Cambrian Reefs Research Articles

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.

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.

Microbial origin of the Ordovician stromatoporoid-like organism Zondarella from the Argentine Precordillera and the post- Cambrian persistence of stromatolite microbialite reefs.

A laminated, dome-shaped structure (with stromatolitic features) forming reefs in the Ordovician of western Argentina was previously described as a probable primitive form of stromatoporoid named as Zondarella. New collected material allowed to realize that the laminae are constituted by well-preserved filaments, assigned to the cyanobacterium AcuasiphonoriaLiu et al., 2016, which forms the lamination seen in “Zondarella” alternating filament density and orientation. According to this, the stromatoporoid status of the growth form named “Zondarella” is no longer sustainable, and is considered to be a skeletal stromatolite. Stromatolites occur widely in Furongian and Early Ordovician rocks, slightly predating the Ordovician cyanobacterial calcification episode, indicated by a many-fold increase in global calcified cyanobacteria diversity during the late Middle to Upper Ordovician. The Acuasiphonoria stromatolites of the mid- to upper San Juan Formation are earliest Dapingian, occurring together with renalcids and Allonema boundstone. Reefs of the Argentine Precordillera prove the persistence of stromatolite reefs without a conspicuous metazoan framework into the early Middle Ordovician. The lower Dapingian reefs bearing calcified cyanobacterial stromatolites of the San Juan Formation are morphologically and dimensionally comparable to lower Cambrian reefs, sharing similar paleoecological features such as community structure and composition; they also record an increase in calcimicrobial diversity and appeared directly above the Basal Dapingian Negative Isotopic Carbon Excursion. This suggests that a replication of similar environmental or oceanographic states, might have triggered the return of comparable ecosystems.

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.

Generalization of regional seismic results within the Aldan-Maya petroleum region

The results of regional geological exploration at the expense of the federal budget within the Aldan-May petroleum region over the past 17 years are analyzed. They made it possible to significantly correct the understanding about the geological structure and assessment of the prospects for oil and gas potential of the study area. Based on the results of these works, 56 prospecting objects were identified for detailed geophysical surveys and exploratory drilling. The initial total resources were estimated for four potential prospective complexes: Jurassic terrigenous, Lower-Middle Cambrian reef, Vendian-Lower Cambrian carbonate and Riphean-Vendian terrigenous-carbonate. According to the estimates given in the seismic reports, the Vend-Lower Cambrian complex has the greatest resource potential. However, the results of the generalization and scientific analysis of geological and geophysical data, carried out at the IPGG SB RAS, allow us to state that the main role in the Aldan-Maya petroleum region will be played by the Riphean complex, where oil fields is expected.

Genetic model for early Cambrian reef limestone-hosted Pb-Zn deposits in the world-class Huayuan orefield, South China: New insights from mineralogy, fluorite geochemistry and sulfides in situ S-Pb isotopes

The Xiangxi-Qiandong metallogenic belt (>300 Mt sulfide ores @∼4.0 wt% Pb + Zn), located at the southeastern margin of the Yangtze Block, is one of the important Pb-Zn producers in China. Although carbonate-hosted Pb-Zn deposits in this district have attracted great attention during past decades, many issues, particularly the ore genesis and geodynamic setting, remain unclear. In this study, we take the Limei and Yutang deposits as typical case studies, using detailed geology, rare earth elements and yttrium (REY) geochemistry of fluorite and sulfides in situ S-Pb isotopes, to propose a new model for early Cambrian reef limestone-hosted Pb-Zn deposits in the district. REY contents (0.47 to 8.67 ppm; mean = 3.99 ppm) in fluorite from Limei and Yutang slightly deplete in LREE and HREE relative to MREE, indicating that the REEs and metals were transported predominantly as chloride complexes in a low-temperature (<200 °C) hydrothermal fluid with high saline. New δ34S values of sulfides (+27.66 to + 35.46‰ in Limei and + 24.80 to + 36.07‰ in Yutang) imply that S2- was originated from evaporite sulfate within ore-hosting strata via thermo-chemical reduction (TSR) as the presence of reducing agents (bioclastic limestone and organic matter). In situ δ34S values show a slightly large range (variation up to 11.27‰) and vary significantly within single grain (up to 4.60‰), which may be inherited from the S composition of H2S in the original fluid. In situ Pb isotopic ratios of sulfides from Limei (206Pb/204Pb = 18.001–18.269, 207Pb/204Pb = 15.645–15.796 and 208Pb/204Pb = 38.243–38.597) and Yutang (206Pb/204Pb = 18.135–18.384, 207Pb/204Pb = 15.714–15.924 and 208Pb/204Pb = 38.347–38.874) positively correlates in Pb-Pb diagrams, indicating the mineralizing metals are mainly derived from Proterozoic basement rocks with an amount of contribution from early Cambrian sedimentary rocks. Mineralogy, fluorite REY geochemistry and sulfides in situ S-Pb isotopes together with previously reported mineralization ages suggest that the mixing of metal-rich fluid from basement rocks and the H2S-rich fluid originated from ore-hosting strata plays an important role in forming the Pb-Zn deposits of Limei and Yutang. This study provides a typical example for better understanding the genesis and geodynamic setting of MVT deposits in the foreland basin, South China.

