Metazoan Reefs Research Articles

Ocean oxygenation and ecological restructuring caused by the late Paleozoic evolution of land plants

The dramatic increase in ocean-atmosphere oxygen levels during the Devonian and Mississippian is increasingly linked to the diversification of land plants, yet the timing and extent of this event remain uncertain. This study uses the redox-sensitive rare earth element cerium (cerium anomaly—Ce/Ce*) to investigate ocean redox conditions during the deposition of globally distributed Paleozoic carbonate strata. Our Paleozoic Ce/Ce* record suggests that Cambrian, Ordovician, and Silurian oceans had relatively low O2 levels (mean Ce/Ce* = 0.86 ± 0.10, 0.91 ± 0.14, and 0.91 ± 0.10 [±1σ], respectively). In contrast, short-lived ocean oxygenation events, possibly related to the diversification of small land plants, likely occurred throughout the Early and Middle Devonian (mean Ce/Ce* = 0.80 ± 0.07 and 0.58 ± 0.14, respectively). “Modern” Ce/Ce* values (<0.36) first occurred during the Late Devonian, suggesting that the main phase of Devonian and Mississippian oceanic oxygenation was related to the evolution of large vascular plants and the first forests. Despite this, the significant variability of Ce/Ce* values during this time suggests that shallow marine settings were susceptible to redox instability, possibly caused by upwelling of anoxic deep waters. This redox instability potentially provides evidence of a mechanism for contemporaneous mass extinction and metazoan reef collapse events. Development of strongly oxic conditions during the Late Devonian may have resulted in the demise of many Paleozoic-type organisms, facilitated the radiation of the modern evolutionary fauna, and established the modern oxygenated ocean-atmosphere system.

Depositional Environment and Ecological Response of Bioconstructions: A Case Study of Southern China (Guizhou Province) in Moscovian-Gzhelian.

From the late Carboniferous to the early Permian, multiple pulses of glaciation and deglaciation have been caused by the LPIA. The Pennsylvanian period experienced phases of recovery, proliferation, and decline, ultimately forming a reef system distinctly different from that of the Mississippian period. During the late Bashkirian to Moscovian, the metazoan reef experienced a limited resurgence, with reef predominantly formed by chaetetid developing in the United States, northern China, and Japan. During the Kasimovian to Gzhelian, the phylloid algal reef dominated the global reef systems. In the late Pennsylvanian, bioconstruction cases and paleoenvironmental proxies in southern Guizhou Province were studied to investigate the composition, recovery, and evolutionary processes of the bioconstructions as well as their response to environmental variations during this period. Several bioconstructions have been reported in the Lumazhai section of Houchang Town, Guizhou Province, southern China, from the Moscovian to the Gzhelian. The upper Carboniferous strata are well-preserved and continuously exposed. The continuous strata, abundant fossils, and diverse bioconstructions provide excellent research materials for exploring the mutual constraints between organisms and their environment. This study identified ten microfacies, whose vertical evolution indicated significant changes in the depositional environment related to relative sea-level fluctuations. Skeletal grains are widely present in these facies. Among them, foraminifera, algae, bryozoans, crinoids, and Tubiphytes are the most common and exhibit distinct distribution characteristics in various environments. Quantitative statistics, CCA and theoretical ecospace have been utilized to examine and interpret environmental impact factors. Quantitative analysis of their relative abundance and distribution patterns provides insights into the complex interactions between organisms and environmental factors. The relative abundances of different organisms and factors controlling their bioconstructions are influenced by relative sea-level changes. CCA analysis reveal that hydrodynamic conditions are the primary influencing factor. Variation trends in average tiering and motility reveal the characteristics of biological communities during environmental changes in phylloid algae and microbial bioconstructions. These bioconstructions are not directly correlated with changes in environmental factors, and the biological communities in phylloid algae mounds and biostromes exhibit similar organism compositions and ecological niches across different environments.

