Rudist Research Articles

Behind the scenes in the Department of Palaeontology, The Natural History Museum: report of two visits by the Geologists' Association, 15 and 22 January, 2000

The Department of Palaeontology of the Natural History Museum is one of the largest palaeontological research institutions in the world. The collections of the Department include about 9 000 000 specimens, covering vertebrates, invertebrates, microfossils and trace fossils. The Geologists' Association visited the Department on 15 and 22 January, 2000. Highlights of the collection were demonstrated, including Mesozoic marine reptiles, dinosaurs, Archaeopteryx, Aepyornis eggs, plants, bryozoans, echinoderms, foraminiferans, rudist bivalves, arthropods, brachiopods and a notable tablecloth.

Species richness and abundance patterns of Tethyan Cretaceous rudist bivalves (Mollusca: Hippuritacea) in the central-eastern Mediterranean and Middle East, analysed from a palaeontological database

Data about the stratigraphical and regional distribution of Cretaceous rudist bivalves in the central-eastern Mediterranean and Middle East from the Barremian to the Maastrichtian have been compiled in a taxonomic databank system. The investigated region comprises northern, central and southern regions of the Cretaceous Tethys. The regionally different quality of taxonomic data is critically discussed. Both species richness and abundance (number of species records) peak in the Early Aptian, Late Cenomanian and Early Campanian. Temporal patterns in species richness are examined in relation to evolutionary innovations of the group. Crises in species richness and abundance during the Early and mid-Cretaceous were coeval with oceanic anoxia associated with platform drowning. Also, regional emergence affected large parts of the Apulian Plate and other central Tethyan carbonate platforms during the mid-Cretaceous period of globally high sea level, so that these crises can be attributed to regional environmental perturbations, induced by either oceanic anoxia or tectonic movements. The Cenomanian–Turonian transition marks an important faunal turnover, as indicated by the demise of taxonomic groups which are characterised by recumbent ecological morphotypes and predominantly aragonitic shells. During the Turonian, they were replaced by associations dominated by largely calcitic Radiolitidae and Hippuritidae, among which elevator morphotypes dominated. The stratigraphy of many post-Turonian deposits in the studied region is too imprecise for a detailed analysis of Late Cretaceous distributional patterns, and the demise of rudists during the Maastrichtian in particular. The regional pattern of species richness and abundance is similar throughout most of the investigated region. A notable exception is the broad north-eastern African shelf from Libya to Lebanon, where rudists were almost completely absent after the Coniacian. This coincided with shallow-water phosphorite and black-shale formation on the extended shelf of this region, indicating the upwelling of nutrient-rich waters and stagnant conditions. During this time (Campanian–Maastrichtian), species-rich associations thrived along the north-eastern Afro-Arabian margin, now exposed in south-eastern Turkey, Iran, the United Arab Emirates (UAE) and Oman. This region forms the single distinct faunal province during the Cretaceous in the studied region, as indicated by an endemism of 40% at the genus level. The low species richness and abundance of Hippuritidae on Turonian–Maastrichtian central Tethyan carbonate platforms, when compared with regions having mixed calcareous–siliciclastic sedimentation, is interpreted to argue against a widespread photosymbiosis in the group. Several aspects of this study highlight the importance of regional tectonic and oceanographic controls of distributional, and perhaps also evolutionary, patterns of rudist bivalves. Their delineation still suffers from the regionally different quality of stratigraphical and taxonomic data.

THE RUDISTS OF SOUTHERN ISTRIA - AN EXAMPLE OF ENVIRONMENTALLY INDUCED SUCCESSION WITHIN SANTONIAN LIMESTONES

Rudist bivalves thrived commonly in the shallow carbonate platforms of the Cretaceous Tethyan realm. The presence of two vertically separated Rudist assemblages along two well preserved Upper Cretaceous sections of cape Mrlera (southern part of the Istrian peninsula) is interpreted as environmentally induced faunal replacement. The first succession begins with pelagic limestones, followed by a mixed radiolitid-hippuritid assemblage, and by monogeneric floatstones-biostromes of radioliotids. The lower pelagic unit represents relatively open marine conditions while the uppermost radiolitid unit originated under more restricted marine conditions.Radiolitid floatstones with rare Gorjanovicia bouquets represent the beginning of the second succession. A mixed radiolitid-hippuritid assemblage, characterised by radiolitid floatstones where rare large Vaccinites individuals occur, replaces the radiolitid assemblages in vertical succession. In both sections, sea level changes were the most important factor causing the replacement of rudist assemblages. Slight differences between the two successions could be attributed to differences in submarine topography. Radiolitids and hippuritids might have occupied different biotopes within subtidal environments of the Adriatic carbonate platform. Hippuritids preferred more stable and deeper subtidal environments. By contrast, radiolitids preferred the shallowest parts of subtidal areas. Therefore, these two major rudist groups may be used for determination of different paleoecological conditions.

