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.

Molluscs are known for their ability to produce a calcified shell resulting from a genetically controlled and matrix-mediated process, performed extracellularly. The occluded organic matrix consists of a complex mixture of proteins, glycoproteins and polysaccharides that are in most cases secreted by the mantle epithelium. To our knowledge, the model studied here—the argonaut, also called paper nautilus—represents the single mollusc example where this general scheme is not valid: the shell of this cephalopod is indeed formed by its first dorsal arms pair and it functions as an eggcase, secreted by females only; furthermore, this coiled structure is fully calcitic and the organization of its layered microstructures is unique. Thus, the argonautid shell appears as an apomorphy of this restricted family, not homologous to other cephalopod shells. In the present study, we investigated the physical and biochemical properties of the shell of Argonauta hians, the winged argonaut. We show that the shell matrix contains unusual proportions of soluble and insoluble components, and that it is mostly proteinaceous, with a low proportion of sugars that appear to be mostly sulfated glycosaminoglycans. Proteomics performed on different shell fractions generated several peptide sequences and identified a number of protein hits, not shared with other molluscan shell matrices. This may suggest the recruitment of unique molecular tools for mineralizing the argonaut’s shell, a finding that has some implications on the evolution of cephalopod shell matrices.

Trilobites are widespread in Early Devonian deposits of north Gondwana; some of the most emblematic ones were collected from the famous latest Emsian (Early Devonian) mudmound locality Hamar Laghdad in south-eastern Morocco. This locality is famous for its trilobites, especially for the conspicuous red-coloured remains of phacopid trilobites with often greenish eyes. Here, we present a taxonomic revision of the previously described trilobites from the so-called Red Fauna of Hamar Laghdad. We introduce the new taxa Harpes hamarlaghdadensis n. sp. and Morocops davidbrutoni n. sp. Phacopids dominate the trilobite assemblage from the Red Cliff at Hamar Laghdad in terms of the number of specimens (represented by Morocops) as a whole. Additionally, we focus on the description of trilobite diversity of this interval compared to Algerian assemblages.

Trilobites are important elements of the Devonian macrobenthos; some of them were collected in the Chefar el Ahmar Formation, from two sections located near Beni Abbes in the Saoura Valley (Ougarta Basin, Saharan Algeria). This formation is characterized by alternations of claystones and limestones, and it is considered to be late Emsian to early Frasnian in age. Only the lower part of this formation has yielded trilobites so far; their presence has been known for a long time. Phacopines clearly dominate the trilobite assemblages, with Austerops, Barrandeops, Chotecops and Phacops s.l. as the main genera. Two new species are described (Austerops salamandaroides sp. nov. and Phacops ouarouroutensis sp. nov.), while some other taxa are presented in open nomenclature. Comparisons are made with closely allied species. These new trilobite occurrences have been analysed in terms of their intra- and interspecific variability and biodiversity. The occurrence of Struveaspis maroccanica, previously known from the Saoura Valley, provides an early Eifelian age, which is also confirmed by the presence of trilobites Thysanopeltis and Koneprusites, and ostracods Bairdiocypris devonica and Bufina ?subovalis.