Heterochrony of tooth root initiation in rabbits

One of the considerable evolutionary modifications to the dentition of rabbits and other lagomorphs has been the transition from teeth of finite growth, characterized by the presence of brachydontic roots in ancestral species, to teeth in which their generative parts operate as cell renewal systems to maintain a continuous growth of teeth in contemporary species. In contemporary lagomorphs evidence for the existence of vestigial roots in the “rootless”, continuously growing permanent cheek teeth has not been described. It was the purpose of this study to examine developing molariform teeth for evidence of such structures. The basis for the examination was serial histological sections of maxillae and mandibles of contemporary rabbits ( Oryctolagus cuniculus L.) harvested from foetuses and postnatals aged from the 15th day postconception to birth and from birth to the 90th postnatal day respectively. The fossil teeth of the Miocene lagomorphs from the genus Amphilagus and Titanomys were used for direct comparisons together with evidence from the literature. It was found that of the 3 root openings which appeared during the early development of permanent molariform teeth, the two lateral ones were homologous with the 2 lateral brachydontic roots present in adult molariform teeth of ancestral lagomorphs. The time required for initiation of the root openings, counted from the day of appearance of the cap stage, was found to be 6 days for deciduous teeth and 3 to 4 days for permanent ones. Four stages in the ontogenesis and evolution of the odontogenic cell renewal system of lagomorphs have been described.