Toothcomb - Anatomical Structure

Anatomical Structure

The toothcomb of most lemuriforms includes six finely spaced teeth, four incisors and two canine teeth that are procumbent (tilt forward) in the front of the mouth. The procumbent lower canine teeth are the same shape as the incisors located between them, but they are more robust and curve upward and inward, more so than the incisors. In the permanent dentition, the canines erupt after the incisors. The crowns of the incisors are also angled in the direction of the forward tilt, and the crowns of both the incisors and canines are elongated and compressed side-to-side. The apical ridge, following along the front edges of the toothcomb teeth, is V-shaped in most lemuriforms, tapering off from the midline. As a result of this dental reconfiguration, the upper and lower incisors do not contact one another, and often the upper incisors are reduced or lost completely.

The French anatomist Henri Marie Ducrotay de Blainville first identified the two lateral teeth of the lemuriform toothcomb as canines in 1840. Canine teeth are normally used to pierce or grasp objects. With modified lower canine teeth, the first lower premolars following the toothcomb are usually shaped like typical canine teeth (caniniform) and assume their function. These premolars are commonly confused with canines. Normally the true canines in the lower jaw sit in front of the upper canines, and in toothcombed primates, the caniniform premolars rest behind it.

The lemuriform toothcomb is kept clean by the sublingua or "under-tongue", a specialized muscular structure that acts like a toothbrush to remove hair and other debris. The sublingua can extend below the end of the tongue and is tipped with keratinized, serrated points that rake between the front teeth.

Among lemurs, the toothcomb is variable in structure. Among indriids (Indriidae), the toothcomb is less procumbent and consists of four teeth instead of six. The indriid toothcomb is more robust and wider, with shorter incisors, wider spaces between the teeth (interdental spaces), and a broader apical ridge. It is unclear whether this four-toothed toothcomb consists of two pairs of incisors or one pair of incisors and one pair of canines. In fork-marked lemurs (Phaner) the toothcomb is more compressed, with significantly reduced interdental spaces. All six teeth are longer, straighter, and form a more continuous apical ridge. In the recently extinct monkey lemurs (Archaeolemuridae) and sloth lemurs (Palaeopropithecidae), the toothcomb was lost and the incisors and canines resumed a typical configuration in the front of the mouth. The aye-aye also lost its toothcomb, replacing it with continually growing (hypselodont) front teeth, similar to the incisors of rodents.

In colugos, the toothcomb has a completely different structure. Instead of individual incisors and canine teeth being finely spaced to act like the teeth of a comb, the biting edge of the four incisors have become serrated with as many as 15 tines each, while the canine acts more like a molar. These serrated incisors are kept clean using the front of the tongue, which is serrated to match the serrations of the incisors. Similarly, the hyracoid toothcomb consists of incisors with multiple tines, called "pectinations". In contrast to the colugos, the size and shape of the tines are more uniform.

The toothcomb of treeshrews is like the lemuriform toothcomb in that it uses interdental spaces to form the comb tines, but only two of its three pairs of lower incisors are included in the toothcomb and the canines are also excluded. The lateral two incisors in the toothcomb are generally larger. In the extinct arctocyonids, all six lower incisors were part of the toothcomb. In African antelopes, the toothcomb is strikingly similar to that of lemuriforms in that it consists of two pairs of incisors and a pair of canines.