Ontogenetic staging of the mammalian dentition, and its value for assessment of homology and heterochrony

Abstract

A closely graded system of ontogenetic staging is presented for changes that occur during early development of the dentition in therian mammals, up to and including early phases of the calcification process. Such a precise staging system is essential to detect differences in the time of onset and subsequent differentiation of individual tooth loci within the jaws of a single species, and within supraspecific taxa. Analysis of the evolutionary significance of these developmentalheterochronies can play a valuable role in the assessment of serial homology in the therian dentition, and it provides additional insight into the nature and pattern of tooth loss during evolution. Following the description and illustration of bud, cap, and bell substages, examples are presented from the ontogeny of the primary dentition in macroscelidids and hyracoids. These demonstrate the use of this staging system for assessing the serial homology of modified teeth such as “premolariform” canines and the role of developmental retardation in the evolutionary loss of primary and secondary teeth.