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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.

Skeletal growth banding has been studied in the vertebral arm ossicles of the shallowwater brittle stars Ophiothrix fragilis, Amphiura filiformis, A. chiajei, Ophiura ophiura and O.albida collected from near Oban. Scanning electron microscopy of ossicle microstructure show the bands reflect differences in stereom porosity and surface relief on the surface of the wing-like fossae of the ossicle. The evidence supports the interpretation of the bands as annual growth markers, with coarse-pored stereom laid down during periods of active growth in summer/autumn whilst finer-pored stereom, that often forms ‘breaking wave’- like ridges, is formed during periods of slow or negligible skeletal growth during the rest of the year. Growth curves fitted to growth ring measurements indicate three contrasting growth strategies amongst the species examined. These data generally support recent interpretations of demographic structure reached from studies of size frequencies in samples collected from other areas in the NE Atlantic range of these species.

The similar proximal and distal attachments to the stereom of primary spine ligament in the echinoid Eucidaris tribuloides are described, from thin sections and SEM studies on frozen and fractured spine articulations and ligaments from decalcified material. The orthogonal structure of the general stereom is modified on the attachment zones where bundles of collagen cylinders enter approximately hexagonally arranged channels. Straps of collagen extend in parallel series between adjacent bundles via regularly placed ports and collagen loops rather than non-striated 'tendons' pass over skeletal trabeculae. The regular pattern of collagen straps is most evident on the proximal and distal attachment zones. Mechanical features of the non-adhesive mode of attachment are considered, together with similarities and differences between insertion of muscle cells and mutable collagenous tissue (ligament) in echinoderms.

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.

Publisher Summary This chapter discusses the growth and survival of postsettlement sea urchins. Growth in echinoids means change in mass, diameter, and shape of the testes, which requires the expansion, calcification, and production of soft tissues. Skeletal growth is based on cellular processes that result in both shape changes during growth and rates of diameter change. Various forces that determine shape during growth have been explored in earlier studies, but shape seems best described as a response to forces generated by skeletal weight. Different skeletal elements of echinoids from a wide range of habitats are analyzed by using X-ray diffraction. Echinoid skeletons consist of ossicles that include spines, elements of Aristotle's lantern, and plates of the test that are attached to each other by small projections and collagen threads. Growth is by calcification around individual plates and the addition of new plates at the aboral ends of the ambulacral and interambulacral rows. Ossicles are calcite that contains various amounts of magnesium and are constructed as a fenestrated stereom that varies in porosity and construction. The resorption of calcite appears to be a general phenomenon in echinoids.

Stylophora are a peculiar extinct group of asymmetrical deuterostomes whose biological affinity has been fiercely debated. Disarticulated skeletal elements of a ceratocystid stylophoran recovered from the earliest Middle Cambrian of Morocco are not only the oldest stylophorans in the fossil record, but their exceptional preservation provides crucial data on the microstructure of its skeleton. Stylophoran plates are constructed of a three-dimensional mesh, termed 'stereom', identical to that of living echinoderms in which stereom microstructure provides a reliable guide to the nature of the investing soft tissues. Using modern echinoderm anatomy to interpret stereom microstructure of stylophoran elements, here we show that the large proximal lumen of their appendage was filled with muscle and that ligamentary tissues bound distal elements firmly together. We find no evidence for a mouth in the proximal lumen and no evidence that the covering plates of the appendage were articulated. Thus, although skeletal structure suggests that stylophorans are echinoderms, their appendage was not a feeding arm but a muscular locomotory organ.

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

Previous models of reversible catch in echi- noid spines, as a property of muscle or of collagen, are briefly reviewed and discussed. This brief review offers a new interpretation of catch in primary spines of Eucidaris tribuloides, viewing the collagen and small muscles of the catch ligament working together as a variable-length ten- don. In the model presented, changes in ligament length when out of catch are accommodated by sliding of dis- continuous, interdigitating and cross-link-stabilized col- umns of collagen fibrils, the muscle layer external to the ligament effecting spine movement. Catch is viewed as a consequence of contraction of small muscles inserted on the collagen columns within the ligament. Ligament shortening tightens the profuse (ca. 30,000/mm2) and highly ordered collagen insertion loops within the ster- eoms of the spine base and test, and catch results from the multiplicative effect of these friction sites in series. New data are presented on novel structural cross-links between collagen fibrils. The cross-links stabilize the liga- ment columns. The central ligament in Eucidaris plays a purely passive mechanical role in maintaining the align- ment of the spine-test articulation. It contains no muscle and neither contracts nor undergoes catch; its insertions are simple, unlike the complex stereom insertions of the main ligament.

