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1.
The Pleistocene Miami Limestone that crops out on the lower Florida Keys is overlain by thin (16 cm or less), discontinuous, Holocene calcareous crusts (caliche) that are usually laminated, composed dominantly of calcite micrite and may or may not incorporate part of the underlying limestone. Both allochems and sparry calcite cement in the former unit contain endolithic algae and fungi, borings and unicellular algae. Biogenic structures identical to those in the Miami Limestone also occur in the calcareous crusts but are somewhat less abundant in the latter unit versus the former unit. The calcareous crusts were formed in the vadose diagenetic environment. Some of the CaCO3 necessary for the micrite that comprises the bulk of the crusts was probably derived from solution of carbonate from a soil cover and some from wind blown salt spray. Most of the micrite, however, was formed by replacement of the uppermost portions of the Miami Limestone. Replacement involved micritisation of allochems and a previously unreported process, sparmicritisation, the degrading recrystallization of sparry calcite to micrite. Minor sparmicritisation was caused by micrite calcification of endolithic fungi or algae within sparry calcite cement or by micrite precipitation in empty borings within such cement. Most sparmicritisation took place by dissolution of sparry calcite and concomitant precipitation of micrite in the space occupied previously by the dissolved spar. Such sparmicritisation is interpreted to be caused by chemical reactions involving the crystals, pore water which is moving slowly but steadily and organic compounds released during bacterial decomposition of fungi, algae or both. It is recognized that sparmicritisation occurs in the marine diagenetic environment and is not, therefore, necessarily indicative of vadose diagenesis. Incomplete sparmicritisation is responsible for some of the clotted textures typically found within calcareous crusts and may explain such textures in many other carbonate rock types. A combination of sparmicritisation and micritisation has probably greatly influenced the porosity of many reefs and, in some cases, led to the formation of ‘micritic reefs’.  相似文献   

2.
Micrite envelopes are a common feature in carbonate sediments and are typically associated with the micrite filling of borings produced by microendolithic organisms. These are referred to as 'destructive micrite envelopes' and have long been recognized as reflecting an important early diagenetic process. Recent analysis of sediments collected from back-reef environments at Discovery Bay, north Jamaica, however, has demonstrated 'envelope' formation on the surfaces of carbonate grains, clearly distinct from the micrite filling of microborings. Such constructive envelopes occur almost exclusively in sediments from grass-bed environments and are always intimately associated with 'biofilms' comprising abundant mucilage, cyanobacteria, bacteria and diatoms. It is suggested that these envelopes represent a product of both biologically mediated micrite precipitation (occurring within the biofilm mucilage and around the biofilm components, i.e. cyanobacteria and diatoms) and associated trapping of carbonate mud and fine-grained sediment. Their recognition only within grass-bed sediments may enable their use as a diagnostic feature of grass-bed environments or vegetation-stabilized substrates in the rock record.  相似文献   

3.
贡云云  姜含璐  倪胜利 《中国地质》2018,45(6):1271-1288
在辽宁金州寒武系长山组顶部,发育一层厚约15 m的生物丘。宏观上,生物丘主要由凝块石和均一石组成;微观上,由致密泥晶组构和各种类型的颗粒组成。在生物丘内部,各种类型的颗粒如钙化微生物、底栖鲕粒、核形石、生物碎屑和凝块等的发育,显示了生物丘复杂的显微组构。其中,三叶虫碎屑表面的泥晶结壳,表现出建设性和破坏性泥晶化作用。泥晶中分散分布的生物碎屑,反映了生物丘形成过程中泥晶较强的黏聚作用。致密泥晶中大量黄铁矿颗粒的发育,反映了异养细菌活动对泥晶形成的贡献。生物丘内部各种类型的颗粒与黄铁矿颗粒的共生,反映了生物丘形成过程中存在复杂的微生物作用,这为微生物沉积作用的研究提供了基础素材,也为生物丘内部各种类型颗粒的研究提供了重要实例和线索。  相似文献   