Cambrian reefs in the lower Poleta Formation: a new occurrence of a thick archaeocyathan reef near Gold Point, Nevada, USA

The lower member of the Poleta Formation is well known for its abundant archaeocyaths, including an exceptionally preserved reef at Stewart’s Mill in Nevada. Large archaeocyath-bearing reefs are relatively uncommon, with most occurring as small nearshore patch reefs. Here, we describe a new 70 m-thick archaeocyath reef from the lower Poleta Formation, herein referred to as the Gold Point Reef, located near the townsite of Gold Point, Nevada. In spite of broad similarities with the Stewart’s Mill exposure, the Gold Point Reef shows an interplay between shoal and reef development over the ~ 70 m exposure that differs from what is observed at Stewart’s Mill. This reef also preserves a small shelly fauna in its uppermost 10 m, dominated by Hyolithellus. Three successive ecological stages of reef development are observed here: the pioneer stage characterized by a single rare frame-building species; the diversification stage involving about nine species of both frame-building and mud-sticking archaeocyaths; and the final foundering stage represented by one to two species of archaeocyaths and facies indicating reef demise, including abundant grainstone/packstone shoals and a silicified upper bed with a settlement of hyolitheminthes. During the apex of reef development here, there are large modular archaeocyaths, which were able to produce thick secondary skeletal tissue for a firm attachment; these archaeocyaths were capable of building large reefs in a variety of settings. Additionally, the carbon isotope profile recorded in the Gold Point Reef shows similarities to nearby exposures of the equivalent lower Poleta Formation, with the exception of a small negative downturn at the top of the reef that is not found in other locations. Combined with the documentation of facies changes over short lateral distances, the isotope data suggest that archaeocyathan reefs may have been time-transgressive across the shelf during deposition of the lower Poleta. Additionally, our work suggests that both here and globally, the specific biotic assemblage reflected the local influences of the environment on reef development, creating highly variable coeval paleocommunities even within a restricted area.

