaptation of calcareous skeletal organisms during climate change.

Reduction in animal abundance and oxygen availability during and after the end-Triassic mass extinction.

The end-Triassic biodiversity crisis was one of the most severe mass extinctions in the history of animal life. However, the extent to which the loss of taxonomic diversity was coupled with a reduction in organismal abundance remains to be quantified. Further, the temporal relationship between organismal abundance and local marine redox conditions is lacking in carbonate sections. To address these questions, we measured skeletal grain abundance in shallow-marine limestones by point counting 293 thin sections from four stratigraphic sections across the Triassic/Jurassic boundary in the Lombardy Basin and Apennine Platform of western Tethys. Skeletal abundance decreased abruptly across the Triassic/Jurassic boundary in all stratigraphic sections. The abundance of skeletal organisms remained low throughout the lower-middle Hettangian strata and began to rebound during the late Hettangian and early Sinemurian. A two-way ANOVA indicates that sample age (p < .01, η2 =0.30) explains more of the variation in skeletal abundance than the depositional environment or paleobathymetry (p < .01, η2 =0.15). Measured I/Ca ratios, a proxy for local shallow-marine redox conditions, show this same pattern with the lowest I/Ca ratios occurring in the early Hettangian. The close correspondence between oceanic water column oxygen levels and skeletal abundance indicates a connection between redox conditions and benthic organismal abundance across the Triassic/Jurassic boundary. These findings indicate that the end-Triassic mass extinction reduced not only the biodiversity but also the carrying capacity for skeletal organisms in early Hettangian ecosystems, adding to evidence that mass extinction of species generally leads to mass rarity among survivors.

Deep-water fossil assemblages from the Ediacaran-Cambrian transition of western Hunan, South China and their biostratigraphic and evolutionary implications

The Ediacaran-Cambrian (E-C) transition was a critical stage for the origin and evolution of early complex life on Earth. Because studies of E-C biotas and environments are relatively fewer for deep-water than for shallow-water facies, we undertook a systematic paleontological analysis of the E-C transition in the deep-water Liuchapo Formation at Longbizui, South China. Two sequential but distinct assemblages were recovered, namely the Lower Liuchapo Assemblage (LLA) and the Upper Liuchapo Assemblage (ULA). The LLA consists mainly of soft-bodied fossils, including Longbizuiella hunanensis gen. et sp. nov., Nenoxites jishouensis sp. nov., Nenoxites irregularis sp. nov., Nenoxites sp., Helanoichnus sp., Shaanxilithes ? sp. and tiny-sized spherical fossils. The ULA is dominated by skeletal organisms comprising the small shelly faunas (SSFs, mainly protoconodonts, hyoliths and Cambroscleritida), sponge spicules, and the first reported Poratusiramus xiangxiensis gen. et sp. nov. Biostratigraphic correlation support a latest Ediacaran age for the LLA, whereas the majority of the ULA belongs to the early Cambrian—thus the change from the LLA to the ULA coincided with the E-C transition. Our results suggest that, during the E-C transition, a biological revolution may have occurred in deep-water settings synchronously with the well-studied faunal turnover in shallow-water environments. At Longbizui, fossil abundance and diversity decreased gradually during the latest Ediacaran and recovered stepwise after the E-C transition. The deep-water fauna, including both benthic and pelagic members, developed a biomineralization ability during the E-C transition. As calcareous and possibly phosphatic skeletal organisms began to flourish in shallow-water environments, siliceous skeleton-forming organisms, represented by the spicule-bearing sponges, colonized the deep seafloor in great abundance. In addition, the siliceous spicules documented in this study record fast evolutionary development during the early history of sponges, one of the earliest-branching metazoan phyla. The evolution of spicules from monaxon to diaxon-triaxon and then to tetraxon and polyactinal forms, as well as their considerable increase in diameter and length, was achieved during the first SSF biozone. • First detailed study of deep-water fossil assemblages of Ediacaran-Cambrian age. • Deep-water fauna may have experienced a turnover during the E -C transition. • Patterns of biodiversity and abundance paralleled those of shallow-marine habitats. • Record of diversification of earliest sponges and other siliceous biomineralizers. • Utility of evolutionary sequence of sponge spicule types as biostratigraphic index.