4.
Pelmatozoans diversified primarily during the Middle and Late Ordovician Period, with Early Ordovician records being much more limited, resulting in many gaps in our knowledge of the early stages of their diversification. Dissociated pelmatozoan ossicles have been found abundantly in one section in the Tonggao Formation (Tetragraptus approximatus Biozone, Floian, Early Ordovician). Most of the ossicles are thecal plates and stem ossicles from hemicosmitoid and glyptocystitoid cystoids. Thecal plates of ‘Cheirocrinus’ sp., Polycosmites sp., and other plates of uncertain affinity are described. A different ossicle type, Pentagonopentagonalis (col.), may represent crinoid remains; this would be one of the earliest occurrences of the class. The thecal ossicles and columnals are all considered, as both sets of data are desirable to determine the most complete estimate of generic diversity. The echinoderm ossicles may have been transported in from shallower water palaeoenvironments and clusters of ossicles may represent coprolites or regurgitates. Estimates of Early Ordovician palaeogeography that place this site at 30°S or near the palaeoequator are supported by the physiological requirements of the primitive echinoderms described herein. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

5.
Large areas of southern Australia and New Zealand are covered by mid‐Tertiary limestones formed in cool‐water, shelf environments. The generally destructive character of sea‐floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates, yet these limestones include occasional units with marine cements: (1) within rare in situ biomounds; (2) within some stacked, cross‐bedded sand bodies; (3) at the top of metre‐scale, subtidal, carbonate cycles; and (4) most commonly, associated with certain unconformities. The marine cements are dominated by isopachous rinds of fibrous to bladed spar, interstitial homogeneous micrite and interstitial micropeloidal micrite, often precipitated sequentially in that order. Internal sedimentation of microbioclastic micrite may occur at any stage. The paradox of marine‐cemented limestone units in an overall destructive cool‐water diagenetic regime may be explained by the precipitation of cement as intermediate Mg‐calcite from marine waters undersaturated with respect to aragonite. In some of the marine‐cemented limestones, aragonite biomoulds may include marine cement/sediment internally, suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences. We suggest that marine cementation occurred preferentially, but not exclusively, during periods of relatively lowered sea level, probably glacio‐eustatically driven in the mid‐Tertiary. At times of reduced sea level, there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters, and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites, which now experienced increased swell wave and/or tidal energy levels, fostering sediment abrasion and reworking, reduced sedimentation rates and freer exchange of sediment pore‐waters. Energy levels were probably also enhanced by increased upwelling of cold, deep waters onto the Southern Ocean margins of the Australasian carbonate platforms, where water‐mass mixing, warming and loss of CO2 locally maintained critical levels of carbonate saturation for sea‐floor cement precipitation and promoted the phosphate‐glauconite mineralization associated with some of the marine‐cemented limestone units.  相似文献   

6.
The geometry and evolution of vertically segmented normal faults, with dip separations of < ca 11.5 m have been studied in a coastal outcrop of finely bedded Cretaceous chalk at Flamborough Head, U.K. Fault trace segments are separated by both contractional and extensional offsets which have step, overlap or bend geometries. The location of fault trace offsets is strongly controlled by lithology occurring at either thin (ca 1 mm-8 cm) and mechanically weak marl layers or partings between chalk units. Fault segmentation occurred during either fault nucleation within, or propagation through, the strongly anisotropic lithological sequence. An inverse relationship between fault displacement and number of offsets per length of fault trace reflects the progressive destruction of offsets during fault growth. The preservation of fault offsets is therefore dependent on offset width and fault displacement. Fault rock, comprising gouge and chalk breccia, may vary in thickness by 1.5–2.0 orders of magnitude on individual fault traces. Strongly heterogeneous fault rock distributions are most common on small faults (< 10 cm displacement) and are produced mainly by destruction of fault offsets. Shearing of fault rock with increasing displacement gives rise to a more homogeneous fault rock distribution on large faults at the outcrop scale.  相似文献   

7.
New material of pterasterid asteroids from the UK chalk is described on the basis of ossicles recovered from washed residues. A new species, Pteraster lyddenensis sp. nov., is erected for oral and adambulacral ossicles and a primary radial ossicle from the Cenomanian Grey Chalk Subgroup of Dover (Kent), and the first UK record of Pteraster kutscheri Gale, 2022 is described from the upper Campanian Chalk of Norwich (Norfolk); both taxa belong to extant groups of Pteraster. Pteraster lyddenensis sp. nov. is the oldest known representative of the genus. The benthic invertebrate fauna of the Cretaceous chalk facies includes a number of extant genera which at the present day dwell in the deep sea. However, their presence was probably due to the low-productivity oceanic palaeoenvironment of the Chalk Sea, simulating deep-ocean conditions, rather than its depth.  相似文献   