Factors regulating the development of elevator rudist congregations

Abstract Upper Cretaceous carbonate platform deposits show a widespread development of distinctive lenticular to tabular bodies of rock (lithosomes) formed by congregations of upright tubular (‘elevator’) rudist bivalves. Here we discuss the factors that regulated the initiation, consolidation and termination of such lithosomes, based on examples in the Santonian of the southern Central Pyrenees. In the study area, rudist congregations developed between phases of sediment influx from neighbouring source areas. For the initiation of settlement, sedimentation rate evidently had to be very low or nil. The first rudist settlers both provided more hard substrates for subsequent recruitment, and fuelled the in situ formation of bioclastic sediment, leading to the embedding and consolidation of the congregation (both positive feedbacks to establishment). Thereafter, rudist density correlated with inferred sediment destabilization at the benthic boundary layer, which in turn affected rudist recruitment. Thus, successful rudist larval settlement declined with increasing numerical density of individuals in the congregation — a crucial negative feedback mechanism. The density of the rudist congregations could then have been maintained at more or less the same level through time by this stabilizing process. Finally, development of the rudist congregations ceased with progressive shallowing, usually involving reworking of their upper parts and/or burial by renewed influxes of sediment.

A preliminary phylogeny for rudist bivalves: sifting clades from grades

Abstract The taxonomic history of the late Jurassic-Cretaceous rudist bivalves (Superfamily Hippuritoidea) is reviewed and a new phylogeny based on cladistic analysis of 32 skeletal characters is proposed. Informative characters include: relative thickness and structures of the outer (calcitic) shell layer; valve asymmetry and attachment to the substrate by either the left or right valve; form of the ligament; and, in the aragonitic inner shell, the dentition and shelly supports (myophores) for the adductor muscles, as well as accessory cavities and blind-ending (pallial) canals. The myocardinal complex (teeth and myophores) is especially important for discriminating clades previously lumped in paraphyletic or even polyphyletic taxa (e.g. Caprinidae, sensu lato , in the bivalve Treatise (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology, Part N. Mollusca 6, Bivalvia , Geological Society of America and University of Kansas). As outgroup, a megalodontid bivalve was used, as this shares three derived traits (massive dentition, modified parivincular ligament and posterior myophores) in common with rudists. The clade of all rudists is united by possession of an outer shell layer of fibrillar prismatic calcite (albeit greatly reduced in a few taxa). Two subclades are distinguished according to the attachment of the shell — either by the left valve (all ‘diceratids’, except Diceras and Valletia , plus the monophyletic requieniids), or by the right valve ( Diceras, Valletia and all other rudists). Monophyly of some previously established families is substantiated. These include Caprinidae, sensu stricto (Skelton, P. W. 1978. Philosophical Transactions of the Royal Society of London, Series B , 284 , 305–318), Radiolitidae (with Agriopleura as sister taxon) and Hippuritidae (with Tepeyacia emerging as a possible sister taxon), while others are resolved into distinct clades. There is some evidence for the Polyconitidae Mac Gillavry (1937), though with negligible bootstrap support.

Skeletal growth rates of Upper Cretaceous rudist bivalves: implications for carbonate production and organism-environment feedbacks

Abstract The skeletal growth rates of late Cretaceous rudist bivalves have been inferred from cyclic variations of isotopic and chemical compositions which are found in sclerochronological profiles of outer shell layers. Annual shell accretion of 11 studied shells from different environmental settings was in the range of less than 10 to 54 mm. CaCO 3 production of individual rudists was calculated to range from 12 to 214 g a −1 , and estimates of annual production of rudist communities assuming dense growth fabrics range from 4.6 to 28.5 kg m −2 . These production rates are not significantly higher when compared to modern mussel or oyster beds but, in contrast to modern analogues, rudist associations were much more important for the sedimentary budget of low-latitude shallow-water depositional environments. Controlling factors for the formation of growth fabrics and carbonate production are evaluated in a case study of rudist formations from a single depositional sequence. From turbulent outer platform environments to lagoonal inner platform settings, a decrease both in carbonate production and size of shells was found among specimens of a single species. No differences in carbonate production are evident when carbonate production in siliciclastic and calcareous environments is compared. Potential feedbacks between rudists and their environment induced by vertical growth, large production of bioclastic sediment and ejected biodeposits are discussed.

