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, in which the carnassials are the sole shearing teeth, in the creodonts other molars have a subordinate shearing function. 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. 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. Though not superficially similar, the triconodont teeth of some early mammals such as eutriconodonts 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. Similarly the "tooth lips" of clevosaurid sphenodontians 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 size of the carnivorous animal the extent to which the diet is carnivorous the size of the chunk of meat that can be