Spherulitic-prismatic Layer Research Articles

Biomineralization in Polychaete Annelids: A Review

Polychaete annelids are a very important group of calcifiers in the modern oceans. They can produce calcite, aragonite, and amorphous phosphates. Serpulids possess very diverse tube ultra-structures, several unique to them. Serpulid tubes are composed of aragonite or calcite or a mixture of both polymorphs. The serpulid tubes with complex oriented microstructures, such as lamello fibrillar, are exclusively calcitic, whereas tubes with prismatic structures can be composed either of calcite or aragonite. In serpulids, the calcareous opercula also have complex microstructures. Evolutionarily, calcitic serpulid taxa belong to one clade and the aragonitic taxa belong to another clade. Modern ocean acidification affects serpulid biomineralization. Serpulids are capable of biomineralization in extreme environments, such as the deepest part (hadal zone) of the ocean. The tubes of calcareous sabellids are aragonitic and have two layers, the inner irregular spherulitic prismatic layer and the outer spherulitic layer. The tube wall of cirratulids is composed of aragonitic lamellae with a spherulitic prismatic structure. In some other polychaetes, biominerals are formed in different parts of the animal body, such as chaetae or body shields, or occur within the body as granule-shaped or rod-shaped inclusions.

Siphuncular Structure in the Extant Spirula and in Other Coleoids (Cephalopoda)

The shell wall in Spirula is composed of prismatic layers, whereas the septa consist of lamello-fibrillar nacre. The septal neck is holochoanitic and consists of two calcareous layers: the outer lamello-fibrillar nacreous layer that continues from the septum, and the inner pillar layer that covers the inner surface of the septal neck. The pillar layer probably is a structurally modified simple prisma layer that covers the inner surface of the septal neck in Nautilus. The pillars have a complicated crystalline structure and contain high amount of chitinous substance. The interspaces between the pillars probably are traversed horizontally by numerous chitinous membranes like in the cuttlebone chambers in Sepia. The connecting ring is composed of similar two layers as that in the extant Nautilus: the outer spherulitic–prismatic layer and the inner chitinous layer. The spherulitic–prismatic layer takes its origin on the outer surface of the septal neck, whereas the inner chitinous layer is the non-calcified continuation of the lamello-fibrillar nacreous layer of the septal neck. The siphuncular structure in Spirula is compared with that in the extant Nautilus, fossil nautilosiphonate nautiloids, and five taxa of coleoids.

Siphuncular structures in Calciosiphonate nautiloid orders Actinocerida, Orthocerida and Barrandeocerida (Cephalopoda)

The calciosiphonate type of the connecting ring was probably present in the oldest known nautiloid cephalopods, plectronocerids, from the Late Cambrian. This type also occurs in the nautiloid orders Actinocerida, Orthocerida and Barrandeocerida that appeared in the Ordovician. The calciosiphonate connecting ring is composed of two calcareous layers: the outer, spherulitic–prismatic layer, and the inner, calcified-perforate layer. The calcified-perforate layer contains cavities that are separated by calcified walls. The cavities open into the shell chamber by pore canals. In Adamsoceras holmi (actinocerid) and Orthoceras scabridum (orthocerid), the cavities have large diameters and thin calcareous walls, whereas in Archigeisonoceras folkeslundense (orthocerid), and probably also in Danococeras gotlandense (orthocerid) and Boionautilus sp. (barrandeocerid), the cavities have small diameters and thick calcareous walls. The structure of the connecting rings is discussed and reconstructed.