Editorial preface to special issue: Paleoecology and evolution of Paleozoic corals and reef ecosystems

The Paleozoic Era was an interval of profound changes in metazoan reef ecosystems, which became magnificent in scale and diversity, owing to the evolution of various skeletal reef builders especially corals and sponges. This special issue comprises 16 papers that improve our understanding on the complex evolution and paleoecology of corals and metazoan reef ecosystems, and their response to environmental changes through the Cambrian to Permian. The case studies present detailed evidence that helps to reconstruct the complicated interactions among reef organisms, the succession of metazoan reefs, and their relations to paleoenvironments. Some key findings address (i) Cambrian reefs and microbe–metazoan reef transition to the Ordovician, (ii) reef component and paleoecological features of Siluro-Devonian coral-stromatoporoid reefs, and (iii) growth characteristics, paleogeography, and reef formation of Late Paleozoic corals and sponges. As such, this special issue will stimulate further work on reef studies and be of interest to a broad community of paleoecologists, to scholars in various fields of the geosciences as well as to reef biologists.

The last episode of coral-stromatoporoid reef and faunal changes across a Devonian-Carboniferous Boundary section of South China

This study reports the final occurrence of coral-stromatoporoid biostromal reef and the faunal and facies change across the Devonian-Carboniferous Boundary (DCB) at Changtanzi section, Longmenshan area of South China. The biostrome occurred in the third member of the Changtanzi Formation and is uppermost Famennian in age, characterized by abundant and high diversity stromatoporoids and tabulate corals, which were subsequently greatly affected with the environmental fluctuations indicated by the end-Devonian carbon isotope excursion. Stromatoporoids and tabulate corals survived and thrived in the uppermost Famennian but their association was eventually terminated in the Lower Hangenberg Crisis (LHC) by extinction of Famennian stromatoporoids. Brachiopods and rugose corals are found to occur as early as in the Upper Hangenberg Crisis (UHC), represented by solitary rugose corals Hebukophyllum sp., Neozaphrentis sp. and Uralinia sp., as well as abundant brachiopods, gastropods, in general representing post-disaster fauna of low species level diversity. This study shows distinctive faunal and lithofacies changes during the Hangenberg Crisis, and examines the strong link between faunal and environmental changes. The result indicates that the environmental fluctuations in the Devonian-Carboniferous transitions caused sudden and significant impacts on marine ecosystems, leading to the disappearance of coral-stromatoporoid metazoan reefs.

Timing and pattern of the Devonian Nehden event: Implication for the biotic recovery following the F–F extinction event

The biotic recovery process following the Frasnian–Famennian (F–F) mass extinction played an important role in reestablishing the late Palaeozoic marine ecosystem. The Nehden event is assumed as the global biotic recovery process in the aftermath of the F–F mass extinction, but the timing and pattern of this event is still in dispute due to lack of consistent geological records. In order to recognize the Nehden event and elucidate its timing and pattern, a newly refined conodont biostratigraphic framework spanning the Palmatolepis minuta minuta Zone to the Pa. rhomboidea Zone (370.8–367 Ma) has been established in the Yangdi section, Guangxi, South China. Based on a detailed analysis of microfacies and carbon isotope of carbonates (δ13Ccarb), the start of the Nehden event has been placed at the upper part of the Pa. m. minuta Zone, marked by deposition of calcareous shales and proliferation of palmatolepid conodonts and ostracods. Two episodes of the Nehden event are recognized in this study according to integrated palaeontological, sedimentological, and geochemical data from South China and other continents. The first phase of the Nehden event is assigned at the upper part of the Pa. m. minuta Zone, characterized by radiation of conodonts, ostracods, and brachiopods, which is coincident with the minimum value of negative δ13Ccarb excursion and climate warming. The second phase is placed at the Pa. termini to Pa. glabra pectinata zones, documented by biodiversification of cheiloceratid ammonoids and phacopid trilobites, which is concurrent with climate warming and the maximum transgression during the early Famennian. The extensive development of microbial bindstone in the lower Famennian succession at Yangdi demonstrates that microbial communities occupied the vacant ecological niches left behind by metazoans after the F–F biotic crisis. The expansion of ecological niches of microbial communities and depauperation of metazoan reefs implies the bioconstructor turnover from metazoan to microbial communities following the F–F extinction. The time equivalence of biotic radiation, climate change and sea-level fluctuation in two episodes of the Nehden event indicates the early Famennian biotic recovery was favored by climate warming and transgression. The results in this study not only offer important data to constrain the timing of the Nehden event, but also provide an insight on the process of biotic recovery following the F–F mass extinction event.

