The origin of the lower fourth molar in canids, inferred by individual variation

Background: An increase in tooth number is an exception during mammalian evolution. The acquisition of the lower fourth molar in the bat-eared fox (Otocyon megalotis, Canidae, Carnivora, Mammalia) is one example; however, its developmental origin is not clear. In some canids (Canidae), individual variation exist as supernumerary molar M₄. This study focuses on the acquisition of the lower fourth molar in canids and proposes that the inhibitory cascade model can explain its origin.

Methods: Occlusal view projected area of lower molars was determined from 740 mandibles obtained from Canis latrans, Nyctereutes procyonoides, and Urocyon cinereoargenteus museum specimens. For each molar, relative sizes of molars (M₂/M₁ and M₃/M₁ scores) affected by inhibition/activation dynamics during development, were compared between individuals with and without supernumerary molar (M₄).

Results: Possession of a supernumerary molar was associated with significantly larger M₂/M₁ score in Canis latrans, M₃/M₁ score in Nyctereutes procyonoides, and M₂/M₁ and M₃/M₁ scores in Urocyon cinereoargenteus compared to individuals of these species that lacked supernumerary molars.

Discussion: We propose that, in canids, the supernumerary fourth molar is attributable to reduced inhibition and greater activation during molar development. In the bat-eared fox, altered inhibition and activation dynamics of dental development during omnivorous-insectivorous adaptation may be a contributing factor in the origin of the lower fourth molar.