Crystallographic control on the substructure of nacre tablets

Nacre tablets of mollusks develop two kinds of features when either the calcium carbonate or the organic portions are removed: (1) parallel lineations (vermiculations) formed by elongated carbonate rods, and (2) hourglass patterns, which appear in high relief when etched or in low relief if bleached. In untreated tablets, SEM and AFM data show that vermiculations correspond to aligned and fused aragonite nanogloblules, which are partly surrounded by thin organic pellicles. EBSD mapping of the surfaces of tablets indicates that the vermiculations are invariably parallel to the crystallographic a-axis of aragonite and that the triangles are aligned with the b-axis and correspond to the advance of the {010} faces during the growth of the tablet. According to our interpretation, the vermiculations appear because organic molecules during growth are expelled from the a-axis, where the Ca–CO3 bonds are the shortest. In this way, the subunits forming nacre merge uninterruptedly, forming chains parallel to the a-axis, whereas the organic molecules are expelled to the sides of these chains. Hourglass patterns would be produced by preferential adsorption of organic molecules along the {010}, as compared to the {100} faces. A model is presented for the nanostructure of nacre tablets. SEM and EBSD data also show the existence within the tablets of nanocrystalline units, which are twinned on {110} with the rest of the tablet. Our study shows that the growth dynamics of nacre tablets (and bioaragonite in general) results from the interaction at two different and mutually related levels: tablets and nanogranules

Restudy of some plectronocerid nautiloids (Cephalopoda) from the late Cambrian of China; discussion on nautiloid evolution and origin of the siphuncle

The sub-class Nautiloidea is divided into two super-orders, Nautilosiphonata and Calciosiphonata, basedon different structural types of the connecting rings.The late Cambrian order Plectronocerida has the calciosiphonate type of connecting ring similar tothat in post-Cambrian orthocerids. It is structurally more complex than the nautilosiphonate connectingring in late Cambrian ellesemerocerid-like nautiloids. The plectronocerid nautiloids, therefore, evolvedfrom the ellesmerocerid-like nautiloids and not vice versa. As indicated by the complex siphuncularstructure in plectronocerids, cephalopod evolution began earlier than previously estimated, probably inthe early Cambrian. The siphuncle in cephalopods originated from a calcareous septum that becamepartially non-calcified and formed the connecting ring

Cameral deposits in Paleozoic cephalopods

Calcareous cameral deposits have been described in several orthocerid and actinocerid nautiloids. According to the prevailing hypothesis, they were secreted during the lifetime of the animal, either by living tissues in the shell chambers, or by precipitation from the cameral liquid. In the present paper, cameral deposits are described in three species of Carboniferous orthocerid-like coleoid (Order Colorthocerida) from USA. The shell walls and septa in these coleoids are very thin and poorly calcified. In one half of the population of the three species, the septa are completely fragmented and there are no cameral deposits. In the other half of the population, the septa are partially fractioned and their surfaces are covered by welldeveloped cameral deposits. In contrast to the septa, the cameral deposits do not show any fractioning. To explain the origin of the cameral deposits, the following hypothetical scenario is the most realistic. After the death of the animals, the shells were accumulated on the sea floor and in one half of the population the septa became fully fractioned by the hydrostatic pressure. In shells of another half of the population, the septa were only partially fractioned. The calcifying bacteria entered the chambers of the dead shells through the porous connecting rings and gave rise to the cameral deposits

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.No funding indicated.</p

Carboniferous coleoids with mixed coleoid-orthocerid characteristics: a new light oncephalopod evolution

Orthocerid-like coleoids with mixed orthocerid-coleoid characteristics are described for the first time from the Carboniferous of USA. The appearance of these coleoids represents transitional morphology between the orthoconic nautiloid and coleoid lineages. This transitional state is based on the new genus Colorthoceras n. gen. with three assigned new species (C. inflata n. sp., C. tubulata n. sp. and C. concavus n. sp.) in the new family Colorthoceridae of the new order Colorthocerida. Orthocerid nautiloid characteristics include a longiconic phragmocone with a well-developed body chamber, and a central, sub-central or sub-ventral siphuncle with endosiphuncular deposits. The shell wall in the new order Colorthocerida is characterized by the coleoid characteristics of a lack of the nacreous layer, with a high content of chitin that created a somewhat semi-elastic shell. The connecting rings are uni-layered, directly continuous from the septal neck, and have a mixed chitinous-calcareous composition similar to that in order Mixosiphonata. The shell wall structure in these unique orthocerid-like coleoids is similar to that in the previously described Carboniferous bactritoid-like coleoids. The evolution of these coleoid characteristics appears to represent an unsuccessful evolutionary experiment, as the diversity of this nautiloid lineage was in gradual decline in the Upper Paleozoic

Characterization of two new superorders Nautilosiphonata and Calciosiphonata and a new order Cyrtocerinida of the subclass Nautiloidea: siphuncular structure in the Ordovician nautiloid Bathmocerass (Cerphalopoda)

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.No funding indicated.</p