Colonial Corals Research Articles

Colonial Rugosa from the Early Devonian (Pragian) of the Zeravshan Range, Tajikistan

Schröder, S., June, 2007. Colonial Rugosa from the Early Devonian (Pragian) of the Zeravshan Range, Tajikistan. Alcheringa 31, 121-151. ISSN 0311-5518. The colonial rugose corals of the Shishkat fauna (Pragian, Kshtut Formation) from the Zeravshan Mountains are compiled and redescribed according to current taxonomic standards. The fauna can be categorized as a so-called ‘Carlinastraea-Fauna’, and a generally comparable generic composition is recognized on a global scale. Some Silurian relictual genera such as Maikottia or Pycnostylus are recorded. Of the 15 taxa discussed, Australophyllum soghdianum sp. nov. is described. All other specimens are assigned to known species or discussed in the context of open nomenclature. The Shishkat-fauna is clearly dominated by the Ptenophyllina (especially Carlinastraea and Spongophyllum), and Ptenophyllidae (Australophyllum, Xystriphyllum and in particular Lyrielasma). A remarkable occurrence of Vepresiphyllum indicates a weak faunal relation to eastern Australia. All other taxa show close affinities to coeval associations in the Urals, but especially to those from the Turkestan Range and the south Fergana Valley. The treatment of the Fasciphyllidae is supplemented by a short rediscription of the Turkestan Fasciphyllum maikottaense Lavrusevich, 1972 because its type material is lost.

Faunal turnover and changing oceanography: Late Palaeozoic warm-to-cool water carbonates, Sverdrup Basin, Canadian Arctic Archipelago

Carbonate sedimentary rocks in the Fosheim–Hamilton sub-basin at the eastern end of the Sverdrup Basin contain a particularly sensitive record of changing palaeoceanography and faunal turnover during the northward movement of Pangaea through the Late Palaeozoic. Carboniferous and earliest Permian inner ramp carbonates are a photozoan association of colonial corals, fusulinids, algae and ooids, with sparse heterozoan elements (crinoids, brachiopods and bryozoans). Based on modern analogues these biotic components reflect warm-water oligotrophic marine environments. Contemporaneous mid-ramp and basin biotas are dominated by a heterozoan fauna (solitary corals, crinoids, bryozoans and brachiopods) that grew in cooler deeper waters that may have been beneath a thermocline. Initial cooling began in the latest Sakmarian with an influx of cool deep water and climatic deterioration allowing cool and warm waters to mix, disrupting the thermocline, and producing a sub-tropical marine biota whose remains were reworked by storms and re-deposited as tempestites. Faunas are heterozoan (crinoids, bryozoans, brachiopods and sponges) with photozoan elements (colonial corals and symbiont bearing fusulinids) prominent only in inner ramp facies. By late Artinskian these photozoan components are only present in shallowest basin-margin environments. Artinskian subtropical waters contained somewhat elevated trophic resources that together with mixing of photozoan and heterozoan elements in the subtropical inner ramp produced the greatest faunal diversity seen in the Sverdrup Basin. The biota was completely boreal in nature, and warm shallow waters with a warm refuge biota, were now supplemented by cool waters derived from the Panthalassa Ocean. Profound change took place in the Kungurian, coincident with localised uplift during the Melville Disturbance and increased siliciclastic input. Neritic, storm-dominated sediments are wholly heterozoan (brachiopods, bryozoans, crinoids and sponges) with no remaining photozoan elements. Nutrient levels are elevated (but not eutrophic) as indicated by phosphate and glauconite, extensive bioerosion, numerous infaunal foraminifers, and an impressive and diverse shelly biota, that during times of high siliciclastic input was replaced by a profuse Zoophycos infauna. Brachiopod and bryozoan faunas are distinctly boreal. Continued cooling, relatively high nutrients, as indicated by abundant glauconite and phosphate, and decreased siliciclastic input eventually resulted in an abundant heterozoan biota (brachiopods, bryozoans, crinoids and sponges) with local gigantism, but reduced brachiopod and bryozoan diversity. These Middle Permian rocks may be the coolest water deposits in the Sverdrup Basin. The faunas are still part of a boreal Arctic province, with strong ties to cool water faunas of Uralian basin. The succession ends with Late Permian biosiliceous spiculitic cherts containing accessory heterozoan components (brachiopods, bryozoans and few crinoids). These sediments, in part coeval with evaporites and terrestrial red beds on the Eastern European Platform, are interpreted to reflect a combination of cool waters, elevated nutrients, and mid-latitude storminess.

