page_title: Machairodontinae
section_title: Skull
hierarchical_section_title: Machairodontinae / Skeleton / Skull
caption_attribution_description: English: the machairodont smilodon fatalis with jaws set at 128 degrees, the maximum possible for this species
context_page_description: Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae. They were found in Asia, Africa, North America, South America, and Europe from the Miocene to Pleistocene living from about 16 million until about 11,000 years ago.
  The Machairodontinae contain many of the extinct predators commonly known as "saber-toothed cats", including the famed genus Smilodon, as well as other cats with only minor increases in the size and length of their maxillary canines. The name means "dagger-tooth", from Greek μάχαιρα, dagger. Sometimes, other carnivorous mammals with elongated teeth are also called saber-toothed cats, although they do not belong to the felids. Besides the machairodonts, saber-toothed predators also arose in nimravids, barbourofelids, Machaeroidinae, Hyaenodonta and even in two groups of metatherians.
context_section_description: The most studied section of the machairodont group is the skull, and specifically the teeth. With a large range of genera, good fossil representation, comparable modern relatives, diversity within the group, and a good understanding of the ecosystems inhabited, the machairodont subfamily provides one of the best means of research for the analysis of hypercarnivores, specialization, and the relationships between predator and prey.
  Machairodonts are divided into two types: dirk-toothed and scimitar-toothed. Dirk-toothed cats had elongated, narrow upper canines and generally had stocky bodies. Scimitar-toothed cats had broader and shorter upper canines and a typically lithe body form with longer legs. The longer-toothed cats often had a bony flange that extended from their lower mandible. However, one genus, Xenosmilus, known only from two fairly complete fossils, broke this mould; possessing both the stout, heavy limbs associated with dirk-toothed cats, and the stout canines of a scimitar-toothed cat.
  Carnivores reduced the number of their teeth as they specialized in eating meat instead of grinding plant or insect matter. Cats have the fewest teeth of any carnivore group, and machairodonts reduce the number even further. Most machairodonts retain six incisors, two canines, and six premolars in each jaw, with two molars in the upper jaw only. Some genera, such as Smilodon, bear only eight premolars with one fewer on the mandible, leaving only four large premolars on the mandible along with two stunted canines and six stout incisors. The canines are curved back smoothly, and serrations are present, but are minor and wear away with age, leaving most middle-aged machairodonts (at about four or five) with no serrations. Hints in the bones such as these help paleontologists to estimate the age of an individual for population studies of an animal long extinct.
  Longer canines necessitate a larger gape. A lion with a gape of 95° could not bear canines that are nine inches long because they would not be able to have a gap between the lower and upper canines larger than an inch or so, not enough to use for killing. Machairodonts, along with the other groups of animals that acquired similar teeth by convergent evolution, needed a way to change their skulls to accommodate the canines in several ways.
  The main inhibitors of a large gape for mammals are the temporalis and masseter muscles at the back of the jaw. These muscles have the capacity to be powerful and undergo a great degree of modification for ranging bite forces, but are not very elastic due to their thickness, placement, and strength. To open the mouth wider, these species needed to make the muscles smaller and change their shape. The first step in this was to reduce the coronoid process. The masseter, and especially the temporalis, muscles insert on this jutting strip of bone, so reduction of this process meant the reduction of the muscles. Less mass for each muscle allowed greater elasticity and less resistance to a wide gape. Changing the shape of the temporalis muscle in this respect created a greater distance between the origin and insertion, so that the muscle became longer and more compact, which is generally a more suitable format for this type of stretching. This reduction led to a weaker bite.
  The skulls of machairodonts suggests another change in the shape of the temporalis muscle. The main constraint to opening the jaws is that the temporalis muscle will tear if it is stretched past a critical degree around the glenoid process when the mouth is opened. In modern felids, the occipital bone extends backward, but the temporalis muscles that attach to this surface are strained when opening the jaw wide as the muscle is wrapped around the glenoid process. To reduce the stretch of the temporalis muscle around the immovable process, machairodonts evolved a skull with a more vertical occipital bone. The domestic cat has a gape of 80°, while a lion has a gape of 91°. In Smilodon, the gape is 128°, and the angle between the ramus of the mandible and the occipital bone is 
caption: Skull of Smilodon fatalis at maximum gape (128°)
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