This study tests the hypotheses that the birefringent calcite and stereom structure of the brittlestar (Ophiuroidea, Echinodermata) endoskeleton polarizes light and that certain brittlestars respond to polarized light. The first hypothesis was tested in Ophioderma brevispinum by examining ossicles from freshly killed specimens under polarized light. This analysis revealed that the lateral arm shields, oral arm shields, arm spines and aboral disk ossicles are dichroic and thus polarize light. The second hypothesis was tested in two orientation experiments under polarized light. The results from the first orientation experiment showed (1) that, under polarized light, animals oriented significantly and unimodally, (2) that, under polarized light with the e-vector perpendicular to that in 1, animals oriented significantly and unimodally to within 18° of the bearing of the animals in 1, and (3) that, under unpolarized light, animals did not orient significantly. The results from the second orientation experiment showed that, under polarized light, animals oriented significantly and unimodally to within 17° of an individual preference previously established under polarized light; but under unpolarized light, animals did not orient significantly to an individual preference established under polarized light. Thus, O. brevispinum orients under polarized, but not unpolarized, light. The unimodal orientation and lack of consistent alignment with the e-vector suggest that polarized light is not used as a directional cue but instead as a signal to sustain oriented behavior. The dichroism of the ossicles of O. brevispinum and the animal’s capacity (though eyeless) to discriminate between polarized and unpolarized light suggest that the mechanism of polarization sensitivity may rely on polarizing filters built from the animal’s skeleton.

The camarodont echinoderms have five distinct mineralized skeletal elements: the embryonic spicules and mature test; spines, lantern stereom and teeth. The embryonic spicules are transient structural elements of the larval skeleton whereas the spines and test plates are permanent structural elements. The teeth are continuously growing structures, matching wear at the incisal adoral end to the rate of new production at the aboral plumula. The mineral in all cases is a high magnesium calcite, but the magnesium content, crystal shape and growth pattern is different in each type of skeletal element. The crystal shape and organization into macro structures depends on the presence of an organic matrix which creates the spaces and controls the environments for crystal initiation and growth. The detailed mechanisms of crystal regulation are not known, but much work has been done on defining the proteins which appear to be involved. Phosphorylated matrix proteins may be of special importance. Biochemical isolation of proteins, construction and analysis of cDNA libraries, and most recently high-throughput proteomic analysis in conjunction with the sequencing of the complete genome have yielded a detailed list of protein components likely to be involved in the mineralization processes. However, the proteome-genome analyses have not yet provided insight into the mechanisms of crystallization, calcite composition, and orientation applicable to all skeletal elements. Although the embryonic pluteus and their spicules are the best studied system, it appears that spicule is not representative of the mature skeletal elements. Now armed with the compositions of most of the proteins involved, the next phase of research will have to focus on the specific localization of the proteins and individual biochemistries of each system with regard to mineral content and placement.

PALEONTOLOGY/EVOLUTION Were the flattened, asymmetric Paleozoic animals known as stylophorans in fact early chordates. This calcichordate hypothesis requires that chordates originally possessed calcite skeletons made of stereom (a microscopic mesh embedded in mesodermal tissues). But, traditionally, stereom has been seen as diagnostic of the echinoderms, and most paleontologists have interpreted stylophorans accordingly. Because they and the other homalozoans lack the fivefold symmetry so characteristic of living echinoderms (such as sea urchins), all four classes of homalozoans usually are placed together near the base of the phylum Echinodermata. Recently, researchers have combined ontogenetic, embryologic, and molecular evidence to identify body-wall homologies among extant echinoderms. David et al. have applied these findings to reinterpret homalozoan morphologies (in particular, the thecal walls, stems, arms, and brachioles). They conclude that homalozoans comprise independently evolved groups of echinoderms, whose asymmetry is derived rather than representative of the phylum's original morphology. Three classes possess brachioles and are placed within the blastozoans (an extinct Paleozoic clade). The controversial stylophorans are most closely releated to crinoids (sea lilies). This new phylogeny is more congruent with the known fossil record. — SJS Paleobiology 26 , 529 (2000).

