The zygomasseteric system (or zygomasseteric structure) refers to the anatomical arrangement of the

masseter muscle In anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it is the ...

and the

zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...

(cheek bone) in the skulls of

rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

. This system plays a crucial role in the diverse

chewing Chewing or mastication is the process by which food is crushed and ground by the teeth. It is the first step in the process of digestion, allowing a greater surface area for digestive enzymes to break down the foods. During the mastication proc ...

mechanics observed across rodent species. The zygomatic arch is modified to accommodate the masseter muscle, a primary muscle responsible for

jaw The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth ...

movement. The masseter muscle itself is often divided into superficial, lateral, and medial components, allowing for a wide range of jaw motion, particularly the

anteroposterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

or propalinal movement (front-to-back chewing motion) characteristic of rodents. 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, reflecting adaptations to different dietary niches and chewing strategies.

Protrogomorphy

The members of this grade include nearly all of the pre-Oligocene rodents of North America and Asia and some of those of Europe. Several lineages survive into the Oligocene or early Miocene, with only one species still alive today, the

mountain beaver The mountain beaver (''Aplodontia rufa'')Other names include boomer, mountain boomer, ground bear, giant mole, gehalis, sewellel, suwellel, showhurll, showtl, and showte, as well as a number of other Native American terms. "Mountain beaver" is a ...

(''

Aplodontia rufa The mountain beaver (''Aplodontia rufa'')Other names include boomer, mountain boomer, ground bear, giant mole, gehalis, sewellel, suwellel, showhurll, showtl, and showte, as well as a number of other Native American terms. "Mountain beaver" is a ...

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

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...

of protrogomorph rodents is unmodified and the

infraorbital foramen In human anatomy, the infraorbital foramen is one of two small holes in the skull's upper jawbone ( maxillary bone), located below the eye socket and to the left and right of the nose. Both holes are used for blood vessels and nerves. In anatomic ...

is small. The superficial masseter originates on the lateral surface of the anterior

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

and inserts along the ventral margin of the angular process of the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

. The lateral masseter inserts here as well and originates from the lateral portion of the zygomatic arch. The small medial masseter originates along the medial surface of the zygomatic arch and inserts along the dorsal portion of the mandible at the end of the tooth row.

Sciuromorphy

This condition is found in most members of the family

Sciuridae Squirrels are members of the family Sciuridae (), a family that includes small or medium-sized rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrel ...

(suborder

Sciuromorpha Sciuromorpha ( 'squirrel-like') is a rodent Order (biology), suborder that includes several rodent Family (biology), families. It includes all members of the Sciuridae (the squirrel family) as well as the mountain beaver species. Traditionally, ...

), and also in members of the

Castoridae Castoridae is a family of rodents that contains the two living species of beavers and their fossil relatives. A formerly diverse group, only a single genus is extant today, ''beaver, Castor.'' Two other genera of "giant beavers", ''Castoroides'' ...

, the

Eomyidae Eomyidae is a family of extinct rodents from North America and Eurasia related to modern day pocket gophers and kangaroo rats. They are known from the Middle Eocene to the Late Miocene in North America and from the Late Eocene to the Pleistocene ...

, and the

Geomyoidea Geomyoidea is a superfamily of rodent that contains the pocket gophers ( Geomyidae), the kangaroo rats and mice ( Heteromyidae), and their fossil relatives. Characteristics Although dissimilar in overall appearance, gophers have been united wit ...

. Relative to the primitive protrogomorphous condition, the superficial masseter remains unchanged. The lateral masseter has shifted forward and upward, behind and medial to the superficial masseter. Here it originates from a wide zygomatic plate developed on the anterior (maxillary) root of the zygomatic arch. This shift of origin changed the direction of pull of the anterior part of the lateral masseter from 30 to 60 degrees, greatly strengthening the forward component of the masseter contraction.

Hystricomorphy

This condition is found throughout the suborders

Hystricomorpha Hystricomorpha (from Greek ὕστριξ, ''hystrix'' 'porcupine' and Greek μορφή, ''morphē'' 'form') is a term referring to families and orders of rodents which has had many definitions throughout its history. In the broadest sense, it ref ...

and

Anomaluromorpha Anomaluromorpha is a clade that unites the anomalures, springhares, and zenkerella. It has alternately been designated as either a suborder or infraorder. Most recently, recognized it as one of five suborders of rodents. Characteristics T ...

. In the suborder

Myomorpha The suborder Myomorpha contains 1,524 species of mouse-like rodents, nearly a quarter of all mammal species. Included are mice, rats, gerbils, hamsters, lemmings, and voles. They are grouped according to the structure of their jaws and molar t ...

, it is found in the superfamily

Dipodoidea Dipodoidea is a Superfamily (biology), superfamily of Rodent, rodents, also known as dipodoids, found across the Northern Hemisphere. This superfamily includes over 50 species among the 16 genera in 3 families. They include the Jerboa, jerboas (f ...

and some fossil

Muroidea The Muroidea are a large Taxonomic rank, superfamily of rodents, including mice, rats, voles, hamsters, lemmings, Gerbillinae, gerbils, and many other relatives. Although the Muroidea originated in Eurasia, they occupy a vast variety of habitat ...

(such as '' Pappocricetodon''). Hystricomorphy is also found in the African

dormouse A dormouse is a rodent of the family Gliridae (this family is also variously called Myoxidae or Muscardinidae by different taxonomists). Dormice are nocturnal animals found in Africa, Asia, and Europe. They are named for their long, dormant hibe ...

Graphiurus The African dormice (genus ''Graphiurus'') are dormice that live throughout sub-Saharan Africa in a variety of habitats. They are very agile climbers and have bushy tails. They primarily eat invertebrates, with other components of their diet inc ...

'', which is a member of the suborder

. In hystricomorphs the medial masseter is enlarged and originates on the side of the rostrum (in extreme cases as far forward as the premaxilla), where it then passes through a greatly enlarged infraorbital foramen to insert on the mandible. This gives an almost horizontal resultant to the muscle contraction.

Myomorphy

This condition is found in the

(

) and most Gliridae (

: in the latter it is often referred to as pseudomyomorphy). suggest that the infraorbital foramen of the extinct

sciurid Squirrels are members of the family Sciuridae (), a family that includes small or medium-sized rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrels. ...

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. Myomorphs combine characteristics found in both the sciuromorphous and hystricomorphous rodents. Both the lateral and medial masseter muscles have migrated, and both a large zygomatic plate as well as a large infraorbital foramen are present. This type gives the greatest anteroposterior component of any rodent zygomasseteric system, which might explain the success of the cosmopolitan Muroidea.

References

{{Reflist Rodent anatomy Skull