Development and collapse of the early Cambrian shallow-water carbonate factories in the Hannan-Micangshan area, South China

The development of carbonate factories has potential importance in studies of paleoenvironment, sedimentology, and reservoir geology, but the mechanisms controlling the growth and demise of the large amount of carbonate sedimentation have not been broadly investigated, especially in terms of deep-time sedimentary records. Here, we provide a high-resolution study of the development and collapse of a series of tropical shallow-water carbonate factories distributed in the Hannan-Micangshan area of the South China Craton during the early Cambrian (Age 3). Unlike modern tropical shallow-water factories, the major carbonate producers in the early Cambrian factories contained ooids, benthic microbial communities (predominantly calcified microbes), and symbiotic microbial-skeletal organisms (i.e., calcimicrobial–archaeocyath bioherms). The ooid factories were mostly distributed in the tidal sand complexes around the Hannan-Micangshan massif; and some of the mixed quartz-ooid deposits may have been remodeled by terrigenous sands sourced from the adjacent fluviodeltaic delivery. The microbial carbonate factory was primarily characterized by thrombolites in terms of the lithological composition, with a small contribution of stromatolites and oncolites (grain sizes mostly of 2–4 cm). The thrombolites formed not only mound architectures individually (length of ~1–5 m, and height of ~0.5–2.5 m), but also small- to large-scale bioherms associated with reef-building skeletal organisms (i.e., archaeocyaths). Nutrient conditions may have played an essential role in regulating the distributions of the microbial and microbial-skeletal factories (mesotrophic conditions), but they did not have a significant effect on the ooid factories. These tropical shallow-water carbonate factories were located in oxygenated aqueous environments with some small transient low-oxygen intervals that corresponded to barrier microbial and microbial-skeletal factories in the study area. Hydrodynamic condition and other controls (e.g., wind) may have played roles in shaping the factory morphology and distribution features in the study area. The chemical index of alteration (CIA; values of 60–65) and petrological features (occurrence of large-scale ooid sand complexes) support a warm and dry climate during the development of these tropical shallow-water factories. Finally, the transition from carbonate-dominated to siliciclastic-dominated depositional systems driven by wetter climate conditions (CIA, close to 70) may have been responsible for the cessation of the shallow-water carbonate factories in the Hannan-Micangshan area, which resulted in a rapid drowning, and subsequently, continuous input of a high volume of siliciclastics in late Age 3 to early Age 4 (Yanwangbian Formation). This study provides clues to understanding the growth and demise of deep-time tropical carbonate factories and serves as a valuable reference for studies of other regions and time periods. • We present a high-resolution study of the tropical shallow-water carbonate factories. • These factories contained ooids, calcified microbes, and skeletal organisms. • Nutrient conditions may have played a key role in regulating factory distributions. • Hydrodynamic condition and wind shaped the factory morphology and distribution. • The shift of the depositional system was likely responsible for factory collapse.

Reconstruction of tectonically disrupted carbonates through quantitative microfacies analyses: an example from the Middle Triassic of Southern Italy