8.
A new species of cladid(?) crinoid, Segmentocolumnus (col.) clarksoni, based on distinctive, disarticulated stem material, is described from the Upper Llandovery Kilbride Formation. Hitherto, this unit has yielded two taxa based on single, nearly complete crinoids. In contrast, S. (col.) clarksoni is known from numerous specimens, including common long pentagonal, pentameric, heteromorphic pluricolumnals with symplectial articulations, broad pentagonal lumina and narrow claustra. A related morphospecies is known from the Ashgill (Upper Ordovician) of Ireland. The fossil echinoderms of the Llandovery (Lower Silurian) are poorly known globally. Where present in this interval, echinoderms are more commonly preserved as disarticulated ossicles and rarely as complete specimens. Complete crinoids have now been identified from nine horizons in the Llandovery of the British Isles, making this one of the better known pelmatozoan faunas from this time interval. However, only two of these occurrences have yielded as many as five or more identifiable taxa. Seven of the nine occurrences are Upper Llandovery (Telychian). Genera are typical of the Silurian or (Upper Ordovician + Silurian); the only remnant Ashgill taxon that did not survive the Llandovery was the morphogenus Segmentocolumnus (col.) Donovan, an ‘extinction’ that probably owes more to taxonomic method than any evolutionary pattern. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

9.
NACI GÖRÜR 《Sedimentology》1979,26(4):605-608
Fragments of echinoderms in the Karaisali Limestone (Miocene) of the Adana Basin, Turkey, show large syntaxial overgrowths which are restricted mostly to their undersides, whilst the upper surfaces are covered by micrite. Although this pendant style of the overgrowths is reminiscent of vadose zone ‘gravitational cement', they appear to have formed in the open voids of sediments in the marine environment, simultaneously with, as well as after, the internal deposition of micrite.  相似文献   

10.
Geochemical signals from speleothems are commonly used in the investigation of palaeoenvironments. In most cases, however, little attention is paid to whether or not these signals are primary or altered by diagenesis. The speleothems of the Castañar Cave (Cáceres, Spain), which are initially formed of calcite or aragonite, have undergone a variety of meteoric diagenetic processes such as micritization and neomorphism (inversion), that collectively modify their primary features (textures, mineralogy, geochemical signals). The mean δ13C and δ18O values of the aragonites in the cave are −8.66 and −4.64 respectively, whereas the primary calcites have mean δ13C and δ18O values of −9.99 and −5.77, respectively. Following the diagenetic process of micritization, the aragonite isotopic signals averaged −7.63 δ13C and −4.74 δ18O and the calcite micrite signals −9.53 δ13C and −5.21 δ18O. Where inversion took place, some secondary calcites after the aragonite show preserved aragonite, whereas others do not. The secondary calcites without aragonite relics show isotopic values slightly higher than those of the primary calcite due to the inheritance of the aragonite signal. Where aragonite relics are preserved, the isotopic signatures are very similar to those of the aragonite micrite.In addition, the stable isotopic values and Sr and Mg contents of the speleothems became also modified by micritization and/or inversion. These diagenetic processes were driven by the changes in composition of the cave waters over time and space, but also, in the case of aragonite, by its initial unstable mineralogy.The present results highlight how important diagenesis is in caves and how the initial features of cave minerals may be lost. These changes alter the geochemical signals shown by speleothems, which may have an impact on the interpretation of the results obtained in palaeoenvironmental studies.  相似文献   

11.
Stromatactis‐bearing mud‐mounds remain an enigmatic reef type despite being common in Palaeozoic ramp settings. Two well preserved Upper Devonian (Frasnian) mud‐mounds in the Mount Hawk Formation crop out side by side in the southern Rocky Mountains of west‐central Alberta and provide an opportunity to develop a new case study that can be compared with the other coeval examples, such as those well‐known ones in southern Belgium, as well as evaluate competing hypotheses for mud‐mound formation. The southern mud‐mound is 46·2 m thick and 38·6 m wide at the base, whilst the northern one is 53·3 m thick and 72·2 m wide at the base, and they exhibit three or four growth stages indicated by interfingering and onlapping geometries with flanking strata. The biota is diverse, but fossils only occupy 10·7% by volume, among which sponge spicules, echinoderms, ostracods, brachiopods and calcimicrobes belonging to Girvanella and Rothpletzella are the most common. Five microfacies are discriminated in the mud‐mounds: biomicrite, clotted micrite, spiculite, stromatolite and laminite, with clotted micrite comprising the largest proportion. There is no internal vertical or lateral palaeoecological zonation, and the presence of calcimicrobes and calcareous algae throughout indicates accretion entirely within the photic zone, in a deeper ramp setting seaward of a large carbonate platform to the east. Stromatactis is abundant and the cavities were mostly due to excavation by currents rather than physical collapse of spiculate siliceous sponges. Formation of lime mud involved a combination of multiple organisms, mechanisms and processes. Cyanobacteria were integral to mud‐mound frame‐building and accretion because they stabilized the surface, often permineralized to form Girvanella and provided organic matter that was decomposed by bacteria. This induced precipitation of micrite, forming early indurated rigid masses, evidenced by the presence of intraclasts, stromatactis cavities, isopachous marine cements, absence of bioturbation and rare synsedimentary brittle deformation. The same microbial components, invertebrate biota and clotted micrite occur in underlying strata, suggesting that there was a protracted period of potential mud‐mound initiation before the exact conditions arose to trigger it. The ramp setting, antecedent sea floor topography and relative sea‐level likely contributed together to control this. This study indicates that mud‐mound formation was controlled by a combination of processes, but they are essentially a microbial buildup.  相似文献   

