Ordovician Paracrinoids from the Baltic: Key Problems of Comparative Morphology of Pelmatozoan Echinoderms

The erect feeding appendages of paracrinoids, brachioles of typical blastozoans and arms of crinoids are morphologically similar in their terminal growth, biserial cover plates, and pinnulation. This is attributed to the inducing effect of the radial ambulacral canal on their growth mode. The uniserial brachioles of Laurentian paracrinoids are homologous to the biserial brachioles of the Baltic Achradocystites and Heckerites, and those of other blastozoans. Based on this assumption, the two Baltic genera, which have a brachiole system plesiomorphic for paracrinoids, and a similar morphology of the theca, are assigned to this class. Brachiolars in brachioles are a new development, homologous to the flooring plates of the food groove and, where present, are the continuations of these plates beyond the theca. The uniserial brachioles of Laurentian paracrinoids evolved from the biserial brachioles as a result of a gradual shift of brachiolars in the neighboring rows and their subsequent fusion in pairs. Brachials in crinoidal arms are a new development that evolved as distal serial growth of radial plates under the induced influence of the incipient radial canals emerging from the closed vestibular cavity, which was an ontogenetic innovation in crinoids. The transformation of a nonorganized small-plated theca into a large-plated, and completely or partly symmetrized theca, or vice versa is possible and results from accelerated or retarded growth of some plate generation in relation to the growth rate of the theca.     

A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa,Echinodermata) and its bearing on the evolution of early crinoids

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well‐preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of Eumorphocystis suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.     

On the sites of secondary podia formation in a juvenile echinoid: growth of the body types in echinoderms

The growth of the adult echinoderm body is addressed here in the echinoid Holopneustes purpurescens in a study of the early development of the secondary podia along the five radial canals of the adult rudiment. At a stage when the first four secondary podia have formed along each radius oral to the primary podium, two podia are on one side of the radius and two are on the other side, all at a different distance from the primary podium. The pattern of the connexions of these secondary podia to the radial canals changes in successive radii in a manner similar to Lovén’s law for skeletal plates and matches the reported sequence in the times at which the first ambulacral skeletal plates form in the adult echinoid rudiment. A similar pattern is described for the reported origins of the secondary podia in apodid holothurians. A common plan for the growth of the body types is described for echinoids, asteroids, holothurians and concentricycloids. The five metameric series of secondary podia formed in echinoderms have a coelomic developmental origin like the single metameric series of somites formed in the axial structures of chordates.     

Oral Region Homologies in Paleozoic Crinoids and Other Plesiomorphic Pentaradial Echinoderms

The phylogenetic relationships between major groups of plesiomorphic pentaradial echinoderms, the Paleozoic crinoids, blastozoans, and edrioasteroids, are poorly understood because of a lack of widely recognized homologies. Here, we present newly recognized oral region homologies, based on the Universal Elemental Homology model for skeletal plates, in a wide range of fossil taxa. The oral region of echinoderms is mainly composed of the axial, or ambulacral, skeleton, which apparently evolved more slowly than the extraxial skeleton that forms the majority of the body. Recent phylogenetic hypotheses have focused on characters of the extraxial skeleton, which may have evolved too rapidly to preserve obvious homologies across all these groups. The axial skeleton conserved homologous suites of characters shared between various edrioasteroids and specific blastozoans, and between other blastozoans and crinoids. Although individual plates can be inferred as homologous, no directly overlapping suites of characters are shared between edrioasteroids and crinoids. Six different systems of mouth (peristome) plate organization (Peristomial Border Systems) are defined. These include four different systems based on the arrangement of the interradially-positioned oral plates and their peristomial cover plates, where PBS A1 occurs only in plesiomorphic edrioasteroids, PBS A2 occurs in plesiomorphic edrioasteroids and blastozoans, and PBS A3 and PBS A4 occur in blastozoans and crinoids. The other two systems have radially-positioned uniserial oral frame plates in construction of the mouth frame. PBS B1 has both orals and uniserial oral frame plates and occurs in edrioasterid and possibly edrioblastoid edrioasteroids, whereas PBS B2 has exclusively uniserial oral frame plates and is found in isorophid edrioasteroids and imbricate and gogiid blastozoans. These different types of mouth frame construction offer potential synapomorphies to aid in parsimony-based phylogenetics for exploring branching order among stem groups on the echinoderm tree of life.     

