Zygomasseteric system

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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. The main types are described as protrogomorphous, sciuromorphous, hystricomorphous, and myomorphous.[1]


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.[2]


Skull of an oriental giant squirrel. Note the classic sciuromorphous shape of the anterior zygomatic region.

The ventral surface of the zygoma tilts and broadens into a zygomatic plate. The lateral masseter extends forwards onto the rostrum. The superficial masseter extends forwards along the zygoma.[2]

This condition is found in most of the family Sciuridae (suborder Sciuromorpha), and also in the Castoridae, the Eomyidae, and the Geomyoidea.[2]


Skull of a nutria demonstrating the hystricognathous lower jaw and hystricomorphous zygomasseteric system.

The medial masseter is enlarged, and passes through an enlarged infraorbital foramen to originate on the side of the rostrum. The superficial masseter originates on the front edge of the zygoma, and the lateral masseter extends over most of its length.[2]

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.[2]


Gerbil skull displaying the myomorphous condition

A combination of the expanded and tilted zygoma of sciuromorphy and the enlarged foramen of hystricomorphy.[2]

This condition is found in the Muroidea (Myomorpha) and most Gliridae (Sciuromorpha: in the latter it is often referred to as pseudomyomorphy). Korth & 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.[2]