Coraux du Barrémien du Sud de la France (Ardèche et Drôme)

Corals from the Barremian of southern France (dépt. Ardèche and Drôme) are described. The rather small fauna of colonial corals encompasses 23 species belonging to 18 genera of both Hexa- and Octocorals. The assemblages from the lower as well as upper Barremian show stratigraphic relationships to those of the Hauterivian and Aptian of the Tethys and the Caribbean province.

Shallow-marine carbonates of the tropical-temperature transition zone: effects of hinterland climate and basin physiography (late Miocene, Crete, Greece)

Abstract In modern oceans, the transition zone between the tropical and temperature carbonate province is gradual and covers a wide latitudinal belt. Little knowledge exists regarding the geological signatures of this zone. This paper describes a late Miocene (early Tortonian-early Messinian) transitional carbonate system that combines elements of the tropical and cool-water carbonate systems (Iraklion Basin, island of Crete, Greece). As documented in stratal geometries, the submarine topography of the basin was controlled by tilting blocks. Coral reefs formed by Porites and occurred in a narrow clastic coastal belt along a central Cretan landmass and steep escarpments formed by faulting. On the gentle dip-slope ramps of those blocks having the widest geographical distribution within the basin, extensive covers of level-bottom communities existed in a low-energy environment. Isolated colonial corals were present in the shallow segments of the ramps. Consistent patterns of landward and basinward shift of coastal onlap in all outcrop studies reveal an overriding control of third- and fourth-order sea-level changes on sediment dynamics and facies distributions over block movements. An increasingly dry climate and the complex submarine topography of the fault-block mosaic kept sediment and nutrient discharge from a central Cretan landmass at a minimum. The skeletal limestone facies therefore reflects oligotrophic conditions and sea surface temperatures near the lower threshold temperature of coral reefs in a climatic position transitional between the tropical coral reef belt and the temperate zone. It is suggested that the recognition of an overall late Miocene aridification trend helps to explain the Mediterranean-wide distribution of shallow-marine carbonates, both cool-water and warm-water, in settings adjacent to uplifting mountain ranges (intramontane basins).

The Santonian-Campanian phosphatic chalks of England and France

Phosphatic chalks were deposited during the mid-Santonian to early Campanian between 85 Ma and 83 Ma in NE France and southern England. The beds contain abundant brown granular phosphate, largely consisting of phosphate-filled and coated foraminiferal tests, phosphatized macrofossil fragments, faecal pellets, phosphatized intraclasts and vertebrate remains. Phosphate contents typically range from 5–20% P 2 O 5 . Beds 1–20 m thick occur within elongate 1 km long, 250 m wide and 30 m deep erosional basins (‘cuvettes’), cut into the underlying Coniacian-Santonian white chalks. Several superimposed cuvettes with consistent orientations may occur. Cuvettes are floored by phosphatized, bored and encrusted hardgrounds, and contain unique faunal assemblages that include ahermatypic colonial corals, and encrusting oyster and worm patches. Belemnites, fish and reptile remains are common. Extensive reworking by currents and biota occurred during the deposition of the phosphate. Gravity flow led to the dislocation and redeposition of basal hardground blocks and the formation of synsedimentary slump folds. Phosphatic chalks accumulated in water depths of between 70 m and a few hundred metres in oxic, current-swept areas of an open epicontinental sea of normal salinity. High surface productivity, moderate organic-carbon fluxes, reduced sedimentation rates and high rates of sediment mixing led to the precipitation of phosphate within hardground surfaces and carbonate grains. Bacterial decomposition of organic matter raised phosphate contents in pore waters and promoted the geochemical conditions necessary for phosphate precipitation. Phosphate precipitates, now transformed to carbonate fluorapatite, include um-sized ovoid bodies and massive coatings interpreted to be of microbial origin. Phosphatization was a rapid process occurring under mixed aerobic/anaerobic redox conditions in the upper decimetre of the sediment column, close to the sediment-water interface. Phosphatization took place predominantly within the confines of the cuvettes, where phosphate grains became concentrated by current winnowing and transport. Phosphatic chalk sedimentation occurred on the NE and northern margins of the Anglo-Paris Basin following regressive events that enabled bottom currents to erode the cuvettes and maintain productivity and the other sedimentary conditions necessary for phosphate accumulation.