THE EARLY RADIATION AND PHYLOGENY OF ECHINODERMS

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.     

Arrays in rays: terminal addition in echinoderms and its correlation with gene expression

The echinoderms are deuterostomes that superimpose radial symmetry upon bilateral larval morphology. Consequently, they are not the first animals that come to mind when the concepts of segmentation and terminal addition are being discussed. However, it has long been recognized that echinoderms have serial elements along their radii formed in accordance with the ocular plate rule (OPR). The OPR is a special case of terminal growth, forming elements of the ambulacra that define the rays in echinoderms. New elements are added at the terminus of the ray, which may or may not be marked by a calcified element called the terminal plate (the "ocular" of sea urchins). The OPR operates in every echinoderm, from the occasionally bizarre fossils of the Cambrian to the most familiar extant taxa. Using the OPR and other criteria of recognition, echinoderm body wall can be divided into two main regions: extraxial components are associated with the somatocoels, axial components (formed in accordance with the OPR) with the hydrocoel. We compare patterns of development in axial regions of echinoderms with those found in the anterior-posterior axes of the earliest echinoderms as well as other invertebrates. Although axial and extraxial skeletons appear to be composed of the same biomineral matrix, the genes involved in patterning these two skeletal components are likely distinct. During development of the axial skeleton, for instance, the genes engrailed and orthodenticle are expressed in spatial and temporal patterns consistent with the OPR. Other genes such as distal-less seem to demarcate early ontogenetic boundaries between the axial rudiment and the extraxial larval body. There is a complex and pervasive reorganization of gene expression domains to produce the highly divergent morphologies seen in the Echinodermata. We integrate morphological and genetic information, particularly with respect to the origins of radial symmetry in the rudiment, and the concomitant development of the rays.     

Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms

SYNOPSIS. The Extraxial/Axial Theory (EAT) of echinoderm skeletalhomologies describes two major body wall types: axial and extraxial.The latter is subdivided into perforate and imperforate regions.Each of the regions has a distinctly different source in earlylarval development. Axial skeleton originates in the rudiment,and develops in association with the pentaradially arrangedhydrocoel according to specific ontogenetic principles. Perforateand imperforate extraxial regions are associated with the leftand right somatocoels respectively, are not governed by ontogeneticprinciples of plate addition, and are products of the non-rudimentpart of the larval body. The morphology of even the most bizarreof the earliest echinoderms can be explored using the EAT. Amongthese, edrioasteroid-like taxa best fit the idea that formsexpressing archimery in the sequential arrangement of axial,perforate extraxial, and imperforate extraxial regions are thefirst echinoderms. Metamorphosis is especially marked in cladesthat have a high axial to extraxial skeleton ratio because structuresdeveloping from the non-rudiment part are suppressed in favorof the developing axial elements during this process. However,inearly echinoderms, extraxial skeleton makes up a far largerproportion of the body wall than axial, implying that metamorphosiswas not as significant a part of the developmental trajectoryas it is in more recently evolved taxa. Echinoderm radiationconsists of a succession of apomorphies that reduced the expressionof extraxial components but increased the influence of axialones, with a concomitant increase in the prominence of metamorphosis.     

What a New Model of Skeletal Homologies Tells Us About Asteroid Evolution

The Extraxial-Axial Theory (EAT) is applied to the body wallhomologies of asteroids. Attempts to characterize major platesystems of asteroids as axial or extraxial, particularly thosethat are highly organized into series, can be problematic. However,the Optical Plate Rule (OPR) is instrumental in establishingthat ambulacrals and terminals are axial. It is equally clearthat the region aboral to the marginal frame is a part of theperforate extraxial body wall (with the possible exception ofthe centrodorsal, which is likely imperforate extraxial). Previouslyestablished EAT criteria, particularly those strongly rootedin the embryologically expressed boundary between axial andextraxial body wall in larvae, suggest that marginals, and perhapsadambulacrals, are extraxial in origin. We also explore theextraxial nature and phylogenetic significance of the odontophore.Our data from both juveniles and adults show that plate andtube foot addition sequences occur according to the OPR, andshed light on poorly known homologies of the asteroid mouthframe. These data indicate that the mouth angle ossicle mustat least contain the first ambulacral, although we cannot ruleout the possibility that the first adambulacral also contributesto the construction of this ossicle. The interpretations providedby the EAT for all ossicles suggest a synapomorphy scheme forsomasteroids, ophiuroids, and asteroids.     

