Zygomasseteric system

Variations in the structure of the zygomatic arch and the masseter muscle's insertion points have led to the classification of rodents into four main zygomasseteric types: protrogomorphous, sciuromorphous, hystricomorphous, and myomorphous,[1]: 29–32  reflecting adaptations to different dietary niches and chewing strategies.

Several lineages survive into the Oligocene or early Miocene, with only one species still alive today, the mountain beaver (Aplodontia rufa).

[2] The molerats (family Bathyergidae) are considered secondarily protrogomorphous since their zygomatic condition is clearly derived from a hystricomorphous ancestor.

The superficial masseter originates on the lateral surface of the anterior maxilla and inserts along the ventral margin of the angular process of the mandible.

[4] This type gives the greatest anteroposterior component of any rodent zygomasseteric system, which might explain the success of the cosmopolitan Muroidea.