The main goal of the paper is the reconstruction of a Middle Triassic buildup cropping out in the central part of the Southern Apennines. Middle Triassic reefs of the western Tethys realm are well known in the Northern and Southern Alps. In contrast, few studies of the Anisian–Ladinian carbonate platforms of the southern Apennines are available, due to the diagenetic alteration and tectonic disruption that hinder their paleoenvironmental and stratigraphic reconstruction. In an attempt to fill this gap, and to improve the knowledge on the Anisian–Ladinian carbonates of central Mediterranean area, this research is focused on a carbonate buildup cropping out in the “La Cerchiara” area, Sasso di Castalda (Basilicata, Southern Italy). The buildup, affected by intense tectonic deformation associated with the development of the Apennine thrust and fold belt, was studied using a statistical evaluation of the quantitative microfacies data. The research enabled a reconstruction of the original stratigraphic relationships of the various buildup fragments. A positive linear regression between the sample positions vs the percentage of autochthonous carbonates indicates an increase of the autochthons carbonate toward the top of the succession. The allochthonous fabrics (packstone/wackestone) at the base of the section (Unit IIIa) pass gradually upward into autochthonous (boundstones) facies (Units IIIb, I), consisting of microbialites (clotted peloidal micrite, microbial-derived laminae, and aphanitic micrite), microproblematica and cyanobacterial crusts, with few encrusting skeletal organisms. Statistical data suggest that units IIIa, IIIb, and I are in stratigraphic order while unit II appears to have been moved by tectonic dislocation from its original position at the base of the succession. The absence of metazoan reef framework, and the richness of micro-encrusters, autochthonous micrite and synsedimentary cements, suggest a mud-mound style of growth for the carbonate bodies of the Southern Apennine during the Anisian.

Facies analysis of the Upper Ordovician Xiazhen Formation, southeast China: Implications for carbonate platform development in South China prior to the onset of the Hirnantian glaciation

The Upper Ordovician Zhe-Gan Platform was a short-lived carbonate platform that formed near the northern margin of the Cathaysian Land of the South China Block. The Zhe-Gan Platform is less well understood than the adjacent Yangtze Platform due to the absence of detailed sedimentological study as well as the structural complexity of the area. The aim of this study was to reconstruct the Zhe-Gan Platform based on detailed facies analysis of the middle to upper Xiazhen Formation. A total of 24 shallowing-upward depositional cycles (C1–24) are identified and subdivided into three types based on the vertical association of facies: (1) mixed carbonate–clastic subtidal cycles; (2) carbonate subtidal cycles; and (3) peritidal-capped subtidal cycles. The reconstructed depositional model indicates a ramp-type, mixed siliciclastic–carbonate platform, generally similar to other Late Ordovician carbonate platforms characterised by dominant skeletal grains, less common ooids, and an absence of skeletal barrier reefs. The newly developed Late Ordovician ramp-type carbonate platforms could have been induced by the evolution of skeletal organisms and the accompanying palaeoceanographic changes prior to the Hirnantian glaciation. The complex palaeogeography of the South China Block would have influenced the co-occurrence of the Xiazhen carbonate platform and black shales of the adjacent Yangtze Platform on the same block during the Late Ordovician.

Cryptic growth strategies of the Cambrian coralCambroctoconus: flexible modes of budding and growth in immediate response to available space

AbstractThe Cambrian coralCambroctoconusoccurs selectively in the crypts of calcimicrobeEpiphytonreefs of the Zhangxia Formation (Miaolingian) in Shandong Province, North China. These cryptobionts preferentially grow laterally and/or downward, in some cases showing pendent growth from the ceilings of the reef framework. The upside‐down growth is clearly shown by the downward‐oriented aperture and the presence of fork‐like holdfasts at its base. The coral aggregations reveal two cryptic growth strategies: one involves the prevalence of far smaller individuals in vertically narrow spaces; the other involves the predominance of modular individuals in large cavernous spaces. Offset individuals frequently appear projecting downward and are connected to each other in a chain‐like fashion. Individuals often undergo rejuvenescence laterally and downward, when the growth directions may change. Therefore, the size and modularity of individuals and the direction of budding depend largely upon the cryptic microenvironments available at the time. The cryptobionts make the best use of vacant spaces by modifying the location of budding and altering their growth directions through rejuvenescence. The cryptic growth strategies found inCambroctoconusindicate a certain degree of morphological variability in the modules. More importantly, they indicate that the individuals flexibly and exquisitely utilized the microbial crypts that were still predominant even during the middle Cambrian. These modes of cryptic growth demonstrate the age‐specific exploitation of niches by sessile skeletal organisms that was facilitated by the development of a firm attachment structure.

