Sedimentology and stromatoporoid palaeoecology of Frasnian (Upper Devonian) carbonate mounds in southern Belgium

Da Silva, A.‐C., Kershaw, S. & Boulvain, F. 2011: Sedimentology and stromatoporoid palaeoecology of Frasnian (Upper Devonian) carbonate mounds in southern Belgium. Lethaia, Vol. 44, pp. 255–274. Stromatoporoids are the most abundant large skeletal organisms in middle Frasnian carbonate mound environments of southern Belgium. They occur in environments ranging from flank and off‐mound, mound core, shallow mound and restricted mound. A detailed log and comprehensive sampling of stromatoporoids in a single section cutting through all middle Frasnian mound levels in La Boverie–Rochefort Quarry, near Rochefort and Dinant reveals a stromatoporoid assemblage comprising 10 genera; 472 samples, containing an overall total of 3079 stromatoporoids (including complete and fragmented specimens) have been studied. The following list gives abundance using numbers of specimens and areas of total stromatoporoid area on outcrop surfaces (% number; % area in cm²): Actinostroma (0.4; 9.2), Amphipora (15.5; 1.7), Atelodictyon (0.2; 4.4), Clathrocoilona (0.3; 0.5), Euryamphipora (13.7; 0.7), Idiostroma (2; 1.9), Salairella (1.2; 9.6), branching Stachyodes (43.2; 59.1), laminar Stachyodes australe (1.9; 1.3), Stictostroma (4.8; 13.1) and Trupetostroma (0.2; 0.8), showing that Stachyodes is approximately half of the total assemblage. Deeper environments contain more abundant low profile forms, shallow water facies contain more domical and bulbous forms; branching forms are ubiquitous. Low profile stromatoporoids are likely to have been important sediment stabilizers that may have led to expansion of the carbonate factory, and they may have therefore contributed to the structural building of the mounds. Stromatoporoid‐coral intergrowths are observed in only Stictostroma suggesting that there is a close biological relationship between them; however, stromatoporoid skeletons in almost all cases appear to be unaffected by the presence of intergrown corals, suggesting they were commensals. □Frasnian, Late Devonian, mounds, palaeoecology, stromatoporoid.     

Paleoecology of Sphenothallus on an Upper Ordovician hardground

A hardground from the Upper Ordovician Dillsboro Formation near Dillsboro, Indiana, U.S.A., preserves an assemblage of encrusting and boring fossils on both top and bottom surfaces. The slab is inferred to have been an undercut ledge, and the dominant fossils of the assemblage, holdfasts of the tube-building worm Sphenothallus and trepostome bryozoans, are prevalent on both sides. The clumping of Sphenothallus holdfasts has been statistically demonstrated using a nearest-neighbor technique. Sphenothallus has also been shown to withstand overgrowth in interactions with bryozoans.     

Middle Ordovician reefs of Norway

The Middle Ordovician reefs of Norway were the first to develop in the western part of the Balto-scandian epicontinental sea and are the earliest coral-stromatoporoid reefs so far reported in Europe. Small patch reefs in the Steinvika Limestone, Langesund-Skien district, consist mainly of algae, echinoderms, corals and stromatoporoids. Bryozoans, molluscs, arthropods and brachiopods are also present. The reefs developed on pelmatozoan-rich substrates and are organically zoned, consisting of a pioneer community of stemmed echinoderms and sheet algae, a high-diversity intermediate community dominated by fasciculate corals and a low diversity climax community of massive corals and stromatoporoids. These communities are interpreted as the seral stages of an autogenic ecological succession. Small patch reefs are also present in the laterally equivalent Mjøsa Limestone, Toten and Nes-Hamar districts. These are organically very similar to those in the Steinvika Limestone and developed in an identical way. A large complex, consisting of several reefs, is also present in the Mjøsa Limestone. Unlike the reefs elsewhere, which developed within shallow inshore areas, this complex developed at the outer edge of the inshore shelf. The outstanding feature of the complex is the main reef forming the offshore limit which is totally dominated by stromatoporoids and lacks a sequential development. This is due to the influence of the harsher environment at the shelf edge.     

