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Carnassials are large teeth found in many carnivorous mammals, used for shearing flesh and bone in a scissor- or shear-like way. In the Carnivora, the carnassials are the modified fourth upper premolar and the first lower molar. These teeth are also referred to as sectorial teeth.
Carnassial teeth are modified molars (and in the case of carnivorans premolars) which are adapted to allow for the shearing (rather than tearing) of flesh to permit the more efficient consumption of meat. These modifications are not limited to the members of the order Carnivora, but are seen in a number of different mammal groups. Not all carnivorous mammals, however, developed carnassial teeth. Mesonychids, for example, had no carnassial adaptations, and as a result, the blunt, rounded cusps on its molars had a much more difficult time reducing meat. Likewise, neither members of Oxyclaenidae nor Arctocyonidae had carnassial teeth.
On the other hand carnivorous marsupials have carnassial teeth. Both the living Tasmanian devil (Sarcophilus harrisii) and the recently extinct Tasmanian wolf (Thylacinus cynocephalus) possessed modified molars to allow for shearing, although the Tasmanian wolf, the larger of the two, had adaptation more similar to the modern dog. The Pleistocene marsupial lion (Thylacoleo carnifex) had massive carnassial molars. A recent study concludes that these teeth produced the strongest bite of any mammal in history. Moreover, these carnassial molars appear to have been used, unlike in any other known mammal, to inflict the killing blow to the prey by severing the spinal cord, crushing the windpipe or severing a major artery. Like these true marsupials, the closely related borhyaenids of South America had three carnassial teeth involving the first three upper molars (M1-M3) and the second through fourth lower molars (m2-m4). In the borhyaenids the upper carnassials appear to have been rotated medially around the anteriorposterior axis of the tooth row in order to maintain tight occlusional contact between the upper and lower shearing teeth.
Creodonts have two or three pairs of carnassial teeth, but only one pair performed the cutting function: either M1/m2 or M2/m3, depending on the family. In Oxyaenidae, it is M1 and m2 that form the carnassials. Among the hyaenodontids it is M2 and m3. Unlike most modern carnivorans, which the carnassials are the sole shearing teeth, in the creodonts other molars have a subordinate shearing functions.
In modern carnivorans the carnassial teeth pairs are found on either side of the jaw and are composed of the fourth upper pre-molar and the first lower molar (P4/m1). The location these carnassial pairs is determined primarily by the masseter muscle. In this position, the carnassial teeth benefit from most of the force generated by this mastication muscle, allowing for efficient shearing and cutting of flesh, tendon and muscle.
The scissor-like motion is created by the movement between the carnassial pair when the jaw closes. The inside of the fourth upper pre-molar comes into contact with the outer surface of the first lower molar, thus allowing the sharp cusps of the carnassial teeth to slice through meat.
The length and size of the carnassial teeth vary between species, taking into account factors such as:
- the size of the carnivorous animal
- the extent to which the diet is carnivorous
- the size of the chunk of meat that can be swallowed.
Evolution of carnassial teeth
The shape and size of sectorial teeth of different carnivorous animals vary depending on diet, illustrated by the comparisons of bear (Ursus) carnassials with those of a leopard (Panthera). Bears, being omnivores, have a flattened, more blunt carnassial pair than leopards. This reflects the bear's diet, as the flattened carnassials are useful both in slicing meat and grinding up vegetation, whereas the leopard's sharp carnassial pairs are more adapted for its hypercarnivorous diet.
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