Morphology and biomechanical implications of isolated discocystinid plates (Edrioasteroidea, Echinodermata) from the Carboniferous of North America

Detailed examination of isolated thecal plates belonging to three discocystinid edrioasteroids, Spiraclavus nacoensis Sumrall, Hypsiclavus kinsleyi Sumrall, and Giganticlavus bennisoni Sumrall and Bowsher, reveals striking similarity in morphology among these species. Stereom observed in the ambulacral floor plates indicates that ligamentous connective tissue opened the ambulacral cover plates and muscle tissue closed them. The ambulacral floor plates are interpreted as rigid supports for the oral surface with the interambulacral areas acting as flexible integuments of plates. The aboral surface is interpreted as flexible and highly contractile. All discocystinid thecal openings are consistent in morphology with adaptations for thecal pressurization. Extension and contraction of the theca was accomplished by pumping water in and out of an inflatable sac associated with the periproctial opening. The pedunculate zone is interpreted as passively expanding and contracting by relaxing of mutable collagenous tissue and stiffening when the theca was in the desired position. All of these features illustrate that discocystinid edrioasteroids have highly–evolved morphology and function.     

Functional morphology and evolution of the cystoid Echinosphaerites

Echinosphaerites , known from the Lower and Middle Ordovician, has branched biserial brachioles. Such features are so far unknown in the Rhombifera. Echinosphaerites had a skeletal meshwork like that of other blastozoan echinoderms, with a fine outer mesh layer and an inner coarse mesh layer. During evolution the number and location of brachioles, including the pattern of brachiole branching, changed by increase in the number of brachioles and increased complexity of the branching pattern. The exothecal pore structures increased in the complexity of patterns of tangential canals. The pattern of skeletal growth in Echinosphaerites is discussed. The Echinosphaeritidae and Caryocystitidae families are closely related and show parallel development.     

Modularity and heterochronies in the evolution of metazoa: Paleontological aspect

All organisms are formed of more or less independent elements, modules. Paleontology deals with morphological modules preserved in the fossil state and allows their evolution within taxa of different levels to be reconstructed. Modularity provides organisms with the ability to evolve, since changes in one module does not influence others, nor disturb the integrity of organism. Each organism may have unique modules represented by a single copy and serial modules developing according to a certain symmetry type. Serial terminal growth is characteristic of ambulacra of echinoderms, in which it is combined with alternating appearance of structures on the right and left of the symmetry plane. The morphology of the solute Maennilia estonica, which has been investigated in detail, shows that the growth model for the brachiola is similar to the model for ambulacra of sea urchins, but without an ocular plate. Probably, the hydrocoel initially induced the appearance of a skeleton necessary for its activity and organized its development according to its own model of terminal growth. Subsequently, the axial skeleton appearing following this pattern could have organized the growth of adjacent parts of the extraxial skeleton following the same model to form a united module. The fusion of modules could have resulted from heterochronies. Extant and extinct material connected with the change in the anteroposterior axis in evolutionary and ontogenetic development of echinoderms provides a prominent example of heterochronies. Heterochronies were the mechanism connecting characters into an integrated ensemble of the body plan. Archaic diversity reflects an attempt to create a new body plan. Various manifestations of archaic diversity show that the emergence of a new higher taxon is connected with the combination of a number of characters united in an integrated complex forming the body plan which is stable from the moment of appearance due to strict recursive relationships between its modules rather than with the acquisition of an individual character, even if it is very important.     

