Jaw type and jaw-closing muscles

A distinguishing feature between carnivores and herbivores is the posterior end of the jaw, the so-called angle (Fig. 1). It is the boney bit you can feel just below your ears. In carnivores and most omnivores, the angle is quite small. In herbivores and humans, it is expanded and convex.

Skulls of a carnivore (dog), herbivore (horse), and a human

Fig. 1:  Skulls of a carnivore (dog), herbivore (horse), and a human. Dashed lines mark the level of the tooth row. Crosshairs mark the jaw joint. In carnivores, the jaw joint lies on the same level as the tooth row, which results in a scissor-like jaw motion; horizontal movement of the mandible (lower jaw) is, however, not possible. In herbivores and humans, the jaw joint lies above the tooth row, allowing for side-to-side and backward-and-forward jaw movement [1].

The size of the angle has to do with those groups’ differing relative proportions of the two main jaw-closing muscles (adductors): musculus temporalis and musculus masseter (Fig.2).

The masseter muscle attaches at the zygomatic arch and the outer side of the lower jaw. It is usually larger and stronger than the temporalis in herbivores to allow side-to-side motion of the jaw for grinding fibrous plant matter. In carnivores, the temporalis muscle is larger than the masseter to pull the lower jaw up with great speed and power. The masseter is relatively small in carnivores and mostly aids in stabilising the jaws during closing. Consequently, the angle ­– the part of the jaw where the masseter attaches – is small or even absent, but enlarged in typical herbivores.

The two main jaw-closing muscles

 

Fig. 2:  The two main jaw-closing muscles (adductors), masseter and temporalis, in dogs, horses, and humans.

Figure 3 gives an overview of the relative proportions of masseter and temporalis in several mammals, including humans. 

Jaw-closing muscles of several carnivores (red), omnivores (orange), herbivores (green), and humans

Fig. 3:  Jaw-closing muscles of several carnivores (red), omnivores (orange), herbivores (green), and humans (black); data from [2]. Note how much larger the temporalis muscle (dark grey) is in carnivores and omnivores compared to herbivores. Although the temporalis is slightly larger than the masseter muscle in humans, the distribution is nearly identical to that of the dromedary – an avid herbivore. The third adductor, the pterygoid, merely assists in the fine control of jaw movement, but does not add significantly to the bite force.

What about humans?

  • Humans – like herbivores – have an expanded, convex angle.
  • Their jow joint – like that of herbivores – lies above the tooth row, allowing for side-to-side and backward-and-forward jaw movement.
  • The temporalis muscle is slightly larger than the masseter muscle, and the latter is not as well developed as that of most herbivores. However, this is also observed in some herbivores, e.g., the dromedary.
REFERENCES:
 
[1] Schwenk, Kurt. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.

[2] Turnbull, William D. Mammalian masticatory apparatus. Vol. 1088. Field Museum of Natural History, 1970.