JMBA2 BIODIVERSITY RECORDS 2005

The known geographical distribution of the colonial scleractinian coral Cladocora debilis Milne Edwards & Haime, 1849 has been extended with new record from the eastern Adriatic Sea. This scleractinian coral was found in spring 2002 on the cliff at the south of Lastovo Island (South Adriatic). The new record reported here is the first confirmation of C. debilis presently living in the Adriatic Sea.

Ontogenetic Changes in the Capacity of the Coral Pocillopora damicornis to Originate Branches

Most colonial corals vary intraspecifically in growth forms, and the diversity in branching morphology is especially striking. While the effects of environmental factors on growth forms have been studied, the genetic control of coral branching patterns has received little attention. The discovery of ontogenetic changes in the capacity to originate branching would set the stage for studies of how branch formation is genetically controlled. During experiments investigating contact reactions in the coral Pocillopora damicornis, we observed that young colonies derived from settled planulae and colonies regenerated from adult branch tips assumed different growth forms. Young colonies formed at least one branch from the central region of the colony, while colonies regenerated from adult branch tips (3-5 mm long) did not form branches during the 9-month observation period. This pattern was invariable, regardless of the types and outcomes of the contact experiments or the orientation of the branch tips. However, some fragments taken from 1- or 2-year-old colonies formed branches. This suggests that the rate of branch formation in P. damicornis colonies decreases with age. These findings will facilitate investigations of the mechanism of coral branch formation at the molecular level.

Diagenesis of growth bands in fossil scleractinian corals: identification and modes of preservation

Light and dark bands in fully recrystallized fossil hermatypic corals are generally interpreted to represent annual growth increments reflecting a photosymbiotic life style—an interpretation of far reaching significance in palaeoecology. In this paper we describe annual growth bands in the colonial coral Porites in a perfect (aragonite and microstructures retained) and fully recrystallized (sparry calcite mosaic) style of preservation from sediments of Late Miocene age (Crete, Greece). Analysis of a continuous spectrum of transitional preservational stages shows that in Miocene Porites preservation of the growth banding was controlled by preferential dissolution of the high-density band associated with cementation by drusy calcite spar during freshwater diagenesis/shallow burial diagenesis. Marine precipitates (pelletoidal Mg-calcite) preferentially accumulated along tabulate dissepiments producing an additional growth rhythmicity. Massive Porites had annual growth rates of ∼4.0 mm, whereas in ramose branching Porites, a conspicuous banding is formed by concentrations of marine micropelletoidal cement along dissepiments at ∼1.8 mm spacing. If taken as annual growth increments, these bands represent very low extension rates, however, they may rather reflect subannual forcing functions (i.e., lunar cycles). An identical scenario of precipitation and concentration of pelletoidal carbonate along dissepiments and dissolution-controlled documentation of growth bands can be inferred for Late Jurassic microsolenids. Therefore, growth bandings in fossil corals potentially reflect both, monthly and annual cycles. Consequently, care must be taken when using coral growth bands in palaeoecology and palaeoclimatology.

Intraspecific Contact and Egg Production in the Massive Coral Goniastrea Aspera in Okinawa, Subtropical Japan