Characterization of two new superorders Nautilosiphonata and Calciosiphonata and a new order Cyrtocerinida of the subclass Nautiloidea; siphuncular structure in the Ordovician nautiloid Bathmoceras (Cephalopoda)

Based on differences in the siphuncular structures, the subclass Nautiloidea is divided into two new superorders: Nautilosiphonata and Calciosiphonata. The first superorder is characterized by the nautilus-type of connecting rings, and the second superorder by calcified-perforate type of the connecting rings. A new order Cyrtocerinida is erected for the families Bathmoceratidae, Cyrtocerinidae and Eothinoceratidae, previously included in the order Ellesmeroceratida. The siphuncular structure in the Ordovician nautiloid Bathmocerasholmi n. sp. is described. It is characterized by (1) connecting rings that are composed of an outer, calcareous, spherulitic–prismatic layer and an inner, fibrous, chitinous layer, and (2) prominent siphuncular ridges that originate from the inner surfaces of the connecting rings. The structure of the siphuncular ridges in Bathmoceras is compared with that of the actinosiphonate lamellae in the Silurian oncocerid nautiloid Octamerella.

Characterization of nautiloid orders Ellesmerocerida, Oncocerida, Tarphycerida, Discosorida and Ascocerida: new superorder Multiceratoidea

Multiple scars of shell muscles in the body chamber are characterized in five Palaeozoic nautiloid orders: Ellesmerocerida, Oncocerida, Tarphycerida, Discosorida and Ascocerida. The number of the scars is highly variable, from about 5 pairs in tarphycerids to about 60 pairs in ascocerids. The shell muscles that were attached to the scars in ellesmerocerids, oncocerids, dicosorids and ascocerids are here interpreted as the longitudinal mantle muscles and not as the head/funnel muscles. The interpretation of the multiple scars in tarphycerids is still unclear. In all five orders, the connecting ring structure is of the Nautilus type. The outer spherulitic-prismatic layer of the connecting ring is much thicker than that of the layer in Nautilus. It is highly porous in ellesmerocerids, oncocerids and tarphycerids and similar to that in Nautilus, but less porous and more dense in discosorids and ascocerids. The inner fibrous-organic layer of the connecting ring in all five orders is not preserved, but it has probably been considerably thinner than that in Nautilus. In some oncocerid taxa, the siphuncle contains actinosiphonate deposits of complicated structure. The ontogenetic growth of these deposits is reconstructed. On the basis of the presence of multiple muscle scars and the Nautilus type of connecting ring, the ellesmerocerids, oncocerids, discosorids and ascocerids are included in the new superorder Multiceratoidea. It is still unclear whether tarphycerids also belong to this superorder.

Siphuncular structure in Silurian discosorid and ascocerid nautiloids (Cephalopoda) from Gotland, Sweden: implications for interpretation of mode of life and phylogeny

The connecting ring structure is studied in the discosorids Phragmoceras and Trimeroceras, as well as in the ascocerid Choanoceras. These taxa have the Nautilus type of connecting rings, composed of an outer spherulitic–prismatic layer and an inner organic fibrous layer, the latter destroyed by diagenesis. As indicated by its dense granular structure and high resistance against diagenesis, the spherulitic–prismatic layer was probably less porous and had higher mechanical strength than the corresponding layer in Nautilus. Discosorids and ascocerids were closely related. In both Phragmoceras and Choanoceras, the spherulitic–prismatic layer of the connecting ring is thickened on the inner surface of the septal neck forming a bullette, and multiple muscle scars surround the base of the body chamber. Mature shells of discosorids, ascocerids and oncocerids occur in large numbers in shallow-water carbonate facies at a few localities on Gotland. It is probable that the corresponding palaeoenvironments have been the spawning grounds for these nautiloids. Modified body chambers may have been used for the internal brooding of eggs.

Silurian oncocerid Octamerella (Cephalopoda) from Gotland, Sweden

An actinosiphonate deposit with radial lamellae occurs in the siphuncle of several genera classified within the Oncocerida. Unique structural preservation of the connecting ring and the actinosiphonate deposit is here described in the oncocerid Octamerella sp. from the Silurian of Gotland, Sweden. The connecting ring of Octamerella is of the Nautilus type. It consists of an outer porous spherulitic–prismatic layer and an inner thin fibrous organic layer, the latter destroyed by diagenesis. The actinosiphonate deposit consists of two, structurally different, spherulitic–prismatic sublayers. The thin outer sublayer has a fragile porous structure and low mechanical strength, similar to the outer spherulitic–prismatic layer of the connecting ring. The inner spherulitic–prismatic sublayer that forms the actinosiphonate lamellae has a more rigid, less porous structure and higher mechanical strength than the outer sublayer. Each lamella is longitudinally traversed by a calcareous central plate that has a solid structure and consists of thin acicular crystallites that are orientated parallel to the long axis of the plate. The secretion of the actinosiphonate deposit probably began immediately after a new siphuncular segment was completed. The connecting ring together with the actinosiphonate deposit functioned as a reservoir and transport route for the cameral liquid improving the buoyancy regulation.

