Tissue diversity and evolutionary trends of the dermal skeleton of Silurian thelodonts

Previously described scale morphotypes of Silurian thelodonts, constrained by their representation as isolated dermal denticles are reassessed to provide a more robust character basis for their inclusion in future phylogenetic studies. As relatively common microfossils, thelodonts are important biostratigraphical markers, but their interrelationships with geologically younger species known by complete skeletons are still unresolved. We examined scales of 21 known morphotypes from north-eastern Europe, Siberia and central Asia and described their distinct tissue arrangements considering (1) thickness and direction of dentine tubules, (2) presence or absence of a pulp canal, (3) number and position of pulp canals, (4) the presence or absence of a distinct outer crown layer and (5) the extent of Sharpey's fibres penetrating the scale base. We correlated the traditional thelodont scale type morphologies with these distinct scale histologies, as found in Silurian thelodonts. In addition, a new histological type for thelodont scales, the Talimaalepis type, is described to represent a new taxon, from the Early-Mid Silurian. Our study suggests that, through time, there is a general trend of increasing complexity in thelodont dermal tissue structures. Three types of dentine and internal scale organisations were distinguished in Silurian species studied, namely (1) irregular, thin tubular dentine; (2) irregular, thick tubular dentine, with two subtypes as a function of pulp canal development and (3) regular, tubular dentine (orthodentine).