Virtual Systematics with Annotate: The Mexican archaeocyaths example

Digitization and online publishing of museum specimen data are happening worldwide. Studies based solely on online data become increasingly accessible. The current events, for example, reducing our transport-related carbon footprint or the COVID-19 pandemic, provide key opportunities to highlight the full value of digitized collections and their related tools, which allow us to continue our research from home or at least without travelling. Are existing data resources and tools adequate for engaging in a research project from beginning to end? To address this issue, we propose to use the Mexican archaeocyaths digitized collection from the Museum National d’Histoire Naturelle, Paris, France (MNHN) and the freeware Annotate in order to describe and identify all the archaeocyaths from the Mexican Cambrian reef. Archaeocyaths are aspiculate sponges that lived during the Cambrian Period. They were the first animals to build reefs. In the MNHN collection, they are found as thin-sections with several archaeocyaths per thin-section (Fig. 1). Multiple individuals are grouped under a single collection number and a single species name. The list of species in the thin-section is only captured on the paper label, and cannot currently be found online. To study an archaeocyaths' reef, the archaeocyaths have to be described and identified one by one, and the location of each specimen has to be accuratly captured. Is it possible to do this with Annotate? Can a palaeontologist use only digitized specimens and Annotate to study a complete fauna of a given time and space? Annotate is an image annotation tool for the natural sciences. It allows users to measure, count, and tag all the morphological structures of an organism. Photos may be imported from the Recolnat database or users may import their own photos. Users can measure lengths, surfaces, and angles, count occurrences and add points of interest. Users can also tag the different individuals to identify them. Morphological terms may be imported as a standardized list from Xper2 or Xper3. Xper3 is a web platform that manages descriptive data and provides interactive identification keys. The results of the measurements and annotations can be exported into CSV format (comma-separated values) or into a structured descriptive data (SDD) format. To identify an archaeocyath to genus level, we need to identify morphological structures and count the occurrence of some of them, and for an identification to the species level, we need to measure different additional parts. The standardized list of morphological terms has been imported from the archaeocyaths genera knowledge base and the list of measurements has been created directly in Annotate. Lengths (e.g., pore size, cup diameter), counts (e.g., number of septae, number of pores) and points of interest (e.g., tumuli, canals, septa) are easy to use. What are the key lessons learnt to remember at the end of this study? The digitized archaeocyaths from Mexico have been identified as easily with Annotate as if a microscope and thin sections were used. The CSV export provided quick access to statistics calculations. The main difference between a microscope and Annotate is the working time. Some functionalities of Annotate are not optimized, their uses are time consuming. For instance, the importation of photos is not really appropriate for archaeocyaths studies. Two sections (transversal and longitudinal) per specimen are necessary to see all the morphological structures. These two parts of the same rock are packed together with one collection number. While users can easily switch from one section to another with a microscope, they can not with Annotate. Annotate allows only one photo per collection number from Recolnat, but not images of the two sections and their metadata. The main difference between a microscope and Annotate is the working time. Some functionalities of Annotate are not optimized, their uses are time consuming. For instance, the importation of photos is not really appropriate for archaeocyaths studies. Two sections (transversal and longitudinal) per specimen are necessary to see all the morphological structures. These two parts of the same rock are packed together with one collection number. While users can easily switch from one section to another with a microscope, they can not with Annotate. Annotate allows only one photo per collection number from Recolnat, but not images of the two sections and their metadata. Although Annotate is not an intuitive tool to use it is still very powerful however, some training is required to fully take advantage of it, and there is no documentation available. This freeware has great potential as it can assist researchers in their work and proposes an alternative to the need to travel around the world to study a fossil.

Geological and biological reasons for the collapse of reef formation, Paleozoic

Paleozoic reef formation developed cyclically, and its global termination has been caused by the biological reasons — biotic crises and mass extinctions near the borders early and middle Cambrian, Ordovician and Silurian, Frasnian and Famennian, Serpukhovian and Bashkirian, Permian and Triassic. The Early Cambrian reef formation has ended along with disappearance of archaeocyathid. In the later stages reefs were much more difficult ecosystems, and they stopped developing before the full extinction of the reef-building communities. The interruption of reef formations within the separate stages have been connected with the geological and paleogeographic reasons – volcanism, regression, climate aridization etc.

INCREASE IN CARBONATE CONTRIBUTION FROM FRAMEWORK-BUILDING METAZOANS THROUGH EARLY CAMBRIAN REEFS OF THE WESTERN BASIN AND RANGE, USA

During the early Cambrian, organisms with robust skeletons began to integrate with microbialite reef structures. Specifically, archaeocyathan sponges were among the first metazoan reef-builders. Here we investigate the transition from microbial-dominated reef environments to metazoan-based reefs from strata in the western Basin and Range of California and Nevada. This study integrates point count data from petrographic thin sections with stable carbon isotopic and elemental composition of carbonates. From the earliest reef bearing formations to the latest, metazoan framework contribution increases from zero to 29.7%. This increase is linked to the addition of new framework-building organisms, namely coralomorphs, as well as an increase in archaeocyath body size. Correspondingly, Shannon's diversity increases from 0.652 to 1.492. However, skeletal contributions from additional organisms within the reefs (e.g., trilobites, echinoderms) appear unchanged and their diversity is not correlated with framework-builder diversity. A positive carbon isotopic excursion within the Lower Poleta Formation correlates with decreases in the abundances of uranium and molybdenum that suggest a global change in organic carbon burial as opposed to localized or diagenetic factors. This allows for chemostratigraphic correlation to published carbon isotopic data and provides a proposed regional age constraint of roughly 517 million years. Overall, early Cambrian reefs at this location exhibit a pattern of increasing metazoan contribution during the transition from microbial- to metazoan-based reef support, however, diversity remained low until additional organisms evolved to inhabit these ecosystems.