Growth bands are evident in sections of the radial shields of Astrobrachion constrictum and on direct examination of dissected material. These bands result from the sequential addition of scales to the radial shields, a phenomenon not typical of ophiuroid radial shields. The rate of addition of these scales and the pattern of calcite deposition within them was studied using the skeletal growth marker calcein, and the validity of using the radial shield scales as age markers was examined. Individual brittle stars, injected intra-coelomically with calcein solution in the field at 6-month intervals, were collected and transverse fluorescent growth marks were found on radial shield scales. Measurement of the amount of stereom laid down between and after injections indicated the amount of growth in successive 6-month periods. Growth rate, as determined by measurement of the length of the radial shields, was faster during winter/spring, but the number of new radial shield scales increased during summer/autumn. Deposition of calcite within the scales also varied seasonally, greater amounts of large-pored stereom being deposited in winter. Growth bands were not laid down annually; as many as 11 bands appeared in a single year. Larger brittle stars did not appear to deposit new radial shield scales; 23 mm may be the maximum disc diameter for A. constrictum. Additional key words: Calcein, stereom Astrobrachion constrictum FARQUHAR 1900 is a euryalinid ophiuroid that lives in a mutualistic relationship on black coral colonies of the genus Antipathes (Grange 1991). Both the brittle star and black coral are normally found in deep water around southeast Australia and New Zealand (Baker 1980), but in New Zealand's Fiordland, a habitat with little current or wave action and low light intensity, as in far deeper waters, accommodates both organisms within SCUBA diving depths. This provides a unique opportunity to study a member of the little-known ophiuroid order Euryalida. In many ophiuroids, skeletal elements are large relative to body size and, in some species, arm and disc ossicles exhibit bands that have been interpreted as annual growth markers (Gage 1990a,b; Dahm 1993; Dobson & Stancyk 1994; Law 1995; pers. obs.). Skeletochronometry, the measurement of growth bands in skeletal elements (Stump & Lucas 1990), has become an important tool in the aging of ophiuroids. Examination of the anatomy of Astrobrachion constrictum (unpubl. obs.) revealed no apparent growth bands in arm ossicles, the stereom being, using the nomenclature of Smith (1980), of a uniform, labyrinthic nature. However, transverse bands were present in the radial shields (Stewart 1995). The drugs tetracycline hydrochloride and calcein (2,4-bis-[N,N'-di-(carbomethyl)-aminomethyl] fluorescein) have both been used successfully to leave a fluorescent mark on the calcified elements of a variety of living organisms, including fishes (Wilson et al. 1987; Tsukamoto 1988), brachiopods (Rowley & McKinnon 1995), and echinoderms (Kobayashi & Taki 1969; Taki 1972a,b; Pearse & Pearse 1975; Ebert 1984, 1988; Stump & Lucas 1990; Dobson & Stancyk 1994; Medeiros-Bergen & Ebert 1995). Tetracycline incorporated at the site of calcification fluoresces yellow/green when viewed under ultraviolet (UV) light and calcein fluoresces green under UV (Suzuki & Mathews 1966; Kobayashi & Taki 1969; Seeto 1992). Calcein is reportedly less toxic than tetracycline (Wilson et al. 1987; Rowley & McKinnon 1995) and, after trials were conducted to establish if the chemical would be incorporated in the skeletal elements of A. constrictum (Stewart 1996), it was selected as the preferred marker for the present study. The purpose of this study, the first description of skeletal growth bands in a euryalid, was to examine patterns of growth and form in the radial shields of A. constrictum, determine the mechanism of band formation, provide information on the periodicity of the This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 05:15:37 UTC All use subject to http://about.jstor.org/terms

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

Specimens of Democrinus conifer (A. H. Clark, 1909) dredged from about 280 m were fixed aboard ship for transmission electron microscopy. The most important new contributions to stalk histology are the following: an exact description of the different cell types of the stereom spaces; a demonstration of the haemal channel; description of the radial aggregations of cells in the central canal; discovery of coelomic nerves; and discovery of nerve tracts running in association with the aboral extension of the axial organ. The collagenous ligaments of the stalk are separated into three types anatomically (and possibly also functionally). The stalk contains no trace of an axial sinus derived during ontogeny from the anterior coelom (=axocoel). In the roots, the central canal contains a root nerve, but no extensions of the haemal channel or of the coelomic tubes; therefore, roots of bourgueticrinid sea lilies are not homologous to cirri of isocrinid sea lilies or feather stars. The chambered organ and axial organ of D. conifer closely resemble the same organs in feather stars that have previously been described by electron microscopy. The functional implications of our structural results are: (1) cells in the stereom space appear to be a major site of nutrient reserves, (2) the abundant cells with lipid-rich organelles could make the sea lily body more buoyant, and (3) the absence of muscles or other cells specialized for contractility indicates that the stalk of bourgueticrinid sea lilies cannot bend actively.

