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Living crinoids are exclusively passive suspension feeders and benthic as adults. However, in the past they adapted to a broad range of ecological niches. For instance, the stratigraphically important middle Paleozoic scyphocrinoids are hypothesized to have been planktonic, employing their inferred gas-filled globular, chambered structure at the distal end of the stem, the so-called lobolith, as a buoyancy device with the crinoid calyx suspended below it. Here, we evaluate this hypothesis using evidence from skeletal micromorphology and theoretical biomechanical modeling. Lobolith walls are typically composed of ossicles, which are exclusively composed of constructional labyrinthic stereom. In plates from the distal side of the lobolith, this stereom extends into microperforate stereom layer, forming wavy ridges and spines. No microscale adaptations for preventing gas leaks and/or ingress of water (such as internal and external imperforate stereom layers) are known. Furthermore, theoretical calculations suggest that the scyphocrinoid tow-net mode of feeding would have resulted in small relative velocities between the towed filter and the ambient water, thus making it an ineffective passive filter feeder. We suggest that the lobolith of these crinoids acted as a modified holdfast rather than as a floating buoy. Its globular shape and distally positioned microspines served as adaptations for living in unconsolidated sediments, analogous to iceberg- and snowshoe-like strategies used by some mollusks and brachiopods. Like modern isocrinids, scyphocrinoids could have maintained an upright feeding posture by extending the distal portion of the stalk along the bottom. In this recumbent posture, the distal part of the stalk with the lobolith might have functioned as a drag anchor. As a consequence of the ~3-m-long stem, even with this posture, the benthic scyphocrinoids could have risen to the highest epifaunal tier in the Paleozoic.

Three incomplete thecae of the stylophoran Lagynocystis cf. pyramidalis (Barrande, 1887) are described from the Upper Ordovician of the Baltic Paleobasin for the first time. These specimens of Lagynocystis are the first records of this genus from the Ordovician outside peri-Gondwana, thus significantly expanding its paleogeographical distribution of this genus. Due to the preservation of the specimens the microstructure of the stereom is studied for the first time in this genus.

A hardground surface in the middle Silurian (Wenlock: Sheinwoodian) Massie Formation is well exposed at the New Point stone quarry in Napoleon, southeastern Indiana, USA, where it is densely encrusted by pelmatozoan echinoderm attachment structures. Among the most common attachment structures encrusting the hardground are terminal dendritic radix structures attributable to the monobathrid camerate crinoid Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites. Radicles belonging to both of these genera are often modified by secretion of secondary stereom, resulting in swollen, irregular calcitic masses that extend onto the substrate and can engulf nearby bioclasts. Herein, we describe a specimen that resembles a portion of a swollen radicular attachment structure; however, close inspection reveals that is actually an interaction between radicles of Caryocrinites and Eucalyptocrinites, as indicated by the presence of both a trilobate lumen, characteristic of Caryocrinites, as well as a pentalobate lumen, characteristic of Eucalyptocrinites. Hence, what appears to be part of a single structure is actually parts of two interwoven structures belonging to entirely different taxa. Increased attention to echinoderm attachment structures consisting of overgrowths and secondarily mineralized radicles may result in recognition of additional specimens that represent analogous biotic interactions rather than simple holdfast occurrences.

Holocrinid crinoids (Holocrinus) are among the oldest, if not the oldest, post-Paleozoic crinoids, which are widely considered the stem-group isocrinids for the post-Paleozoic monophyletic subclass Articulata. Owing to their phylogenetic and stratigraphic position, the morphology and paleobiology of these crinoids have received considerable attention. Although the holocrinid stalk is similar to that of fossil and living isocrinids, it lacks flat and rigid synostosial articulations, which are sites specialized for autotomy. Holocrinids were thus long considered sessile, without the ability to re-attach. However, taphonomic data, such as specific pattern of preferred disarticulation at the distal facet of nodals, indicated that Holocrinus was likely capable of stalk autotomy and relocation. This trait, that is considered an anti-predatory adaptation against benthic predation, has recently served as example of fundamental change in benthic marine communities, such as the Mesozoic marine revolution (MMR). Here I describe stereom microstructure of the stalk of Early-Middle Triassic Holocrinus. The holocrinid stalk is mainly constructed of straight stereom passageways passing through the columnals, that are diagnostic of long through-going ligaments. At the distal symplexial nodal facet, however, fine and dense stereom, in which pores are irregular in outline and show no alignment, is observed. The latter stereom type strongly resembles synostosial stereom in living isocrinids, that is associated with short ligaments, specifically designed for autotomy. These findings thus strongly support stalk shedding abilities in holocrinids. These results emphasize that microstructural observations, coupled with taphonomic data, may provide more insights into functional inferences than macromorphological data alone.

The spines of echinoids are common palaeontological objects, but rarely supply more than this minimal information, because they are difficult to identify to genus or species. Some taxa, particularly cidaroids, may be named based on the external morphology of their spines, but this is of no assistance when identifying spines in thin section. Yet, the internal complexities of the stereom of the echinoid spine may be unique to a particular species or, at least, genus or family. A scanning electron micrograph (SEM) atlas of the internal structure of the primary spines of 14 of the most common species of regular echinoids from shallow–water environments of the tropical western Atlantic Ocean is presented herein. This demonstrates the stereom structure of the spines of each of these species by reference to multiple images. Plates of SEMs are supplemented by brief descriptions and comparisons. This atlas is intended as a tool that can be applied to diverse aspects of palaeontological and sedimentological studies in thin section.

Uintacrinoids (Uintacrinoidea) are among the most bizarre of all the crinoids. These stratigraphically important Late Cretaceous crinoids are characterized by an atypical large stalkless globular cup without cirriferous centrodorsal and extremely long arms. Owing to this unusual morphology, the mode of life of these crinoids has confused palaeontologists for more than a century. While uintacrinoids have been commonly regarded as pelagic forms, either swimming actively or floating passively by means of a putative buoyancy mechanism, some recent authors have favoured a benthic mode of life. However, evidence supporting a benthic lifestyle hypothesis is still limited. Here we describe the stereom microstructure of Marsupites and Uintacrinus. In Marsupites, thecal plates are typically comprised of coarse to medium, clearly structural, galleried stereom, which alternates with zones of fine and deeply penetrating galleried stereom oriented perpendicular to the plate margins. The latter stereom type corresponds to the external ribbed ornament of the theca, and is indicative of collagenous sutural fibers for binding adjacent plates. In Uintacrinus, thecal plates are almost entirely composed of fine galleried stereom diagnostic for collagenous sutural fibers. In both taxa, a perforate stereom layer, that is known to increase plate strengthening and resistance to abrasion in some Recent echinoderms, is developed most externally, and also produces a characteristic interior growth banding pattern. No skeletal adaptations for maintaining buoyancy, such as skeletal lightening and hollow architecture, that are known to occur in nektonic and pseudoplanktonic crinoids, are observed. Instead, the stereom organization in uintacrinoids closely resembles those observed in Recent benthic crinoids. Our findings thus strongly support a benthic mode of life of uintacrinoids. Their widespread distribution within a short time interval can be best explained by having a very long-term planktonic larval stage as previously hypothesized by some authors.