Back-reef and lagoonal communities, Givetian (Middle Devonian) in Guangdong, South China: Their role in global Devonian reef development

Back-reef and lagoonal communities played an important role in the construction of global reef ecosystems and shallow water carbonate factories during the Devonian Period, but their reef-building significance is not fully understood. This study describes a previously undocumented Givetian stromatoporoid biostromal sequence, 12 m thick and comprising 7 biostromes, in the Tangjiawan Formation of Yaozhai, Guangdong Province, Southeast China. Large bulbous or columnar stromatoporoid Actinostroma and delicate dendroid stromatoporoid Amphipora are abundant, with a comparatively smaller presence of tabulate corals, rugose corals, brachiopods, gastropods, calcimicrobes, and tubeworms. The dominant Actinostroma-Amphipora assemblage, together with leperditicopid ostracods, parathuramminid foraminifers, and calcispheres, indicates a shallow, low-energy back-reef to lagoonal setting. Taphonomic analysis shows a common occurrence of overturned skeletons, and the deposit is interpreted as a parabiostrome constructed in a low-energy environment and affected by episodic high-energy events. Statistical data of similar global assemblages from the Lochkovian to the Famennian based on 153 localities show a relatively rare occurrence of back-reef and lagoonal communities from the Lochkovian to the Eifelian, peaking during the Givetian and Frasnian together with metazoan reefs then declining sharply in the Famennian after the Kellwasser Event. Although autobiostromes have rarely been documented, the densely packed reworked bulbous/domical and dendroid stromatoporoids justify the existence of low-diversity but highly abundant metazoan ecosystems in back-reef to lagoonal settings. This study thus highlights our understanding of the back-reef and lagoonal facies and the important role they played in the expansion of large-scale carbonate factories and reef ecosystems during the Middle to Late Devonian.

Reconstructing the feeding ecology of Cambrian sponge reefs: the case for active suspension feeding in Archaeocyatha.

Sponge-grade Archaeocyatha were early Cambrian biomineralizing metazoans that constructed reefs globally. Despite decades of research, many facets of archaeocyath palaeobiology remain unclear, making it difficult to reconstruct the palaeoecology of Cambrian reef ecosystems. Of specific interest is how these organisms fed; previous experimental studies have suggested that archaeocyaths functioned as passive suspension feeders relying on ambient currents to transport nutrient-rich water into their central cavities. Here, we test this hypothesis using computational fluid dynamics (CFD) simulations of digital models of select archaeocyath species. Our results demonstrate that, given a range of plausible current velocities, there was very little fluid circulation through the skeleton, suggesting obligate passive suspension feeding was unlikely. Comparing our simulation data with exhalent velocities collected from extant sponges, we infer an active suspension feeding lifestyle for archaeocyaths. The combination of active suspension feeding and biomineralization in Archaeocyatha may have facilitated the creation of modern metazoan reef ecosystems.

Stromatolites from the Majiagou Formation in the Ordos Basin, Northwestern China

Stromatolites declined remarkably in the Middle Ordovician, which has been speculatively related to the concomitant proliferation of metazoan reefs during the Great Ordovician Radiation Event, however, they are flourish and widely distributed in the Middle Ordovician (Darriwilian) Majiagou (Machiakou) Formation in the Ordos Basin, northwestern China. The stromatolites in the Ma-5 Member of the Majiagou Formation in the Ordos Basin exhibit significant morphological diversity, and are ready to be divided into wavy and domal stromatolites from the south and west part, columnar in the Northeast part, accompanied with rarely hemispheroid and conical ones. The stromatolites in the Ma-5 Member distribute in a subcircular ring belt, which can be subdivided into three sedimentary divisions, mostly near the boundary between the gypsum-bearing dolomite flat and gypsum salt lake, of which, the north-central division, with its thicker stromatolite-bearing strata and more complex stromatolite morphologies, was the most favorable for the establishment of stromatolites, where grazing animals were absent and burrowing animals few. Some calcified microorganisms were recovered from the stromatolites, which substantially increases their known diversity for this interval and suggestive of a severe undersampling of the middle Ordovician calcified cyanobacteria fossil record in the Ordos Basin.