Parameterization of stromatoporoid shape

Kershaw, Stephen & Riding, Robert 1978 07 15: Parameterization of stromatoporoid shape. Lethaia , Vol. 11. pp. 233–242. Oslo. ISSN 0024–1164.
Stromatoporoid cross-sectional shape can be considered as a product of the interaction of three morphological variables: the relative proportions of basal, vertical and diagonal dimensions. The gross arrangement of internal lamination can be superimposed upon the resulting outlines. These shapes are taken to represent cross sections in any vertical plane through the centre of the coenosteum. This simple parameterization scheme is presented in triangular arrays which include the stromatoporoid morphotypes laminar, domical and bulbous, and varieties of them together with forms not utilized by stromatoporoids. Dendroid and irregular forms are too complex to be readily included in the scheme. Smooth and ragged varieties of laminar and domical forms are distinguished and related to sedimentation on the flanks of the coenosteum. Two types of mutual arrangement of latilaminae within the coenosteum are recognized: enveloping, where they completely overlap previous latilaminae, and non-enveloping, where they do not. Laminar-domical-bulbous forms represent a series generated mainly by reduction of the basal dimension. They commonly exhibit enveloping latilaminae except in ragged varieties and some extended domes. Measurement of defined dimensions in the field allows stromatoporoid morphotypes to be plotted onto the triangular arrays and provides a rapid method of displaying the range of forms present.     

Intracoenosteal variation in a Devonian stromatoporoid

Fagerstrom, J. A. & Saxena, K. M. L.: Intracoenosteal variation in a Devonian stromatoporoid.
Statistical analysis of four different features of coenosteal morphology measured in sixteen randomly distributed vertical and tangential thin sections from a specimen of Syringostroma sherzeri (Grabau) from the Detroit River Group near Ingersoll, Ontario, indicates that although the degree of variation is very high, the variation in a single section is representative of the variation in the entire coenosteum. This discovery may therefore be regarded as a partial vindication of current practice in stromatoporoid taxonomy of baaing identifications on examination of one vertical and one tangential sectior. Furthermore, this study indicates that certain morphological features are less variable than others and are therefore of greater value as taxonomic criteria. In cases where the number of measurements per section is small, tests of the significance of differences based on a comparison of the medians, rather than means, is preferred because such tests do not assume that the data are normally distributed.     

Silurian brachiopods in a stromatoporoid bioherm

Ludlovian limestone at Ljugarn, Gotland. repreqents a shallow marine environment of stromatoporoid biohenns and flanking areas of mud and stromatoporoid debris. Brachiopods were restricted to surfaces of the debris deposits. Lissatrypa sulcata, a smooth atrypacean, dominated the brachiopod fauna, with individuals living beak downward in high density clusters, developing weaker pedicle attachment and a sulcus during ontogeny. Surrounding low density areas of Lissatrypa included small, pedicleattached, non-sulcate shells. Howellella, Spirigerina, and Morinorhynchus? lived in isolated clusters near low density Lissatrypa populations, and 10 other very rare brachiopod species may have been non-reproducing members of the community. Brachiopcd species diversity, and thus number of niches, was lower in the biohermal enviroment than in bathymetrically equivalent, level-bottom communities.     

Improving the parameterization of stromatoporoid shapes – a detailed approach to stromatoporoid morphometry

Eight specimens of Devonian stromatoporoids with well visible latilaminae arrangement exposed on polished slabs were subjected to a detailed morphometric analysis. The studies have revealed that the so far used stromatoporoid parameterization method leaves a broad field of uncertainty concerning the exact definitions of particular parameters. The possible ways of making the measurements and the propositions of more precise definitions of the parameters used in the method, both of the growth form above the sea bottom and of the whole skeleton are therefore presented in this paper. The B_m - basal length of the skeleton, has been defined as a straight line joining the two ends of the basal surface, which have been pointed basing on the stromatoporoid's overall shape, latilaminae arrangement and angular relation to the main growth axis position in ontogenetical development, length. The B_n - basal length of the growth form above the sea bottom, is to be measured along a straight line joining the ends of the last visible latilamina. From among a number of possible definitions of the V_m - vertical height of the whole skeleton, a straight line joining the initial growth nucleus and the highest point on the specimen's surface above the B_m line has been selected. The V_n - vertical height of the growth form above the sea bottom is defined as the height of the point on the stromatoporoid surface highest above the B_n measured perpendicularly to it. A parameter of burial ratio, BR=(V_m-V_n)/V_m, is introduced.     

Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square‐meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.     

Paleoecology of Boring Barnacles

The morphology of acrothoracican boreholes primarily expressesa mechanical mode of boring. Paleozoic representatives raspedonly in the vertical plane, while Mesozoic forms developed anadditional horizontal rasp to carve out the cavity for the ovaries.Boring barnacles strongly prefer shells of live hosts to inorganicand dead substrates, and epibenthonic shells to other ones,but otherwise show little host specificity. Only a few speciesseem to have developed close commensal relationships with specifichosts, such as burrowing echinoids, nectonic belemnites, andhermit crabs.     