Tubercles and spines of 33 species of extant irregular echinoids were studied to provide data for interpreting tubercle structure and arrangement in fossil echinoids. Tubercle morphology is analysed functionally. It is possible to infer much about the posture, movement and function of the associated spine from tubercle morphology. The development and direction of areole enlargement and the structure of the platform are the most important features in this respect. Surface stereom porosity is used to differentiate tubercles and miliaries, which bear either spines or pedicellariae, from granules, which have no such appendages. Tubercles of regular echinoids are usually radially symmetrical and can be broadly separated into fixed-pivot and sliding-pivot systems. In irregular echinoids, spines are usually modified for a particular function and tubercle morphology is correspondingly varied. The power stroke of the oral spines is radial in pygasteroids and holectypoids, whereas, in clypeasteroids and cassiduloids, it is posterior. This is reflected in their different burial and locomotory behaviour. Oral spine and tubercle arrangement in Cassidulus resembles that found in spatangoids and this again is reflected in its behaviour. Spine and tubercle differentiation becomes quite pronounced in certain clypeasteroids. It is argued that lunules and notches are modifications which allow sand dollars to feed on the organic-rich surface layer of sediment while remaining infaunal. Tubercle and spine diversity is most pronounced in the spatangoids and is described in detail for Echinocardium. Each group of spines with a particular function is associated with morphologically distinct tubercles. Tufts of spines are readily recognizable from the tubercle arrangement. The considerable variation in the arrangement of spines and tubercles within the anterior ambulacrum is thought to reflect the varying emphasis placed on adoral transportation of sediment. Fasciole arrangement shows no correlation with depth of burial or grain size of the substratum. Aboral tubercle density is correlated with sediment grain size. To maintain a water-filled space between the tip of the spines and the test surface, echinoids increase spine density and develop a uniform covering of spines with either distally swollen or spatulate tips. In spatangoids, a more effective coverage is achieved by development of curved or oblique aboral spines, and the aboral mucous coat prevents fine particles from falling between spines and clogging the burrow. The structure and arrangement of tubercles can be extremely useful in palaeobiological studies.

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.

Circular marks, flush with the test or slightly depressed, exist on the test surface of various echinoid species. Fifty-six species belonging to regular and irregular echinoids were examined in order to describe the diversity and structure of these marks and to discuss their origins, with particular emphasis being put on the spatangoids Heterobrissus niasicus and Maretia planulata. Investigations combine statistical, light and electron microscopical methods. The marks correspond to surrounding ordinary tubercles in their size, their distribution on the test, their structure and their microstructure (stereom meshwork). Marks recolonized by miliaries and marks overlapping each other attest that they were made during the life of the sea urchins. This hypothesis is strengthened by comparisons between marks and artificially extracted tubercles. The microstructure of numerous marks displays original patterns with blunt broken surfaces or concentric structures suggesting that these marks result from skeletal fracture and resorption processes. From the structure and distribution of these marks it is argued that they are formed by the natural removal of tubercles. Two possible origins are retained: they are scars resulting from a traumatic extraction of spines and tubercles, or they are due to an autotomy of tubercles and associated spines, a process ontogenetically controlled and always combined with sparse or heterogeneous tuberculation. The marks resulting from these two processes are not randomly distributed either on the test or between different groups of echinoids. Their distribution and abundance are strongly involved in the ornamentation of the test.

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.

Sea lilies of the order Isocrinida (Metacrinus rotundus) were dredged from Sagami Bay, Japan, and their stalks were studied by transmission electron microscopy. New contributions to stalk histology are: an exact description of the different cell types in the stereom spaces; a demonstration of the haemal channel; and the discovery of (1) coelomic nerves, (2) collateral stalk nerves and (3) nerve tracts running with the aboral extension of the axial organ. Within the stalk, there is no structure derived from the axial sinus (=axocoel), and the widely accepted homology between the crinoid stalk and the larval asteroid stalk is thus open to serious question. The stalk has collagenous ligaments of two main types (intercolumnal ligaments and peripheral through-going ligaments); however, there are no central through-going ligaments comparable to those in stalks of larval feather stars and of bourgueticrinid sea lilies. The absence of muscles or other cells specialized for contractility indicates that the stalk of isocrinid sea lilies cannot bend actively. The cirri, which project from the stalk of M. rotundus, contain no muscle cells; however, the epithelial cells lining the oral and aboral coeloms of each cirrus contain a presumed contractile apparatus that is a bundle of 5 nm cytoplasmic filaments oriented parallel to each other and to the long axis of the cirrus.

