Patterns of molar development and loss of the hypoconulid in treeshrews (Scandentia)

Mammalian teeth are complex and extremely morphologically diverse. Two models for dental development, the Patterning Cascade Model (PCM) and the Inhibitory Cascade Model (ICM), outline controls for some of this diversity. However, these models have only been assessed in extant taxa with derived molars, such as primates and seals. Thus, the evolution of developmental controls on mammalian molars remains uncertain. In this study, we examine the PCM and ICM in a sample (n = 41 specimens, 11 species) of extant treeshrews (Scandentia), mammals that retain the ancestral tribosphenic molar characteristic of early mammals. Treeshrew molar morphology, including third molar hypoconulid presence and their relative molar size, is notably variable. According to the PCM, closely spaced mesial (earlier-developing) cusps inhibit later-developing cusps such as the hypoconulid. Therefore, we predict that taxa lacking the hypoconulid will be characterized by more closely spaced mesial cusps. According to the ICM, taxa are expected to have molar proportions where the second molar is the average size of the first and third. Previous work has demonstrated that taxa which lack the third molar hypoconulid (i.e., cercopithecin primates) do not follow predictions of the ICM. Our results suggest that hypoconulid presence among treeshrews is explained by the PCM. Although treeshrews molars also follow the prediction of the ICM, third molar size does not explain hypoconulid presence. Overall, our results provide greater insight into models of molar development, and their implications for studying the evolution of the tribosphenic molar.