Variations of redox conditions in the atmosphere and Yangtze Platform during the Ediacaran-Cambrian transition: Constraints from Cr isotopes and Ce anomalies

The Ediacaran-Cambrian transition was characterized by the evolution from Ediacaran soft-body to Cambrian skeletal organisms, and fluctuations of the marine carbon cycle. Marine redox changes should have played an essential role in both ecological and environmental evolutions; however, temporary changes and spatial distribution of redox conditions in the Ediacaran-Cambrian oceans remain elusive and highly debated. In this study, we analyzed chromium (Cr) isotope and cerium (Ce) anomaly profiles in carbonates from the Upper Ediacaran-Lower Cambrian Dengying and Zhujiaqing formations at the Xiaotan section, South China, in an attempt to provide new insights helping unravel Earth's surface redox changes during the Ediacaran-Cambrian transition. Our results show that these carbonates record generally positive δ53Crauth values (up to 1.15‰) with fluctuations and prominent negative Ce anomalies (Ce/Ce⁎ values down to 0.35), implying a broad increase in oxygenation in the atmosphere and the inner shelf of the Yangtze Platform during the Ediacaran-Cambrian transition. The first appearance and subsequent diversification of the Cambrian animals may have benefited from this oxygenation event. The pronounced fluctuations of δ53Crauth and δ13Ccarb values can be explained by episodic oxidation of a deep-water DOC pool in a stratified water column across the chemocline through upwelling, releasing light carbon isotopes and organic-matter-bound isotopically unfractionated Cr (III) into shallow waters.

APPLICATION OF RAMAN SPECTROSCOPY IN COMPARISON BETWEEN CRYPTIC MICROBIALITES OF RECENT MARINE CAVES AND TRIASSIC PATCH REEFS

ABSTRACTMicrobialites are common carbonate structures in cryptic niches of marine environments throughout geological time. In this research we compare the microbialites of small bioconstructions (biostalactites) of modern submarine caves of Sicily with those developed in small crypts of Carnian patch reefs of the Dolomite Mountains (Heiligkreuz Formation, Alpe di Specie) using Raman spectroscopy, a method that allows in situ determination of the organic content of microbial components. This methodology partly solves the uncertainty of geomicrobiological studies that use bulk measurements (i.e., biomarker analyses), which make it difficult to associate mineral precipitates with a specific microbial process. In the modern marine caves, the complex biotic relationships among skeletal organisms (mainly serpulids) and microbial communities produced biostalactites in which microbially induced biomineralization is the consequence of autotrophic and chemoheterotrophic bacterial activities. Sulfate-reducing bacteria, fed by metazoan organic matter, flourish in millimetric oxygen-depleted cavities of the skeletal framework, and induce autochthonous micrite deposition and early stabilization of the biostalactites. Similar processes have been interpreted to induce the deposition of the microbialites in the Upper Triassic patch reefs of the Dolomites. These small shallow water reefs, made up mainly of scleractinian corals, sponges and red algae, hold a skeletal framework rich in millimeter- to centimeter-size cavities, ideal cryptic niches for growth of microbial communities. Specific sulfate-reducing bacteria biomarkers are first identified using bulk measurements obtained by solvent extraction. The subsequent in situ characterization of organic compounds through micro-Raman spectroscopy indicates the same biogeochemical signatures of the microbialites within the cryptic cavities of the biostalactites of modern marine caves as those inside the skeletal framework of Carnian patch reefs. These data, showing the same processes in Triassic and modern cryptic microenvironments, is evidence that the microbially mediated precipitation in confined environments is a process independent of geological time that further investigation may be able to test.