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.

Three structures in the dermis of the dorsal arm plate (DAP) of the brittlestar, Ophiocoma wendti, appear to comprise a photoreceptor system. The upper surface of the DAP bears transparent, knob-like, microscopic structures which are expanded peripheral trabeculae (EPT) of the calcite stereom. The EPT are part of the photoreceptor system and can facilitate light transmission through the DAP by decreasing light refraction, reflection and absorption that occur at stereom/stroma interfaces. Bundles of nerve fibres located below the EPT are a second component of the system, and may function as primary photoreceptors. The intensity of light impinging on the putative sensory tissue is regulated by the diurnal activity cycle of chromatophores, the third element of the system. During the day the chromatophores cover the EPT and thereby shade the nerve fibres. At night they retract into inter-trabecular channels, uncovering the EPT and thereby exposing the nerve fibres to transmitted light. Thus, the transparent stereom may play a role in photoreception, in addition to its generally recognized skeletal function. Although ciliated cells that may be sensory are present in the epidermis of Ophiocoma wendti, they do not appear to be photoreceptors. Functional analogues of the brittlestar photoreceptor system in other echinoderms are discussed, emphasizing the relationship between photosensitivity and the transparency of the stereom in several classes of Echinodermata.

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 in the deep-sea irregular sea urchins Echinosigra phiale (family Pourtalesiidae) and Hemiaster expergitus (family Hemiasteridae) was studied from deep-sea samples taken during the years 1973 to 1985 from two stations at 2900 and 2200 m depth in the Rockall Trough (N.E. Atlantic Ocean). Growth zones, similar to those described from sea urchins in shallow water, are present as a series of wide white bands separated by narrow, dark rings in the calcite stereom of the test plates after heating to 350°C. In shallow water, such growth zones seem to result from seasonally varying growth rates. In the supposedly constant conditions in the deep sea, a seasonal growth pattern is unexpected but may occur in response to recently discovered annual pulses in downward flux of detritus from the euphotic zone, providing a seasonally varying food supply for such deposit-feeding species living in the bottom sediment. On this assumption, growth curves were fitted to counts of growth zones (as representing age in years), in the larger lateral and ventral test plates of E. phiale and H. expergitus. The opportunity was also taken to fit growth curves derived from counts of growth zones in samples of the inshore spatangoids Spatangus purpureus and Echinocardium pennatifidum. Plots of counts against test length of Echinosigra phiale and H. expergitus, although scattered and not clearly asymptotic, indicate, growth to be slower than in the two inshore spatangoids, and than in the coastal species Echinocardium cordatum, for which there are good recent growth data, available.

Adults of Comatilia iridometriformis, A. H. Clark, 1909, a deep-water feather star, retain the following attributes limited chiefly to juveniles of other species in its family, the Comasteridae: small size (maximum arm length about 30 mm), large radial ossicles, incomplete pinnulation, slender cirral ossicles and coarse stereom. This is the only known instance of paedomorphosis (retention of juvenile characters in the adult) among extant crinoids. Observations from the submersible “Alvin” in about 580 m indicate that C. iridometriformis lives associated with ahermatypic, bankforming corals (Lophelia prolifera and Enallopsammia profunda). The crinoids cling to dead coral and other solid substrates, but not to living coral. The crinoids appear to prefer slow, locally variable and multidirectional water movements. Although some other crinoids brood early developmental stages externally or within brood pouches, C. iridometriformis is the only crinoid that appears to brood giant embryos and larvae within the ovarian lumen (although poor histological fixation left some doubt about the exact location). Larvae approaching the cystidean stage were the most advanced brooded forms observed; release from the ovary probably occurs at about this stage. Stalked pentacrinoid larvae are sometimes found on the cirri of the adult crinoids. C. iridometriformis is also peculiar in being the only brooding member of the family Comasteridae and the only internally brooding feather star not found in middle or high southern latitudes. It is suggested that: paedomorphosis in this feather star results from accelerated sexual maturation (progenesis), the potentially unstable habitat may contribute to selection for rapid growth; and neither food nor space may be limiting. Moreover, brooding may be both a design constraint (due to small size) and a way to prevent loss of progeny from a discontinuous habitat where current flow tends to be unidirectional.