Isotopic and chemical intra-shell variations in low-Mg calcite of rudist bivalves (Mollusca-Hippuritacea): disequilibrium fractionations and late Cretaceous seasonality

Isotopic (δ13C, δ18O) and elemental (Mg, Sr, Mn, Fe) compositions were analysed in sclerochronological profiles of several shells of late Cretaceous rudist bivalves from Greece, Turkey, Somalia and the Arabian Peninsula. The preservation of original compositions of low-Mg calcite of outer shell layers is indicated by constant and high Sr, generally low Fe and Mn, and the preservation of fibrous-prismatic ultrastructures. Cyclic variations in δ18O and Mg are interpreted to reflect seasonal temperature/salinity cycles and, thus, annual growth increments. In shells of Torreites, amplitudes of correlated δ13C and δ18O cycles cannot be related to reasonable palaeotemperatures or salinity. This isotopic pattern reflects vital fractionations of an extent which is unknown from modern bivalves. In contrast, almost identical ranges and amplitudes of δ18O cycles are observed in 13 shells of five species from Santonian-Campanian localities in central Greece and northern Turkey, suggesting that seasonal variations in environmental conditions were recorded without significant vital fractionations. The effect of seasonal salinity changes on δ18O of the shells is evaluated, and mean palaeotemperatures are constrained within the range of 30–32.5 °C. The annual range of temperature was estimated to be 7 °C, assuming a constant salinity. This agrees with other isotopic proxies of Late Cretaceous palaeotemperatures, and with global circulation models which predict higher low-latitude sea-surface temperatures than the present ones.

Using critical path method to analyse the radiation of rudist bivalves

It has been suggested that certain families of rudist bivalves (superfamily Hippuritoidea) were ‘preadapted’ to inhabit different environments and radiated to fill them. This scenario was based on a hypothesis of a geometric constraint of shell growth, a differential increase in the number of rudist genera possessing uncoiled shells relative to those possessing spirogyrate (i.e. coiled) shells, and a speculative phylogeny. In this paper, these ideas are reformulated and tested by comparing a postulated sequence of structural changes, expressed according to critical path method of management theory, and phylogeny, inferred from cladistic analysis. The hypothesis of constraint is unfalsified, and the cladogram is consistent with a proliferation of rudists possessing uncoiled shells. The proliferation may be attributed to the origin of an invaginated ligament, which should be considered as a direct adaptation.

Aptian (Lower Cretaceous) rudist bivalves from NE Spain: Taxonomic problems and preliminary results

The rudist species Toucasia carinata ( Matheron, 1842 ), Pseudotoucasia santanderensis ( Douvillé, 1889 ), P. catalaunica Astre, 1932 (Requieniidae), Horiopleura baylei ( Coquand, 1865 ), Polyconites verneuili ( Bayle, in de Verneuil et al. 1860) (“Caprotinidae”), and Caprina parvula ( Douvillé & O'Gorman, 1933 ) (Caprinidae) from the Aptian of NE Spain are described. Taxonomical important sections are illustrated and taxonomical problems are discussed. The study reveals that we still know very little about the ontogenetic character variation within closely related species and genera. This can hamper straightforward determinations and phylogenetic interpretations. Implicitly it can also curtail the value of these taxa for biostratigraphical and palaeobiogeographical purposes. Improvements in these fields will be largely connected to intensified basic taxonomic work, that is, to the study and illustration of morphological variations within these groups.