Analysis of the environmental impacts affecting Cambrian reef building and carbonate settings during the Miaolingian and Furongian epochs: A hypothesis for consideration

The Miaolingian and Furongian epochs of the Cambrian period have been identified as a time of limited metazoan reef development. The aim of this paper is to improve understanding of the biological and geochemical conditions that affected reefs during this interval, and to propose a hypothesis for understanding why metazoan reef development was inhibited. To address these issues, a global dataset of fossil occurrences (N ​= ​25,307) spanning Cambrian Stage 4 to the early Ordovician (Tremadocian) was extracted from the Paleobiology Database, Paleoreef Database, and a review of the primary literature. Findings show that the proportion of reefs constructed by metazoans fell from 40% in the Wuliuan age to 0% in the Drumian age, with reefs being overwhelmingly dominated by microbial ecosystems through the remainder of the Cambrian. The proportion of skeletal material constructed from carbonate fell from 85% in the Wuliuan age to 63% in the Drumian age across all the fossil occurrence data, before recovering. These findings suggest that environmental conditions may have not been favorable to carbonate organisms, but this does not fully explain the prolonged reduction of metazoans within reefs throughout this interval. A hypothesis proposed here is that Miaolingian to Furongian metazoan reef abundances were low because of two factors: (1) shallow water anoxia – and other factors such as elevated temperatures and ocean acidification – caused the extinction of metazoan reef builders in the late-early Cambrian and (2) deep water anoxia and marine regression, resulted in a loss of habitat. These inhibiting conditions were not necessarily concurrent but are inferred to have collectively suppressed the growth of metazoan reefs until the Early Ordovician when more shelf space for new reef development occurred. This hypothesis provides a first step in exploration of these conditions during the middle and late Cambrian and for reef development in general.

High-temperature combustion event spanning the Guadalupian−Lopingian boundary terminated by soil erosion

A major bio-crisis in the Guadalupian−Lopingian transition (Capitanian−Wuchiapingian, middle−late Permian), possibly driven by the volatile eruption of the Emeishan large igneous province (LIP), was marked by the first-order collapse of global metazoan reefs and decline of fusulinid foraminifera, but with only minor impacts on other marine invertebrates (brachiopod, crustacea, other foraminifera). To assess the exact cause of this event, we conducted geochemical analyses of the shallow marine strata at the global stratotype section and point of the Guadalupian−Lopingian (G−L) boundary (GLB) in Laibin, South China, which corresponds to the last step of the Capitanian bio-crises during Emeishan volcanism. Here, we detect evidence for high temperature combustion of organic matter in air spanning the GLB (indicated by enriched coronene) that was terminated by a soil erosion event accompanied with terrestrial vegetation collapse at the mass extinction level (evidenced by enriched dibenzofuran) and a carbon isotope perturbation (a 2.5–3.5‰ negative shift of δ13Ccarb). Molybdenum data indicates oxic seawater during the combustion event, likely reflecting regression. These findings imply that large volatile volcanic eruptions of the Emeishan LIP may have caused these environmental extreme events and mass extinctions, and that relatively lower magnitude of volcanism related to Emeishan LIP may have led to impacts on terrestrial−nearshore ecosystems.

