The tessellated pattern of dermal armour in the Heterostraci

The earliest and most primitive heterostraeans possessed a tessellated carapace. Isolated dentine tubercles scattered in the skin formed primordia around which concentric rings of further tubercles were laid down. This pattern of growth produced a characteristic terrazzo. From this stage the gradual elimination of tesserae can be traced in several groups. Beginning with areally growing or cyclomorial tesserae, the individual units appear simultaneously or synchronomorially, thereafter they become fused into a system of large discrete plates. Finally these synehroriomorial units appear earlier and earlier in ontogeny with progressively wider zones of cyclomorial growth being added on to them. Thus a pattern of cyclomorial plates is eventually produced. In the psammosteids there was a redevelopment of tesserae so that the latest stages were comparable to the very early stages, although the tesserae were developed on an already existing pattern of large plates. The possible origin of the tessellated pat tern of dermal armour is discussed. The apparent macular nature of dermal structures is not considered to be an inherent property of skin but instead due to simple physico-chemical factors