Introduction

Eruption is the process whereby each tooth is carried from its developmental position within the jaw to its functional position in the mouth; movement is mainly axial in direction although rotation, tilting and lateral movement are also common components. Tooth emergence, the term used to distinguish the actual appearance of the tooth in the mouth from other phases of the process, occurs at particular times and in a particular sequence. Tooth emergence can be precocious or delayed, it may be incomplete or the tooth may not emerge at all, remaining buried in the jaw. The sequence and chronology of eruption differ to some extent from one taxon to another (Owen, 1866/1868).

In animals with high-crowned hypsodont teeth, such as horses and bovids, continuous eruption of the teeth as the crowns are gradually worn down is an integral feature of the masticatory apparatus (Ainamo, 1970; Ainamo and Talari, 1976). Even in non-hypsodont dentitions where tooth-wear is a less predominant feature, it is likely that a process known as continuous eruption occurs. This process has been most studied in man but some supportive observations have also been made on the cheek tooth dentitions of rats, mice and hamsters (reviewed by Miles, 1961), where there is strong evidence that, as age advances, recession of the gingival and alveolar crest margins (passive eruption) occurs leaving the crowns of the teeth more and more exposed.