The effect of intraspecific contact (Contact) on egg production was examined in the massive coral Goniastrea aspera in Okinawa, subtropical Japan. The contact was non-aggressive without damaging soft tissues each other. Within Contact colonies, polyp volume, polyp fertility (%polyps with gonad), and NE/PV (number of eggs per polyp volume) were significantly smaller in marginal (Mg) polyps without direct intraspecific contact than other polyps, but no difference was found between non-marginal and Mg-Contact (marginal with direct intraspecific contact) polyps. Comparisons of non-marginal polyps (non-marginal and Mg-Contact polyps were combined in Contact colonies) between Non-Contact and Contact colonies showed that fertility and NE/PV were significantly larger in Contact colonies than in Non-Contact colonies, but polyp volume were not different significantly. Further analyses dividing colonies at Non-Contact maturation colony size (60 polyps) revealed that fertility and NE/PV were significantly larger in Contact colonies than in Non-Contact colonies only in the small colonies (<60 polyps), indicating that the intraspecific contact promoted sexual maturation at smaller colony size; one polyped Contact coral was also reproductive. The lack of correlation between polyp volume and NE/PV in the small Contact colonies, and the similarity of NE/PV in non-marginal and Mg-Contact polyps within a colony, suggest that the maturation at smaller size in Contact colonies is realized by reproductive integration of polyps at the colony level. The present results show that size-structured populations such as colonial corals may show phenotypic diversity in key demographic parameters, such as reproductive output, dependent on ecological conditions.

CLASSIFICATION AND CONTROLS OF INTERNAL BANDING IN PALAEOZOIC STROMATOPOROIDS AND COLONIAL CORALS

: Palaeozoic corals and stromatoporoids exhibit a variety of internal banding phenomena, many of which have been commonly interpreted as annual growth bands. We evaluate bands through analysis of colonial corals and stromatoporoids from three stratigraphic intervals: Upper Ordovician of Manitoba Canada, and Llandovery–Wenlock and Ludlow of Gotland, Sweden. Banding features are divided into four categories: (1) absence of banding; (2) density banding formed by variation in density or form of elements; (3) growth-interruption banding indicating growth cessation and regeneration; and (4) post-mortem banding caused by compaction or diagenesis. For discrimination of band types, it is essential to examine internal structures and skeletal margins in thin sections or acetate peels. Species vary considerably in degree and type of banding; each has a distinct pattern of variation. We propose criteria to determine if banding is consistent with seasonally induced growth variation: (1) consistency in band character and thickness; (2) continuity of skeletal growth; (3) marginal features; and (4) evidence of diagenetic alteration. Density bands in tabulate and rugose corals probably represent annual growth variations, but results for stromatoporoids are more ambiguous; although stromatoporoids commonly show banding, unequivocal density banding is poorly developed and growth interruption generated most stromatoporoid banding. Cerioid rugose and tabulate corals possess the thickest density bands; the thinnest bands are in stromatoporoids and heliolitid tabulates.

Scleractinian assemblages under sediment input: their characteristics and relation to the nutrient input concept

In the geological record, scleractinian-dominated “turbid-water” bioconstructions that accumulated under substantial terrigenous input, along with shallow neritic marls rich in well-preserved corals are common. These occurrences indicate that beyond some threshold, the ecologic and taphonomic effects of sedimentation prevail over the effects of nutrification. Field data and experiments on recent corals show that some taxa cope well with terrigenous turbidity and sedimentation, and acclimate to increased heterotrophy. Colonial corals resilient to sediment input commonly are massive to platy forms of high skeletal plasticity and with large polypars, to effectively reject sediment, or are branched species providing a very small sedimentation area cleaned by feeble currents, or small-polyped massive taxa that tolerate intermittent veneering by sediment. These corals may comprise “siltation assemblages” of recent turbid-water reefs and level-bottoms, in siliciclastic environments. Conversely, excess input of even low amounts of dissolved inorganic nutrients into clear waters adversely affects many coral species. Fossil turbid-water bioconstructions (TWB) were situated in inner shelf to shore zone settings or near storm wave base on shelves or ramps, did not stack into reef complexes, mostly show a cluster to segment fabric, are scarce in or devoid of frame pores with marine cement, and lacked a high carbonate slope. Relative to fossil “clear-water” buildups preserved in pure limestone successions, fossil TWB contain (moderately) diverse coral assemblages, with an increased proportion of sediment-resistant forms. Aside from coral behaviour that is elusive in fossils, the sediment resistance of corals was mainly determined by colony shape and polypar size, but small-polyped, sediment-tolerant corals became common since the Early Cretaceous. Late Jurassic to Cretaceous turbid-water coral assemblages are dominated by plocoid or thamnasterioid forms; corresponding Cainozoic assemblages consist mainly of cerioid and plocoid taxa. In fossil TWB, bioerosion and encrustation are similar or higher than in contemporaneous clear-water buildups. Under higher sediment input, accumulation of discrete TWB was quenched, and level-bottoms of both corals and soft-substrate biota formed. In the resulting “coral marls”, depending on turbidity, sedimentation and substrate stability, the coral fauna consists largely of both solitary corals and (mainly) sediment-resistant pseudocolonial/colonial forms, or is dominated by solitary corals. Many corals show growth “anomalies” resulting from partial mortality due to episodic, rapid sedimentation and unstable substrate. In coral marl environments, because of terrigenous input, nutrient levels probably were elevated, yet macroboring and encrustation are very scarce. Beyond some threshold of terrigenous sediment input and accumulation, the ecologic and taphonomic effects of sedimentation and unstable substrate prevail over the effects of nutrient enrichment. During the Meso-Cainozoic evolution of scleractinians, increasing photoautotrophy and progressive invasion of oligotrophic environments is in contrast to a Late Cretaceous to Cainozoic increase in the relative number of reefs in marginal-marine/siliciclastic settings. This trend may be explained by second-order sea-level fall and shrinking of epicontinental carbonate seas and isolated platforms, driving corals to more narrow, attached shelves, and by a wider total trophic range in which distinct coral assemblages can thrive, as a result of coral evolution and diversification.