One of the most profound environmental changes thought to be reflected in chemical composition of numerous geological archives is Mg/Ca ratio of the seawater, which has varied dramatically throughout the Phanerozoic. Echinoderms that today typically form high magnesium calcite skeletons are increasingly being utilized as a proxy for interpreting secular changes in seawater chemistry. However, accurate characterization of the diagenetic changes of their metastable high magnesium calcite skeletons is a prerequisite for assessing their original, major-element geochemical composition. Here we expand the existing models of diagenesis of echinoderm skeleton by integration of various analytical methods that up to now rarely have been used to assess the diagenetic changes of fossil echinoderms. We validated the preservation of a suite of differently preserved echinoderm ossicles, mostly crinoids, ranging in age from the Cambrian through Recent. In 13 of 99 fossil echinoderm ossicles we found well-preserved porous microstructure (stereom), non-luminescent behaviour or blotchy dark color in cathodoluminescence, and distinct nanostructural features (layered and nanocomposite structure). Moreover, in representatives of such preserved samples, distribution of sulphates associated with organic matter is identical to those in Recent echinoderms. Only such ossicles, despite of local micrometer-scale diagenetic changes, were herein considered well-preserved, retaining their original major-element skeletal composition. By contrast, majority of samples show transformation to the stable low magnesium calcite that leads to obliteration of the primary geochemical and micro/nanostructural features and is accompanied with increase in cathodoluminescence emission intensity. Using only well-preserved fossil echinoderm samples, we found purely random variation in Mg/Ca in echinoderm skeletons through the observed time series; any periodicities in echinoderm skeletal Mg/Ca ratio which might be related to the secular transitions in calcite and aragonite seas were not confirmed. These findings suggest that, in contrast to some groups of organisms with relatively weak control over their biomineralization (such as algae, sponges, and bryozoans), in which polymorph mineralogies consistently changed according to the seawater type, application of fossil echinoderms, in particular crinoids, to seawater Mg/Ca reconstructions is unreliable. These data emphasize a key-role of physiological factors (the so-called vital effects) in echinoderm biomineralization.

Exceptionally well-preserved ossicles of xenomorphic stalked crinoids (Echinodermata) were found into the Late Ypresian clay of the Tuilerie de Gan (Pyrenees-Atlantiques, southwestern France). Three kinds of columnal articulation correspond to three different genera, including the new genus Eocenocrinus. These columnals are attributed to Paraconocrinus romanensis, Democrinus londinensis and Eocenocrinus hessi n. gen., n. sp. Associated benthic fauna as well as columnal characters related to water depth confirm an increasing depth to an epibathyal environment as previously suggested by mollusk and foraminifer associations. Paraconocrinus romanensis associated with Democrinus londinensis mark the maximum water depth, which did not exceed 300 m. Distal columnal series with proximal roots of P. romanensis and D. londinensis were collected with their ossicles connected, indicating that these species lived on a muddy bottom. E. hessi n. gen., n. sp. is suspected to have been attached on hard substrate. Without diagenetic crystallizing, the crinoid stereom and its micro-biocorrosion appear as in extant specimens. We attribute to Eocenocrinus n. gen. large columnals previously described from northeastern Italy and recognize two species: E. bayani n. gen., n. sp. (Middle Lutetian) and E. didymus (Priabonian). E. hessi n. gen., n. sp. may represent the oldest known species in the family Phrynocrinidae.

Drilling predation of cassid gastropods (tonnacean) on echinoids is common in marine environments but is rarely documented. Tests of the minute clypeasteroid Echinocyamus pusillus OF Muller (1776) were collected from the Mediterranean Sea (Isola del Giglio, Italy). Besides general morphology, features pertaining to the morphology and distribution of predatory boreholes were examined. Furthermore, borehole frequencies among different samples sites were compared. Cassid gastropods are assumed to be the originators of the boreholes. A total of 1061 tests were analysed for drilling rates with 15.3% drilled with predominantly single boreholes. The borehole morphology is strongly affected by the microstructure of the skeleton; the borehole outline is irregular if drilled within areas where ambulacral pores are present. Vertical borehole morphology is influenced by stereom density. The size frequencies of non-drilled and drilled specimens show significant differences. Comparisons of borehole size with test size ...

Middle Miocene tests of Clypeaster from L’Arrabassada (Tarragona, north-east Spain) show evidence of intense endoskeletozoan colonization, preserved as borings and associated carbonate secretions that allow gastrochaenid bivalves to be identified as the colonizers. Two modes of occurrence have been recognized for these bivalve dwelling cavities; ‘intrastereom clavate borings’ which are restricted to the echinoid stereom, and ‘semi-endoskeletal dwellings’, which penetrate across the test wall and extend as carbonate crypts into the sediment fill of the internal test cavity. Their size, density and position rule out a syn-vivo relationship with the echinoids and demonstrate that colonization was post mortem. Because of the endurance of clypeasteroid skeleton and the pronounced bell-shaped morphology of Clypeaster, the tests of these echinoids provided the most suitable substrates for hard-bottom colonizers on an otherwise sandy seafloor. The scenario described from Tarragona can be extended to other Neogene and Quaternary strata elsewhere; there is ample evidence for the long-term utilization of tests of Clypeaster by gastrochaenid bivalves in shoreface palaeoenvironments.

The chemical composition and microstructure of the calcite cuticles of eleven species of phacopine trilobites have been investigated by electron beam imaging, diffraction, and microanalysis, and results reveal that the lenses of their schizochroal eyes differed significantly in chemical composition from the rest of the cuticle in vivo. Apart from the eye lenses, most cuticles are inferred to have escaped extensive recrystallisation because their constituent crystals are sub-micrometre in size and have a preferred orientation that is consistent between species. Their current compositions of ~ 1.4 to 2.4 mol% MgCO3 are likely to be close to original values, although as they commonly luminesce and contain detectable manganese and iron, some diagenetic alteration has taken place. The associated lenses have a microstructure that is suitable for focusing light, yet are optically turbid owing to the presence within calcite of micropores and crystals of microdolomite, apatite, celestite and pyrite. The microdolomite indicates that lenses recrystallised from an original high-Mg calcite composition and this is supported by the presence of nanometre-scale modulated microstructures in both the calcite and dolomite. These lenses currently contain ~ 1 to 6 mol% MgCO3, and by comparison with the proportion of magnesium lost from echinoderm stereom in the same thin sections, may have contained ~ 7.5 mol% MgCO3 in vivo. In some samples, more extensive diagenetic alteration is evidenced by recrystallisation of the cuticle including lenses to coarse equant calcite or enrichment of the cuticle, but not necessarily the lenses, in magnesium accompanying replacement by a Mg–Fe phyllosilicate. The phacopine trilobites had to modify partition coefficients for magnesium considerably in order to grow lenses with contrasting compositions to the rest of their cuticles, and such a strong vital effect on biomineralisation suggests that incorporation of magnesium was essential for functioning of their calcite optical systems.