Spatio-temporal distribution of the Cambrian maceriate reefs across the North China Platform

Maceriate reefs, characterized by centimeter- to decimeter-scale branching structures, flourished during the Cambrian to Early Ordovician on Laurentia, and Gondwana (the North China Platform). Dendritic and columnar maceriate reefs were newly discovered from the northwestern and central parts of the North China Platform. The maceria structures are composed of sponges (e.g., keratosans and lithistids) and microbial components, including Girvanella, Renalcis, and Tarthinia, whereas the interstitial material consists of lime mud with bioclasts and intraclasts. The first appearance of maceriate reefs was diachronous across the North China Platform, younging from the northwest (Drumian Age, Miaolingian) to the southeast (Jiangshanian Age, Furongian). In general, thrombolites and dendrolites were dominant in the microbialite-dominated reefs of the Miaolingian and microbe–sponge maceriate reefs were dominant in the Furongian. This dramatic shift to microbe–metazoan reef assemblages resulted from a combination of factors associated with the Miaolingian–Furongian boundary interval, including global transgression and a shift from widespread anoxia of the SPICE event to more oxygenated post-SPICE conditions. The North China examples suggest that sea level and paleoenvironmental conditions may have played a major role in this transition and its timing.

Global cooling initiated the Middle-Late Mississippian biodiversity crisis

During the Mississippian period, metazoan reefs and other marine faunas gradually recovered from the Late Devonian mass extinctions and reached a peak in the late Visean (~334–332 Ma). Faunal diversity started to decline from the latest Visean (~332–330 Ma) through Serpukhovian (~330–323 Ma), with significant genera/species losses and ecosystem reconstruction. This Middle-Late Mississippian biodiversity crisis (M-LMBC) was thought to have been caused by global cooling associated with the late Paleozoic Ice Age (LPIA), but existing sedimentological and temperature proxy data suggest that the global cooling event—that marks the onset of the main glaciation phase of LPIA—happened either ~4 Myr before or ~ 1–5 Myr after the initial biodiversity decline at ~332 Ma. Here, we report oxygen isotope data of diagenetically screened, well-preserved brachiopod calcite (δ18Ocalcite) from late Visean-Serpukhovian (or Middle-Late Mississippian; ~334–323 Ma) strata in South China where biodiversity data are well documented. The δ18Ocalcite data reveal a ~ 2.0‰ positive shift from −4.6 ± 0.2‰ to −2.7 ± 0.5‰ with an estimated ~4.7–5.5 °C drop in sea surface temperature (SST) during ~332.5–331.5 Ma in the late Visean. This cooling event coincides with fast decline of metazoan reef abundance, followed by decrease of benthic faunal diversity. The δ18Ocalcite data, in combination with calibrated sedimentological and biodiversity data, demonstrate the coupling between late Visean (~332 Ma) onset of the main glaciation phase of the LPIA and initiation of the M-LMBC.

Catastrophic event sequences across the Permian-Triassic boundary in the ocean and on land

This review forms the preface of a special issue dealing with Environmental stresses and biotic responses during the Palaeozoic-Mesozoic transition, and develops an improved understanding of the sequence of catastrophe events associated with the Permian-Triassic (P–Tr) transition in particular. The 16 papers that comprise this special issue mostly focus on the Paleo-Tethys and Panthalassia regions, and enable us to recognize seven distinct phases of biotic and environmental evolution that may be precisely correlated between marine and terrestrial environments. Phase 1 represents the normal marine environments and biota that existed at 252.104 Ma, prior to the onset of major environmental perturbation. Phase 2 is marked by the onset of a negative carbon (δ13Ccarb) isotopic excursion, and coincides with a significant decline in radiolarian diversity, the collapse of communities in shallow metazoan reefs and deep-water settings, and the loss of rainforests on land (including the disappearance of Gigantopteris flora). Phase 3 is characterized by a pulse of biodiversification on the eve of the end-Permian extinction (EPE). Phase 4 comprises the EPE event itself, dated to 251.941 Ma, and coincides with an extreme episode of widespread anoxia, biodiversity loss, community structural collapse, and volatile volcanic eruptions. Phase 5 is marked by the renewed onset of oxic conditions in all oceanic environments except those in microbialite-rich shallow platform settings where abundant pyrite framboids signal dysoxia. Phase 6 includes the earliest Triassic extinction (ETE) event, dated to 251.880 Ma, and associated with euxinic to anoxic conditions and ocean acidification. An extended phase of deoxygenation and acidification returned to the marine settings during Phase 7, possibly driving a calcium crisis. The terrestrial extinction event (Phase 2) probably commenced c. 60 kyrs (in South China) or 370 kyrs (in Sydney basin) before the marine P–Tr extinction, the latter consisting of two distinct events (Phases 4 and 6), spaced c. 60 kyr apart, each being marked by both a carbon isotope excursion and ecologic collapse. The environmental changes associated with these three extinction phases in the ocean and on land can be attributed to the effects of volcanic emissions (e.g., CO2, SO2, halogens and metals) during the eruption of the Siberian Traps Large Igneous Province and convergent margin volcanism related to Tethyan tectonics.

