Teeth and dentitions

Vertebrate teeth may be used as weapons in fighting and self-defence, but they also provide
the vertebrates the first tool for feeding, making it possible to trap and swallow prey and, especially in the case of mammals, to render food more suitable for digestion in the gastrointestinaltract. The teeth consist of a crown (protruding tothe mouth) and root (embedded or attached to the bone). The crown is composed of an externalmineralized enamel (or enameloid) layer, the hardest mineralized tissue, and an innermineralized dentin which surrounds the pulpal cavity filled by living cells capable of dentinregeneration and sensory function. In the roots of the mammalian teeth, the dentin and pulpare surrounded by a mineralized cementum and a periodontal ligament that attaches the teethto the surrounding bone.

The whole dentition is composed of units of separate teeth of serial homology, i.e. having a
common evolutionary origin, and has been regarded as an example of merism (Butler, 1995;
Weiss et al., 1998). Teeth in various vertebrates may reside on the surfaces of mouth or pharynx, but during the evolution they became restricted to a horseshoe-shaped dental arch liningthe oral cavity . The fish and reptile teeth may be replaced several,even hundreds of times during the lifespan of the animal (polyphyodont), but mammals may replace some of them only once (diphyodont dentition).

The teeth in different parts of the mouth may have specialized forms in lower vertebrates, but the specialization to trueheterodonty with distinct tooth classes, as seen in modern mammalian species, started onlyduring the reptilian evolution . The heterodonty of mammals íncludes morphologicallyand developmentally distinct tooth classes or types: the anterior incisors, caninesand usually multicusped postcanine teeth, premolars and molars . In theancestral mammalian dentition, three primary incisors, one primary canine and four primarypostcanine teeth (premolars) developed in each jaw quadrant .

These could be replaced(in this case also called deciduous) with successional (secondary) teeth developingfrom the dental lamina lingual to the predecessor . In addition, three or more molars may develop posterior to these teeth and have nodeciduous predecessors. Together with the unreplaced and secondary teeth they constitutethe permanent dentition. Luckett and Butler considered the molars (e.g. humanpermanent molars) as primary teeth. Teeth may be regarded as primary or secondaryaccording to whether they develop from the surface epithelium or the dental lamina . Most mammalian species, though, have reduced dentitions as they have lost some orseveral of the ancestral teeth. Thus, mouse and other muroid rodents develop in each quadrantonly one incisor and three molars, which are not replaced, the molars presumably beinghomologous to the posterior-most postcanine teeth (molars) of other species .

In human dentition, two primary incisors, a primary canine and two primary postcanineteeth (called primary or deciduous molars) are replaced with two permanent incisors,a permanent canine and two permanent premolars, and in addition three permanent molarsdevelop without deciduous predecessors . Thus, during evolution, Homosapiens has lost one of the incisors and two anteriormost premolars.