With a few notable exceptions, the teeth of extant reptiles are unspecialized; that is, most teeth look alike, and the dentition is called homodont (Latin for "alike teeth"). The teeth may vary considerably in size along the length of the tooth-bearing bones, especially in snakes, because the teeth are deciduous and are replaced regularly. This type of dentition is called polyo-dont. Teeth are present on the bones of the upper and lower jaw and on other bones forming the roof of the mouth (palatine and pterygoid). If teeth are ankylosed (cemented by calcification) to the inside of jawbones, the dentition is pleurodont. This is the condition of all snakes and most lizards. If the teeth are ankylosed to a bony ridge along the jawbones, as in some lizards, the dentition is acrodont. Crocodilian teeth are situated in sockets, as are the teeth of mammals, and this dentition is called thecodont. The most spectacular type of tooth specialization in extant reptiles involves the fangs of venomous snakes. These fangs are hollow, elongated teeth on each side of the front of the upper jaw, although some species have solid, grooved fangs on each side of the rear of the upper jaw. In front-fanged snakes, venom is forcefully injected through the fangs and exits into the prey through slitlike openings on the lower anterior face of each fang. In rear-fanged snakes, venom runs under little pressure along the grooves and enters prey as the rear fangs successively embed themselves into prey during swallowing. Among the front-fanged species are those with folding fangs that are normally held parallel to the roof of the mouth and rotated down into position as needed. Other front-fanged snakes have less mobility associated with their fangs, which are therefore always in the biting position. The fangs typically are much longer in species with folding fangs than in species with fixed front fangs. With the exception of fangs, most teeth in extant reptiles are used to grip prey, although some lizards have specialized, blunt teeth that crush snail shells. Some extinct reptiles had far more specialized tooth patterns than do the surviving groups.
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