Rise of clathrodictyid stromatoporoids during the Great Ordovician Biodiversification Event: insights from the Upper Ordovician Xiazhen Formation of South China

AbstractClathrodictyids are the most abundant stromatoporoids in the Upper Ordovician Xiazhen Formation (middle to upper Katian) of South China. A total of nine species belonging to four clathrodictyid genera are identified in the formation, includingClathrodictyon idenseWebby and Banks, 1976,Clathrodictyoncf.Cl. microundulatumNestor, 1964,Clathrodictyoncf.Cl.mammillatum(Schmidt, 1858),Clathrodictyon megalamellatumJeon n. sp.,Clathrodictyon plicatumWebby and Banks, 1976,Ecclimadictyon nestoriWebby, 1969,Ecclimadictyon undatumWebby and Banks, 1976,Camptodictyon amzassensis(Khalfina, 1960), andLabyrinthodictyon cascum(Webby and Morris, 1976). The clathrodictyid fauna in the Xiazhen Formation is very similar to those from both New South Wales and Tasmania, although the latter two Australian regions do not share any common clathrodictyid species during the Late Ordovician. The paleobiogeographic pattern indicates that the northward drift of South China resulted in a favorable environment for the migration of clathrodictyids from other peri-Gondwanan terranes to South China. In addition, these peri-Gondwanan clathrodictyid species hosted various endobionts, representing a variety of paleoecological interactions. The high abundance and species-level diversity of clathrodictyid species presumably increased the substrate availability of suitable host taxa, judging from the diverse intergrowth associations between clathrodictyids and other benthic organisms. These paleoecological interactions between stromatoporoid and other organisms are known from the Late Ordovician and became more abundant and widespread in the Siluro–Devonian. Overall, the Late Ordovician clathrodictyid assemblage in South China demonstrates one of the highest species-level diversities among all peri-Gondwanan terranes and represents a precursor of the complex, clathrodictyid-dominated communities of later metazoan reefs during the Great Ordovician Biodiversification Event.UUID:http://zoobank.org/6063c47d-cb77-4a03-98cf-b2354656dea6

Keratose sponge fabrics from the lowermost Triassic microbialites in South China: Geobiologic features and Phanerozoic evolution

The Permian-Triassic mass extinction (PTME) strongly devastated marine ecosystems, and, consequently, sponges, especially the reef-building clades, suffered dramatic losses in biodiversity. The Early Triassic therefore was believed to be an evolutionary gap for sponges. Microbialites spread over shallow marine carbonate settings across the entire low-latitude Tethys region following the PTME and occupied the ecospace that the pre-extinction metazoan reefs left. Here, we report putative keratose sponge consortia from the microbialites near the Permian-Triassic boundary in the Xiushui, Laolongdong, and Dongwan sections, South China. The putative keratose sponges exhibit vermiform, filamentous textures forming maze-like networks. Within the keratose sponge-microbial fabrics, the calcified sponge skeleton might firm the overall framework of microbialite, promoting construction of the sponge-microbial build-ups. The coeval occurrence of putative keratose sponges in both eastern Palaeotethys and central Neotethys regions indicates that sponges may have widely spread and played important roles in constructing metazoan-microbial reefs in earliest Triassic. Besides, several characteristics are conducive to keratose sponge surviving the stressful environments after the PTME. Global dataset shows that keratose sponges mostly confined to tropic and temperate zones. Keratose sponges overall flourished in coincidence with occurrence abundance peaks of microbial reefs during the Phanerozoic history, and they seem to be particularly abundant and widespread in the Cambrian-Ordovician, Late Devonian and the aftermath of the end-Permian mass extinction.