Shallow-buried Pleistocene Madrepora-dominated coral mounds on a muddy continental slope, Tuscan Archipelago, NE Tyrrhenian Sea

Subfossil azoxanthellate deep-sea coral mounds occur at 355–410 m on the continental slope of the NE Tyrrhenian Sea between Gorgona and Capraia islands, Tuscan Archipelago. These low-relief patch reefs are at present buried by a thin muddy drape. Their age is latest Pleistocene. The colonial scleractinian Madrepora oculata is the major frame builder, in association with the solitary coral Desmophyllum dianthus and the colonial coral Lophelia pertusa. These NE Tyrrhenian Madrepora-dominated coral mounds represent one of the few known Mediterranean examples of deep-coral colonization of a muddy, low-gradient continental slope.

Paleoecology of an early Miocene, rapidly submerging rocky shore, Motuketekete Island, Hauraki Gulf, New Zealand

More than 70 macrofossil taxa (including 14 bivalves, 6 gastropods, 8 corals, 4 echinoderms, and 10 barnacles) are recorded from early Miocene (Otaian) Kawau Subgroup strata (Cape Rodney Formation and Motuketekete Limestone, lower Waitemata Group) at Motuketekete Island, Hauraki Gulf, north of Auckland City. Both in situ and transported fossils occur in deposits of greywacke boulder conglomerate, cobble to pebble conglomerate/sandstone, bioclastic calcareous grainstone, and an allochthonous breccia debris event unit, which correspond to lithofacies A, C, D, and E, respectively, of Ricketts et al. Greywacke boulders accumulated at the base of a greywacke paleocliff or sea stack that was planed off at its top to form a shore platform during the Miocene. A >2 m long, 16 cm thick coral colony grew atop a mixed substrate of boulders, pebbles, and sand, and exhibits two successional regrowth phases following debris‐influx events. Boulders and cobbles bored by pholadid bivalves (Parapholas aucklandicum Powell) are common in these basal bouldery talus deposits. A diverse suite of macrofossils, including chaliciform corals, occurs in somewhat finer grained deposits that buried the greywacke basement and boulder talus pile, and indicates either slightly deeper or more turbid conditions in the shallow photic zone. The cross‐bedded, bioclastic Motuketekete Limestone overlies these coarse‐grained Cape Rodney Formation units. Its fauna indicates deepening, with replacement of shallow by offshore taxa (e.g., Crenostrea with Bathylasma), as the Waitemata Basin underwent rapid tectonic subsidence and redeposition of sediments with the onset of subduction along the Hikurangi convergent margin. A newly identified lens of “upper breccia” (lithofacies E) in the Motuketekete Limestone contains rounded blocks of colonial coral and Tertiary siltstone. The Motuketekete occurrence of lithofacies E extends the known geographic range of this geologically instantaneous deposit; it records a regional tectonic event that is interpreted as an avalanche/debris flow triggered by faulting. The breccia appears to be a reliable marker for local lithostratigraphic correlation in lower Waitemata strata exposed north of Auckland. Appendices provide a systematic analysis of the barnacle fauna and the description of a new gastropod species, Bolma (Bolma) ballancei. The overall biotic content of the Kawau Subgroup, especially those taxa associated with hard substrate community development, indicate warm subtropical conditions similar to those found elsewhere in northern New Zealand during the early Miocene.