Facies analysis of an Upper Cretaceous high-energy rudist-dominated carbonate ramp (Matese Mountains, central-southern Italy): subtidal and peritidal cycles

Upper Cretaceous shallow-water limestones rich in rudists have been studied in the Matese Mountains. They show a preponderance of skeletal components (molluscs and benthic foraminifers) and a lack of non-skeletal grains. The most prominent sediments of the studied stratigraphic interval are rudist-dominated fine- to coarse-grained lithologies. Most of the Senonian successions are characterized by massive neritic limestones rich in molluscs (mostly rudists and subordinate gastropods) and a few corals. The sediments were generated in situ on shelves where rudist bivalves were the primary sediment producers. These sediments were actively moved by storms, waves and swells. The finer fractions were probably winnowed out and deposited in deeper water. Well-bedded dolomitized mudstones/wackestones, microbial laminites and benthic foraminifer wackestones characterize some successions where they rhythmically alternate and testify to deposition in tidal-flat environments. The lithofacies, faunal and taphonomic characters and the sedimentary structures are typical of a ramp-like open shelf with local peritidal deposition. The vertical lithofacies organization consists of numerous decimetre- to metre-scale shallowing-upward subtidal and peritidal cycles. A complete transition from peritidal to subtidal cycles has been recognized and documented. The peritidal cycles are usually bounded by desiccation surfaces and/or dolomitized crusts followed by shallow-subtidal to peritidal facies of the next cycle. The subtidal cycles are typified by rudist floatstones passing to large-scale cross-bedded bioclastic packstones/grainstones. Current and storm structures are distinctive features; in shallower-water environments the effects of submarine erosion prevail; hardgrounds commonly developed and the resulting cycles are usually truncated. The formation of the peritidal cycles appears to have been controlled by accommodation space in response to fluctuations in relative sea level. The formation of the subtidal cycles was determined by sedimentation rates in response to fluctuations in the zones of fair-weather and storm-wave reworking, in turn controlled by relative sea level. This indicates that sediment production and supply were important controlling factors in the cycle formation during the early Senonian stratigraphic interval. The recognition of similar depositional styles in other early Senonian successions cropping out in the southern Apennines suggests that this type of sedimentation is a recurring attribute of the Upper Cretaceous open shelves.

The Great Pearl Bank Barrier of the Arabian Gulf as a Possible Shu’aiba Analogue

ABSTRACT Within the Arabian Peninsula, the Shu’aiba Formation is one of three Cretaceous carbonate formations in which rudist bivalves are an important component. The favourable hydrocarbon reservoir properties of these carbonates are primarily attributed to the presence of the rudists and their associated debris, which accumulated along the margins of an intra-shelf basin. The rudist banks caused differentiation of an earlier carbonate platform into lagoon, back-bank, bank, fore-bank and open marine environments. Understanding of the orientation of these banks has been significantly assisted by micropalaeontological analysis of the rudist-associated sediment, but may be additionally enhanced by the study of Recent large bivalves, such as the ‘fan mussel’ Pinna spp. The depositional geometries of the rudist-dominated facies of the Shu’aiba Formation may be better understood by studying the Great Pearl Bank Barrier, located on the southern flank of the Arabian Gulf, as this may present a Recent analogue for variations in sedimentation and bioclast distribution. The Great Pearl Bank Barrier complex includes a submarine ridge that extends for approximately 200 kilometers between the Qatar Peninsula and Abu Dhabi, and lies in water depths of less than 8 meters, together with a deep lagoon and barrier flank facies. The submarine barrier complex and the back island lagoons consist primarily of bivalve shells, sands and mud, in which are embedded locally dense populations of the large bivalve species Pinna bicolor Gmelin and P.mururicata (Linnaeus). These forms may serve as Recent counterparts for the extinct Aptian constratal elevator rudists, such as Glossomyophorus costatus Masse, Skelton and Sliskovic, with a form that resembles Pachytraga sp., and Agriopleura blumenbachi that characterise the back-barrier and lagoonal facies, respectively, of the Shu’aiba Formation in the region. The oysters that have colonised the barrier crest have a clinging habit and may occupy a niche that equates with the Aptian recumbent rudist Offneria murgensis.

