Zygomasseteric system

The zygomasseteric system (or zygomasseteric structure) in rodents is the anatomical arrangement of the masseter muscle of the jaw and the zygomatic arch of the skull. The anteroposterior or propalinal (front-to-back) motion of the rodent jaw is enabled by an extension of the zygomatic arch and the division of the masseter into three distinct parts.

Types of zygomasseteric system
The main types are described as protrogomorphous, sciuromorphous, hystricomorphous, and myomorphous.

Protrogomorphy
This is the primitive condition, found in the family Aplodontiidae and in various fossil groups. The rostrum is unmodified and the masseter originates on the ventral zygomatic surface. The Bathyergidae is considered secondarily protrogomorphous since its zygomatic condition clearly derived from a hystricomorphous ancestor.

Sciuromorphy
The ventral surface of the zygoma tilts and broadens into a zygomatic plate. The masseter lateralis extends forwards onto the rostrum. The masseter superficialis extends forwards along the zygoma.

This condition is found in most of the family Sciuridae (suborder Sciuromorpha), and also in the Castoridae, the Eomyidae, and the Geomyoidea.

Hystricomorphy
The masseter medialis is enlarged, and passes through an enlarged infraorbital foramen to originate on the side of the rostrum. The masseter superficialis originates on the front edge of the zygoma, and the masseter lateralis extends over most of its length.

This condition is found throughout the suborders Hystricomorpha and Anomaluromorpha. In the suborder Myomorpha, it is found in the superfamily Dipodoidea and some fossil Muroidea (such as Pappocricetodon). Hystricomorphy is also found in the African dormouse Graphiurus, which is a member of the suborder Sciuromorpha.

Myomorphy
A combination of the expanded and tilted zygoma of sciuromorphy and the enlarged foramen of hystricomorphy.

This condition is found in the Muroidea (Myomorpha) and most Gliridae (Sciuromorpha: in the latter it is often referred to as pseudomyomorphy). Korth and Emry (1991) suggest that the infraorbital foramen of the extinct sciurid subfamily Cedromurinae may have allowed for the passage of the masseter muscle. If true, this subfamily would represent an additional example of myomorphy in the rodent suborder Sciuromorpha.