Analogous biomineralization processes between the fossil coral Calceola sandalina (Rugosa, Devonian) and other Recent and fossil cnidarians

Abstract

The current work represents a distinctive study about the biomineral properties of exceptionally good preserved skeletons of Calceola sandalina from the Middle Devonian of Couvin (Belgium), Smara (Morocco) and (Algeria) and their relation in the evolution of biomineralization of cnidarians. Structural and crystallographic analyses of the skeletons have been done by petrographic microscopy, electron scanning microscopy (SEM), atomic force microscopy (AFM), electron backscatter diffraction (EBSD), computer-integrated polarization microscopy (CIP) and electron microprobe analysis (EMPA). Calceola skeletons have many similarities with other cnidarians, mainly with other Palaeozoic corals as Syringoporicae: The microcrystals are composed of co-oriented nanocrystals that remind to mesocrystals, suggesting a biocrystallization process by particle attachment (CPA). The relationship between the nanocrystals and microcrystals suggest a growth mode similar to mineral bridges. A similar model was described for Syringoporicae corals (Tabulata) and it is similar to the coordinated-growth mode described in scleractinians and molluscs. Calceola skeletons show also a convergent structure with scleractinian forming Rapid Accretion Deposits (RAD), which share some structural and chemical properties. These evidences suggest analogous processes of biomineralization derived from a stem group of cnidarians. The results of this paper highlight the value of biomineralization studies in fossil organisms to understand the evolution of biomineralization mechanism through Phanerozoic.