Restricted morphospace occupancy of early Cambrian reef-building archaeocyaths

AbstractThe evolution of novel morphologies can signify expansion of a clade into new niches. This can be studied in the fossil record by investigating the morphospace occupancy of organisms, with small morphospaces signifying low morphological disparity and more diffuse morphospaces suggesting a broader range of morphologies adapted to different environments. Morphological disparity of many taxa (arthropods, crinoids, etc.) from the Cambrian to modern intervals have been studied in this manner. However, no study has investigated this in archaeocyaths, which, as reef builders, can have a disproportionate effect on early Cambrian biodiversity relative to their frequency. Here, we collect morphological data on archaeocyathan sponges, mostly from Laurentia. More than 600 museum specimens and 400 field samples were measured for traditional morphometric characters and discrete gross morphological characteristics. We find that archaeocyaths have an average cup/individual (body) diameter of 10.6 mm. This is significantly smaller than a selected group of modern demosponges and lithistid sponges that measure 94.1 mm and 66.8 mm in diameter, respectively, and each has a larger size variance. Archaeocyathan gross morphologies are also highly constrained to a few simple morphologies (three to six categories), while modern demosponges and lithistids are more diverse (nine categories each). These data indicate that Laurentian archaeocyaths were restricted in their morphological disparity, potentially due to limitations imposed by having a robust calcareous skeleton while still maintaining a large intervallum cavity space to facilitate passive entrainment. The fact that these Cambrian reef builders were restricted in their morphological complexity may have had a strong influence on the biodiversity of early Phanerozoic ecosystems. Furthermore, a clade limited to only a few specific morphologies is at an increased risk of extinction.

Sedimentology of reefal buildups of the Xiann\xfcdong Formation (Cambrian Series 2), SW China

The reefs in the Xiannüdong Formation (Cambrian Series 2) are the oldest archaeocyathan–microbial bioconstructions in China, but the details of their microbial structures have not been previously described. However, a new section at Tangjiahe site, northern Sichuan Province, contains very well-preserved microbial fabrics that provide these details, and is described in this study. The Tangjiahe section contains three levels of reefal buildups that were constructed by a consortium of archaeocyaths and calcimicrobes in varying proportions. The lowest (oldest) reefal buildup is a calcimicrobial biostrome, possibly in the form of a wide mound with a low relief (unconfirmed due to outcrop limitation), which was formed by Epiphyton with rare small archaeocyaths, and is sandwiched by flat-pebble conglomerates. The middle reefal buildup is a high-relief calcimicrobial mound, enclosed by oolites, that was built by intergrown Renalcis and Tarthinia. Archaeocyath fossils are uncommon, and were bound into the framework by microbial carbonates. The uppermost (youngest) reefal buildup is a low-relief archaeocyathan mound lacking calcimicrobes but partly having microbially-clotted textures attached on archaeocyaths. Calcimicrobes built or aided archaeocyaths to form the framework of Tangjiahe reefs. The three buildups formed in low-energy lagoons behind ooid shoals, and the environment was nutrient-rich due to terrigenous influx from adjacent lands. Tangjiahe reefs thus resemble most Early Cambrian reefs, in settings consistent with eutrophic, calm environments, and are characterized by the domination or aid of calcimicrobial components in framework construction.

Depauperate skeletonized reef-dwelling fauna of the early Cambrian: Insights from archaeocyathan reef ecosystems of western Mongolia