Variabilité architecturale du test chez le genre Collyrites (Echinoidea, Disasteroidea) au Jurassique moyen

The aim of this paper is to study the evolution of the architecture of the « apical system–periproct complex » of the genus Collyrites (Echinoidea, Disasteroidea) for species from the Bathonian and Callovian stages (Middle Jurassic). The studied material comes from several localities of the Paris basin. The apical system of the genus Collyrites is subdivided in two parts: (i) an anterior part composed by four genital (1, 2, 3, 4) and three ocular (II, III, IV) plates, called trivium; (ii) a posterior part composed by two ocular plates (I, V) and one genital plate (5), without gonopor, which circle the periproct, called bivium. The genital plate 5, which may collapse in the periproct, is not always visible. The Bathonian–Callovian transition is marked by a subdivision of the bivium in two parts: the periproct breaks up from the posterior ocular plates (I, V). These morphological changes are associated with architectural modifications. The trivium stays relatively stable during the Bathonian and the Callovian, but a supplementary plate may be inserted into the trivium. The bivium shows important modifications linked to the separation of the periproct and the ocular plates (I and V). Such a separation is marked by a strong development of supplementary plates, these ones keeping in connection with the periproct and the ocular plates I and V. The supplementary plates are more and more developed whereas the distance between the periproct and ocular plates increases. The connection between the trivium and the bivium is similarly provided by supplementary plates. The size of these plates seems to significantly increase between the Bathonian and the Callovian. Moreover, some specimens from the Bathonian and the Callovian may have an atypical architecture with a supplementary genital plate or a genital plate with two pores. The “extraxial axial theory” allows to recognize two types of skeleton: (i) an “axial skeleton” corresponding to ocular plates and plates of the ambulacra and interambulacra; (ii) an “extraxial skeleton” corresponding to the genital and supplementary plates, and the periproct. Architectural modifications between the Bathonian and the Callovian is a result of a more important development of the “extraxial skeleton” while the “axial skeleton” shows few modifications during this time interval.     

Early stalked stages in ontogeny of the living isocrinid sea lily Metacrinus rotundus

The early stalked stages of an isocrinid sea lily, Metacrinus rotundus, were examined up to the early pentacrinoid stage. Larvae induced to settle on bivalve shells and cultured in the laboratory developed into late cystideans. Three‐dimensional (3D) images reconstructed from very early to middle cystideans indicated that 15 radial podia composed of five triplets form synchronously from the crescent‐shaped hydrocoel. The orientation of the hydrocoel indicated that the settled postlarvae lean posteriorly. In very early cystideans, the orals, radials, basals and infrabasals, with five plates each in the crown, about five columnals in the stalk, and five terminal stem plates in the attachment disc, had already formed. In mid‐cystideans, an anal plate appeared in the crown. Late cystideans cultured in the field developed into pentacrinoids about 5 months after settlement. These pentacrinoids shared many crown structures with adult sea lilies. On the other hand, many features of the stalk differed from those in adult isocrinids, while sharing many characteristics with the stalk of feather star pentacrinoids, including disc‐like proximal columnals, high and slender median columnals, synarthrial articulations developmentally derived from the symplexial articulations, limited formation of cirri only in the proximal columnal(s), and an attachment disc. On the basis of these findings, phylogenetic relationships among extant crinoid orders are discussed.     

Skeletal development in <Emphasis Type="Italic">Acropora palmata</Emphasis> (Lamarck 1816): a scanning electron microscope (SEM) comparison demonstrating similar mechanisms of skeletal extension in axial versus encrusting growth