12.
Caliche profile formation, Saldanha Bay (South Africa)   总被引：3，自引：0，他引：3  
A sequence of gradational lithification events can be observed in caliche profiles, in the Saldanha Bay area (South Africa), from friable lightly cemented aeolian calcarenites or littoral shelly deposits through an intermediate semi-indurated zone to an upper strongly indurated zone (calcrete). Lightly cemented sediment fabrics exhibit bridge and meniscus cements, micritic druses and vadose compaction phenomena. The middle semi-indurated zones exhibit coated grains in which irregular borings and/or tubules with tangential acicular fibres contribute to coated grains. Random networks of acicular fibres also occur in void spaces. In fully indurated upper layers of the caliche profiles, fabrics of micriteand microspar (in voids) occur in complex brecciated macro-fabrics. The features represent changes in a sequence from the friable primary sediments to the calcretes. Fresh-water vadose flushing leaches grains and causes formation of meniscus and bridge cements and uneven druses. In the middle zone, inorganic processes are aided by the action of micro-organisms; fungi, bacteria or algae which produce tubules and irregular borings; the overall effect of which is to break down original detrital carbonate particles and enclose them in a crypto-crystalline micrite. The acicular fibres probably result from evaporation of supersaturated solution. Mechanical processes cause fracturing, which repeated many times gives complicated brecciated fabrics within the upper indurated zone.  相似文献   

13.
In order to understand the post-depositional history of carbonate rocks of Guri Member (Lower to Middle Miocene), three stratigraphic sections were selected in north Bandar-Abbas in southeast of Iran. Sampling was carried out, analyzed for selective parameters such as oxygen and carbon isotopic compositions (δ18O and δ13C) and interpreted in the present study. We recognized several diagenetic processes including micritization, cementation, neomorphism, compaction, dissolution, silicification, dolomitization, fracturing and vein filling. Some of the diagenetic processes occurred at different conditions, so in order to achieve precise interpretation, samples from different carbonate components such as, micrite, fracture cement, solution pore cement, intergranular cement, and some biotic allochems were analyzed. In this study micrite samples were subdivided into two groups including micro-spary and micrite. They were recognized under Cathodoluminescence microscope. In addition, micrite samples were classified into five groups based on their depositional environments: supratidal, lagoon, coral bar, open sea, and open basin. There were minor changes in stable isotope ratios based on the sedimentary environments, stratigraphy successions, and micro-spary or micrite properties. In this study, similar calcite cements in petrography studies were differentiated by stable isotope data. Those calcite cements have formed in different diagenetic environments such as meteoric and burial cements. Paragenetic sequence of carbonate rocks were interpreted by integration of petrographic and isotopic studies. We have reconstructed diagenetic models of Guri Member into four stages including marine, meteoric, burial, and uplifting.  相似文献   

