|The Digestive System|
THE TEETH OF VERTEBRATE ANIMALS
The teeth of lampreys are analogous in function, but are different from other toothed vertebrates in their structure and development. Lamprey teeth are cornified epidermal structures. They do not contain either enamel or dentin.
The teeth of all other vertebrates have a covering of enamel, and an underlying dentin layer which grows from odontoblasts of the dermis layer. Most fish, amphibians, and reptiles have simple pointed teeth. Low, flat surfaced teeth for crushing or grinding are found in very few fish and reptile species. The ratfish, and some of the lungfishes have tooth plates. These tooth plates consist of broad enamel surfaces with roughened or jagged ridges separted by grooves.
All mammals, and a few non-mammalian vertebrates (crocodilians and some fossil reptiles and birds)have thecodont dentition (teeth are set in sockets in the jaw bone). Most fish, amphibians, and reptiles have their teeth either resting on the surface of the jaw bone (acrodont dentition), or more broadly attached to the jaw on its rim and inner wall (pleurodont dentition).
Nonmammalian vertebrates are typically homodont, meaning that all of the teeth are of the same shape, as in sharks and crocodiles. Mammals are typically heterodont. This means that they have a variety of tooth shapes. Most mammals have incisors, canines, premolars, and molars.
Early appearances of the heterodont tooth condition are found in fossil forms of early mammals. The triconodont cheek teeth appeared in some of the earliest prototherian mammals. A triconodont tooth had three points set in a straight line. Later prototherians had the trituberculate tooth shape, with the cusps or points offset from one another and interlocking with the cusps of the opposing jaw.
The dentition of the modern mammals can be described by dental formulas. The dental formula of a mammal lists the number of each of the tooth types, incisors, canines, premolars, and molars, from one side of the upper jaw, above a similar list from the lower jaw.
Incisors and canines are fairly consistent in shape among mammals. The incisors of rodents are unique in that they grow continually. Also rodents and many other herbivores lack canines and even some premolars. The premolars of some carnivores are modified for cutting meat from the bones of prey. This shearing tooth shape is known as secodont.
Molars of most mammals are of a type known as bunodont. They have low blunt cusps covered with enamel. The selenodont teeth of ungulates, and the lophodont teeth of elephants have folds of enamel which run deep in the tooth between areas of the dentin. As these teeth wear they quickly loose the enamel of the surface, but the folds of the enamel persist. The areas of exposed dentin wear away more quickly than the enamel. The surface of the tooth is then left with ridges of enamel separated by grooves of dentin.
Humans and most other mammals are diphyodont, meaning that they grow two sets of teeth. As we age the deciduous teeth (baby teeth, milk teeth) are lost and replaced with permanent teeth. A few mammals are considered toothless because they lack permanent teeth. Some of these species do grow deciduous teeth in infancy, but loose them as they get older. These species are sometimes referred to as monophyodont.
Most other vertebrates are polyphyodont. They loose teeth and replace them repeatedly throughout their life.
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