New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Abstract

Numerous new cases of preserved shell microstructure were discovered in molluscs from the Middle Cambrian Gowers Formation (Ptychagnostus atavus ⁄ Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been calcitic semi-nacre, a type of microstructure known in brachiopods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotrochozoans underscores a similar mechanism of biomineralization. Moreover, these observations suggest a prevalence of calcite-shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre-like microstructures in many of these fossils reveals how widespread these strong (fractureresistant) microstructures were in Middle Cambrian molluscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.

Deep conservation of bivalve nacre proteins highlighted by shell matrix proteomics of the Unionoida Elliptio complanata and Villosa lienosa

“…The comparison of these bivalve-conserved protein sets with those previously detected in gastropod nacre confirms the global dissimilarity of the nacre SMP sets of bivalves and gastropods (at least those of the abalone) [19]. Bivalves and gastropods exhibit different kinds of nacres (respectively, sheet and columnar nacres), the homology of which is still the subject of a debate about parsimony principles [14], crystallographic and microstructural textures [11,57], along with palaeontological and molecular evidence [12,13,20]. Figure 4 summarizes all available information on the occurrence of protein domains observed from the shell proteome of various mollusc models, comprising nacre and prismatic shell layers of bivalves (Unionoids, Pinctada and Mytilus), cross-lamellar layers of gastropods (Lottia and Cepaea), and, finally, fibrous layers of brachiopods, taken as an outgroup.…”

Chronostratigraphic and palaeogeographic significance of an early Cambrian microfauna from the Heraultia Limestone, northern Montagne Noire, France

“…If the large polygons consisted of reproductions of outer epithelium cells, they should be preserved in patches on the internal mold that might vary from specimen to specimen. Moreover, the observation of Vendrasco et al (2010) that "polygonal textures are often on the same horizontal plane as imprints of other types of shell microstructure and endoliths, indicating that phosphate precipitated directly on the surface of the inner shell" is confirmed by the identification, on specimens from the Montagne Noire, of another type of microstructural imprints (lamello-fibrillar) in the same plane as the polygonal texture. Vendrasco et al (2010) further point out that the polygons are often distinct, with high relief.…”

Nacre in Molluscs from the Ordovician of the Midwestern United States

“…This is similar to what is expected in nacre: 64°, 90°, 116° and 122° [1], although measurements of these angles in tablets from our photographs and those in the literature show a more continuous distribution from 98° to 152° (n = 54). The measured angles in Isorthoceras differ from that of calcite rhombohedra, 78° and 102°, expected if the shell microstructure were calcitic semi-nacre [3]. The tablet moulds interconnect in places, but where separate the width of individual tablet moulds ranges from about 6 µm to 14 µm (mean 10 µm; median 9.1 µm; standard deviation 3.0 µm; n = 21).…”

“…During the Early Palaeozoic radiations of metazoans, the range of shell microstructures in molluscs expanded in step with increasing disparity of overall shell form [1][2][3]. In at least some lineages the evolutionary transition in the shell was to more fracture-resistant crystal/organic configurations [4], correlating with an apparent increasing selective pressure from progressively more efficient predators.…”