14.
In freshwater environments such as river and stream bottoms, rocks and submerged vegetation are covered with a biological felt (also called a periphyton, microbial mat, biofilm, etc.) that is susceptible to calcification. Compilation of an extensive bibliography and our own observations have allowed the identification of 44 species of Coccogonophyceae, 122 Hormogonophyceae, 2 Chrysophyceae, 35 Chlorophyceae, 3 Xanthophyceae, 2 diatoms, and 3 Rhodophyceae that grow on calcareous tufa and coat vegetation. Diverse genera include species that are also calcified but impossible to determine because they lack reproductive organs. Crystals have been described from 74 species in the literature and we have observed 53 others. They can be classified into 10 groups: (1) platelets on cell walls (Volvocales, analogues of coccolithophorids) (2) crystals in mucilage (Synechococcus, diatoms, Hydrurus) and calcified stalks (Oocardium) (3) sheaths containing crystals in the form of simple or three-branched needles, dendritic crystals, and crystals with box-work fabric (Geitleria, Scytonema) (4) sheaths containing calcite spherulites (5) stalks intersecting a large crystal (Cymbella) (6) micrite tubes (Phormidium, Schizothrix) (7) isolated rhombohedra (Zygnema, Scytonema), rhombohedra in clusters or chains (Nostoc parmelioides) (8) sparite platelets (Vaucheria) or isodiametric crystals (Scytonema, Chaetophora) (9) large crystals crosscut by many parallel filaments (Rivularia, Batrachospermum), and (10) fan-like crystals (Phormidium). These crystals can be arranged in clusters or form regular laminations. They can transform into isodiametric sparite crystals to form fan-like or radial palisadic structures. Knowledge of primary crystals and their diagenetic transformations is necessary to correctly interpret freshwater stromatolites. The latter always result from intense calcification and are a diagenetic transformation of a biological felt made of many prokaryotic and eukaryotic algal species, small invertebrates, and organic and mineral debris.  相似文献   

15.
The Upper Cretaceous chalks of southern England are a thick sequence of rhythmically bedded, bioturbated coccolith micrites, deposited in an outer shelf environment in water depths which varied between 50 and 200–300 m. The products of sea floor cementation are widely represented in the sequence, and a series of stages of progressive lithification can be recognized. These began with a pause in sedimentation and the formation of an omission surface, followed by (a) growth of discrete nodules below the sediment-water interface to form a nodular chalk, erosion of which produced intraformational conglomerates. (b) Further growth and fusion of nodules into continuous or semicontinuous layers: incipient hardgrounds. (c) Scour, which exposed the layer as a true hardground. At this stage, the exposed lithified chalk bottom was subject to boring and encrustation by a variety of organisms, whilst calcium carbonate was frequently replaced by glauconite and phosphate to produce superficial mineralized zones. In many cases, the processes of sedimentation, cementation, exposure and mineralization were repeated several times, producing composite hardgrounds built up of a series of layers of cemented and mineralized chalk, indicating a long and complex diagenetic history. Petrographic study of early cemented chalks indicates lithification was the result of the precipitation of small crystals on and between coccoliths and coccolith fragments. By analogy with known occurrences of early lithification in Recent deeper water carbonates, the cement is believed to have been either high magnesian calcite or aragonite, and more probably the former. The vast scale of operations involved in the cementation process precludes carbonate in expelled pore fluids as the source of cement, whilst quantities of aragonite incorporated in sediment are also inadequate. This, plus the observed association of horizons of early lithification with pauses in sedimentation associated with omission surfaces suggests seawater as a source of cementing materials. Stratigraphic studies indicate that processes of early lithification leading to hardground formation proceeded to completion in intervals to be measured in tens or hundreds of years. Regional studies suggest that early lithification characterized relatively shallow water phases associated with regional regression over the whole of the area, whilst in detail, the distribution of mature mineralized hardground complexes is strongly correlated with sedimentary thinning and condensation over small areas and the buried flanks of massifs. Early cementation in more basinal areas is typically in the form of nodular developments and incipient hardgrounds, whilst day contents in excess of a few percent appear to have inhibited early lithification. The striking rhythmicity of hardgrounds and nodular chalks is no more than a particular expression of the overall rhythmicity of chalk sequences. The stage of early lithification reached in any instance is dependent on sediment type, the time interval represented by the associated omission surface and the degree of associated scour and erosion (if any). Chalk hardgrounds differ from most others described in the geological literature in their widespread distribution (individual hardgrounds may cover up to 1500 km2), the presence of striking glauconite and phosphate replacements of lithified carbonate matrices, their frequently sparse epifaunas, and boring infaunas dominated by clionid sponges. These differences reflect the deeper water shelf setting of the chalk, and the more open marine, oceanic circulatory system, both strikingly different from the setting of other, shallower water hardgrounds. Litho- and biostratigraphic variation in the chalk sequences of the area studied are summarized in an appendix.  相似文献   

