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The Cambrian coral Cambroctoconus occurs selectively in the crypts of calcimicrobe Epiphyton reefs 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 in Cambroctoconus indicate 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. [ABSTRACT FROM AUTHOR]

Abstract The Gzhelian (Late Pennsylvanian) reefs of the Amushan Formation in eastern Inner Mongolia, North China, were formed by filamentous microbes and phylloid algae. Loosely accumulated prostrate phylloid algae provided substrates for growth of filamentous microbes to construct a rigid reef framework. Filamentous microbes grew parallel to the surfaces of phylloid algae, forming micritic encrustations. Subsequent developed stage of the filamentous microbes, growing perpendicularly to the phylloid algae surfaces, produced the peloid-A aggregations (concentrated areas of peloids of A type). The remaining spaces in the phylloid algal–microbial frameworks were filled by microbes that grew perpendicularly to the phylloid algae surfaces, generating peloid-B aggregations (concentrated areas of peloids of B type). Microbial sediment with various proportions of phylloid algae is a significant character of the Gzhelian buildups in the subtropical North China. North China buildups have similar reef-building organisms to the typical Tethyan buildups, but have a different construction model. As such, the subtropical North China buildups are considered as a sub-type of the tropical Tethyan buildups. [ABSTRACT FROM AUTHOR]

The Cambrian Series 3 Zhangxia Formation in Shandong Province, North China, includes small-scale lithistid sponge-microbial reefs. The lithistid sponges grew on oolitic and bioclastic sediments, which were stabilized by microbial activities. The relative abundances of microbial components (e.g. calcimicrobe Epiphyton and stromatolites) vary among the reefs. However, the microbial components commonly encrusted or bound the lithistid sponges, formed remarkable encrustations on the surfaces of the sponges. Epiphyton especially grew upward and downward. The lithistid sponges thus provided substrates for the attachment and development of microbes, and the microbes played essential roles as consolidators, by encrusting reef-building sponges. Additionally, the lithistid sponges were prone to degradation via microbial activities and diagenetic processes, and were thus preserved as micritic bodies, showing faint spicular networks or abundant spicules. Such low preservation potential within the reef environment obscured the presence of the sponges and their widespread contribution as reef-building organisms during the Cambrian. During the prolonged interval after the demise of archaeocyaths, purely microbial reefs, such as stromatolites and thrombolites have been considered to be the principal reef builders, in association with rare lithistid sponge-microbial associations. However, recent findings, including those from Shandong Province and Korea, suggest that the lithistid sponge-bearing reefs were more extensive during the Epoch 3 to the Furongian than previously thought. These lithistid sponge-microbial reefs were precursors of the sponge-microbial reefs that dominated worldwide in the Early Ordovician. [ABSTRACT FROM AUTHOR]

The Cambrian–Ordovician interval was an intriguing period of geologic time, when reef transitions occurred along with important biodiversification events. This study examines the secular reef transitions in North and South China during the Cambrian and Ordovician in terms of changes in reef construction and related geobiological changes. Microbial reefs in the Terreneuvian have not been well documented in South China. The Cambrian Series 2 interval saw the almost simultaneous appearance of reef-building skeletal archaeocyaths and calcimicrobes. The widely developed archaeocyath–calcimicrobe reefs had totally disappeared by the middle of Series 2 in South China and were followed by purely calcimicrobial reefs. During the Miaolingian and Furongian, skeletal-reef builders were commonly thought to have been inhibited owing to elevated sea-water temperature, which caused lowered oxygen. However, microbial reefs containing lithistid sponges and keratosan sponges were more common in this interval in North China than previously thought. The first skeletal-dominated reefs were established during the Early Ordovician, coincident with a decline in stromatolite reefs in the central Yangtze Platform. During the Middle and Late Ordovician interval, a variety of skeletal reef-builders with diversified calcimicrobes contributed markedly to reef construction in limited areas of both North and South China. The appearance and increase of skeletal-dominated reefs are considered to have been related to lowering global sea-water temperature. Recently reported Late Ordovician cement-rich reefs in both North and South China may contain further evidence of global cooling; however, from an ecological perspective, the novel niches and biological interactions created by early skeletal-dominated reefs may have promoted the further development of reef-building organisms. Cambrian and Ordovician reef successions in North and South China are distributed discontinuously; however, integrating information from both North and South China could yield improved understanding of reef evolution and background environments through these intervals. • Four intervals of Cambrian–Ordovician reef transitions in China are identified. • Calcimicrobial diversity changed dramatically throughout the intervals. • Stromatolite decline coincided with increasing diversity of metazoans. • Data from North and South China will improve understanding of reef succession. [ABSTRACT FROM AUTHOR]