Siphuncular structure in the orders Tarphycerida and Barrandeocerida (Cephalopoda: Nautiloidea)

Abstract: The siphuncular structure is described in two Silurian taxa, Boionautilus tyrannus and Cumingsoceras complanatus, currently placed in the Tarpycerida. Tarphycerids have the Nautilus type of connecting ring that is composed of an outer, thick, spherulitic-prismatic layer and an inner glycoprotein layer, the latter was destroyed by diagenesis. However, both Silurian specimens have the connecting ring of the calcified-perforate type, previously known to occur in orthocerids, actinocerids, plectronocerids and now also in barrandeocerids. In this type, the inner layer of the connecting ring is calcified and perforated by pore canals. Boionautilus and Cumingsoceras are therefore classified with barrandeocerids and not with tarphycerids.

Siphuncular structure in the Recent Nautilus, compared with that in Mesozoic nautilids and ammonoids from Madagascar

The connecting ring in the Recent Nautilus is composed of two layers: a fibrous, organic inner layer, and a porous, spherulitic-prismatic outer layer. The organic layer is an uncalcified continuation from the nacreous layer of the septal neck. In the Mesozoic nautilids Cymatoceras and Paracenoceras from Madagascar, that have the Nautilus type of siphuncles, the inner fibrous-organic layer of the connecting ring has been destroyed by diagenesis. The outer spherulitic-prismatic layer is twice as thick as in Nautilus, which indicates that, in addition to jet-powered swimming, they also used short-term buoyancy changes for vertical migrations, and that the mode of life of these nautilids was somewhat different from that of the Recent Nautilus. Moreover, the Madagascar nautilids lack the auxiliary ridge by which the connecting ring in Nautilus is firmly attached to the inner surface of the preceding septal neck. The siphuncle in Nautilus can therefore withstand a much higher hydrostatic pressure than that in the fossil nautilids that lacked this deposit. In ammonoids the fibrous-organic layer of the connecting ring has a solid, semi elastic structure. In all 11 ammonoid taxa from Madagascar here studied, this layer is preserved whereas the nautilids from the same stratigraphic horizons have lost this layer. This clearly indicates that the fibrous-organic layer of the connecting ring in nautilids is chemically different from that in ammonoids.

Crystalline structure, orientation and nucleation of the nacreous tablets in the cephalopod Nautilus

The nacreous tablets in the Nautilus shell have similar crystalline structure as the tablets in the gastropod Gibbula shell. Etching with Mutvei’s solution reveals that each tablet is composed of vertical crystalline columns that are structurally similar to the acicular crystallites in the outer spherulitic-prismatic layer of the shell wall. The columns are attached to each other to form numerous vertical crystalline lamellae, oriented parallel to the longitudinal axis of the tablet. It is still unknown whether or not the orientation of the vertical lamellae corresponds to that of the crystallographic a- or b-axis. The orientation of the crystalline lamellae in the adjacent tablets is parallel in some nacreous laminae, but random in other laminae. Similar large variation was found in the nacreous tablets of the gastropod and bivalve shells. The nucleation sites of the nacreous tablets are predominantly situated on the peripheral portion of the upper surface of the preceding tablet, both in the shell wall and septa. The “aragonite-nucleating proteins” in the central portion of the crystal imprints of the organic interlamellar sheets, described by several writers, have therefore a negative correlation with the nucleation sites of the nacreous tablets.

