Comparison of accumulation patterns in layered dentinal tissue of some Odontoceti and corresponding patterns in baleen plates and ear plugs of Balaenopteridae

The data of the present examination show that the teeth of delphinids reach their ultimate diameter before eruption from the gum. However, growth in length continues during the first three or four years after birth, thereafter the increase in length is neglegible. In the same period the width of the dentinal growth layers decreases rapidly to approach an asymptote when about 10 layers are deposited. In Phocoena phocoena and Delphinus delphis the greatest thickness of the first dentinal layer is about 350 μm and never exceeds this figure. The decrease in width of the layers can only be expressed by a straight line in a logarithmic plot. Due to this decrease the layers are not comparable to each other. Secondary layers can only be seen in the first three or four layers. In P. phocoena the numbers of hollows in the densitometer records of the cementum, representing the dark lines, are in accordance with the numbers of dentinal layers that could be counted. However, the part in the records representing the dentine can not be divided into “growth periods”, due to the decrease in width of the growth layers. Hence, the records of the teeth are incomparable to those of ear plugs and baleen plates of baleen whales, as these latter continue to grow at a nearly constant rate. The formation of growth layers in the teeth of Sotalia fluviatilis differs from that in the teeth of other delphinids examined so far. The formation of the first seven or eight layers is the same as in all other species. However, the process of dentine formation then changes as also the structure of the dentine, so that no real layers are formed.