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.

Summary 1. Living echinoderms are characterized by an extensive water vascular system developed from the larval left hydrocoel, a complex, multi-plated endoskeleton with stereom structure, and pentamery. Fossil evidence shows that stereom evolved before pentamery, but both were acquired during the Lower Cambrian. 2. Cladistic analysis of Lower Cambrian genera reveals very few characters in common between carpoids and true echinoderms, and that the split between them was the first fundamental evolutionary dichotomy within the Dexiothetica. 3. Helicoplacoids are stem group echinoderms with spiral plating and three ambulacra arranged radially around a lateral mouth. They are the most primitive echinoderms and the first to show a radial arrangement of the water vascular and ambulacral systems. Unlike later echinoderms, their skeleton shows no dorsal/ventral (aboral/oral) differentiation. They were probably sedentary suspension feeders. 4. Camptostroma is the most primitive known pentaradiate echinoderm and, in our view, possibly a common ancestor of all living groups. It had a short conical dorsal (aboral) surface with imbricate plating, a ridged lateral wall and a slightly domed ventral (oral) surface with five curved ambulacra in a 2-1-2 arrangement inherited from the triradiate pattern of the helicoplacoids. Interambulacral areas bore epispires and the CD interambulacrum contained the anus, hydropore and/or gonopore. All parts of the theca had plates in at least two layers. 5. All other echinoderms belong to one of two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa. 6. Stromatocystites is the earliest known eleutherozoan and differs from Camptostroma in having a test with only one layer of plates and having lost the dorsal elongation. In Stromatocystites the dorsal surface is flat and the plating tesselate. Stromatocystites was an unattached, low-level suspension feeder. 7. The lepidocystoids are the earliest known pelmatozoans. They differ from Camptostroma in having an attached dorsal stalk which retained the primitive imbricate plating, and by developing erect feeding structures along the ambulacra. In Kinzercystis, the ambulacra are confined to the thecal surface and erect, biserial brachioles arise alternately on either side. Lepidocystis has a similar arrangement except that, the distal part of each ambulacrum extends beyond the edge of the theca as a free arm. 8. Pelmatozoans diverged more or less immediately into crinoids, with multiple free arms composed of uniserial plates, and cystoids sensu lato, which retained brachioles. Gogia (Lower to Middle Cambrian) is the most primitive known cystoid and differs from Kinzercystis principally in having all plating tesselate, while Echmatocrinus (Middle Cambrian) is the most primitive known crinoid and differs from Lepidocystis in lacking brachioles and in having more than five free arms with uniserial plates. 9. Post Lower Cambrian differentiation of pelmatozoan groups proceeded rapidly, exploiting the primitive suspension-feeding mode of life. Maximum morphological diversity was reached in the Ordovician, but thereafter crinoids progressively displaced cystoid groups and reached their peak diversity during the Carboniferous. The eleutherozoans were slower to diversify, but by the Arenig the earliest ‘sea-stars’ (in reality, advanced members of the eleutherozoan stem group) had reversed their living orientation and had begun to exploit a deposit-feeding mode of life. These in turn led to the ophiuroids, echinoids and holothuroids. 10. The basic echinoderm ambulacrum was already present in the helicoplacoids. It had biserial, alternate flooring plates and complexly plated sheets of cover plates on either side. The radial water vessel lay in the floor of the ambulacrum, external to the body cavity, and gave rise ventrally to short, lateral branches (fore-runners of tube feet) that were used to open the cover plate sheets, and dorsally was connected to internal compensation sacs which acted as fluid reservoirs (and were preadapted for a role in gaseous exchange). Plating on the cover plate sheets was organized and reflected the positions of the lateral branches from the radial water vessel. In Camptostroma, the cover plate sheets had biserially aligned rows of cover plates associated with the lateral branches. 11. Brachioles arose by extension of the lateral branches of the radial water vessel and associated serially aligned cover plates found in Camptostroma. They bear a single alternate series of cover plates. In Lepidocystis the ambulacra extended beyond the edge of the oral surface as true arms. Brachial plates of arms are homologues of primary ambulacral flooring plates, and arms bear multiple series of cover plates. Uniserial ambulacral plating is a derived condition and evolved independently in crinoids, paracrinoids and isorophid edrioasteroids. Pinnules in crinoids arose independently in inadunates and camerates by a progressively more unequal branching of the arms. Thus all parts of the subvective system in crinoids are internally homologous, whereas in cystoids, brachioles and arms (or ambulacra) are not homologous structures. 12. The position of the hydropore is the best reference point in orientating echinoderms. Carpenter's system of identifying ambulacra by letters, arranged clock-wise in oral view with the A ambulacrum opposite the hydropore, is consistent in all echinoderm classes. In all Lower Cambrian pentaradiate echinoderms the anus, gonopore and hydropore lie in the CD interambulacrum and this is accepted as the primitive arrangement. In helicoplacoids we tentatively suggest that the A ambulacrum spiralled down from the mouth while the two ambulacra that spiralled up represent the B + C and D + E ambulacra combined. 13. The pelmatozoan stem arose from a polyplated stalk, via a meric stem to a true column with holomeric (single piece) columnals. This happened independently in the crinoids and the cystoids. 14. Our analysis of echinoderm phylogeny leads us to recommend the following changes to the higher level classification of echinoderms: The phylum Echinodermata includes only those groups with radial symmetry superimposed upon a fundamental larval asymmetry. It has a stem group that contains the triradiate helicoplacoids and a crown group to which all other (pentaradiate) echinoderms belong. The crown group contains two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa, and the Pelmatozoa contains two superclasses, the Crinoidea which are extant and the Cystoidea, which are extinct.

