Evolution of dental morphology and dental formulae is an important topic in mammalogy. Carnivora is a successful taxon in terms of dietary divergence. The molar row of Carnivora is separated into two parts: in the lower molars, the anterior part of the first molar possesses a shearing function that benefits a carnivorous diet, whereas the posterior part of the first molar and the remaining molars possess a grinding function that benefits an omnivorous diet. The shape of the first molar, relative size of molars, and the number of molars are important for dental function. According to the result of a comparative morphological analysis of museum specimens and gene-manipulated mice as well as molecular evolutionary analysis, all three factors can be changed by molecules that represent a developmental model, the inhibitory cascade model (such as BMP7). The model can also explain the increase and decrease of molar number in Carnivora. Among all mammals, the modern platypus is a unique mammal, which has lost teeth but still masticates using horny pads. However, their direct ancestor Obdurodon possessed teeth. Comparative cranial morphological analysis revealed that Obdurodon had a larger infraorbital foramen than the modern platypus, indicating that Obdurodon possessed a less-developed maxillary nerve that passes sensory information from the bill to the brain. Additionally, the modern platypus has a thin maxilla that is unable to house a tooth root because of a well-developed infraorbital canal. These results indicate that loss of teeth in the modern platypus is a resulted of amplification of electroreception.