Carbonate mud mounds,conglomerates, and sea-level history in the Katian (Upper Ordovician) of central Sweden

The Katian (Upper Ordovician) facies succession of the Siljan district, central Sweden, records some of the most prominent environmental changes in the Ordovician of Baltoscandia. These changes include two separate phases of major sea-level drawdown that were of basinwide and presumably global importance. The first regression and lowstand terminated an entire generation of carbonate mud mounds (the Kullsberg Limestone) and resulted in the formation of polymict carbonate conglomerates (Skålberg Limestone) belonging to the Amorphognathus superbus Zone. New stable isotope data from the Amtjärn quarry shows that this is immediately after the peak of the Guttenberg Carbon Isotope Excursion (GICE), which reaches a δ¹³C peak value at 3.3‰ in the uppermost Amorphognathus tvaerensis Conodont Zone. A second major regression and sea-level lowstand is manifested by palaeokarst morphologies in the Slandrom Limestone, which formed close in time to the comparably minor Waynesville positive carbon excursion in the basal Amorphognathus ordovicicus Conodont Zone. The widespread exposure associated with this latter lowstand terminated carbonate production in much of the basin, and, during the subsequent flooding, organic-rich, graptolitic shale formed across most of Baltoscandia. The two corresponding sequence boundaries are amalgamated at the top of truncated carbonate mud mounds in the Siljan district, resulting in a pronounced Middle Katian hiatus in the immediate mound areas.     

Phylogenetic relationships of Early–Middle Ordovician ostracods of Baltoscandia

Phylogenetic analysis of the Early and early Middle Ordovician (Tremadoc and Arenig) ostracod species of Baltoscandia suggests a polyphyletic origin for the suborder Beyrichiocopa. Binodicopes, leiocopes and eridostracans are excluded from the beyrichiocopide clade. An independent origin from the basal ostracods is suggested for the binodicopes and eridostracans. The palaeocopes form a strongly supported monophyletic clade. Within this suborder, the ctenonotellid and the tetradellid families together form a monophyletic clade. The tetradellids are paraphyletic, being a stem-group for the ctenonotellids. Nanopsis nanella , the earliest known ostracod from the Tremadoc, is a basal palaeocope. The early eridostracans Conchoprimitia and Incisua , with their uncomplicated carapace morphology, might be the most primitive ostracods.     

Identification of calcified coccoid cyanobacteria forming stromatoporoid stromatolites

A simple technique is described enabling the identification in scanning electron micrographs (SEM) of combined Iractured and etched stromatoporoid sections, the cyanobacteria-hkc coccoid aggregates building the entire stromatoporoid structure. The coccoid aggregates from stromatoporoids are closely related to extant calcified coccoid cyanobacteria (Plcurocapsales) forming stromalolitic mats in Laguna Mormona (Baja California) and Sabkha Gavish (Sinai). Stromatoporoids, stromatolites, cyanobacteria.     

AnOistodus venustus-like conodont species from the Middle Ordovician of Baltoscandia

Ordovician conodont specimens resemblingOistodus venustus Stauffer, 1935 have been reported from many areas. There is increasing evidence, however, that several lineages with homeomorphic conodont elements have erroneously been referred to one and the same species. I have investigated Baltoscandian conodont elements of this kind in order to find out about their origins and phylogenetic relationships with morphologically comparable elements from other areas. A natural grouping of finds from the Middle and lower Upper Ordovician of Baltoscandia is here described as belonging to a new species of a new genus,Venoistodus balticus n. gen., n. sp. The new species probably evolved paedomorphically fromDrepanoistodus forceps (Lindström, 1955) in the Early Ordovician.     

Faunal distribution and colonization strategy in a Middle Ordovician hardground community

The attached fauna of one of the many hardgrounds from the Galena Group (Trentonian Substage) of the Upper Mississippi Valley is described. The fauna is composed of three principal elements, viz. (1) borers, including Cicatricula retiformis ichnogen. et ichnosp. nov., (2) pelmatozoans with encrusting holdfasts, and (3) bryozoans. Analysis of the distribution of members of each population on the hardground shows that most are strongly aggregated. The nature of, and reasons for, such aggregations are considered in the light of comparable Recent shallow-water marine populations. The community on this hardground, and those on other Galena Group hardgrounds, are immature. This is a consequence of frequent and damaging scour, which these organisms were poorly adapted to resist.     

Paleoecology of benthic community replacement

The literature of community paleoecology is filled with examples in which long-term environmentally-controlled faunal transitions are misidentified as forms of ecologic succession. This has obscured a fundamental community-level process - community replacement - involving gradual to abrupt substitution of one benthic community for another as a result of subtle to sharp changes in habitats over subevolutionary time. In gradually changing environments, replacement takes place through conformational reorganization of species-abundance distributions within established communities, yielding sequences of slightly different fossil associations. Environments that change very rapidly drastically feature a different type of community replacement involving species turnover, wherein environmental tolerance limits of community members are closely approached or exceeded. Paleoecologists should be alert to the strong likelihood that many temporal transitions involving autochthonous fossil associations are, in fact, community replacement sequences.