Pathological Changes of Archaeocyathids

One of the unique groups of skeletal organisms of the lower Cambrian is the archaeocyathids, a group restricted to the early Cambrian and showing stages of their evolution. A longitudinal section through an archaeocyathid can show ontogenetic stages from a juvenile cup to the complete development of species characters. This feature allows the study of ontogenetic changes in the group, revealing various heterochronies and other patterns that can be directly linked to evolutionary reconstructions. Archaeocyaths from various regions include a large number of damaged and deformed cups. It is shown that deformations and pathological changes originating in the Tommotian (Nochoroicyathus sunnaginicus Zone) could be manifested in the general evolution of the group.

From rolling stones to rolling reefs: a Devonian example of highly diverse macroids

The upper carbonate member of the Aisemont Formation (Upper Frasnian, Belgium) yielded a surprisingly diverse assemblage of macroids varying in size, shape and composition, from simplest microbial oncoid to complex, polytaxic circumrotatory macroid. A descriptive classification combining composition and geometry of macroids is proposed. It suits for sedimentological and palaeobiological analyses. Five categories forming a continuum are distinguished as follows: (1) simple oncoids, consisting of microbial coating of various nuclei; (2) composite oncoids, involving several encrusting taxa and microbes-induced precipitates; (3) simple circumrotatory macroids, constructed by one or few different organisms and forming free-rolling particle; (4) polytaxic circumrotatory macroids, constructed by diverse organisms, usually in sequential coating and form free-rolling object; and, (5) polytaxic domed macroids, polytaxic assemblage of encrusting organism forming domed or columnar anchored bioconstructions. The dominant (in number and volume) encrusting organisms are the tabulate corals and stromatoporoids that both possessed the ability to attach to any hard substrate. Microbes also contributed to the formation of macroids although the volume they occupy is relatively limited. Skeletal invertebrates likely grew during the good season when environmental conditions were suitable for the growth, then partly or totally died and was colonized by microbes during the bad season. The surface of the macroid (after rolling or not) was subsequently re-coated with skeletal organisms when suitable conditions re-established. Final burial of the macroid occurred during the good season as their outermost surface is rarely coated with microbes. Geopetal sediment filling of cavities displays various directions indicating the rolling of the objects during their formation. Periodical rolling or tilting may be related to seasonal storms and the resulting assemblage formed genuine rolling reefs of crucial importance for birth palaeoecological and sedimentological analysis.

Cryptic serpulid-microbialite bioconstructions in the Kakoskali submarine cave (Cyprus, Eastern Mediterranean)

The biostalactites from the Kakoskali cave in Cyprus represent a new example of the complex biotic relationships between skeletal organisms and microbial communities in building bioconstructions of cryptic marine environments. Biostalactites are mainly constituted of polychaetes of the family Serpulidae and, to a lesser degree, foraminifers and bryozoans. Within the skeletal framework of these organisms, two types of microcrystalline calcite (micrite) have been recognized: autochthonous and detrital micrite. The autochthonous fraction is syndepositionally lithified and occurs as clotted peloidal and, subordinately, aphanitic (structureless) textures, suggesting the presence of heterotrophic microbial activities thriving on decaying metazoan organic matter. This fraction is limited to the protected portions of the bioconstructions, especially in the inner and lower parts. The presence of iron and manganesiferous oxidizing bacteria is suggested by the deposition of ferromanganesiferous crusts and Frutexites-like structures. These microbial-induced biomineralizations are the main evidence of carbonatogenetic and Fe–Mn, autotrophic and chemoheterotrophic, bacterial activities. The Kakoskali cave is frequently visited by divers who, during their immersions, resuspend the fine bottom sediment, which later covers the surface of the bioconstructions, disturbing the delicate equilibrium of the biotic association. This perturbation, which is also caused by strong waves and currents, during winter months, reflects on the bioconstruction morphologies, community composition, and colonization pattern. Bioconstructions exhibit an upper smooth surface, produced by few taxa (e.g., polychaetes, foraminifers), hosting a low number of living individuals, and a lower comparably rough surface, colonized by a more abundant community showing a higher species richness. The ratio surface roughness/smoothness is related to micrite sediment type: the upper part is mainly characterized by loose detrital micrite while the internal and lower parts by syndepositional cemented autochthonous micrite.