Reef-dwelling organisms in the early Cambrian include typical members of the Cambrian Fauna such as trilobites, chancelloriids, and hyoliths. However, in comparison to later Ordovician reefs, Cambrian reefs often do not appear as diverse. Here, we investigate early Cambrian microbial-archaeocyathan reefs of the Salaagol Formation in the Zavkhan Basin of Mongolia as well as perform a literature survey of 40 early Paleozoic reefs to understand the timing of reef biodiversity. By using point count analysis, we can quantify the carbonate contribution of framework builders and reef-dwelling organisms in these ecosystems. We find that Mongolian reefs contained several genera of framework building archaeocyaths (5.0% by volume) and associated microbial forms (5.4%), but are unevenly distributed through the formation. Archaeocyathan contribution increases are potentially concurrent with negative shifts in carbon isotopic composition, similar to other localities in Gondwana. However, reef-dwelling biota was minimal (0.3%) throughout the Salaagol Formation and did not correlate to changes in framework builders or geochemical proxies such as U, V, and carbon isotopes. A literature survey of Ediacaran through Ordovician reefs suggests that there is a trend of increasing reef-dweller abundance, functional richness, and skeletonization through time. The changes in abundance (17.9% to 28.8% considered “frequent”) and functional richness (3.7 to 6.0 functional groups) could be due to an increase in productivity in the oceans during the Cambrian, thereby expanding the base of the food web during the lead up to the Ordovician Radiation. The trend of increasing skeletonization could be due to the evolution of more robust skeletons among reef dwellers in order to protect themselves from increased predation in reef environments. These trends suggest that, at least during the early Paleozoic, increases in biological diversity are not necessarily concurrent with the establishment of a framework reef.

Палеогеография как инструмент реконструкции кембрийского рифообразования на востоке Северо-Тунгусской нефтегазоносной области: от истории исследований к перспективам

Палеогеография как инструмент реконструкции кембрийского рифообразования на востоке Северо-Тунгусской нефтегазоносной области: от истории исследований к перспективам

First evidence of middle to late Cambrian deposition by first palynological data from the Torgau–Doberlug Syncline (subsurface Central Germany, Mediterranean shelf of Gondwana)

Cambrian to Ordovician sequences in Germany are known from only a few regions of limited geographic extent. One of the largest of these regions is represented by the Torgau–Doberlug Syncline (TDS) where fossiliferous early to mid-Cambrian strata occur in the subsurface. Cambrian skeletal fauna and archaeocyathan-calcimicrobial reef mounds from this area indicate a West-Gondwanan Mediterranean palaeogeographic affiliation. However, organic-walled microfossils were not studied so far. New palynological research on the old and recently sampled, but hitherto non-investigated drilling core Wis BWA 1686-81 provided a moderately diverse acritarch microflora. The moderately diverse acritarch assemblage is dominated by diverse leiosphaerids and the stratigraphically important genera Cristallinium, Eliasum, and Timofeevia associated with specimens representing most probably the “galeate” acritarch plexus (particularly Cymatiogalea, Priscogalea, and Stelliferidium) and a few further acritarch genera. This microflora points to a biostratigraphic position within the Cambrian Series 3 to possibly early-Furongian age including indicating for the first time that there was deposition in this palaeogeographic region continuing to the latest mid-Cambrian. The hitherto assumed sedimentary gap from early mid-Cambrian until Ordovician which seems to be common in whole Saxo-Thuringia is now slightly shorter. The results support the model of a connected Cambro-Ordovician “Mediterranean shelf” with closely related internal areas and with well comparable early Palaeozoic sequences from Spain in the Southwest of Europe until East-Central Europe at least with the TDS as their today northernmost occurrence.

Cambrian Reefs in the Western North China Platform, Wuhai, Inner Mongolia

AbstractMid to late Cambrian thrombolites and maze‐like maceriate reefs from the western North China Platform, Wuhai, Inner Mongolia, northwestern China, occur in the middle of a succession dominated by thin‐bedded lime mudstone‐shale/marlstone alternations, and are laterally surrounded by limestone conglomerate and/or grainstone. Thrombolite, characterized by meter‐scale lenticular mounds composed of millimeter‐ to centimeter‐scale mesoclots and wackestone matrix, occurs in the lower middle part of the sequence. Thrombolite mesoclots are composed of microstromatolites with alternating dark gray and light gray micritic laminae. The maze‐like maceriate reefs occur in the middle to the upper part of the sequence, commonly forming lenticular mounds up to 1 m thick. They are characterized by centimeter‐ to decimeter‐scale branched maze‐like structures, whose biogenic portions (maceria) are selectively dolomitized. The maceriae are composed of poorly preserved microstromatolites and siliceous sponges. Inter‐macerial sediments consist of lime mud and scattered bioclasts. These Wuhai reefs are generally similar to but older than various other Cambrian reefs previously reported from the Shandong region, northeastern China.

