Carnassial pair in Sabre-Tooth Cats (Smilodon)

    The shearing action of the teeth in modern Carnivora is generated when the inner surface of the fourth upper pre-molar (P⁴) passes outside the outer surface of the first lower molar (M₁). The (P⁴ / M₁) combination is called the carnassial pair. This particular combination is characteristic of both modern groups of Carnivora, Feliformia and Caniformia, and is thus a synapomorphy of the Order. The shearing teeth of extinct carnivores (Miacidae) are a different combination, which indicates separate ancestry. Miacid Sabre-tooth cats are not the direct ancestors of modern cats.