Orthodentine‐osteodentine and osteocytic‐anosteocytic transition inferred from Silurian stem chondrichthyan dermal jawbones with ever‐white teeth overgrown by brown denticles

Background: The dentigerous jawbones of a 423-million-year-old stem chondrichthyan (ischnacanthid acanthodian) carry a characteristic combination of multi-row non-shedding white teeth with brownish overgrowing denticles. The color distinction makes it a good model for observing the interaction between two odontode lineages, but the nature of the opaque white material and its relationship to other tissues is unknown. Results: Synchrotron microtomography demonstrates that the white tissue is a lightly mineralized orthodentine. By contrast, the orthodentine on the denticles is highly mineralized and almost vitreous. The brownish color of the denticles is from the osteodentine that fills the pulp of both teeth and denticles. The continuation of dentine tubules through orthodentine and osteodentine indicates their production by the same population of odontoblasts. The teeth are added sequentially, while the same generation of denticles is added synchronously, both associated with bone apposition. The overgrowth of denticles at the growing front of a tooth row will terminate its tooth addition, and the timing of this event determines the number and length of the tooth rows. Osteocyte lacunae in the latest bone layers appear to be infilled by bone matrix, which explains the scarcity of lacunae in the matured bone layers. Conclusions: Heterochronic shift of the mineralization processes could be a potential mechanism to produce the peculiar white tissue. It suggests the invention of novel dental tissues may only require modification of the developmental process, which may provide an explanation for why vertebrates have already acquired a great diversity of dental tissues during their early evolution.