The Bonaparte hypothesis and the evolution of dental homologies in monotremes

Monotremes occupy a pivotal position in mammalian evolution, yet the origin and homology of their dentition remain highly debated. This study revisits and updates the ‘Bonaparte hypothesis’, which proposes that monotreme dental patterns originated from Gondwanan dryolestoid ancestors, particularly meridiolestidans. Drawing on recent fossil discoveries from South America and Australia, including new monotreme and meridiolestidan material, a comprehensive comparative analysis of dental and mandibular morphology is presented. Multiple lines of evidence reveal notable similarities in cusp arrangement, crown organisation, root structure, and mandibular features between monotremes, meridiolestidans, and Northern Hemisphere dryolestoids. These include the structure of triakididrepanons, patterns of root fusion, development of cingulids, and the presence of a hypertrophied mandibular canal. The results support the plausibility of complex dental transformations, potentially involving the fusion of adjacent tooth units, as originally proposed by Bonaparte. Although the hypothesis remains difficult to test due to the fragmentary fossil record and the lack of developmental evidence, it is consistent with current data and compatible with models proposing a Gondwanan origin of tribosphenic dentition. These findings highlight the importance of Gondwanan taxa in early mammalian evolution and underscore the need for new fossil discoveries to further resolve the origins of monotreme dental morphology.