Late Aptian Radiolitidae (rudist bivalves) from the Mediterranean and Southwest Asiatic regions: taxonomic, biostratigraphic and palaeobiogeographic aspects

The onset of the Radiolitidae in the mid-Aptian was subsequently followed by a diversification and a geographic spreading of the group during the late Aptian. From the Mediterranean and Southwest Asian regions are recorded four species of Eoradiolites and one species of Praeradiolites , all well defined. Moreover at least two species of Eoradiolites and two species of Praeradiolites , with a poorly defined taxonomic status, are also present. The geographic distribution of the Mediterranean faunas runs from Southern France, Spain, Italy, Bosnia, East Serbia, Lebanon to Algeria. The southwest Asian faunas are documented from Western China, North India, Pakistan, Afghanistan and Iran. The age of the radiolitid-bearing localities is discussed and/or documented after micropalaeontological assemblages. Mediterranean and Southwest Asian faunas are remarkably different: the former group is dominated by typical cellular shelled forms ( Eoradiolites katzeri-E. plicatus being the cardinal taxa) whereas the latter is dominated by acellular forms ( Eoradiolites gilgitensis being the cardinal taxon). Palaeogeographic reconstruction of the late Aptian palaeoenvironments place the Mediterranean fauna on the northern and the southern margin of the “Mediterranean Seuil” without any significant dissimilarities in composition, while the southwest Asian faunas are restricted to the northern margin of Tethys. These two bioprovinces are both located in the tropical domain of the northern hemisphere. The existence in southern Russia of a wide marine embayment connected to high latitude cool waters suggests a possible thermal barrier between the Southwest Asian and Mediterranean faunas, close to the Caucasus. The corresponding faunal groups, with common origin, could have derived from isolation of founder populations coupled with some adaptive divergence during the rapid dispersion of the family at an early stage of evolutionary maturity.

A Late Cretaceous terebratulid brachiopod from Jamaica, and its significance for Mesozoic brachiopod palaeobiogeography and evolution

Recent fieldwork has accumulated collections of terebratulid brachiopods from Late Cretaceous strata on Jamaica referrable to the genus Dyscritothyris Cooper. This is confirmed by the preparation of serial sections of the internal structures of one well-preserved specimen. The only other record of this genus is from the Late Cretaceous rocks of Cuba. The specimens from Jamaica and Cuba are considered to belong to the same species, confirming oceanographic links between these islands during the Late Cretaceous, already apparent from similarities between their rudist bivalve and echinoid faunas. The small-sized Dyscritothyris is considered to be a Late Cretaceous derivative of the Cretaceous terebratulid Capillithyris which has been described primarily from Europe. Such a relationship would support models for the dispersal and evolution of brachiopods during the continued opening of the Central Atlantic Ocean during the Cretaceous.

The rudists re-examined

The rudists re-examined

The rudist Amphitriscoelus (Bivalvia-Hippuritacea) in the lower Cretaceous of southwestern Mexico

The emended descriptions of the genus Amphitriscoelus and the species A. waringi Harris and Hodson are given, and a new species is proposed (A. pluriloculata). These rudist bivalves are derived from a thick Lower Cretaceous sequence of carbonate sediments interlayered with siliciclastic, volcaniclastic, and volcanic rocks in southwestern Mexico, near Huetamo, in the State of Michoacan. The species belong to a recently discovered rich fauna containing other rudists, nerineid gastropods, orbitolinid foraminifers, and calcareous algae. The fauna shows a close affinity with the Amphitriscoelus fauna of northern South America. Similar assemblages are also present in Texas, Cuba and other Mexican localities. The wide distribution of the fauna allows interpretation of paleogeographic relations among all the regions as well as interpretation of paleoecological similarities. The existence of an homogeneous large faunistic province during the Early Cretaceous is suggested.

Rudists: Bivalve Morphospace Abandoned and Re-Entered

Rudists: Bivalve Morphospace Abandoned and Re-Entered

Stable isotope sclerochronology of rudist bivalves: Growth rates and Late Cretaceous seasonality

The almost complete translation of shell accretion to vertical growth was a successful morphogenetic adaptation of rudist bivalves, which replaced coral-algal-hydrozoan communities in most Tethyan shallow-marine environments during the Cretaceous. Growth rates of rudist shells are, therefore, important in reconstructions of Cretaceous shallow-marine benthic communities. Carbon and oxygen isotopic compositions were analyzed in sclerochronological profiles through the shells of two Late Cretaceous rudist bivalves from Greece and Turkey. Values of δ 13 C vary between +0.8‰ and +2.2‰ in the hippuritid Vaccinites ultimus and between +0.8‰ and +2.6‰ in the radiolitid Gorjanovicia cf. costata . Ranges in δ 18 O are −5.3‰ to −2.9‰ and −4.6‰ to −2.1‰, respectively. A distinct cyclicity of δ 13 C and δ 18 O in both shells reflects Late Cretaceous seasonality and indicates annual vertical-growth increments of 45 mm in V. ultimus and 54 mm in Gorjanovicia . The concentrations of minor elements are similar to those of extant bivalves with low-Mg calcite shells.