The asteroid fauna from the Late Oxfordian marls (bifurcatus Zone, stenocycloides Subzone) of Savigna includes 11 taxa, distributed between 8 families, of which 1 (Plumasteridae) is new. A goniasterid of distinctive morphology, Hessaster longimarginalis, is described as new, as is an asteriid, Savignasterias villieri. The material was recovered by surface picking and processing over 1,000 kg of sediment, and includes 28 partial individuals and approximately 2,000 isolated ossicles in excellent preservation, which show fine details of the stereom architecture and additionally allow ontogenetic changes to be described. Detailed comparison with extant asteroids enables diverse ossicle types to be assigned precisely to individual taxa. The phylogenetic relationships of individual fossil species with living taxa are determined using characters derived from numerous skeletal elements, and it is shown that the benthopectinid and pterasterid species present in the Savigna fauna are basal to their respective families. In comparison with other assemblages collected from Jurassic clay facies, the Savigna asteroid fauna is unusually diverse. Importantly, the fauna includes elements typical of present day deep sea environments (bathyal and abyssal), living abundantly in Oxfordian shelf seas of about 50 m depth.

The clypeasteroid echinoid skeleton is a multi-plated, light-weight shell construction produced by biomineralization processes. In shell constructions, joints between individual elements are considered as weak points, yet these echinoid skeletons show an extensive preservation potential in both Recent and fossil environments. The remarkable strength of the test is achieved by skeletal reinforcement structures and their constructional layouts. Micro-computed tomography and scanning electron microscopy are used for microstructural and volumetric analyses of the echinoid's skeleton. It is shown that strengthening mechanisms act on different hierarchical levels from the overall shape of the skeleton to skeletal interlocking. The tight-fitting and interlocking plate joints lead to a shell considered to behave as a monolithic structure. The plate's architecture features distinct regions interpreted as a significant load-transferring system. The internal support system follows the segmentation of the remaining skeleton, where sutural layout and stereom distribution are designed for effective load transfer. The structural analysis of the multi-plated, yet monolithic skeleton ofEchinocyamus pusillusreveals new aspects of the micro-morphology and its structural relevance for the load-bearing behaviour. The analysed structural principles allowE. pusillusto be considered as a role model for the development of multi-element, light-weight shell constructions.

The early Middle Cambrian Monastery Creek Phosphorite (Beetle Creek Formation, Queensland, Australia) contains an assemblage of disarticulated echinoderm ossicles that are exquisitely preserved. Amongst this material we recognize pelmatozoan brachials, radials, basals and holomeric columnals. Although we cannot reconstruct the complete animal with precision, these elements represent the oldest known pelmatozoan with crinoid-like appendages. Key elements include isotomously to heterotomously branched uniserial appendage plates with a tripartite adoral food groove, a longitudinal central canal interpreted as housing entoneural nerve, and differentiated articulation facets. There are also epispire-bearing radials bearing one to four arm insertion-facets, each one pierced by a central neural canal. These canals run internal towards the oral area beneath the external food groove. Co-occuring material includes single truncated cone-shaped basals and holomeric columnals, both with a similar articulation pattern, and irregular, epispire-bearing thecal plates. This mosaic of crinozoan (uniserial isotomous to heterotomous arms with neural canal), blastozoan (epispire-bearing thecal plates, appendage leading to oral thecal food groove without direct connection with body cavity) and apomorphic characters (circumoral instead of basal entoneural plexus) is unexpected and demonstrates that crinoid-like pelmatozoans with uniserial, branched arms appeared significantly earlier than the Tremadocian, when the first articulated crinoid skeletons are found. It also raises questions about the polyphyletic appearance of feeding appendages among pelmatozoan echinoderms.

Calcite isocrinid ossicles from the Middle Jurassic (Bathonian) clays in Gnaszyn (central Poland) show perfectly pre− served micro− and nanostructural details typical of diagenetically unaltered echinoderm skeleton. Stereom pores are filled with ferroan calcite cements that sealed off the skeleton from diagenetic fluids and prevented structural and geochemical alteration. In contrast with high−Mg calcite skeleton of modern, tropical echinoderms, the fossil crinoid ossicles from Gnaszyn contain only 5.0–5.3 mole% of MgCO3. This low Mg content can be a result of either a low temperature environ− ment (ca. 10C) and/or low Mg/Ca seawater ratio. Both conditions have been proposed for the Middle Jurassic marine en− vironment. Occurrence of Mg−enriched central region of stereom bars of Jurassic columnal ossicle of Chariocrinus an− dreae is consistent with the concept of magnesium ions involvement in earliest growth phases of calcium carbonate biominerals. Key wor ds: Echinodermata, Crinoidea, calcite, nanostructure, geochemistry, AFM, NanoSIMS, Jurassic.

Previously undescribed material in the Natural History Museum Basel provides the basis for a reappraisal of the enigmatic Middle Jurassic to Early Cretaceous genus Cyclocrinus D’ORBINGY known only from mostly single columnals and attachment discs. Only one of the previously described Jurassic species, C. rugosus, is recognized despite wide variation in morphology of columnals ranging from Bajocian to Oxfordian in age. A small percentage of columnals are axillary, show facets for side branches or are arched, which has led to the assumption that Cyclocrinus columnals represent radicular cirrals and should be assigned to the Order Bourgueticrinida. Both hypotheses are rejected. The prevalence of single columnals is explained by the restriction of loose, galleried (α) stereom to an area near the facets and around the axial canal; through-going ligament fibres responsible for the preservation of pluricolumnals were thus largely absent from much of the stereom. Cyclocrinus variolarius (SEELEY) from the Albian of England hardly differs from the Jurassic C. rugosus. The ordinal position of Cyclocrinus and of the family Cyclocrinidae SIEVERTS-DORECK is left open though the form has some resemblance to the Early Jurassic millericrinid genus Amaltheocrinus KLIKUSHIN.