Massive cryptic microbe-sponge deposits in a Devonian fore-reef slope (Elbingerode Reef Complex, Harz Mts., Germany)

A massive occurrence of microbial carbonates, including abundant sponge remains, within the Devonian Elbingerode Reef Complex was likely deposited in a former cavity of the fore-reef slope during the early Frasnian. It is suggested that the formation of microbial carbonate was to a large part favored by the activity of heterotrophic, i.e., sulfate-reducing bacteria, in analogy to Quaternary coral reef microbialites. The Elbingerode Reef Complex is an example of an oceanic or Darwinian barrier reef system. In modern barrier reef settings, microbialite formation is commonly further facilitated by weathering products from the central volcanic islands. The Devonian microbialites of the Elbingerode Reef Complex occur in the form of reticulate and laminated frameworks. Reticulate framework is rich in hexactinellid glass sponges, the tissue decay of which led to the formation of abundant micrite as well as peloidal and stromatactis textures. Supposed calcimicrobes such as Angusticellularia (formerly Angulocellularia) and Frutexites, also known from cryptic habitats, were part of the microbial association. The microbial degradation of sponge tissue likely also contributed to the laminated framework accretion as evidenced by the occurrence of remains of so-called “keratose” demosponges. Further typical textures in the microbialite of the Elbingerode Reef Complex include zebra limestone, i.e., the more or less regular intercalation of microbial carbonate and cement. Elevated concentrations of magnesium in the microbialite as compared to the surrounding metazoan (stromatoporoid-coral) reef limestone suggests that the microbialite of the Elbingerode Reef Complex was initially rich in high-magnesium calcite, which would be yet another parallel to modern, cryptic coral reef microbial carbonates. Deposition and accretion of the microbialite largely occurred in oxygenated seawater with suboxic episodes as indicated by the trace element (REE + Y) data.

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.

Revision of thelodonts, acanthodians, conodonts, and the depositional environments in the Burgen outlier (Ludlow, Silurian) of Gotland, Sweden

ABSTRACT Ludfordian strata exposed in the Burgen outlier in eastern Gotland, Sweden record a time of initial faunal recovery after a global environmental perturbation manifested in the Ludfordian Carbon Isotope Excursion (LCIE). Vertebrate microfossils in the collection of the late Lennart Jeppsson, hosted at the Swedish Museum of Natural History, hold the key to reconstruct the dynamics of faunal immigration and diversification during the decline of the LCIE, but the stratigraphic relationships of the strata have been debated. Historically, they had been placed in the Burgsvik Formation, which included the Burgsvik Sandstone and the Burgsvik Oolite members. We revise the fauna in the Jeppsson collection and characterize key outcrops of Burgen and Kapellet. The former Burgsvik Oolite Member is here revised as the Burgen Oolite Formation. In the Burgen outlier, back-shoal facies of this formation are represented and their position in the Ozarkodina snajdri Biozone is supported. The shallow-marine position compared to the coeval strata in southern Gotland is reflected in the higher δ13Ccarb values, reaching +9.2‰. The back-shoal succession includes high-diversity metazoan reefs, which indicate a complete recovery of the carbonate producers as the LCIE declined. The impoverishment of conodonts associated with the LCIE in southern Gotland might be a product of facies preferences, as the diverse environments in the outlier yielded all 21 species known from the formation. Fish diversity also returned to normal levels as the LCIE declined, with a minimum of nine species. In line with previous reports, thelodont scales appear to dominate samples from the Burgen outlier.