Regular and flexible modes of division and hystero‐ontogenetic growth in the Silurian rugose coral Stauria favosa

New modules arise in colonial corals as the result of asexual reproduction. The Silurian rugosan Stauria favosa ordinarily exhibits cerioid coralla with a characteristic cross‐shaped axial structure and a typical pattern of parricidal increase. Quadripartite increase at the sites of the four protosepta is most common, whereas cases of tripartite increase are rare. Parental protosepta are transformed into dividing walls, where the four protosepta first appear with a definite polarity in offset corallites. Daughter corallites inherit metasepta as metasepta, and catasepta as catasepta, within the same quadrants as those of the parent. Metasepta are inserted serially, following Kunth's rule, as is characteristic of rugosan protocorallites. As each daughter corallite derived immediately from the same parent is arranged with identical polarity, it grows equally and evenly both individually and as a group. Daughters thus form protosepta and metasepta under strict phylogenetic and developmental constraints. However, individual corallites grow and reproduce autonomously, by using all available skeleton and space of the parent. Although each module cannot modify essential modes of division, flexibility of the system was via changes in the density and arrangement of corallites, and regulating modes of growth, in tandem with adjacent corallites within the corallum. It is probable that regularity, due to constraints of several origins, as well as flexibility are typical of other rugosan colonies and played an important role in growth dynamics between corallites and corallum.

Toe-of-slope deposits of a Givetian reef-rimmed platform: provenance of calcareous density-flow deposits (Rabat-Tiflet-Zone, Morocco)

Givetian subaqueous density-flow deposits reveal the existence of a peritidal carbonate platform in sedimentary basins preserved within the Rabat-Tiflet-Zone of Morocco. The calcareous component assemblage displays a photozoan carbonate production mode of the neritic source environments. Characteristic elements of the allochthonous faunal association are colonial tabulate corals, stromatoporoids, crinoids, bryozoans and thick-shelled brachiopods. Active growing reefs and cortoid sand shoals at the platform margin as well as periplatform carbonates at the uppermost slope settings contributed bioclastic and lithoclastic lime debris to the toe-of-slope of the carbonate apron. Bipartite cobble rudstone beds are interpreted as deposits of hyperconcentrated density flows, which cannot be maintained on very low-angle slopes for as long as more dilute flows and represent short run-out distances. Beds consisting of mostly well-organized pebbly grainstones, packstones and grainstone-wackestone couplets are deposits of surge-like concentrated flows and turbidity flows.

Milieux coralliens du Dogger près de Betioky (Madagascar) : la fin d'une exception

Abstract Corallian outcrops of the Dogger in the Betioky area (Madagascar) have been previously interpreted as barrier reefs or atolls involved in a large carbonate platform. This statement appeared exceptional for these times. A field study demonstrates that, in fact, they are only sparse solitary or colonial corals, whose growth occurred with some difficulty. They developed in meadows strongly marked by terrigenous inputs, in a subsiding context, and during brief episodes favourable to the genesis of ooids. To cite this article: B. Lathuiliere et al., C. R. Geoscience 334 (2002) 1169–1176.

Corals not serpulids: mineralized colonial fossils in the Lower Jurassic marginal facies of South Wales

Poorly preserved colonial corals occur near the base of the Lower Jurassic marginal facies at Southerndown, South Wales. Previously they have been interpreted as serpulid colonies, despite a dissimilarity to any serpulids known from elsewhere in the Lias or the few known extant colonial serpulids. However, local preservation of fine detail reveals evidence, in the form of corallites, septa and tabulae, that they are scleractinian corals of the Suborder Faviina, Family Stylophyllidae. These coral specimens occur in close association with barite-galena veins in the underlying Carboniferous Limestone and adjacent Lias marginal facies. Their widespread misindentification as ‘serpulid reefs’ is a consequence of coarse replacive mineralization by barite, which has largely obscured the diagnostic characters.

