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Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified fourth upper premolar and the first lower molar. These teeth are also referred to as sectorial teeth.
On the other hand, carnivorous marsupials have teeth of a carnassial form. 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 dentition more similar to the dog. The Pleistocene marsupial lion ( Thylacoleo carnifex) had massive carnassial molars. A recent study concludes that these teeth produced the strongest bite of any known land 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 Borhyaenidae 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 anterior-posterior axis of the tooth row in order to maintain tight occlusional contact between the upper and lower shearing teeth.
Creodonta had 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.Gunnell, Gregg F. (1998). "Creodonta." pp. 91–109 at p. 91 in C.M. Janis, K.M. Scott, and L.L. Jacobs ( eds.) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, Cambridge. . In Oxyaenidae, it is M1 and m2 that form the carnassials. Among the Hyaenodontidae it is M2 and m3. Unlike most modern carnivorans, in which the carnassials are the sole shearing teeth, in the creodonts other molars had a subordinate shearing function. The discussion concerning creodont carnassials is found at page 321. The fact that the two lineages developed carnassials from different types of teeth has been used as evidence against the validity of Creodonta as a clade.Matthew, William Diller (August 1909). "The Carnivora and Insectivora of the Bridger Basin, Middle Eocene". Memoirs of the American Museum of Natural History 9. pp. 289–576.Morlo, M., Gunnell G., and P.D. Polly. 2009. What, if not nothing, is a creodont? Phylogeny and classification of Hyaenodontida and other former creodonts. Journal of Vertebrate Paleontology 29(Supplement 3): 152A.Polly, P.D. 1994. What, if anything, is a creodont? Journal of Vertebrate Paleontology 14(Supplement 3): 42A.
Modern carnivorous bats generally lack true carnassial teeth, but the extinct Necromantis had particularly convergent teeth, in particular M1 and M2, which bore expanded heels and broad stylar shelves. These were particularly suited for crushing over an exclusively slicing action.Gunnell GF & Simmons NB, Evolutionary History of Bats: Fossils, Molecules and Morphology, Cambridge University Press, 2012.
Though not superficially similar, the triconodont teeth of some early mammals such as are thought to have had a function similar to those of carnassials, sharing a similar shearing function. Eutriconodonts possess several speciations towards animalivory, and the larger forms such as Repenomamus, Gobiconodon and Jugulator probably fed on vertebrate prey.Zofia Kielan-Jaworowska, Richard L. Cifelli, Zhe-Xi Luo (2004). "Chapter 7: Eutriconodontans". Mammals from the Age of Dinosaurs: origins, evolution, and structure. New York: Columbia University Press. pp. 216–248. . Similarly the "tooth lips" of clevosaurid such as Clevosaurus are described as "carnassial-like". A lineage of pycnodont fish also developed carnassials eerily convergent with those of modern carnivorans.
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 occludes. The inside of the fourth upper pre-molar closely passes by 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 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. During the Late Pleistocene – early Holocene a now extinct hypercarnivorous wolf ecomorph existed that was similar in size to a large extant gray wolf but with a shorter, broader palate and with large carnassial teeth relative to its overall skull size. This adaptation allowed the megafaunal wolf to predate and scavenge on Pleistocene megafauna.
Cheetah, Homotherium and Barbourofelis, have relatively elongated blade-like shape carnassials, with reduced lingual cusps. This may have been an adaptation to consume quickly the flesh of a prey before larger and stronger predators arrive to take it from them, either from other species or from their own group.
Carnassial teeth infections are common in domestic dogs. They can present as abscesses (a large swollen lump under the eye). Extraction or root canal procedure (with or without a crown) of the tooth is necessary to ensure that no further complications occur, as well as pain medication and antibiotics.
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