Growth of the skeleton of regenerating spines of the sea urchin, Strongylocentrotus purpuratus, was studied with the light and scanning electron microscopes during the formation of a growth ring or cycle. Growth was initiated about three days after fracture and was linear between 5 and about 40 days after fracture, with a mean rate of 0.16 mm/day. There-after, a decline in growth rate was observed, being attributed to abrasion. The new skeleton first appeared as minute, conical "micro-spines"︁ on the fractured surface of the spine shaft initiating regeneration of the inner zone of meshwork. Subsequent growth of micro-spines of both the developing inner zone of meshwork, and an outer zone of radiating wedges, formed a conical fenestrated skeleton on the fractured surface of the shaft. Further deposition of micro-spines along the shaft, initially at the level of fracture, formed meshwork which gradually became solidified externally resulting in a new cycle about 60 days after fracture. In contrast, a new cycle was initiated at the milled ring in non-fractured spines during total regeneration on bare tubercles, demonstrating that growth of spines also takes place in the absence of fracture. Experiments conducted in vitro demonstrate that spine regeneration is not a polar phenomenon.

The relationship between the calcite skeleton and associated cells in the spines of Echinus esculentus is described. The cell bodies lie in channels within the skeleton and cytoplasmic processes extend from them toward the calcite. Swollen regions of the cytoplasmic processes contain organelles such as mitochondria and various vesicles. The plasma membranes of the cytoplasmic processes appear to spread over the calcite surface which is thus topologically intracellular. A considerable portion of the stereom space is extracellular and contains an extracellular fluid and collagen fibres.

