The endocochleate coleoid cephalopod Spirula spirula, the only present-day representative of the order Spirulida, secretes a coiled shell consisting of a series of chambers divided by septa and connected by a siphuncle. It is the shell closest to those of Recent and extinct ectochleate cephalopods: nautiloids, ammonoids. Therefore, its study may help to understand which characters remained unchanged or became transformed during the evolution of endocochleates. We have carried out detailed observations on the different structures composing the Spirula shell, with the aim of reconstructing their morphology, distribution, and mutual relationships. Alongside this, we also review the previous profuse terminology. Taking into account the additional information provided by growth lines and crystal orientations, we propose mechanisms for the secretion of the shell structures. All these mechanisms are integrated in a consistent way into a general model of chamber formation. The periostracum is secreted within a distinct periostracal groove. The outer shell layer is secreted externally to the periostracum by the soft tissues lining the shell externally. The inner shell layer is produced by the shell wall mantle, whereas the septa and the siphuncle are made periodically by a differentiated septal/siphuncular mantle. The most adoral septal mantle edge changes from secreting septal to inner shell wall material to produce the mural flap. The adapical ridge is formed by passive precipitates from cameral fluid residues trapped by surface tension, whereas the fibrous prismatic deposits of the connecting ring are biominerals produced remotely within mantle secretions. Homologies with Nautilus and Sepia are discussed.

Gymnolaemata bryozoans produce CaCO₃ skeletons of either calcite, aragonite, or both. Despite extensive research, their crystallography and biomineralization patterns remain unclear. We present a detailed study of the microstructures, mineralogy, and crystallography of eight extant cheilostome species using scanning electron microscopy, electron backscatter diffraction, atomic force microscopy, and micro-computed tomography. We distinguished five basic microstructures, three calcitic (tabular, irregularly platy, and granular), and two aragonitic (granular-platy and fibrous). The calcitic microstructures consist of crystal aggregates that transition from tabular or irregularly platy to granular assemblies. Fibrous aragonite consists of fibers arranged into spherulites. In all cases, the crystallographic textures are axial, and stronger in aragonite than in calcite, with the c-axis as the fiber axis. We reconstruct the biomineralization sequence in the different species by considering the distribution and morphology of the growth fronts of crystals and the location of the secretory epithelium. In bimineralic species, calcite formation always predates aragonite formation. In interior compound walls, growth proceeds from the cuticle toward the zooecium interior. We conclude that, with the exception of tabular calcite, biomineralization is remote and occurs within a relatively wide extrapallial space, which is consistent with the inorganic-like appearance of the microstructures. This biomineralization mode is rare among invertebrates.

Serpulid polychaetes are marine worms that secrete calcium carbonate tubes in which they live. Despite extensive previous research on their microstructures, there are no crystallographic data and their biomineralization process remains unclear. Here, the microstructures of the tubes of seven serpulid species were studied, including their chemical composition, mineralogy and crystallography, using X-ray diffraction, Raman and Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam, electron backscatter diffraction, and thermogravimetric analysis. Generally, serpulid tubes have a high amount of organic matter (~ 7.5 wt%), consisting of chitin and proteins, and the calcite is always present as medium to high magnesium calcite. Three main microstructures were identified: granular-prismatic and lamello-fibrillar calcite, and fibrous aragonite. They all displayed an axial texture, which is stronger in the lamello-fibrillar calcite, with the c-axis aligned with the elongation axis of the crystals. These findings demonstrate that only some instances of the granular-prismatic and the lamello-fibrillar calcite are biogenic (primary) microstructures. Conversely, other instances of the granular-prismatic calcite and the fibrous aragonite are a consequence of a recrystallization process (i.e. secondary). Replacement may occur on either primary or secondary calcitic microstructures (replaced by aragonite). Secondary microstructures retain remnants of the previously replaced microstructures, such as vestigial crystals or major growth increments. The high-Mg nature of the calcite favors recrystallization. The plywood arrangement of the lamello-fibrillar calcite is hypothesized to result from the ordering of a chitin fibrillar precursor into a cholesteric liquid crystal phase, with the calcite subsequently growing by oriented nucleation onto the organic fibrils.

Exceptionally preserved 135-million-year-old ammonoids from the Neuquén Basin at the Andean foothills revealed a fossilised structure never recorded before. Ammonoids are cephalopods that inhabited the oceans for about 400 million years until they became extinct 66 million years ago. Their shells are composed of aragonitic layers bounded externally by an organic periostracum. The latter plays an essential role in initiating shell biomineralisation and protecting minerals from dissolution and abrasion. Here we describe a preserved periostracum in Cretaceous ammonoids, an extremely fragile yet flexible layer, with an approximate thickness of 2 µm and an internal horizontal lamination. The external surface appears mostly smooth, while the internal surface displays a reticulated appearance, interpreted as the casts of aragonite prisms of the calcareous shell. Our results reveal that the ammonoid periostracum contains proteins, polysaccharides, and lipids, consistent with the composition of the periostracum in modern-day molluscs. This study sheds light on a previously little-known organic structure in ammonoids. Its morphological and chemical characteristics allow us to establish that it is a highly conservative structure among molluscs. Furthermore, we show that such a delicate organic structure can be preserved for 135 million years in favourable environmental conditions, opening up the possibility of future discoveries.