Based on rich material (381 specimens examined) from two Cambrian echinoderm faunas, the early Cambrian Balang fauna and middle Cambrian Kaili fauna in Guizhou Province, South China, the taphonomy of gogiid echinoderms is described in detail, and the preservation of stereomic microstructure and organic remains of Cambrian gogiid echinoderms is reported here for the first time. Taphonomic considerations include entombment patterns, decay sequences, individual-specific diagenetic histories, unusual burial postures, selective disarticulation patterns, and post-mortem elongation. In particular, five categories of gogiid entombment patterns are proposed to describe the multi-directional orientations recorded at the burial time of articulated gogiids. Gogiid-bearing slabs of Guizhou material primarily (70%) display the type 2 entombment pattern (articulated gogiids preserved with fan-shaped brachioles); thus, most Guizhou gogiids were buried with brachioles preserved in feeding posture during obrution events. Balang gogiid faunas contain the oldest evidence of palaeoecological interactions among echinoderms and other indigenous taxa. In addition to pre-burial and post-burial decay, other potential causes for unusual disarticulation patterns exhibited by the gogiids from the lower Cambrian Balang Formation include pre-burial bio-disturbance and post-burial bioturbation based on ichnogenera, including Rusophycus and Planolites. Chemical analyses reveal that carbon, calcium, manganese and iron are the major elements responsible for the variety of colours exhibited by Guizhou gogiids. Three-dimensional stereomic microstructure (mean stereom pore size = 8.4–8.7 μm; average trabecular thickness = 4.5–4.6μm) occurs on the external surfaces of thecal plates in two gogiid species. Stereom preservation in calcite suggests that the dissolution of calcareous echinoderm plates, yielding characteristic mouldic preservation, is sub-Recent (after lithificaiton and exposure of gogiid-bearing, marine sedimentary successions on or near the land surface).

The alternation between abiotic calcite and aragonite precipitation in shallow seawater and the oscillation of seawater Mg/Ca ratio throughout the Phanerozoic are both popular hypotheses. Echinoderms with well-preserved stereom provide new empirical evidence that supports both these hypotheses. Analyses are reported from 29 specimens, Cambrian to Eocene age, containing ossicles preserved either as Mg calcite, 3.3 to 8 mole% MgCO3, or as mixed calcite and dolomite, 5 to 12.5 mole% MgCO3. Echinoderms with high mole% MgCO3 occur in the Early Cambrian and late Carboniferous to Triassic; low values come from the Silurian and Jurassic to Cretaceous. The average composition of echinoderms with preserved stereom does not fall below 3.5 mol% MgCO3 during the Phanerozoic and reaches its highest mean value of 16.0 mol% MgCO3 today. An empirical partition coefficient of 0.03182 for modern tropical echinoids is used to indicate the Mg/Ca ratio of ancient seawater from the Mg/Ca ratio of fossil stereom; some assumptions are involved and a likely error is calculated from the Mg2+ variation of modern echinoids. High mean seawater Mg/Ca ratios are calculated for early Cambrian (3.3) and the late Carboniferous to Triassic (2.3) but never reached today's value of 5.2; low Mg/Ca ratios (1.1) are indicated from Jurassic to Cretaceous echinoderms. The 29 echinoderm samples plot close to first-order Mg/Ca seawater oscillations derived from geochemical models and Mg/Ca ratios determined from fluid inclusions, but considerable discrepancies exist when shorter (106 Myr) time intervals are considered. Further data and improved understanding of Mg partitioning is required before accurate secular variation in the Mg/Ca of seawater can be determined from echinoderms. However, they are an underused resource in this context and provide an excellent seawater archive.

Echinoderm skeletons are described within a hierarchical framework ranging from complete organisms to the ultrastructural level. They consist of numerous elements which can be isolated, connected by soft tissue or locked together in rigid structures. The top level considers skeletons as a whole with all associated elements and the basic symmetry of the echinoderms. The next level deals with the structural analysis and modeling of echinoids with respect to the growth parameters and stress resistance of the corona. The flexibility and movement has also been studied for the stalks and arms of recent and fossil crinoids. The next level deals with the elaborate morphology and types of symmetry found in single skeletal elements. The numerous types of stereom architectures found within the elements of all echinoderms are highly correlated to specific functions. A high number of recent studies concern the last hierarchical level on ultrastructure and biomineralization. Lightweight aspects of the skeleton are especially present at the level of conjoined plates, single elements and the stereom.

New information pertaining to the biostratinomy of Uintacrinus assemblages has been derived from re-examination of museum collections and analysis of in situ material. Individuals are preserved as thin lenses in dense aggregations with articulated calyces and arms only on the lower surface, in contrast to disarticulated material on the upper surface. Calyces may be imbricated within a lens and are mostly compressed laterally. Specimens also may be preserved oral side up or down. Some specimens displaying the oral side retain soft-part preservation of the tegmen, anal tube, and ambulacra. A thin, jet black organic lamination is visible beneath calyx plates. SEM analysis reveals the presence of possible microbial spherules and filaments on this lamination. Dense aggregations reveal a number of new preservational features, including marginal indentations and lacunae that suggest cohesive behavior of the layer prior to burial. SEM also reveals well-preserved stereom in articulated crinoids on the lower surface, in contrast to the upper surface in which the stereom is apparently infilled with calcium carbonate. Some specimens retain a black, organic lamination within the crinoidal layer itself that may represent remnants of a microbial mat. It is proposed that a microbial mat of necrolytic origin provided cohesion and that microbial sealing during decay may help to explain other instances of similar crinoid preservation, including both benthic and pelagic forms, in which articulation is confined to lower surfaces.

The stereom of living Echinus esculentus skeletons is composed of Mg calcite (interambulacral plates 6 mole % MgCO3; spines 4 mole % MgCO3) and is compared with the stereom of Pennsylvanian echinoderms from the Holder Formation, Dry Canyon. Some Dry Canyon echinoderms had their pore system (55% volume) filled by magnesian ferroan calcite cement before the stereom transformed (type 1) into small ( 20 µm diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns. The margins of some (type 2) ossicles set in a clay matrix are Fe-rich and contain barite crystals; the Fe2+ and Ba2+ were transferred from the surrounding clay hundreds of microns into these ossicles. Type 1 transformation was catalyzed by internal water from the stereom; the presence of a ferroan calcite crystal casket confined alteration to the skeletal Mg calcite. Type 2 transformation involved both stereom and pore-filling Mg calcite cement. The unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased. The transformation of Dry Canyon echinoderms was probably triggered, or accelerated, by rising temperature caused by burial. The bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original, major-element chemistry of echinoderm skeletons. Minor amounts of Mg calcite cement that occur in Dry Canon (type 1) ossicles indicate the chemistry of porewater close to the sediment-water interface. Fossil echinoderms are preserved in many ways, and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions. The stereom is destroyed in most fossil echinoderms, and even in those where the stereom shape is preserved its internal structure is changed. Echinoderm Mg calcite does not transform with perfect textural preservation.

