Three-dimensional virtual histology of Silurian osteostracan scales revealed by synchrotron radiation microtomography

We used propagation phase contrast X-ray synchrotron microtomography to study the three-dimensional (3D) histology of scales of two osteostracans, Tremataspis and Oeselaspis, members of a jawless vertebrate group often cited as the sister group of jawed vertebrates. 3D-models of the canal systems and other internal structures are assembled based on the virtual thin section datasets and compared with previous models based on real thin sections. The primary homology framework of the canal systems in the two taxa is revised and new histological details are revealed based on the results of this work. There is no separation of vascular canals and lower mesh canals in the Tremataspis scale, contrary to previous results. The secondary upper mesh canals have a limited distribution to the anterior region of the Tremataspis scale. The upper and lower mesh canal systems of Tremataspis have different geometries, inferred to reflect different developmental origins: we interpret the upper system as a probable epithelial invagination, the lower system as entirely vascular. Oeselaspis has no equivalent of the upper mesh canal system. The upper mesh canal system of Tremataspis may have been sensory in function. In Oeselaspis, numerous polyp-shaped structures opening from the canal system onto the surface of the scale resemble the innervation tracts for neuromast organs. The growth of the Oeselaspis scale proceeds by addition of small odontodes containing unmineralized lacunae, which may further mineralize and become more compact. Our results highlight that 3D-histological investigation on scales and other dermal skeletons of osteostracans is necessary to fully appreciate the diversity of skeletal histologies in the group. Traditional 3D-models based on thin sections alone are not reliable and should no longer be used as the basis for homology assessments or functional hypotheses.