Hypodontia and Supernumerary

Teeth fall into a series of continuous morphogenetic fields (incisor, canine and molar) within which gradients of size and shape exist. Supernumerary teeth may develop within a field where the odontogenic capacity for tooth formation is abnormally strong. Tooth agenesis may occur in regions where the usual capacity for tooth formation is reduced or lacking. In general, reduced tooth size, reduction in complexity and total absence are different degrees of the same thing. Both supernumerary and missing or reduced teeth often occur at the periphery of a morphogenetic series. Among modern humans, third molars are the most commonly missing or reduced tooth type, followed by upper lateral incisors, upper or lower second premolars and lower first incisors. The proportion of a population with one or more missing third molars ranges from almost none to 30% and while clearly genetically determined, this strongly suggests genetic drift has a large part to play. Supernumerary teeth may be either supplemental (identical to normal teeth) haplodont (have conical crowns with single roots) or be tuberculate (more complex occlusal surfaces but simple conical roots). Supernumerary or missing teeth have been reported in 5% of Gorilla, 4.3% of Pan and 8,5% of Pongo museum specimens. Agenesis of one or more third molars is very rare in great apes but most rare in Gorilla. Agenesis or reduced peg-shaped upper lateral incisors and lower second premolars all occur in great apes. Supernumerary incisors and premolars in great apes are often either supplemental or haplodont teeth. Distomolars (M4), supernumerary premolars and incisors, which all occur in modern humans, have also all been reported in great ape collections. Among great apes, supplemental M4s, some even larger than M3, are relatively common in Gorilla and Pongo but more rare in Pan where they tend to be diminutive and conical.