Heterodont dentitions

According to Butler (1967), Bateson in 1894 first noticed that teeth resembled vertebrae, digits, and other serial structures. He called this phenomenon ‘merism’ and described the properties of a meristic series. The term is still used today to describe a heterodont dentition as a metameric system, i.e. different teeth have different morphologies and functions along the tooth row (Weiss, 1990).

The teeth of placental mammals have evolved from teeth possessing a single cusp (homodont). Moreover, morphological differences among the different classes of teeth, e.g. between canines and premolars, are more abrupt in some primate taxa than in others. Generally, the differences among classes of teeth are more distinct in the human dentition (Scott and Turner, 1997) than in the dentitions of many non-human primate taxa, particularly in the posterior teeth. In studying heterodont teeth through the years various explanations have been offered regarding the control of their complex development. Butler (1939) proposed the morphogenetic field theory in which there are three developmental fields: incisor, canine, and molar. Within each field there is a polar tooth affecting tooth development; for example, the upper first incisor is more stable than the second incisor and in both the upper and lower jaws the first molar is more stable than the second molar, which in turn is more stable than the third molar.