LITHISTID SPONGE-MICROBIAL REEF-BUILDING COMMUNITIES CONSTRUCT LAMINATED, UPPER CAMBRIAN (FURONGIAN) ‘STROMATOLITES'

Laminoid, weakly fused sponge spicule networks are intercalated between convex stromatolitic laminae in an upper Cambrian (Furongian) reef. Sponges are small, millimeter to centimeter-sized lithistids that encrusted automicritic laminae and in turn were encrusted by microbial biofilms, eventually leading to columnar, crudely laminated ‘stromatolites'. Weakly fused desmas, now replaced with fine, blocky calcite, possess an arcuate geometry along the medial to distal ray axes. Minimal decay and separation of sponge tissue from these spicules produced arcuate, filament-like cavities that obscure the former presence of spicule networks when viewed in cross-section. Further deterioration produced peloidal networks and voids. These observations have important implications when reconstructing middle to late Cambrian reef-building communities. Until recently, these periods were assumed to be virtually devoid of calcified metazoan reefal components. An increasing number of recent studies, however, are demonstrating that metazoan reef-builders were more prevalent. This paper adds a unique element to the nature of framebuilding by demonstrating that lithistid sponge-microbial reef-building communities constructed laminated ‘stromatolites,' and thus may have had a flourishing ecology within late Cambrian microbial buildups.

TALES FROM THE CRYPT: EARLY ADAPTATION OF CRYPTOBIONTIC SESSILE METAZOANS

Cryptic ecosystems of modern and ancient reefs contain substantial amounts of biodiversity. It remains uncertain, however, when and how metazoans adapted to such space. Early Cambrian reef systems witnessed the rise and fall of the earliest known cryptic sessile metazoans. Subsequent Middle Cambrian to Early Ordovician microbial-dominated reefs were generally devoid of true frame-building metazoans, as well as cryptic sessile fauna. The Early Ordovician microbial-siliceous sponge patch reefs of the Dumugol Formation, Korea represent one of the oldest in situ spiculate sponge-bearing cryptic communities exploiting intraskeletal cryptic environments. Less than half of these small millimeter- to centimeter-scale crypts contain low-diversity sessile cryptic assemblages of spiculate sponges and microbialites. The cryptic sponges that attach to the walls of the cavities or on top of internal sediments do not show any skeletal distortion at their contacts with host organisms. The spiculate sponges occur both in open spaces as well as in crypts, and are interpreted to be pioneers of intraskeletal crypts after the death of the cavity-providing organisms. The behavior of occupying transient cryptic habitats is interpreted as an incipient stage of sessile metazoan adaptation to a cryptic space by an opportunistic member of the epibenthic community. This resulted in the extension of the open surface community into crypts which occurred far in advance of the eventual establishment of obligate cryptic forms. The present study provides a critical link for establishing the origin and evolutionary history of early cryptic sessile metazoan adaptation.

The late early Cambrian microbial reefs immediately after the demise of archaeocyathan reefs, Hunan Province, South China

The archaeocyath-bearing reefs that first appeared in South China in the Atdabanian were last seen in the lower Toyonian (e.g., in the Tianheban and Jindingshan formations). The upper Toyonian–lower Amgan Qingxudong Formation of Hunan Province examined in this study includes large-sized reefs formed solely by calcimicrobes of Epiphyton, Kordephyton, Girvanella, Hedstroemia, and Renalcis. During the middle and upper Cambrian, small-scale stromatolite reefs were formed. The Qingxudong reefs are therefore interpreted to be the most well-developed microbial reefs of the post archaeocyath-bearing reefs in the late early Cambrian. The transition from archaeocyath-bearing to purely microbial reefs was caused by a decline in the diversity of metazoans, and was accompanied by a reduction in the abundance of reefs, possibly related in part to late early Cambrian regression. The late early Cambrian reefs of South China contain a well-preserved record of reef succession, and provide important insights into the factors that led to the demise of archaeocyath-bearing reefs and the subsequent long-lasting hindrances to the development of skeletal-dominated reefs during the middle and late Cambrian.

Cambrian reef characteristics and their successions in Shandong Province, North China

Cambrian reef characteristics and their successions in Shandong Province, North China