Interambularcral plates of echinoid Heterocentrotus trigonarius, composed of Mg calcite 1 (≈14 mol% MgCO3), were heated in three timed series of experiments at 300°C. Dried plate fragments and fragments with added water were heated separately in pressurized bombs. X-ray powder diffractometry, unit cell dimensions, and phase compositions are used to monitor reaction progress. After 10 h heating in the bombs dolomite (43.5 mol% MgCO3) and Mg calcite appear (4–7 mol% MgCO3); by 20 h all Mg calcite 1 is consumed, and at 120 h dolomite composition has evolved to ≈47 mol% MgCO3 and calcite to ≈2 mol% MgCO3. Whole plates heated at 300°C in an open muffle furnace develop dolomite (≈42 mol% MgCO3) and Mg calcite 2 (≈6 mol% MgCO3) after 10 h and remain compositionally invariant throughout subsequent heating to 620 h. Limited skeletal water catalyzes the early reaction but escapes from the open furnace and consequently reaction ceases after ≈10 h. The experimentally produced dolomite has relative Mg-Ca ordering of 75% to 79%. The stabilization of echinoid Mg calcite by heating at 300°C to a mixture of dolomite and calcite occurs through a dissolution/precipitation reaction. The alteration fabric produced within the stereom consists of irregularly shaped, branched dolomite crystals > 5 μm homoaxially set in a calcite 2 (bomb) or Mg calcite 2 (furnace) matrix. Round and tubular pores 1 to 5 μm are randomly distributed throughout this fabric. The stereom pore system remains intact during furnace heating but is destroyed during heating in bombs. The texture of experimentally stabilized echinoid skeletons is different from that of fossil echinoderms that are composed of microrhomic dolomite homoaxially set in a single calcite crystal.

Echmatocrinus from the Middle Cambrian Burgess Shale of British Columbia was originally described as the earliest crinoid(?) known from the fossil record. Recently, Conway Morris and Ausich & Babcock have questioned whether Echmatocrinus is in fact an echinoderm, comparing it instead to cnidarians with a polyp-like body and pinnate tentacles, and other authors are beginning to use this reinterpretation. We studied the well-preserved holotype of Echmatocrinus brachiatus, two paratypes, and 18 new specimens recovered from different levels in the Burgess Shale sequence at three localities. All are preserved as pyrite films in dark shale with relatively little relief, suggesting a lightly skeletized body. Complete specimens have a long, slightly tapering, large-plated attachment stalk, a conical cup or calyx with numerous small to medium-sized irregular plates, and 7–10 short arms with heavier plating and (in the holotype) soft appendages alternating from opposite sides of several arms. Several morphologic features indicate that Echmatocrinus is an echinoderm and has crinoid affinities: (1) Sutured plates, shown by darker depressed sutures, slightly raised plate centers, and oriented plate ornament, cover all major parts of the body; (2) reticulate surface ornament in the pyrite film on the plates of all specimens matches the ornament in the Burgess Shale edrioasteroid Walcottidiscus, an undoubted echinoderm, but not the pyritized surfaces of other metazoans in the fauna; (3) this distinctive ornament may represent the surface expression of microporous stereom; (4) possible ligament or muscle pads are present between the arm ossicles to fold and unfurl the more heavily plated arms. Within the echinoderms, only crinoids commonly have a calyx attached by a stalk or stem to the substrate and bear erect, moveable, uniserial arms for feeding. Although Echmatocrinus shows some resemblance to octocorals in overall body shape as an attached suspension feeder, almost all the details are different, indicating that Echmatocrinus is most likely unrelated to this group. All complete specimens of Echmatocrinus are attached to hard substrates, either another fossil or skeletal debris. The new specimens indicate that Echmatocrinus was twice as common (about 0.02%) in the Burgess Shale fauna as previously recorded and represents one of the earliest attached, medium-level, skeletized, suspension feeders or microcarnivores in the fossil record.

Taphonomic study of echinoderms provides useful information on sedimentary conditions before, during, and after burial. Taphonomic studies of Recent echinoderms indicate that much skeletal disarticulation occurs within a few days after death. However, experiments also indicate that within a short period after death echinoderm carcasses remain rather resistant to disarticulation, and thus may be transported a considerable distance by currents; following periods of a few hours of decay, more delicate portions of echinoderm skeletons are readily disarticulated. Some skeletal modules (e.g., crinoid pluricolumnals) may resist disarticulation for periods of months in quiet- and or cool-water environments. Anoxia promotes intact preservation by excluding scavenging metazoans. Echinoderm ossicles may undergo minor abrasion and/or corrosion if left exposed, and less dense stereom corrodes much more rapidly than dense plates, such as echinoid spines. However, heavily abraded ossicles may indicate prefossilization and reworking.Various groups of echinoderms (e.g., pelmatozoans, asterozoans, echinoids) have differing propensities for degradation and, therefore, produce different arrays of preserved fossil material primarily depending upon the relative rates of burial, bottom-water oxygenation, and turbulence. Echinoderms may be divided into three groups based upon the relative ease of skeletal disarticulation. Type 1 echinoderms include weakly articulated forms (e.g., asteroids and ophiuroids) that rapidly disintegrate into individual ossicles. Type 2 includes those echinoderms whose bodies contain portions in which are more tightly sutured, as well as portions in which the ossicles are somewhat more delicately bound (e.g., crinoids, regular echinoids). Such echinoderms display more varied taphonomic grades from fully intact to mixtures of isolated ossicles and articulated modules. Type 3 comprises those echinoderms (e.g., irregular echinoids) in which major portions of the skeleton are so resistant to disarticulation that they may be broken across sutures rather than coming apart at plate boundaries.Comparative taphonomy of particular types of echinoderm skeletal remains leads to recognition of distinctive taphofacies that characterize particular depositional environments. Taphofacies include two types of characteristic modes of fossil preservation: event taphonomic signatures and background taphonomic signatures. Depending upon normal conditions of environmental energy and rates of sedimentation, the background condition of various types of echinoderms for a given facies may range from articulated, unabraded skeletal modules (in Types 2 and 3) to highly corroded and/or abraded ossicles. Conversely, the occurrence of fully intact fossil echinoderms provides unambiguous evidence of rapid and deep burial of benthic communities. Such well-preserved fossil assemblages can provide a wealth of information regarding the paleobiology of echinoderms, as well as the nature of the depositional events and burial histories.This paper presents a preliminary classification and characterization of background and event aspects of echinoderm taphofacies for carbonate- (9 taphofacies, including reefs and hardgrounds) and siliciclastic-dominated (5 taphofacies) environments. In each case, we recognize a spectrum of echinoderm taphofacies that coincides with a gradient of environments, ranging from nearshore, high energy shoreface through proximal and distal storm-influenced shelf, to deeper ramp and dysoxic basinal settings. Most taphofacies also feature particular styles of obrution (smothered bottom) Lagerstätten. These range from scattered lenses of articulated fossils in some high energy sandstone and grainstone facies to bedding planes of articulated, pyrite coated specimens in dark shales. We classify and discuss the genesis of these types of Lagerstätten and list typical examples. Finally, we present a simple model that integrates the occurrence of various echinoderm taphofacies with concepts of cyclic and sequence stratigraphy.