16.
In bioerosion, as in trace fossils as a whole, deeply emplaced structures have greater survival value than shallow structures. That is to say, tiering (the relative depth to which rasping, etching and boring organisms penetrate their substrate) is of paramount importance for the preservation potential of individual trace fossils. An Entobia ichnofacies is established for trace fossil assemblages dominated by deep tier borings and arising from long-term bioerosion, such as occurs on sediment-free submarine cliffs or hardgrounds. A Gnathichnus ichnofacies comprises assemblages containing all tiers, including superficial sculptures produced by radulation that have very little preservation potential. Such assemblages occur in short-term bioerosion situations as on shell surfaces and hardgrounds buried early by sedimentation. Correspondence to: R. G. Bromley  相似文献   

17.
韦龙明 《沉积学报》1995,13(3):89-97
碳酸盐颗粒泥晶化由真菌类和藻类的穿孔所引起，颗粒泥晶化可划分为四个阶段６种类型，泥晶套和泥晶铸型分别代表泥晶化的成长、成熟阶段。丰富的泥晶化颗粒为浅滩标志；泥晶化的深度与沉积速度成反比；泥晶化均匀程度与颗粒翻转次数成正比；颗粒泥晶化类型组合与风暴沉积有关；暴露环境出现溶蚀孔洞或负鲕。菌藻的泥晶化作用可加速海水及成岩压实作用对颗粒的破碎和细化并产生内碎屑和灰泥；鲕粒泥晶化后转变为辐射鲕和假鲕。  相似文献   

18.
The Yamama Formation is the main Lower Cretaceous (late Berriasian–Valangenian) carbonate reservoir in southern Iraq. Petrographic study from thin-section examination shows that the skeletal grains included calcareous algae from both red and green algae. Red algae is concentrated in the upper part of the Formation, and the most important of this algae species is Permocalculus ssp. Green algae is less common, and its concentration is in the middle part of the Formation. The most species found in the Yamama Formation is dasycladeans, and both small and large species of benthonic foraminifera such as Nautiloculina, Textularia, Trocholina, Pseudocyclammina, and Everticyclammina are also present. The non-skeleton grains included oolites, pellets, and micrite. Six cyclic type microfacies have been recognized for Yamama Formation in Ratawi-3 (Rt-3) and Ratawi-4 (Rt-4) Wells, namely peloidal packstone–grainstone, algal wackestone–packstone, oolitic–peloidal grainstone, bioclastic wackestone–packstone, foraminiferal wackestone, and mudstone microfacies. The latter has been divided into two submicrofacies: argillaceous lime mudstone and fossiliferous lime mudstone. The lateral extension of these microfacies has been identified by integrating the thin-section data and well logs’ character variations with similar characteristic for microfacies. The Yamama Formation was affected by five diagenetic processes, which are micritization, cementation, recrystallization, silicification, and stylolites. The Yamama Formation was deposited during a regressive period within the outer ramp, shoal, and inner ramp setting.  相似文献   

19.
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20.
Callocystites fresti sp. nov. is characterized by an elongate theca, protuberant periproct and raised ambulacral facets. Ambulacral branching in Callocystites is isotomous, but left‐ and right‐handed branches occur because the ambulacral groove branches in the same direction twice, interrupting the regularly alternating branching to brachioles. Branching in Strobilocystites does not interrupt the alternation of brachioles and lateral food grooves always pass adoral to the first brachiole of each branch. Sphaerocystites branched as in Strobilocystites. Other morphological features suggest Callocystites, Sphaerocystites and Strobilocystites are not closely related. Callocystites has open infra‐lateral and lateral circlets and five ambulacra; Sphaerocystites and Strobilocystites have closed/modified circlets and four ambulacra. In Callocystites and Sphaerocystites the B and D ambulacra differ from the others and they have one hydropore; in Strobilocystites all four ambulacra are the same and the hydropore is double. All three genera have large, globular thecae. Ambulacral branching was a response to increased food‐gathering requirements. The following are phylogenetically informative callocystitid characters: truly open vs closed/modified plate circlets; number of pectinirhombs; number of ambulacra; presence vs absence of the ‘B D different’ pattern of primary brachioles; single vs double hydropore. In echinoderms with five ambulacra in a 2–1–2 pattern ambulacral homologies are unambiguous: the single amb is A (III). In echinoderms without this pattern or with less than five ambulacra the hydropore or a single gonopore is best for orientation. Both always occur in the CD (V‐I) interambulacrum. The position of the periproct is unreliable. Different patterns and numbers of ambulacra in Palaeozoic echinoderms render a single developmental sequence improbable. Cystoids (blastozoans) had radial water vessels in their ambulacra and added new ambulacral plates terminally. Functional morphological considerations suggest tube feet were essential for food gathering. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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