Paleontological discoveries suggest that the first mineralizing individuals within the group that originated the Mollusk phylum had spicule structures, on the cuticle surface, probably made of aragonite. The composition is corroborated by the acicular (needle shaped) architecture, organized in rosettes (rose shaped). The genus Chancelloria sp., belonging to the Coeloscleritophoran group, is the origin of the phylum Mollusca. The genus in question was, once discovered, associated with sponges.
Figure 3.3 – Acicular mineral structures. On the right, a fossil of Chancelloria sp., with acicular rosettas preserved on the surface of the epiderm.
With the fusion of the spicules, comes the overlapping sheet structures, similar to scales.
Figure 3.4 – The fusion of mineral needles along one direction form sheet like structures (similar to scales), that overlap, covering the epidermis of the organisms. In a: shell (fossil) of Purella sp, seen laterally and from beneath. In b: detailing of the transition from a acicular to leafed architecture, along the growth lines of the shell; in the amplification, the detailing of the fusion of the needles
Figure 3.5 – Transition of the mineralized structures of the leaf type to the tubercular type in Ilsanella sp.
The leaf structures fuse, augmenting their width, forming tubercular structures. In some species, as Ilsanella sp. the transition can be observed over the internal face of the shell.