In recent years, the importance of seasonal dating in archaeology has been widely acknowledged. To this end, relatively sophisticated methods of determining the season of death of cockles (Chione stutchburyi) from coastal midden sites in New Zealand, have been developed. These methods can only be applied when the midden deposits contain cockles and those middens are normally found in the vicinity of lagoons, sandy beaches, or estuaries. However, the Echinodermata are endemic to the rocky shore sub-tidal zone, and recent research in New Zealand on one species of echinoid (Evechinus chloroticus) has defined growth patterns which are seasonally dependant. This research has provided the basis for exploring a method of determining seasonal dates for midden deposits containing Echinoidea. This method, when fully developed, should prove useful for extracting seasonal data from middens which do not have any cockles, and will provide a further means of cross-checking seasonal dates. Hocken Library Anthropology Department University of Otago November, 1972 Recent work by Dix (1969, 1972) on the ecology and biology of the echinoid, Evechinus chloroticus (Val.), has shown that this species has a seasonally determined growth pattern. The possibilities of applying the results of this work to deduce the seasonality of archaeological deposits have been explored initially by Jones (1970). This line of research has been pursued further by the present authors to solve practical problems associated with the seasonal dating of coastal midden sites. A brief summary of Dix's work is given. His research involved an examination of the apical plates of Evechinus. By grinding these plates and staining them with a solution of lactophenol plus ferrous ions, he highlighted a series of bands which he classified according to color ("dark" or "light") and width ("wide," "medium," or "narrow"). Specimens collected over the span of 18 mo demonstrated that each of the 6 classes of band tended to dominate at a certain time of the year (Table 1). Our research concentrated on the interambulacral plates, since these are the parts of the Echinoid exoskeleton which are most frequently found in archaeological contexts. Studies were conducted on modern specimens of Evechinus collected in June and September (southern hemisphere, winter and spring, respectively). Sets of individual coronal plates were ground to a flat surface, thereby removing the tubercles to which the spines of the animal are attached. The staining method used by Dix was found to be difficult to implement. 98 [Vol. 39, No. 1, 1974 This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 05:02:03 UTC All use subject to http://about.jstor.org/terms Table 1. Classification of banding as proposed by Dix with the months during which these bands are most pronounced. Dix classification of banding Range of time over Optimum months brought up by staining technique which bands appear for appearance of these bands Narrow Dark End Mth 8-BegMth 4 Nov-Dec Dark Medium Dark Beg Mth 9-Beg Mth 5 Feb-March Bands Wide Dark End Mth 12-Beg Mth 7 March-April Narrow Light Beg Mth 3-Beg Mth 9 May-June Light Medium Light End Mth 4-Beg Mth 11 Aug-Sept Bands Wide Light Beg Mth 6-Beg Mth 3 Aug-Sept However, it was discovered that the growth laminae can be highlighted by using ordinary medical X-rays, the X-ray negatives being printed directly onto hard paper. The series of black and white bands thus obtained (Fig. la) can be directly correlated with the color bands described by Dix. The reasons for the differential banding obtained by the staining method are not clear, but it is obviously related to the differential absorption of the stain by different areas of the calcite stereom of the plate, and the bands may reflect underlying changes in the density of the calcite stereom. The denser parts of the stereom tend to absorb more of the stain by capillary Fig. la. Ground interambulacral plates of Evechinus. June sample. Measurements were taken along transect AA to the suture. action. Alternatively, the same effect could possibly be explained by different alignments of the crystal axis of the individual calcite crystals. In these circumstances, it is evident that a positive print of an X-ray plate reveals the same, but clearer, banding detail as does the staining method. Fig. la depicts the banding of a June sample; it is assumed that each band is associated with a single year. When the white band widths are measured (along the transect AA), it is found that the bands decrease approximately logarithmically with age. Hence, with any modern or archaeological sample, it is possible to extrapolate the full width of the last band (had

Element abundance distribution, systematic compositional variations, environmental and genetic factors, diagenesis (recrystallization to low-Mg calcite)

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.

At the end of the 19th century among others Schwendener (1874) and Haberandt (1909) described anatomy of plants as a mechanical system. Emphasis was given to “specifically mechanical” cells or “stereids” which, in their entirety, form the “stereom”, a cellular framework to which the stiffness and bearing capacity are ascribed. The contribution of turgor to rigidity of cells with thin cell walls was disregarded in these studies, as in the later work of Mer-genthaler-Paulen (1952), who calculated the moment of inner forces being half that of external forces. Since these forces are commonly supposed to be in equilibrium, they should have been equal. G. Helmcke and F. Otto initiated the present study by drawing our attention to this unsatisfactory level of knowledge.

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.

Growth dynamics of the primary spine of the cidaroid sea urchin Phyllacanthus imperialis was assessed for the first time using pulsed Mg-26-labeling and NanoSIMS isotopic imaging. The sea urchin was incubated twice (for 48 h) in artificial seawater with elevated level of Mg-26. After each labeling event, the sea urchin was returned for 72 h to seawater with natural isotopic abundance of Mg-26. NanoSIMS ion microprobe was subsequently used to visualize the labeled regions of the spine with submicrometer lateral resolution. The growth of the new skeleton was restricted to the distal-most and peripheral portions of the spine. Skeletogenesis involved mostly the deposition of continuous thickening layers and lateral growth involving bridges between previously formed trabeculae. The timescale of formation of individual thickening layers (ca. 1 mu m in width) on the stereom trabeculae was on the order of 1 day. Longitudinal growth occurred mainly at the periphery in the form of small portions of the thickening deposits or more massive microspines that appeared to branch and fuse with those above and below. These microspines were found to grow at about 10 mu m/day. These results reveal that the skeletal growth of a juvenile cidaroid spine is complex and highly heterogeneous, with different extension rates depending on the stage of the stereom development and/or direction of the growth fronts. The growth pattern observed here at the submicrometer scale provides direct evidence supporting the earlier suggestions that the lamellar structure of echinoderm stereom is formed by periodic deposition of continuous mineral layers. (C) 2014 Wiley Periodicals, Inc.