Many Acropora palmata colonies consist of an encrusting basal portion and erect branches. Linear growth of the skeleton results in extension along the substrate (encrusting growth), lengthening of branches (axial growth) and thickening of branches and crust (radial growth). Scanning Electron Microscopy is used to compare the mechanisms of skeletal extension between encrusting growth and axial growth. In encrusting growth, the distal margin of the skeleton lacks corallites (which develop about 1 mm from the edge); in contrast, in axial growth, axial corallites along the branch tip form the distal portion of the skeleton. In both locations, the distal margin of the skeleton consists of a lattice-like structure composed of rods that extend from the body of the skeleton and bars that connect these rods. An actively extending skeleton is characterized by sharply pointed rods and partially developed bars. Distal growth of rods (and formation of bars) is effected by the formation of new sclerodermites. Each sclerodermite begins with the deposition of fusiform crystals (that range in length from 1 to 5 μm). These provide a surface for nucleation and growth of spherulitic tufts, clusters of short (<1 μm long) aragonite needles. The needles that are oriented perpendicular to the axis of the skeletal element (rod or bar), and perpendicular to the overlying calicoblastic epithelium, continue extension to appear on the surface of the skeleton as 10–15 μm wide bundles (of needle tips) called fasciculi. However, some crusts that abut competitors for space have a different morphology of skeletal elements (rods and bars). The distal edge of these crusts terminates in blunt coalescing rods, and bars that are fully formed. Absence of fusiform crystals, lack of sharply pointed rods and bars, and full development of sclerodermites characterize a skeletal region that has ceased, perhaps only temporarily, skeletal extension.     

Flume studies using 1:1 scale models of Series 2 and basal Series 3 Cambrian gogiid eocrinoids from Guizhou Province,China to determine feeding posture and mode of attachment

In Series 2 and 3 Cambrian of Guizhou Province, China, most echinoderms inhabited deeper/quieter water and were attached directly to siliciclastic substrate or biodetritus by biogluing (extrusion of extensible collagen). Feeding postures of abundant long stalked gogiids (e.g., Sinoeocrinus) from these beds were interpreted to have heeled over in the current from the thin flexible distal end of the stalk, with the brachioles streaming in a loose bundle, down current from the theca. To test these and other feeding posture assumptions, 1:1 scale models (holdfast, stalk, and theca) of three genera were carved from soft rubber and brachioles were modeled from braided fishing line. By varying current velocities long stalked flume models did not significantly heel over. Brachioles, both straight and spiraled, extended vertically from the theca in an (elliptical) cone and distally curved downstream. Disrupted flow around straight brachioles (Sinoeocrinus) kept them somewhat evenly spaced. Spiraled brachioles (Guizhoueocrinus, Globoeocrinus) are initially straight and angle outwards so that each proximal end defines a sector over the theca; this spacing keeps the brachioles free from tangling distally. Biogluing the animal to the bottom or to biodetritus seems to be correctly interpreted from the morphological evidence. Superglue was used as the proxy gluing agent for the models, success was limited. The dewatered, siliciclastic, non-bioturbated, seafloor could be only partly reconstructed and the somewhat viscous glue did not deeply penetrate the illite substrate. It is probable that bioglue had low viscosity, penetrated the sediment easily, and was able to agglutinate a large three dimensional anchoring body of sediment without (as is commonly observed) disrupting bedding.     

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

Living vertebrates are divided into those that possess a fully formed and fully mineralised skeleton (gnathostomes) versus those that possess only unmineralised cartilaginous rudiments (cyclostomes). As such, extinct phylogenetic intermediates of these living lineages afford unique insights into the evolutionary assembly of the vertebrate mineralised skeleton and its canonical tissue types. Extinct jawless and jawed fishes assigned to the gnathostome stem evidence the piecemeal assembly of skeletal systems, revealing that the dermal skeleton is the earliest manifestation of a homologous mineralised skeleton. Yet the nature of the primitive dermal skeleton, itself, is poorly understood. This is principally because previous histological studies of early vertebrates lacked a phylogenetic framework required to derive evolutionary hypotheses. Nowhere is this more apparent than within Heterostraci, a diverse clade of primitive jawless vertebrates. To this end, we surveyed the dermal skeletal histology of heterostracans, inferred the plesiomorphic heterostracan skeleton and, through histological comparison to other skeletonising vertebrate clades, deduced the ancestral nature of the vertebrate dermal skeleton. Heterostracans primitively possess a four‐layered skeleton, comprising a superficial layer of odontodes composed of dentine and enameloid; a compact layer of acellular parallel‐fibred bone containing a network of vascular canals that supply the pulp canals (L1); a trabecular layer consisting of intersecting radial walls composed of acellular parallel‐fibred bone, showing osteon‐like development (L2); and a basal layer of isopedin (L3). A three layered skeleton, equivalent to the superficial layer L2 and L3 and composed of enameloid, dentine and acellular bone, is possessed by the ancestor of heterostracans + jawed vertebrates. We conclude that an osteogenic component is plesiomorphic with respect to the vertebrate dermal skeleton. Consequently, we interpret the dermal skeleton of denticles in chondrichthyans and jawless thelodonts as independently and secondarily simplified. J. Morphol. 276:657–680, 2015. © 2015 The Authors Journal of Morphology Published by Wiley Periodicals, Inc.     

