An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils

Abstract. A tiny neomenioid postlarva (Neomeniomorpha, or Solenogastres) collected from the water column 3 to 6 m above the east Pacific seamount Fieberling Guyot has 6 iterated, transverse groups of spicules and 7 regions devoid of spicules between the transverse groups and the anterior-and posteriormost spicules. Three pairs of ventral, longitudinal zones with columns of single spicules, each pair with its own distinctive spicule morphology, lack transverse iteration. The 7 regions bare of spicules are compared to shell fields in developing polyplacophorans, and spicule arrangement is compared to sclerite arrangement on the Cambrian fossils Wiwaxia corrugata and Halkieria evangelista and to the spines and shell plates of the Silurian Acaenoplax hayae. The term iteration is used to denote processes that result in both metameric segments and repeated ectodermal skeletal structures. Iterative morphogenesis was probably present in bilateral animals before the Cambrian. Comparisons of iterated ectodermal skeletal structures among fossil and extant forms are suggested to indicate evolutionary relationship.     

SPICULES IN SILURIAN TABULATE CORALS FROM CANADA, AND IMPLICATIONS FOR THEIR AFFINITIES

Abstract:  Specimens of Favosites from upper Llandovery strata of Anticosti Island show three types of calcite structures, herein interpreted as spicules, preserved within their calices and on top of the last tabula. This is stratigraphically younger material, some 50 m higher than fossils described two decades earlier, in which calcified polyps, each with 12 retracted tentacles, were noted. These more recently found structures show striking similarities in form and position to point, collaret and capstan spicules found in the soft tissues of modern pipe corals, i.e. the Octocorallia (Alcyonacea). Where preserved in a distinct pattern on top of the calcite tabulae, the spicular sclerites in Favosites occur in a particular sequence. Twelve individual, or sometimes six pairs of, triradiate point spicules have shrunk to a circlet near the middle of the calice (resting on the last, outermost, tabula). Surrounding the point spicules are 3–6 circlets of curved, usually perforated, lenticular collaret spicules; and surrounding these are scattered, much smaller, capstan spicules. The spicules display variability, probably ontogenetic, in their form and relative sizes; and they are more similar in form to calcareous spicules of alcyonacean corals than to those known from calcareous sponges. Structures with 12-fold radial symmetry in Heliolites, originally described by one of us as 'septal florets', consist of elements that are considered comparable with the point spicules found in Favosites . They have been recognized in ten species of Heliolites from Silurian (Wenlock–Ludlow) strata in the Canadian Arctic islands.     

Skeleton construction upon local regression of the sponge body

We previously revealed that the mechanism of demosponge skeleton construction is self-organization by multiple rounds of sequential mechanical reactions of player cells. In these reactions, “transport cells” dynamically carry fine skeletal elements (spicules) on epithelia surrounding the inner body space of sponges (basal epithelium (basopinacoderm) and the endodermal epithelium (ENCM)). Once spicules pierce ENCM and apical pinacoderm, subsequently they are cemented to the substratum under the sponge body, or connected to other skeleton-constructing spicules. Thus, the “pierce” step is the key to holding up spicules in the temporary periphery of growing sponges’ bodies. Since sponges can regress as well as grow, here we asked how skeleton construction occurs during local regression of the body. We found that prior to local basopinacoderm retraction (and thus body regression), the body became thinner. Some spicules that were originally carried outward stagnated for a while, and were then carried inwards either on ENCM or basopinacoderm. Spicules that were carried inwards on ENCM pierced epithelia after a short transport, and thus became held up at relatively inward positions compared to spicules carried on outwardly extending basopinacoderm. The switch of epithelia on which transport cells migrate efficiently occurred in thinner body spaces where basopinacoderm and ENCM became close to each other. Thus, the mechanisms underlying this phenomenon are rather mechanical: the combination of sequential reactions of skeleton construction and the narrowed body space upon local retraction of basopinacoderm cause spicules to be held up at more-inward positions, which might strengthen the basopinacoderm's attachment to substratum.     

Spicule structure and affinities of the Late Ordovician hexactinellid‐like sponge Cyathophycus loydelli from the Llanfawr Mudstones Lagerstätte,Wales

Exceptionally well‐preserved specimens of the reticulosan sponge Cyathophycus loydelli from the Sandbian (Late Ordovician) Llanfawr Mudstones Formation of Llandrindod, Waes, UK, have been examined using scanning electron microscopy (SEM). The specimens include exquisitely detailed pyritized spicules, and granular pyritization of surrounding soft tissues. Spicules frequently show axial canals of diameter similar to those of modern siliceous sponges, with hexagonal symmetry typical of modern demosponges rather than hexactinellids. In one case, the axial filament is also preserved. The largest spicules (ray diameter >20 μm) show a complex structure, with a laminar external region similar to that of the extant hexactinellid Monorhaphis. Some spicules preserve sub‐micron detail of the spicule surface, resembling the reticulate collagenous sheath of Monorhaphis. The hexagonal symmetry of the canal confirms that at least some Reticulosa are not crown‐group hexactinellids, but stem‐group Hexactinellida or Demospongea, or stem‐group Silicea. This suggests that a square canal is a sufficient diagnostic feature of total‐group Hexactinellida, but that hexagonal canals were more widely distributed among Early Silicea and were probably not restricted to demosponges. Alternatively, comparison with the structure of modern verongiid fibres suggests that these may be homologous with the outer layers of Cyathophycus spicules, and Cyathophycus may instead be a stem‐group demosponge. The preserved detail of the surface layer shows that pyritization can preserve certain material with extraordinarily fine resolution.     