Frutexites from microbial/metazoan bioconstructions of recent and Pleistocene marine caves (Sicily, Italy)

Frutexites-like structures were recorded in small bioconstructions from Recent and Pleistocene marine caves. The skeletal/microbialite frameworks hosting Frutexites form in cryptic environments characterized by poor light availability and reduced water circulation leading to confinement, oligotrophic conditions and the development of typical cave communities. Biostalactites and biotic crusts develop in the more confined parts of the both Recent and Pleistocene caves. They are composed of skeletal organisms engulfed in fine micrite sediments deposited in situ via microbial metabolic activity. The Frutexites, composed of Iron and Manganese oxides, are confined to microcavities or microfractures of the bioconstructions. They grow from an original discontinuity surface into the surrounding consolidated micritic sediments. A model based on the increase of the microporosity of micritic sediments due to the dissolution of the original material and precipitation of ferromanganesiferous compounds was proposed. This process occurs during the syndepositional diagenetic stage due to the intrusion in the bioconstruction framework of acidic continental water rich in Iron and Manganese. Mesophilic Fe–Mn autotrophic and chemoheterotrophic bacteria flourish on the microcavities and induce the precipitation of Fe and Mn oxides.

Facies associations in warm-temperate siliciclastic deposits: insights from early Pleistocene eastern Mediterranean (Rhodes, Greece)

AbstractDiverse, abundant and usually well-preserved communities of skeletal organisms occur in the lower Pleistocene (Gelasian) siliciclastic deposits of the Greek island of Rhodes. Benthic foraminifers, molluscs and bryozoans have been studied in four measured and sampled sections located in the northern part of the island. Among these bottom-dwelling organisms, numerous extant taxa are good environmental indicators and, combined with field observations and sedimentological data, they provide information on the probable conditions in which they developed. The siliciclastic deposits of the Kritika Formation have been divided into 14 different bio- and lithofacies, which have been further grouped into four facies associations corresponding to four different environmental settings: (1) continental to fluviatile; (2) brackish-water (lagoonal/deltaic); (3) infralittoral (0–20 m); and (4) upper circalittoral (depths of 20–40 m, but also down toc.50–60 m). Among the marine facies associations, several characteristic biocoenoses have been recognized: soft-bottoms (fine to coarse sands and gravels); seagrass meadows; biogenic calcareous crusts on drowned beachrock slabs; red algal rhodoliths; and bivalve shell beds. In the studied sections, 13 superimposed genetic sequences have been documented. The repetition of similar facies associations within each sequence suggests: (1) a possibly eustasy-controlled, cyclic sedimentation; (2) a general subsidence of Rhodes during the deposition of the studied facies associations; and (3) a mostly constant range of environmental conditions (i.e. sedimentation rates and temperature) throughout the Gelasian.

Untangling intricate microbial–sponge frameworks: The contributions of sponges to Early Ordovician reefs

The Early Ordovician witnessed an increased influence of skeletal organisms on the construction of microbial-dominant reefs, which preceded the Middle to Late Ordovician expansion of skeletal-dominant reefs. Skeletal frameworks of Early Ordovician reefs built by stromatoporoids, bryozoans, pelmatozoans and calathiids have recently been reported. In this study, we document the occurrence of localized Archaeoscyphia frameworks and additional unidentified non-anthaspidellid spiculate sponges that played diverse roles in the construction of microbial–siliceous sponge reefs preserved in the Early Ordovician Dumugol Formation, Korea which have not been previously described in coeval reefs elsewhere. Two types of boundstone textures are recognized: microbial-dominated boundstone and sponge–microbial boundstone. Microbialites were primarily responsible for reef construction (making up about one third of reefs), both as framework builders and as encrusters of other reef components. The Archaeoscyphia and spiculate sponges contributed in various ways to reef development. Archaeoscyphia, constituting about one fourth of the Dumugol reefs, mainly constructed sponge–microbial boundstone, together with encrusting microbialites. Clusters of Archaeoscyphia individuals sporadically formed centimeter-sized sponge framestone (3% of reefs) with primary cryptic spaces. Spiculate sponges (4% of reefs), a previously unknown component of Early Ordovician reefs, also played diverse roles, such as encrusting other constituents, dwelling in cryptic spaces, stabilizing reef-flank sediments and even building frameworks. Such framework-building siliceous sponges, together with previously reported skeletal frameworks in other Early Ordovician reefs, reflect an early phase of the transition from microbial- to skeletal-dominated reefs in the early Paleozoic.