An exhumed Palaeozoic underwater scenery: the Visean mud mounds of the eastern Anti-Atlas (Morocco)

About one hundred carbonate mud mounds, covering an area of 440 km 2 in the eastern Anti-Atlas of Morocco, constitute one of the largest mound agglomerations known so far from Lower Carboniferous settings. They occur within a 4000-m-thick succession of shales with intercalated bedded limestones, sandstones, and siltstones. According to conodont and goniatite biostratigraphy, mound formation started in the early Gnathodus texanus Zone and terminated during the G. bilineatus Zone of the Visean stage. Individual mounds are a few metres to 30 m high, have base diameters of up to 300 m and are concentrated in several parallel, WNW–ESE running belts. From their lithology and facies relationships, four types of mounds can be distinguished: (1) massive crinoidal wacke- or packstones without stromatactis; (2) massive crinoidal wacke- or packstones with rare stromatactis; (3) similar to (2), but allochthonous; and (4) biodetrital (skeletal) grainstone mounds. While carbonate deposition in types (1) to (3) was probably triggered by microbial precipitation, type (4) is the result of a predominantly mechanical accumulation of skeletal debris. Biota in the four types comprise a great variety of invertebrates, among which crinoids, sponges, and bryozoans are most common. Diagenesis of the mound carbonates was dominated by recrystallization of micritic matrix and organic remains and late burial cementation. Oxygen and carbon isotope data of brachiopod and crinoid ossicles, matrix, and early marine cements plot in a large field and do not allow definite conclusions about the composition of the ambient seawater. Microbial activity and the absence or scarcity of green algae, colonial corals and coralline sponges suggest deposition of the mounds in moderate water depth close to the lower limit of the photic zone. D 2001 Elsevier Science B.V. All rights reserved.

Palaeocology of Hard Substrate Faunas from the Cretaceous Qahlah Formation of the Oman Mountains

Skeletal encrusters and carbonate hardgrounds are rare in siliciclastic sands and gravels because of high levels of abrasion and sediment movement. An exception to this is the Maastrichtian Qahlah Formation of the Oman Mountains, a sequence of coarse siliciclastic sediments deposited on a shallow marine shelf above wavebase and at an equatorial palaeolatitude. This unit contains intercalated carbonate hardgrounds and other hard substrates which were encrusted and bored. The hard substrates, comprising carbonate and silicate clasts, calcareous bioclasts (mollusc shells and coral fragments) and wood, supported a diverse encrusting and boring fauna dominated in biomass by the oyster Acutostrea. There are twelve bryozoan species and at least two serpulid worm species, most living cryptically. Other encrusters on exposed surfaces include the agglutinated foraminiferan Placopsilina and several species of colonial corals. Borings in the carbonate clasts and shells are predominantly those of bivalves (Gastrochaenolites), with subsidiary clionid sponge (Entobia) and acrothoracican barnacle (Rogerella) borings. The woodgrounds are thoroughly bored by teredinid bivalves (Teredolites). Of the common substrate types, carbonate hardground clasts support the greatest number of taxa, followed by chert clasts, with limestone rockground pebbles being depauperate. Clast composition and relative stability probably explain these differences. Individual clasts probably had variable and typically long colonisation histories. Detailed palaeoecological interpretation is constrained by taphonomic loss, time‐averaging and clast transportation and reorientation. Evidence from the Qahlah Formation shows that tropical rocky‐shore biotas in the Cretaceous were not impoverished as previously believed.

Jurassic corals as emersion indicators

In Recent coral reefs, emersion has resulted in the development of special growth forms of colonial corals which have been named microatolls. These colonies grow almost exclusively in a horizontal direction. In the upper part of the colony, subaerial exposure leads to the decay of the living body. The growth then stops and the skeleton is later colonized by various boring and encrusting organisms. Here we show the first record of a massive Mesozoic coral colony displaying the main features of emersion. The colony has been collected in perireefal Oxfordian (Jurassic) limestones from the Jura Mountains (France). It is situated exactly at the expected place in a shallowing-upward sequence between infralittoral buildups and supralittoral limestones. We suggest that such growth structures could be more common than previously thought in ancient coralliferous sediments and add some new details to discriminate between these colonies.