8 fig.; Echinoderm skeletal debris from the Early-Middle Cambrian boundary Micmacca Breccia of Morocco includes the oldest known holomeric columnals. The original calcite of these ossicles is coated and replaced by iron oxides, occasionally overlain by a late coating of silica, and preserves with high fidelity fine details of their three-dimensional microstructure. Irrespective of their external morphology, columnals can be divided into three groups based on the distribution of stereom microfabrics, which we suggest indicates the presence of at least three different holomeric stemmed taxa. One of these columnal types has well-developed galleried stereom perpendicular to its articulation facets, a sure sign that long, penetrative collagen bundles bound columnals together, as in modern stemmed crinoids. This columnal morphology also shows a primitive type of interlocking articulation, which we term parasymplexy and which may have helped to counter torsional stresses. The two other columnals either lacked fibrous connective tissue or had shallow, non-penetrative fibers between columnals.

SUMMARY Miocidaris keyserlingi is found in the Upper Permian Ford Formation, Zechstein Cycle 1, of the Sunderland district. It is confined to reef core and fore-reef talus slope facies and is absent from higher energy reef-crest and reef-flat habitats. Well preserved material permits the description of stereom microstructure and tooth construction. M. keyserlingi has teeth identical in structure to those of Recent cidarids and this contrasts with tooth structure in both the Carboniferous archaeocidarids and the early Jurassic euechinoids. Hemipyramid morphology and perignathic girdle structure also demonstrate the close relationship between M. keyserlingi and Recent cidaroids. Euechinoids and cidaroids must have diverged prior to the Upper Permian and at least two separate lineages of echinoids passed across the Permo-Triassic boundary.

Laboratory experiments on regular echinoids indicate that low water temperatures retard organic decomposition far more effectively than anoxia, and that the primary role of anoxia in the preservation of articulated multi-element calcareous skeletons may be in excluding scavenging organisms. When tumbled at 20 rpm, specimens that were first allowed to decay for two days in warm seawater (30°C) disintegrated more than six times faster than specimens treated at room temperature (23°C) and more than an order of magnitude faster than specimens treated in cool water (11°C). In contrast, the effects of aerobic versus anerobic decay on disintegration rates were insignificant. The longer the period that specimens were allowed to decay before tumbling, the greater the rate at which specimens disintegrated, until a threshold time that appears to mark the decomposition of collagenous ligaments. This required a few days at 30°C, about two weeks at 23°C, and more than 4 weeks at 11°C forStrongylocentrotus. Up until this threshold, coronas disintegrate by a combination of cross-plate fractures and separation along plate sutures; cross-plate fractures thus can be taphonomic in origin and are not necessarily related to predation. Specimens decayed for longer-than-threshold periods of time disintegrate virtually instantaneously upon tumbling by sutural separation only. Undisturbed coronas can remain intact for months, sufficient time for epibiont occupation. Rates of disintegration were documented semi-quantitatively by recognizing seven stages of test disarticulation, and quantitatively by tensometer measures of test strength and toughness. The effects of temperature and oxygen on decay and the existence of a decay threshold in disintegration should apply at least in a qualitative sense to many other animals whose skeletons consist of multiple, collagen-bound elements.Regular echinoids should still be perceived as taphonomically fragile organisms, but our results suggest the potential for latitudinal as well as bathymetric gradients in the preservation of fossil echinoid faunas. Echinoid preservation under any given set of conditions should also be a function of taxonomic differences in test construction (particularly stereom interlocking along plate sutures) as suggested by previous workers, although our experiments indicate that these effects should only be significant among post-threshold specimens. A survey of regular echinoids from Upper Cretaceous white chalk facies of Britain substantiates the basic experimental patterns, yielding examples of all disarticulation stages and significant taxonomic differences in quality of preservation. A diverse array of borers and encrusters on fossil coronas also corroborates the post-mortem persistence of some tests on mid-latitude seafloors.

O’Malley et al. (2013) report the extraction and characterization of organic molecules from Mississippian crinoids. Chemical analyses of organic compounds of Paleozoic age can provide valuable information on ancient organisms, but are generally a challenging task requiring great caution in the treatment of samples and the interpretation of data. From the view of an organic geochemist, some statements require comment. The comparison of the fringelite pigments discovered by Blumer (1951, 1960) with hypericin is obsolete. In a revision of the structures (Wolkenstein et al., 2006), one of the fringelite pigments was identifi ed as hypericin. To avoid confusion, the closely related fossil crinoid pigments, rather, should be termed fossil hypericinoids. Reference should be clearly made that polycyclic quinones including hypericin have been proven in the Jurassic crinoid Liliocrinus and in the Triassic crinoid Carnallicrinus, and that early cementation of the stereom microstructure is an important factor for the preservation of organic material in fossil crinoids (Wolkenstein et al., 2006). O’Malley et al. give the impression that the crinoids shown in their fi gure 1 are from the Mississippian of Indiana. However, fi gure 1 actually shows a crinoid plate from the Mississippian Maynes Creek Formation at Le Grand, Iowa (Gahn and Baumiller, 2004). Although the fi gure shows an example of color differences among different crinoid species, it should be indicated that these crinoids are from a different locality from those used as samples. O’Malley et al. state that the crinoids they analyzed originate from two different localities in southern Indiana (Morgan and Monroe Counties). However, it is not indicated from which locality the individual samples originated. A detailed provenance is of particular relevance when comparing the mass spectra of only three crinoid samples (their fi gures 3A–3C). Moreover, analysis of three different samples is not suffi cient to support a taxon specifi city of organic molecules and to determine if these molecules indicate phylogenetic relationships as stated. To validate a correlation of the observed data with taxa, more than one sample of the same species should be analyzed. Another major concern is that crude extracts were analyzed, not isolated compounds as suggested in the title. Chromatographic separation is of crucial importance for the analysis of complex mixtures of unknown compounds, which are typically present in extracts from fossils. Without chromatographic separation, the spectroscopic properties of individual compounds cannot be correlated. For example, it is not possible to evaluate if a compound with a specifi c molecular mass also shows fl uorescence properties. Furthermore, crude organic extracts were obtained by extraction with organic solvents (acetone/methanol 4:1 and methanol), but in the legend of their fi gure 3, it is indicated that for excitation-emission matrix (EEM) spectroscopy, samples were analyzed in water. Considering the different solvent properties, it is rather unlikely that the mixture of compounds dissolved in water is identical to the mixture of compounds in the crude organic extract. O’Malley et al. state that the study specifi cally “seeks to determine whether the coloration distinction among fossil species results from individual crinoids possessing unique organic molecules” (p. 347). However, the ultraviolet-visible light spectra of the fossil samples (their fi gure 2) show that only very little absorbance occurs within the range of visible light (400–800 nm). The spectra thus indicate that the extracted fossil compounds are not responsible for distinctive coloration differences of the crinoids. EEM spectroscopy is generally used for characterization of dissolved organic matter in natural waters based on differences in the fl uorescence properties of broad groups of compounds (e.g., humic-like fl uorescence) (Coble, 1996), not for the determination of specifi c functional moieties. Comparing the EEM spectra in detail, signifi cant differences can be observed between the fossil crinoids and also with the reference compound, which do not necessarily support the presence of quinone-like compounds in the fossil samples. Using electrospray ionization time-of-fl ight (ESI-TOF) mass spectrometry for analysis of mixtures, it should be recognized that ESI is an ionization technique most suitable for the detection of chargeable and polar compounds and that suppression of ions may occur (Pramanik et al., 2002). Compounds occurring in trace amounts may give very intense signals if they ionize very well, whereas other compounds, even if they are abundant, may give no signal at all. Therefore, no relative abundances of compounds can be determined from ESI spectra of mixtures and the phrase “most abundant molecular species” (p. 348) is misleading. It is very diffi cult to interpret the ESI-TOF spectra in their fi gure 3 as

