Introduction

A fundamental question in developmental biology is how patterns are established, and in evolutionary biology how patterns are changed. The pattern of vertebrate dentitions (tooth morphology, number, and location) is of obvious importance to the survival of individual organisms and a great diversity of patterns exists among vertebrate taxa (Peyer, 1968). Furthermore, the abundance of teeth and jaws in the fossil record provides a tremendous amount of information about the evolutionary changes that have occurred in these patterns. However, despite the accumulation of histological and molecular information on the development of individual teeth (Thesleff et al., 1996; Ruch, 1987), little is known about the control of the pattern of the dentition as a whole during embryonic development. Such information is crucial for understanding the mechanisms of evolutionary change in the vertebrate dentition. In this chapter, we review aspects of the evolution of dentition patterns, models that have been proposed for the developmental mechanisms responsible for these patterns and candidate genes for controlling them. Although the dentitions of mammals constitute only a small portion of the diversity observed in vertebrates, we will concentrate on this group because of the much greater availability of developmental and genetic data. For the purposes of this chapter, ‘dentition pattern’ refers to the number, location, and arrangement of differently shaped teeth, while ‘dental pattern’ is used to describe the crown structure of individual teeth, e.g. location, shape and size of cusps.

Axial definitions for the adult dentition and embryonic jaws

Various axial definitions for the adult dentition and embryonic mandible have been used in the literature (Table 11.1). However, these definitions can be inconsistent or confusing.