Extant monoplacophorans (Tryblidiida, Mollusca) have traditionally been reported as having an internal nacreous layer, thus representing the ancestral molluscan condition. The examination of this layer in three species of Neopilinidae (Rokopella euglypta, Veleropilina zografi, and Micropilina arntzi) reveals that only V. zografi secretes an internal layer of true nacre, which occupies only part of the internal shell surface. The rest of the internal surface of V. zografi and the whole internal surfaces of the other two species examined are covered by a material consisting of lath-like, instead of brick-like, crystals, which are arranged into lamellae. In all cases examined, the crystallographic c-axis in this lamellar material is perpendicular to the surface of laths and the a-axis is parallel to their long dimension. The differences between taxa relate to the frequency of twins, which is much higher in Micropilina. In general, the material is well ordered, particularly towards the margin, where lamellae pile up at a small step size, which is most likely due to processes of crystal competition. Given its morphological resemblance to the foliated calcite of bivalves, we propose the name foliated aragonite for this previously undescribed biomaterial secreted by monoplacophorans. We conclude that the foliated aragonite probably lacks preformed interlamellar membranes and is therefore not a variant of nacre. A review of the existing literature reveals that previous reports of nacre in the group were instead of the aragonitic foliated layer and that our report of nacre in V. zografi is the first undisputed evidence of nacre in monoplacophorans. From the evolutionary viewpoint, the foliated aragonite could easily have been derived from nacre. Assuming that nacre represents the ancestral condition, as in other molluscan classes, it has been replaced by foliated aragonite along the tryblidiidan lineage, although the fossil record does not presently provide evidence as to when this replacement took place.
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Serge Gofas (Departamento de Biología Animal, Universidad de Málaga), Anders Warén, and Karin Sindemark Kronestedt (Swedish Museum of Natural History) contributed in an essential manner by providing specimens. X-ray diffraction data of Fig. S2 are joint unpublished results of A.C. with Alejandro Rodríguez-Navarro (Departamento de Mineralogía y Petrología, Universidad de Granada). TEM sections of Fig. S3 are from the original material of the late Hiroshi Nakahara, kindly ceded by Mitsuo Kakei (School of Dentistry, Meikai University). J.R-R. is grateful to the Spanish Junta de Andalucía for his research grant. The study has been funded by Projects CGL2004-00802 and CGL2007-60549 (Ministerio de Educación y Ciencia) and by Research Group RMN190 (Junta de Andalucía).
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Nacre in bivalves (a, b), gastropods (c, d), and the cephalod Nautilus (e, f). a View of the growth surface of the bivalve Atrina pectinata; growth fronts are diffuse, i.e., they are composed by many crystals and different growth stages. b Semimature nacre in the bivalve Pinctada margaritifera; it is characterized by polygonal tablets. c Growth surface of the gastropod Perotrochus caledonicus, showing a characteristic tower-like growth. d Mature (fractured) nacre of the gastropod Bolma rugosa. e Growth surface of nacre of a juvenile specimen of Nautilus pompilius, close to the aperture; growth is towered, similar to gastropods. f Growth surface of the same specimen as in e, in a more internal location; growth is more similar to that of bivalves. Arrows indicate the growth direction of the shell (JPG 884 KB)
Pole figures for the bivalves Pteria hirundo (a), the cephalod Nautilus belauensis (b), and the gastropod B. rugosa (c) obtained by EBSD (a) and X-ray diffraction (b, c). In all three cases, the 001 pole figure implies that the c-axis is approximately perpendicular to the shell surface. The 010 pole figure in a and the 110 pole figure in b indicate that the b-axis is parallel to the growth direction of the shell (arrows), whereas the circular distribution in the 110 pole figure of c implies that crystals have their b-axis disoriented (JPG 348 KB)
TEM sections of the nacre of the gastropod Monodonta labio (a, b) and of the bivalve Pinctada radiata (c). a, b In gastropods, the interlamellar membranes extend between the boundaries of the plates of adjacent towers which are in a similar growth stage. c In bivalves, the interlamellar membranes on top of a given lamella of crystals extend freely beyond the edge of the biomineralization front, marked by the position of the last formed crystal (lfc); the adoral end of the interlamellar membrane (ime) is also indicated. Arrows indicate the growth direction (JPG 636 KB)
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Checa, A.G., Ramírez-Rico, J., González-Segura, A. et al. Nacre and false nacre (foliated aragonite) in extant monoplacophorans (=Tryblidiida: Mollusca). Naturwissenschaften 96, 111–122 (2009). https://doi.org/10.1007/s00114-008-0461-1
- Foliated aragonite