Shell microstructure of Styliolina clavulus (Tentaculita) from the Middle Devonian of Czech Republic: implications for phylogenetic affinities and biomineralization of thin-walled tentaculitids

Based on shell microstructure, pelagic tentaculitids have been affiliated with molluscs in the past. Species of the genus Styliolina were among the most important pelagic tentaculitids in the Devonian. The shell wall of Styliolina clavulus is single-layered and composed of thin calcareous lamellae that are parallel to the wall of the shell. The thickness of individual lamellae varies about two times but the thickness of each lamella in the shell wall is constant. The structure is clearly laminar and is similar to bryozoan, brachiopod and molluscan calcitic regularly foliated structures. Based on the structure of S. clavulus alone and the reinterpretation of published SEM images, it is impossible to rule out molluscan affinities for styliolinids, as both regularly foliated and semi-nacreous structures are known in molluscs. However, considering the whole shell structure spectrum of tentaculitoids, one should conclude that the structure of S. clavulus fits well within the group of tentaculitoids and lophophorates in general. The biomineralization of styliolinids was likely advanced and organic matrix controlled; the secreting epithelium was located within the shell and secretion likely took place lamella by lamella.