Characterization of actinoceratoid cephalopods by their siphuncular structure

Partially phosphatized connecting rings show well-preserved structural details in the actinoceratoid Adamsoceras holmi (Ormoceratidae) from the Lower Ordovician (Kundan) of Estonia. Each connecting ring is composed of an outer, thin, spherulitic-prismatic layer, and an inner, thick, calcareous, lamellar layer, the latter being traversed by numerous large pores. The calcareous lamellar inner layer occurs in the connecting rings of three other actinoceratoids: Eushan-tungoceras pseudoimbricatum, Rayonnoceras solidiforme, and Huroniella sp. In Huroniella sp. and R. solidiforme, this layer is traversed by similar large pores as in Adamsoceras holmi, but in E. pseudoimbricatum the pores were probably too narrow to be recognized. The described structural type of the connecting rings, together with well-developed annular endosiphuncular deposits, are here considered as characteristic for actinoceratoids. The inflated connecting ring in actinoceratoids had a great surface area, which increased the number of pores across it. The permeability and emptying rates of the cameral liquid through the connecting ring could therefore have been as high or higher than that in Nautilus. In being composed of numerous calcified lamellae, the mechanical strength of the connecting ring against hydrostatic pressure could have also been the same or higher than that in Nautilus.

A new Middle Ordovician torticonic nautiloid

Abstract A new, dextrally coiled torticonic nautiloid, Pakrioceras holmi, n.gen., n.sp., is described from the lowermost Middle Ordovician (Viruan) Aseri Stage of northern Estonia. This is the next to oldest torticonic cephalopod hitherto described. On the basis of the posterior (‘ventral’) position of the siphonal tube and the occurrence of the thick spherulitic-prismatic layer of the connecting rings, Pakrioceras is assigned to the family Estonioceratidae of the order Tarphycerida. Studies with scanning electron microscopy and energy-dispersive analysis of X-rays (EDAX) show that aragonite in the spherulitic-prismatic layer was substituted by calcium phosphate during early diagenesis. This layer is therefore ultrastructurally well preserved and consists of bundles of acicular crystalline elements, separated by interspaces. Its ultrastructure is identical to that in the corresponding layer of the Recent Nautilus.

Dart formation in Helix aspersa (Mollusca, Gastropoda)

Dart formation in Helix aspersa has been investigated by SEM of isolated darts at progressive stages in their development, and by histology of dart sacs at the same times. Dart formation begins at the tip of a tubercle where a small group of epithelial cells secrete an organic material filling a small CaCO3 cone that is the first mineralized part of the shaft. Subsequent secretory activity by an increasing area of the tubercle epithelium results in an increase in the diameter and anterior lengthening of the shaft. Continued secretion by the tubercle and dart sac epithelium produces the flare and finally the corona. A pattern of deposition is also evident in the fine structure of the mineral. In the shaft and vanes there is an inner layer of spherulitic prismatic structure which is covered by a layer of irregular patches of simple prismatic structure. The outermost layer of the shaft and vanes has a continuous simple prismatic structure. Two layers are present in the flare, an inner granular amorphous layer and an outer spherulitic prismatic layer. The corona consists of a single rarefied prismatic layer. A mechanism of dart formation is suggested that involves two types of organic matrix, calcifying and non-calcifying. Measurements of the calcium content of darts, dart sacs, and collars indicate that the hemolymph is the probable source of calcium for the dart.

Shell repair in Nautilus macromphalus

The shell of Nautilus macromphalus, like that of N. pompilius, consists of an outer spherulitic-prismatic layer, a middle nacreous layer, and an inner semiprismatic layer. No periostracum was observed. When an area of shell 2 cmx2 cm over the living chamber was removed experimentally, it was replaced by shell of normal structure. Regeneration required 45 days. Observations of regenerated shell made with a scanning electron microscope showed that the spherulitic portion of the spherulitic-prismatic layer was formed from crystal grains that developed into spherulites. The nacreous layer was formed by the deposition of stacks of hexagonal crystals which, through lateral growth, made contact with crystals in adjoining stacks and formed continuous layers 1 crystal in thickness. The deposition of all the shell layers during regeneration was probably accomplished by a single mantle region rather than by different mantle regions as in normal shell growth.