Field observations of four common tropical Western Atlantic regular echinoids, Diadema antillarum, Eucidaris tribuloides, Echinometra lucunter and Tripneustes ventricosus, reveal that they are reduced to essentially bleached skeletons, devoid of organic material, within six days of death. This suggests that, in the absence of organic connective tissues, the degree to which stereom interlocks across plate sutures may become an important factor in determining the amount of breakage suffered by the corona in response to biostratinomic processes

The comatulid crinoids Davidaster rubiginosa and D. discoidea live in shallow (5 m) patch reefs off the western coast of the island of San Salvador (Bahamas) and at the shelf edge along the western and southern sides of the island. Their ossicles (plates) occurs as rare, but recoverable, elements in the seafloor sediment. Living specimens of Davidaster were collected, and their arm plates (brachials) and pinnule plates (pinnulars) were subjected to laboratory tests of abrasion and dissolution in order to identify the effects of these processes on the echinoderm's stereom. Ossicles from a Pleistocene reef exposed on the island are rare, but demonstrate the potential for future studies of both biostratinomy and diagenesis

Muscular articulations on the brachials and pinnules of Paleozoic crinoids have traditionally been inferred by the presence of a transverse ridge. We here provide evidence that the microstructure of the stereom, fine meshed where muscles once inserted and coarse meshed where ligaments were emplaced, provides a reliable microstructural criterion for muscular articulation on well-preserved arm ossicles. Partially pyritized ossicles of Aesiocrinus magnificus Miller & Gurley from the Lane Shale of Pennsylvanian age at Kansas City, Missouri exhibit well-preserved stereom in three dimensions under the scanning electron microscope. These brachial and pinnular plates show clearly the distinctiveness of stereom at sites of muscule and ligament attachment. This criterion should be useful in tracing the evolutionary development of flexible arm articulations in several lineages of Paleozoic crinoids.

The diagenetic history of the skeletal elements of Late Pleistocene-Holocene Ophiura sarsi from the shelf off northern Norway (Andfjorden, Malangsdjupet) is elucidated by comparison with natural and induced degradation of the skeletal elements of Recent ophiuroids (brittle stars) and asteroids (sea stars) from Danish waters. Dissolution features ("core-and-rind") in the trabeculae of fossil and Recent echinoderm stereom are initiated during death and early decay of organic tissue in the animals. The trabeculae have a polycrystal­line lamellar ultrastructure and lose their older central part during later stages of dissolution, which are dependant on undersaturation of the sea-water with regard to CaC03• The presence of undersaturated sea-water is supported by palaeoecological studies (Thomsen & Vorren 1984, 1986) implying oxygen deficient periods in the Late Pleistocene and an increased biogenic production in the Holocene. Pyrite framboids are situated in the secondary voids within the trabeculae and in the pore space of the stereom of the Late Pleistocene elements. No pyrite is observed within the polycrystalline lamellar ultrastructure of the trabeculae. The Late Pleistocene "pyritization" took place during oxygen deficient periods at the sediment-water interface or within the reduced zone of the topmost sediment.

The Middle Cambrian genus Cymbionites was originally ascribed to a new subphylum of the Echinodermata, the Haplozoa Whitehouse 1941. It is suggested that what has previously been considered to be the entire skeleton represents only the basal circlet of an eocrinoid. These thick basal plates are composed largely of secondary stereom that welds the circlet together. Scattered amongst the basal circlets are distinctive thecal plates with epispires which are prolonged on the outer surface as shallow grooves perpendicular to the plate suture. Small brachioliferous plates have also been found. Peridionites, the only other fossil ascribed to the Haplozoa, is also considered to be the basal part of an eocrinoid.

Diagenetic alteration of original high-Mg calcite crinoid skeletal material was achieved by reaction of the calcium carbonate with isotopically light meteoric water. The light δ18O values (−10.57 ± 0.74‰, PDB) are a reflection of the water source, whereas the δ13C values (−3.49 ± 2.63‰, PDB) are a consequence of the high reactive-organic-matter content in the sediments of the Brush Creek at Sewickley, Pennsylvania. High Fe (22 970 ± 30 440 ppm) and Mn (3760 ± 1450 ppm) contents suggest further that the diagenetic fluids were reducing waters that reacted with the crinoid ossicles in the shallow-burial marine and meteoric environments.Four different types of pyrite are found within crinoid ossicles. Framboidal pyrite, aggregate pyrite, and nanopyrite are generally found in the pores of the stereom. Micropyrite, which is present in the form of octahedral, dodecahedral, and pyritohedral crystals, and nanopyrite and aggregate pyrite replace the calcitic stereom. The degree of pyritization and the pore filling increase toward the outer periphery of the crinoid ossicles. Pyrite formation of the crinoid ossicles took place in two stages. In stage I, upon death, the crinoids disarticulated and were quickly buried. In this shallow sediment layer (1–10 cm) the pore water was both undersaturated with respect to calcite and oxidizing. This brought about oxidation of the organic tissue in the stroma of the crinoids and selective dissolution of the high-Mg calcite skeletons. With further burial in stage II, in the presence of reactive organic matter local iron was solubilized, marine sulphate was reduced to sulphide, and isotopically light carbon was produced by bacterial action. Reactive iron combined with sulphur to form framboidal and nanopyrite in the pores of the stereoms. With further burial the micropyrite formed in the crinoid ossicles. Termination of the pyritization process came about with depletion of the iron and (or) sulphur, and this process proceeded very rapidly under shallow-burial conditions while the crinoids resided in the microbial sulphate-reduction zone of the marine–phreatic environment.Fossils are preserved in different stages, with brachiopod valves preserved in their original low-Mg calcite mineralogy, whereas molluscs and crinoids show the complete trend from preserved aragonite and high-Mg calcite, respectively, to diagenetic low-Mg calcite. The preservation potential of fossils is closely linked to the thermodynamic stability of the skeletal carbonate in the presence of diagenetic fluids. Carbon/sulphur ratios support the assertion that Brush Creek sediments were deposited in normal marine waters and favour generally oxic redox conditions for Pennsylvanian seawater.

