1993, pp 182-204

An Ontogenetic Assessment of Dental Homologies in Therian Mammals

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Overview

Ontogenetic analysis of the dentition in a wide range of marsupials and eutherians provides valuable criteria for assessing tooth homologies among extant therian mammals. In addition, these developmental data offer evidence for the loss of teeth during mammalian evolution. Such studies can distinguish the deciduous or successional nature of individual teeth, even when replacement does not occur postnatally. Ontogenetic analyses also provide evidence for the formation of vestigial deciduous teeth that do not erupt; these rudiments serve as valuable clues to the homologies of some tooth positions that have been lost during mammalian phylogeny. The continued accumulation of developmental data on the pattern of early budding and differentiation of deciduous tooth germs offers useful criteria for identifying tooth class homologies of highly modified teeth, in addition to their bony and occlusal relationships.

An important finding of this analysis is that epithelial connections among the deciduous tooth, primary dental lamina, oral epithelium, successional lamina, and successor tooth furnish the most useful criteria for assessing the ontogenetic relationships between a deciduous tooth and its successor. This developmental pattern was used to test recent hypotheses of premolar-molar homologies in marsupials, where it was claimed that no true postcanine tooth replacement occurs. Epithelial connections, however, support the traditional hypothesis that the posterior premolar position is occupied by typical deciduous and successional teeth during ontogeny; this falsifies the hypothesis that five molars occurred primitively in marsupials. Finally, ontogenetic data are considered for testing the hypothesis of serial homology between the postcanine dentition of eutherians and marsupials, and for the occurrence of five premolars primitively in eutherians.