Types of locomotion in ophiurans

Motion pictures were taken of the locomotion of two species of ophiurans living in the Sea of Japan:Ophiura sarsi vadicoa Djakonov andAmphipholis kochii Lutken.Ophiura sarsi was found to move with the aid of paddling movements of two pairs of arms: The fifth arm (passive) pointed backward. Ophiurans of this species do not use their ambulacral feet. Three main types of locomotor movements were distinguished inAmphipholis kochii, First, locomotion by the "breast stroke" method, in which one arm (the leading) points forward, two point to the side, and two backward. The two side arms are periodically carried forward, after which the disk and the remaining arms are moved with their aid. In this method waves of flexion and extension of the segments spread along the side arms. Second, displacement by pulling with the leading arm pushing with the hind arms, and third, movement by stepping movements of the ambulacral feet. These three methods of locomotion can be used either independently or in various combinations with each other. The ambulacral feet also provide the link between the active arms and the supporting surface by means of which the ophiurans can move forward.Institute of Oceanology, Academy of Sciencs of the USSR, Moscow. Institute of Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 521–528, September–October, 1976.     

A new cornute (Homalozoa: Echinodermata) from the uppermost Middle Cambrian (stage 3, Furongian) from Northern Iran: Its systematics and functional morphology

A new rare nearly bilaterally symmetrical cornute from the Mila Formation, upper Middle–early Late Cambrian (Series 3, Furongian) from northern Iran is described, Persiacarpos jefferiesi gen. and sp. nov. It resembles Phyllocystis blayaci, but differs by the presence of the distal protrusion of M5 on the posterior end of the theca and by various sizes and patterns of the slightly bulging centralia. The animal appears to be mobile and probably moved in a tadpole-like wriggling manner. It may have fed with ambulacral cover plates open, using cover plates and/or podia for food capture, while the animal was in motion.     

Functional anatomy of the valves in the ambulacral system of sea urchins (Echinodermata,Echinoida)

Summary The water vascular system of sea urchins is examined with special reference to the valves positioned between the radial vessel and the ampullae of the tube feet. The lips of the valve protrude into the ampulla. Thus the valve functions mainly like a check valve that allows the unidirectional flow of fluid towards the ampulla. Each ampulla-tube foot compartment acts as a semi-autonomous hydraulic system. The lumina of the ampulla and the tube foot are lined with myoepithelia except for the interconnecting channels that pierce the ambulacral plate. The contraction of the ampulla results in an increasing hydraulic pressure that protrudes the tube foot, provided that the valve is closed. The retraction of the tube foot results in a backflow of fluid independent of the condition of the valve. The lips of the valve are folds of the hydrocoel epithelium. The pore slit lies in the midline. The perradial faces of the lips are covered with the squamous epithelium of the lateral water vessel. The ampullar faces are specialized parts of the ampulla myoepithelium. Turgescent cells which form incompressible cushions take the place of the support cells. The valve myocytes run parallel to the pore slit and form processes that run along the base of the ampulla and the perradial channel up to the podial retractor muscle. The findings lead to the hypothesis of multiple control of the ampulla-tube foot system: (1) The mutual activity of the ampulla and the tube foot is indirectly controlled by the lateral and podial nerves which release transmitter substances that diffuse through the connective tissue up to the muscle layers. (2) A muscle-to-muscle conduction causes the simultaneous contraction of the ampulla or the podial retractor muscles. (3) The valve muscles are directly controlled by the processes of the valve myocytes which make contact with the podial retractor. In extreme conditions a backflow of hydrocoel fluid towards the radial water vessel occurs.