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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 pairs are found on either side of the jaw, and are composed of the fourth upper pre-molar, and the first lower molar. 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.
- Henry Fairfield Osborn (1907). Evolution of mammalian molar teeth. Macmillan. Retrieved 20 January 2011.
- Cope E.D. (1879). "The Origin of the Specialized Teeth of the Carnivora". The American Naturalist 13: 171–173.
- Savage R. J. G. (1977). "Evolution in carnivorous mammals". Paleontology 20: 237–271.
- "Carnivores". Natural History Collections. Retrieved May 2013.