title: The origin of the lower fourth molar in canids, inferred by individual variation.
abstract: 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 fromCanis latrans,Nyctereutes procyonoides, andUrocyon cinereoargenteusmuseum specimens. For each molar, relative sizes of molars (M2/M1and M3/M1scores) 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/M1score inCanis latrans, M3/M1score inNyctereutes procyonoides, and M2/M1and M3/M1scores inUrocyon cinereoargenteuscompared to individuals of these species that lacked supernumerary molars.
  
  
  CONCLUSIONS
  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.
authors.affiliation: College of Liberal Arts and Sciences, Mie University , Tsu, Mie , Japan.
authors.forename: Masakazu
authors.initials: M
authors.lastname: Asahara
keywords: Dental anomaly; Dental formula; Inhibitory cascade; Supernumerary molar