Modelling Ediacaran metazoan–microbial reef growth

AbstractThroughout the Phanerozoic, sessile metazoans grew in close association with various microbial carbonates to form reefs. The first metazoans with calcareous hard‐parts appeared in the terminal Ediacaran,ca550 million years ago, and these also commonly grew associated with microbial mats, thrombolites and stromatolites, to form the oldest known metazoan–microbial reefs. These hard‐parts also formed the first skeletal (bioclastic) carbonate sediments, which increased to dominate shallow marine carbonate sedimentary production during the Phanerozoic. Here the growth dynamics and sedimentary interactions between Ediacaran microbial–metazoan reefs and their bioclastic products are described based on reef complexes from the Nama Group, Namibia (ca547 Ma), and the first three‐dimensional numerical models are constructed to parametrize these dynamics. These reefs are observed to form large domal mounds and columns neighbouring locally high rates of bioclastic sediment accumulation, and commonly evolve to flat‐topped surfaces overlain by sediment. This model is parsimonious, embodying a single dynamic rule:metazoans attach to microbialite mounds which grow radially into free space shedding bioclasts in proportion to metazoan production. Models evolve mounds into columns due to spatial competition, and produce an oscillating interplay between mound expansion and smothering that results in an inter‐fingering of microbialite mound and detrital sediment. This dynamic is shown to be non‐linear in the proportion of bioclastic sediment produced. Smothering by bioclastic sediment is also demonstrated to both reduce the rate (by volume) of subsequent mound growth, and to overwhelm the growing mat surfaces of stromatolites: after a threshold is reached in the rate of bioclastic sediment deposition, sediment terminates all stromatolite growth. These models show that the general characteristics of field observations can be explained by a single dynamic rule, and that reef‐sourced bioclasts may make an important local contribution to Ediacaran to Cambrian microbial–metazoan reef dynamics – a contribution that remains important throughout the Phanerozoic.

Upper Visean (Mississippian) metazoan-microbial reefs from Guangxi, South China: Insights regarding reef recovery after the end-Devonian extinction

The Mississippian was an important interval for metazoan reef recovery after the collapse of stromatoporoid-coral reef ecosystems during the Late Devonian mass extinctions. Based on field observation and quantitative studies of biotic composition in thin sections, four different reef types were identified in the upper Visean strata in the Gandongzi section of Guangxi Province, South China. These are coral reef, coral-microbial-microproblematica-bryozoan reef, coral-microbial-bryozoan reef, and microbial-bryozoan-coral reef. Their positive topographic relief from one metre to ten metres is either directly observed in the section or by correlation to the coeval non-reef facies in the adjacent Longfeng section. These reefs are constructed primarily of different proportion of colonial rugose corals (Diphyphyllum and Siphonodendron), tabulate corals (Multithecopora and Syringopora), cystoporate bryozoans (Fistulipora), and calcimicrobes (e.g., Garwoodia and Renalcis). Reef dwellers are rich and diversified including crinoids, brachiopods, bryozoans, gastropods, heterocorals, calcispheres, green algae, foraminifers, and ostracods. Their composition and abundance vary among the different reef types. Facies analysis reveals a variable composition, including framestone and bindstone in reef facies, and rudstone, grainstone, packstone, dolomitic crinoidal packstone and dolostone in the strata intercalated with reefs. The microbial reef facies is characterized by abundant spar-filled cavities, which can be classified as stromatactis and fenestral fabrics. In the non-reef facies, geopetal structures are abundant. Relative sea-level changes controlled the growth and demise of the Gandongzi reefs. They commonly grew during relative sea-level rise and died during relative sea-level fall, as evident from increasing and decreasing abundance of micrite, and decreasing and increasing abundance of cortoids, green algae and sparry calcite in reefs and their overlying strata, respectively. However, the final demise of the Gandongzi reef succession was triggered by a dramatic relative sea-level rise, indicated by increased micrite abundance associated with articulated and convex-down brachiopod shells overlying the uppermost reef. The Gandongzi reefs resemble coeval similar reef types on other palaeocontinents, in terms of reef-builder composition and internal structures. The important role of corals in the framework building of the Gandongzi reefs coincided with the late Visean coral reef proliferation, which is a major phase for re-establishment of metazoan reef ecosystems after the end-Devonian mass extinction.