A reconsideration of the relationship between polyaxonid and monaxonid spicules in Demospongiae: new data from the genera Crambe and Discorhabdella (Porifera)

The relationships between different spicule lineages of Demospongiae are revised through the ontogenetic study of the main spicules of the genera Crambe and Discorhabdella. The presence of terminal orifices in the basal spines of the asterose acanthostyles of Discorhabdella, and the actines of the desmas of Crambe have been shown by examining young spicules under high magnification. Thus, the polyaxonid origin of both spicule types is hereby supported by the ontogenetic information, and their homology is also supported by their equivalent arrangement in the skeleton. The current differences in shape between both spicule types are considered the result of a divergent morphological evolution from an ancestral polyactinal corpuscle, by the atrophy/hypertrophy of a different number of actines. Arguments are also presented to support the homology of these two spicule types with the sphaeroclons of Vetulina, and other fossil genera. Moreover, the presence of axial canals inside the tubercles of the tuberose tylostyles of Discorhabdella and Crambe tuberosa indicates that the tubercles are actually atrophied actines as in the case of the hadromerid genus Terpios. According to the ontogeny, the tuberose morphology of these spicules may correspond to the retention of an ancestral characteristic in the Poecilosclerida and Hadromerida; in this case, a monophyletic origin, is suggested between both taxa. From the overall results here presented, the tetraxonid spicule, presently considered by most authors as the primitive morphotype, as well as some monaxons, could be considered as evolving from a polyaxial form.     

皖南早寒武世荷塘组海绵骨针化石

本文报道皖南休宁县早寒武世荷塘组黑色页岩中产出的海绵骨针化石组合,这些海绵骨针化石具有较高的丰度和分异度,它们以二轴四射针、T型针、三轴六射针和三轴五射针为主。骨针形态完整,并保存了内部轴丝、轴管以及同心圈层等微细构造。黄铁矿化在化石的保存中起了重要的作用,化石产出的时代可能为梅树村阶至筇竹寺阶（Tommotian-Atdabanian),这个化石组合证实了海绵动物在早寒武世已开始迅速分异。     

Can the morphology of the integumentary spicules be used to distinguish genera and species of phyllidiid nudibranchs (Porostomata: Phyllidiidae)?

Thirty-eight specimens belonging to four genera and 15 species of the nudibranch family Phyllidiidae were examined to investigate whether the morphology of their integumentary calcareous spicules and/or the occurrence of the spicules within the regions of the body could be used to distinguish genera and species. The spicules were studied separately from five regions of the body of each specimen—the foot, gills, mantle, dorsal pustules (or ridges in Reticulidia) and rhinophores. The mantle itself plus its pustules were found to possess the full complement of spicules in every individual. Four types of spicules were recorded overall—smooth diactines, centro-polytylote diactines, triactines and tetractines. Different regions of the body were found to possess different spicule types: (a) only smooth diactines in the gills, (b) both smooth diactines and triactines in the foot and (c) all of smooth diactines, centro-polytylote diactines and triactines in the mantle, dorsal pustules and the rhinophores. Among the genera, three types of spicules (smooth diactine, triactine, and tetractine) are present in Phyllidia, Phyllidiopsis and Reticulidia, but the form of the spicules is not diagnostic between these genera or between the constituent species. The fourth type of spicule (centro-polytylote diactine) is present exclusively in Phyllidiella, and is diagnostic for that genus. However, we failed to find any difference in spicule form, or composition, or location in the body between the three (closely related) species of Phyllidiella we investigated. Therefore, our key conclusion is that spicule morphology is an extremely important character to tell the genus Phyllidiella apart from all the other genera of the family, but it is not taxonomically informative at the level of species.     

Matrix and Mineral in the Sea Urchin Larval Skeleton

The endoskeletal spicules of sea urchin larvae are composed of calcite, a surrounding extracellular matrix, and small amounts of occluded matrix proteins. The spicules are formed by primary mesenchyme cells (PMCs) in the blastocoel of the embryo, where they adopt stereotypical locations, thereby specifying where spicules will form. PMCs also fuse to form cytoplasmic cords connecting the cell bodies, and it is within the cords that spicules arise. The mineral phase contains 5% Mg as well as Ca, and about 0.1% of the mass is protein. The matrix and mineral form concentric plies, and the composite has different physical properties than those of pure calcite. The calcite diffracts as a single crystal and is composed of well-ordered, but not perfectly ordered, microdomains. There is evidence for adsorption of matrix proteins to specific crystal faces at domain boundaries, which may help regulate crystal growth and texture. Immature spicules contain considerable precipitated amorphous CaCO3, and PMCs also have vesicles that contain amorphous CaCO3. This suggests the hypothesis that the cellular precursor to the spicules is actually amorphous CaCO3 stabilized in the cell by protein. The spicule s enveloped by the PMC cord, but is topologically exterior to the cell. The PMC plasmalemma is tightly applied to the developing spicules, except perhaps at the elongating tip. The characteristics, localization, and possible function of the four identified matrix proteins are discussed. SM50, SM37, and PM27 all primarily enclose the mineral, though small amounts are occluded. SM30 is found in cellular vesicles and is probably the principal occluded protein of the spicule.