Introduction

During the evolution of teeth, different types of periodontal attachment have been developed. It is likely that teeth have either evolved from conodont elements (Figure 12.1) or from denticles of the odontode type (Figure 12.2) in the earliest agnathic chordates (Krejsa and Slavkin, 1987; Smith and Hall, 1990) before the dermal armour and tooth-like placoid scales appeared during the evolution of fishes (Gross, 1966). A new developmental model has been proposed on the basis of separation of odontogenic from osteogenic and chondrogenic neural crest potential (Smith and Hall, 1993). It is explained as a modular development, where each interactive morphogenetic unit may be commited to form either cartilage, bone, odontodes, or teeth (see Chapter 10).

The mode of tooth attachment in lower vertebrates constitutes a rather small spectrum which ranges from the loose fibrous type seen in the elasmobranchs to solid ankylosis between the tooth and the underlying jaw (Peyer, 1968). However, other highly complex types of attachment structures were developed mainly in bony fishes at a relatively late stage in the evolution of teeth. The mammalian periodontium could be regarded as the most complex of all types of tooth attachment, particularly in regard to structure and function (Shellis, 1982; Osborn, 1984; Berkovitz et al., 1992, 1995). However, even in the thecodont mammals there are major differences in periodontal reactivity between rodent, carnivorous, herbivorous and omnivorous dentitions because of apparent differences in the fine structure of the periodontium, in the mode of tooth replacement and in the eruption and growth rate of teeth.