Biogeochemistry of Late Cretaceous Rudist Bivalves: Paleoecologic and Paleoclimatic Implications

Biogeochemistry of Late Cretaceous Rudist Bivalves: Paleoecologic and Paleoclimatic Implications

The Changing Biology of Reef-Building

The close-packing of calcified organisms necessary for reef development is achieved either by larval aggregation (aclonal/solitary metazoans) or by competition for limited or patchy hard substrate (clonal/modular metazoans and encrusting algae). Buildups constructed predominantly by aclonal/solitary metazoans tend to be small, ephemeral soft-substrate communities, with relatively low relief, spatial heterogeneity and biotic diversity. In contrast, frameworks constructed by clonal/modular metazoans or encrusting calcified algae are often sizeable, longlived structures with high topographic complexity, diversity and biomass. Several groups of fossil organisms currently considered as major reef-builders may be re-interpreted either as common cryptobionts (early Cambrian archaeocyath sponges, Paleozoic-Mesozoic sphinctozoan sponges) or as soft-substrate platform dwellers (Paleozoic-Mesozoic stromatoporoid and chaetetid sponges, rudist bivalves). Throughout the Phanerozoic as well as in modern reefs, sessile solitary calcified heterotrophs (filterand suspension-feeders) have been more important members of the cryptos than the open surface, framework-constructing community. Heterotrophs were however more common members of the open surface community during the Paleozoic-Mesozoic than the Cenozoic. Yet most open-surface communities have always been dominated by phototrophic organisms: various calcified algae and microbes during the Paleozoic-early Mesozoic and mixotrophic metazoans, primarily scleractinian corals, from the late Triassic/early Jurassic onwards. The established nomenclature describing reefal fabrics must be revised in the light of these observations. The acquisition of photosymbionts by scleractinian corals in the late Triassic/early Jurassic may have been related to the appearance of dinoflagellates at this time, and was favoured by increased predation pressure from the late Mesozoic onwards. In particular, the appearance of many groups of herbivorous and predatory fish in the Eocene promoted coral domination. Photosymbiosis imparted novel metabolic capabilities to scleractinian corals, allowing them and hence reef communities to invade the previously unexploited low-nutrient, shallow marine environment. INTRODUCTION

Corals to rudists—an environmentally induced assemblage succession

Corals and rudist bivalves commonly coexisted on the shallow carbonate platforms of the Cretaceous Tethyan Realm. Usually, the carbonate platforms show lateral and vertical biotic zonations. In general, the vertical biotic zonation relates directly to a consistent lateral biofacies pattern: coral or coral-rudist assemblages occupy more open zones of the platform, and rudist-dominated assemblages, the more restricted zones. Several corresponding coral-to-rudist assemblage successions in carbonate platforms have previously been widely interpreted as ecological successions. We document some coral-to-rudist assemblage successions, in a well preserved Upper Cretaceous section, in the Tremp area of the South Central Pyrenean Unit, which are interpreted, by contrast, as environmentally-induced faunal replacements. A repeated assemblage succession in the eastern part of the section begins with a coral-dominated assemblage, and passes up gradually to a mixed coral and rudist assemblage that is succeeded by a paucispecific unit of hippuritid elevator congregations. The lower coral-rudist units of such successions represent relatively more open, marine conditions, and the uppermost hippuritid unit relatively more restricted marine conditions. This assemblage succession was formed in response to a change in the depositional setting caused by sediment accumulation. Two consecutive coral-dominated assemblages, overlain by mixed coral and rudist assemblages, constitute a second kind of a sedimentary coral-to-rudist assemblage succession discussed in this paper. Its regular recurrence makes up the uppermost part of the section in the East. The predominance of platy to low domal coral colonies in the lower units suggests that conditions were initially favourable for horizontal coral growth. As the habitat changed, these coral assemblages were replaced by massive-tabular to large domal coral colonies, with the large hippuritid Vaccinites , and were sometimes succeeded, in turn, by clustered hippuritid elevators. The most important factor causing the replacement was probably the increase in sediment flux, associated with turbidity. This change in the depositional environment eventually favoured the establishment of rudist elevators, well adapted to moderate and intermittent to continuous accumulation of sediment.