A rich material of echinoderm fragments from two Middle Cambrian stratigraphical levels on Bornholm are preserved due to phosphatization of the original calcitic stereom. Preservation of echinoderms in this way - not previously recorded from the Middle Cambrian - permits detailed analysis of the three-dimensional stereom structure. Identifiable are fragments of stylophorans and eocrinoids. Stem columnals, most likely from eocrinoids, show a wide and advanced morphological variation indicating articulation similar to that of crinoids. The material from the Exsulans Limestone/Kalby marl (Ptychagnostus gibbus Zone) represents stem-bearing cystoids older than Akadocrinus from Bohemia. The Andrarum Limestone (Sole-nopleura brachymetopa Zone) contains echinoderm fragments from a higher stratigraphical level, a level correlatable with that from which the oldest North American stem–bearing cystoid, Eustypocystis, has been recorded.

Growth zones in echinoid skeletal ossicles are mainly the result of differences in structural characteristics. In the test plates, opaque zones, which appear light in reflected light, dark in transmitted light, and are X-ray dense, have relatively larger trabecules and smaller intertrabecular channels. Translucent zones, which appear dark in reflected light, light in transmitted light, and are less X-ray dense, have relatively smaller trabecules and larger intertrabecular channels. Organic material in the plates, especially when pigmented or charred, enhances the appearance of the growth zones. Opaque zones result from relatively fast plate growth while translucent zones result from relatively slow plate growth; food deprivation leads to the formation of translucent zones. The growth zones appear to be formed seasonally, at least in some cases, probably in relation to seasonal changes in growth rates, and perhaps in relation to seasonal reproductive activity

Mandalacystis dockeryi n. gen. and sp. is known from seven complete or nearly complete thecae, six partial thecae, and numerous separate plates from the Oil Creek Formation in south-central Oklahoma, and a single thecal plate from the Antelope Valley Limestone in Nevada. Details of skeletal microstructure (stereom) are well preserved, permitting scanning electron microscopy for the first time in the Eocrinoidea, and demonstrating that plate thickness on thecal faces is only 60–70 μm. Mandalacystis is characterized by labyrinthine ridges on both faces of the theca, relatively short brachioles, and a normal stem with holdfast (at least in juvenile stages). Based on the normal biserial arrangement of brachiolar plates in the brachioles and the possession of a normal stem, Mandalacystis appears to be one of the most primitive rhipidocystids, closely related to Petalocystites.Mandalacystis dockeryi lived in a storm-dominated, shallow-water offshore habitat on patches of skeletal debris surrounded by soft, terrigenous mud. This setting may have been conducive to the transition from an erect, attached mode of life to the prone, unattached lifestyle of later rhipidocystids.

Discovery of Tremichnus cysticus Brett on a crinoid pluricolumnal from the Willowvale Shale of western New York extends the range of this ichnogenus into the Lower Silurian. Pits attributable to Tremichnus apparently represent embedment sites of parasites or commensalistic, filter-feeding organisms of unknown affinities. Embedment of T. cysticus may have been accomplished by localized inhibition of stereom growth within the pits aided by secretion of substances that stimulated accretion of stereom by the host pelmatozoan in areas immediately surrounding the pits.

All of the major skeletal minerals (calcite, magnesian calcite, aragonite, apatite and opal) and most of the major kinds of skeletal materials (fibrous apatite, spherulitic aragonite, nacre, crossed-lamellar aragonite, foliated calcite, echinoderm stereom etc.) appeared abruptly in many different kinds of organisms at the beginning of the Phanerozoic (570 million years to Recent). Almost all of the clades known to form mineral skeletons originated in the Cambrian Period of earth history (570–505 million years ago) and hardly any kinds of marine organisms developed new types of mineral skeletons after that time. Recent work on the earliest skeletal fossils provides little support for the view that phosphatic skeletons were more common than carbonate skeletons when hard parts first developed.

Middle Pennsylvanian fossils from the Buckhorn Asphalt quarry of Southern Oklahoma are unusual in that early sealing by tar has preserved many of their original textural and mineralogic features and those of the earliest stages of diagenesis. Echinoderm fragments collected from this locality axe well preserved, with original stereom morphology clearly visible optically. One- to five-micrometer anhedral calcian dolomite grains occupy as much as 30 percent of the original stererom of the fragments, as displayed by differential etching and MgK X-my maps. The composition of skeletal fragments is 75 percent Ca.98Mg.02Co3, 25 percent Ca.54Mg.46Co3. Single crystal X-ray di fraction, XRD, EMPA, TEM, and STEM microanalysis were used to characterize the calcian dolomite phase. TEM micrographs revealed an approximate 100 A, "tweed structure" identical to that reported by Reeder and Wenk (1979). Our data imply that magnesium in the calcian dolomites was derived largely from the original magnesian calcite of the echinoderm stereom. Original biogenic high-magnesium calcite is altered via local microdissolution--reprecipitation to anhedral disordered calcian dolomite grains within low-magnesium calcite. The product phases (low-magnesium calcite and calcian dolomite) retain the original crystallographic orientation of the stereom. These products of early diagenesis are apparent precursors to well-defined, ordered euhedral microdolomites. The relationships between the low-magnesium calcite and calcian dolomite, and the chemistry and complex ultrastructure of the calcian dolomites support the kind of low-temperature mechanism of dolomite formation postulated by Gains (1977), Goldsm th and Graf (1958), and others on the basis of hydrothermal experiments.

The multi-plated skeleton of echinoids is made of the stereom, a light-weight construction which resembles a micro-beam framework. Although the two-dimensional design of the stereom has been studied, its spatial architecture is only little known. It is, however, imperative to understand the spatial architecture of the trabecular system in order to interpret its structural principles of this load-bearing construction. The echinoid’s trabecular system is thus analyzed in-depth with respect to eight topological descriptors. The echinoid’s plates are divided into two regions, the center of which consists of an unordered stereom, and the margin which consists of an ordered stereom. The eight trabecular descriptors indicate that the basal topology of the two plate regions are similar. The trabecular system predominantly consists of short and stocky trabeculae that show little tortuosity. The majority of trabeculae intersect in a 3N configuration, where three trabeculae intersect in one common node. Trabeculae in the 3N configuration intersect in an angle of around 120° resulting in a planar and triangular motif. These planar elements, when arranged in an angular off-set, can resist multi-dimensional loads. Results also show that the trabecular orientation perpendicular to the plate’s surface is at an angle of 60°. The trabecular orientation in the plate’s horizontal plane is directional. Both trabecular orientations reflect a construction which is capable of resisting applied loads and can distribute these loads over the entire skeleton. The spatial architecture of the echinoid’s trabecular system is thus considered to be a performative light-weight and load-bearing system.