Large dwellers of the Silurian Halysites biostrome: rhizosessile life strategies of cystiphyllid rugose corals from the Llandovery of Gotland

An exceptionally well‐preserved, unusual biostrome composed of the framebuilding cateniform tabulate coral Halysites catenularius (Linnaeus, 1767) bears an assemblage of the relatively large solitary cystiphyllid rugosan Cystiphyllum visbyense Wedekind, 1927. The corallites of solitary cystiphyllids are embedded within the ranks of the halysitid colonies, which developed on a soft, muddy substrate and in relatively turbid water. The cystiphyllid larvae successively settled mostly on the ranks of halysitid colonies and on colonies of the tiny phaceloid rugose coral Nanophyllum ramosum Johannessen, 1995, whereas calice‐in‐calice recruitment was not successful for these cystiphyllid corals. Further growth of C. visbyense was supported by rhizoid structures, which were most frequently developed on the cardinal (convex) side of the corallite. The process of formation of the rhizoid structures is here discussed and explained in detail, showing that they were formed by the extension of the basal ectodermal tissue of the polyp. The cystiphyllids, which settled on the walls of living corallites of halysitid colonies, used sweeper tentacles to kill the smaller polyps of the colony to maintain the space around them and expand. Hence, they ultimately used the halysitid colonies only as a hard substrate to stabilize their position on the soft muddy sediment.     

Stromatoporoid palaeoecology in the Frasnian (Upper Devonian) Belgian platform,and its applications in interpretation of carbonate platform environments

Abstract: Stromatoporoid faunas in the Frasnian of southern Belgium are abundant in the carbonate platform environments present in this area. Stromatoporoids dominate the large skeletal organisms and occur principally in biostromes. The stromatoporoid assemblage is represented by a small number of taxa. Stromatoporoid genera include Actinostroma, Amphipora, Atelodictyon, Clathrocoilona, Salairella, Stachyodes, Stictostroma, Stromatopora and Trupetostroma which are present in environments ranging from the outer, outer intermediate, inner intermediate and inner zones and associated biostromes. Most large skeletal stromatoporoids are low profile, which reinforces the conclusions of previous studies that low‐profile growth forms were the most successful stromatoporoid forms. These low‐profile forms are likely to have been important sediment stabilisers that may have led to expansion of the carbonate factory. Growth forms vary between facies, indicating some degree of environmental control on form; for example, laminar in the intermediate zone, bulbous and domical in the inner and outer zones. Stromatoporoid taxa vary in occurrence across the environmental gradient from shallow to deep. There is some taxonomic control on growth forms, with some taxa showing more variability than others in different environments.     

江西玉山祝宅上奥陶统下镇组蓝细菌化石及其古环境意义

江西玉山祝宅上奥陶统下镇组发育一层珊瑚生物层,是扬子板块奥陶纪以珊瑚为主的生物碳酸盐岩建造的典型代表,其形成的水深一直缺乏有效的限定。本文研究首次在该珊瑚层中发现蓝细菌,包括Girvanella和Rothpletzella。蓝细菌生活的水体深度不超过50 m,从而将该珊瑚生物层形成时的水体深度限定在50 m以内。珊瑚生物层的下伏灰岩地层产出大型腕足动物,但属种单调,不产任何珊瑚。本文研究在该层灰岩中发现的蓝细菌Hedstroemia和Garwoodia,是两个典型的潟湖相蓝细菌,因此推测该层灰岩中珊瑚的缺失不是因为水太深,而是因为环境的局限。该剖面下部有一套与页岩互层的灰岩,以前被认为是深水相的,但这次在其中发现了蓝细菌Acuasiphonoria,因此也应该是浅水相的。     

Stromatoporoid-coral intergrowths in a Silurian biostrome

Four stromatoporoid species from a stromatoporoid biostrome in the middle Ludlow Hemse Beds, Gotland, Sweden, show intergrowths with syringoporid tabulate and rugose corals, and indicate close relationships between particular coral and stromatoporoid species. The stromatoporoid Clathrodictyon convictum always contains ?Syringopora and this tabulate is rarely found in the other stromatoporoids. C. convictum is also closely associated with Tryplasma flexuosum (rugosa) while Petrozium pelagicum (rugosa) occurs only in the stromatoporoids Plectostroma intermedium and Parallelostroma typicum. The microstructure of ?Syringopora within the stromatoporoids is composed of an inner lamellar layer and an outer radial layer of calcite crystals. Diagenetic alteration has affected the microstructure which differs from recently described Devonian forms having only a radial layer. This shows variability in the structure of the tabulates within stromatoporoids. Information is sparse on the range of such variation and assessment of the relative importance of taxonomic, palaeoenvironmental and diagenetic effects is not possible in the present sample. No evidence is found to prove the precise nature of the relationships; they were not parasitic but may have been mutually symbiotic, or (most probably) commensal. The results suggest that the corals selected the most suitable stromatoporoid species for their requirements. Stromatoporoid morphology may have had an important influence on the association, where corals are more abundantly associated with those stromatoporoid species which adopted a high profile. Overall the associations appear to have allowed the corals to explore higher energy habitats otherwise unavailable to their delicate branching structure.     

Biotic interaction and synecology in a Late Cretaceous coral-rudist biostrome of southeastern Spain

A coral-rudist biostrome exposed in Campanian limestones near the village of Tabernas de Valldigna in south-east Spain was analysed with respect to its palaeontology, sedimentology and palaeoecology. Special attention was given to possible evidence for synecological interactions between corals and rudists. Changes in the rudist shell accretion process are evident in some polished slabs and thin sections and resulted from in vivo contact with coral-colonies. These unusual balcony-like shell protuberances exist where the rudist’s commissure was in contact with corals. They likely represent defence-reactions of rudists against the coral cnidia. Nevertheless, the fossil record of these biotic interactions is rare. This may be due to different growth-rates of rudists and corals, differing shape and size of interacting areas, or different life-spans. In consequence, the discrete ‘window’ of intergroup biotic interaction was small. Sedimentation and resuspension rates were high in the biostrome and corals only established pioneer associations under these unfavourable conditions. A higher diversity of corals is reached, however, when rudists are present. This increase in diversity resulted from the availability of additional ecological niches such as rudist-shell hard substrates and elevation above mobile sediment surface. Rudists on the other hand, received support from stabilisation of their shells through coral encrustation and framework building. In consequence, both groups benefited from their co-existence.     

湖南泥盆纪棋子桥期层状礁的对比研究

湖南泥盆纪棋子桥期的生物层状礁分布很广,类型也很复杂。根据它们不同的古生态特征,可分为６种类型。不同类型分布在不同的古地理位置上,分别形成不同的沉积环境。某些类型的层状礁与其后形成的过缘礁关系密切,对寻找边缘具有重要的批示意义。