Origin and impact of the oldest metazoan bioclastic sediments

The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth’s history. The impact of bioclast origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index fossil Cloudina from the Itapucumi Group, Paraguay. Skeletal concentrations include both dense accumulations of parautochthonous, disarticulated specimens (“Type 1 deposits”) and in situ specimens preserved as loosely packed assemblages (“Type 2 deposits”). At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. The simple fabric and geometry of these accumulations are typical of Cambrian-style shell beds. Despite their Precambrian age, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes.

Fossil assemblages associated with submerged beachrock beds as indicators of environmental changes in terrigenous sediments: Examples from the Gelasian (Early Pleistocene) of Rhodes, Greece

The lower Pleistocene Kritika Member of the Rhodes Formation is composed of predominantly siliciclastic deposits that are sometimes intercalated with hardbottoms formed by drowning of former beachrock beds. A wide variety of sclerobionts occur in this environment, very different from the poorest soft‐bottom populations occurring in other levels of the Kritika Member. The most characteristic, abundant and frequently diverse are the encrusting forms: coralline algae, bivalves, serpulid worms, and bryozoans. Numerous other groups of sessile and vagile organisms are associated with these crust‐forming communities. A number of skeletal organisms (sometimes including zooxanthellate coral colonies) formed small carbonate buildups on top of several beachrock beds. Boring organisms also occur. Evidence for a two‐step deepening succession over the previously formed beachrock is provided by palaeontological and sedimentological data. The rapidly cemented sandy‐conglomeratic beds were first colonised by a relatively shallow‐water (around 20m) biocoenosis of mostly encrusting organisms. Another association, with encrusting and erect organisms, was later established in a deeper environment (20–40m). A comparison with Holocene beachrock occurring on the coast of Rhodes was also undertaken. Their submerged part provided habitat for poorer but similar biotas to those occurring in the Kritika Member. The lower Pleistocene beachrock and associated organogenic fossil assemblages indicate repeated drowning episodes during deposition of the Kritika Member and provide help in sequence stratigraphic interpretations of these siliciclastic deposits. Notably, they can be used as indicators of relative sea‐level changes.

New Finds of Skeletal Fossils in the Terminal Neoproterozoic of the Siberian Platform and Spain

A current paradigm accepts the presence of weakly biomineralized animals only, barely above a low metazoan grade of organization in the terminal Neoproterozoic (Ediacaran), and a later, early Cambrian burst of well skeletonized animals. Here we report new assemblages of primarily calcareous shelly fossils from upper Ediacaran (553–542 Ma) carbonates of Spain and Russia (Siberian Platform). The problematic organism Cloudina is found in the Yudoma Group of the southeastern Siberian Platform and different skeletal taxa have been discovered in the terminal Neoproterozoic of several provinces of Spain. New data on the morphology and microstructure of Ediacaran skeletal fossils Cloudina and Namacalathus indicate that the Neoproterozoic skeletal organisms were already reasonably advanced. In total, at least 15 skeletal metazoan genera are recorded worldwide within this interval. This number is comparable with that known for the basal early Cambrian. These data reveal that the terminal Neoproterozoic skeletal blo...