Diversity of the dermal skeleton in Ordovician to Silurian vertebrate taxa from North America: Histology, skeletogenesis and relationships

All three Ordovician agnathans from Cañon City, Colorado, USA, possess dentine and an enamel or enameloidcovering layer. Two have extensive bony trabeculae of aspidin below the tubercles, as part of the tesserae, whereas the other has cellular bone of attachment as the scale base. Each of the three has a different and distinctive dentine and tissue arrangement. Two have enameloid caps (Astraspis and Vertebrate Indeterminate A), the former is above a microtubular dentine, the latter above a cellular mesodentine. In Eriptychius the dentine is a macrotubular form, none conforms to the type of orthodentine as found in later heterostracans.