A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton , Renalcis , and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ13C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation. • Stromatolites without calcimicrobes occur in the late Ediacaran. • Calcimicrobe-bearing thrombolites first appeared in the earliest Cambrian. • Calcimicrobial reefs formed after a strong negative shift in δ13C values and SSFs. • Microbial reefs changed almost synchronously with fluctuations in marine conditions. [ABSTRACT FROM AUTHOR]

Calcimicrobes together with archaeocyaths became significant in reef construction during the early Cambrian; however, purely calcimicrobial reefs have not been as well documented as archaeocyath-bearing reefs. Here we describe the age-specific features of calcimicrobial reefs of the Zhushadong Formation (Cambrian Series 2, Stage 4) at the Sunmayu and Jinhe sections in Shandong Province, North China. The calcimicrobial reefs of the Sunmayu section are constructed mainly by Epiphyton , Kordephyton , and tubiform microbes. The reefs are in some cases dominated by Amgaina , passing from a framework of Kordephyton , Epiphyton , and tubiform microbes to dendritic growth of Epiphyton , and finally to aggregations of Amgaina. Calcimicrobial reefs in the Jinhe section are likewise constructed mainly by Epiphyton , Kordephyton , and tubiform microbes, but with subordinate Tarthinia and Bija. Epiphyton and Kordephyton as well as Amgaina were thus the main framework builders in the Zhushadong calcimicrobial reefs. Tubiform microbes contributed as encrusters, enhancing the framework rigidity, whereas Tarthinia and Bija played subordinate roles as encrusters and consolidators. Interreef deposits of the Sunmayu reefs are of peloidal or ooidal grainstone; these reefs are inferred to have formed in a high-energy shoal setting. In contrast, interreef deposits of the Jinhe reefs are of peloidal and intraclastic wackestone or grainstone with bioclasts such as trilobites and hyoliths. Tubular cavities, possibly representing burrows, are common in the spaces among frameworks. These calcimicrobial reefs are interpreted to have formed in a subtidal setting, where the water energy was lower than that in the Sunmayu section. In North China, reefs of Cambrian Epoch 2 are limited, as development of the carbonate platform began only toward the end of that epoch. The reefs of the Zhushadong Formation are therefore among the earliest on the North China Platform. The calcimicrobial reefs contain various kinds of calcimicrobes, and the calcimicrobial assemblages are similar to or more diverse than those of coeval archaeocyath-bearing reefs during this period of archaeocyath decline. Calcimicrobes thus flourished at that time and contributed greatly to the construction of reefs. These purely calcimicrobial reefs provide further information on the spatial and temporal changes in calcimicrobial diversity and reef construction during the period before and after the extinction of archaeocyaths. • Zhushadong reefs in the Jinhe and Sunmayu sections contain various calcimicrobes. • The Jinhe reefs developed in a lower-energy setting than the Sunmayu reefs. • Calcimicrobes contributed to the construction of reefs without archaeocyaths. • The reefs are some of the oldest calcimicrobial reefs in North China. • The reefs show calcimicrobial diversity around the time of archaeocyath extinction. [ABSTRACT FROM AUTHOR]