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 a superficial, lateral and medial muscle. The four main types are described as protrogomorphous, sciuromorphous, hystricomorphous, and myomorphous. [1]

Contents

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 ( Aplodontia rufa ). [2] The molerats (family Bathyergidae) are considered secondarily protrogomorphous since their zygomatic condition is clearly derived from a hystricomorphous ancestor. [3] The rostrum of protrogomorph rodents is unmodified and the infraorbital foramen is small. 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] The lateral masseter inserts here as well and originates from the lateral portion of the zygomatic arch. [4] 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. [4]

Sciuromorphy

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

This condition is found in most members of the family Sciuridae (suborder Sciuromorpha), and also in members of the Castoridae, the Eomyidae, and the Geomyoidea. [3] Relative to the primitive protrogomorphous condition, the superficial masseter remains unchanged. [2] 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. [4] 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. [2]

Hystricomorphy

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

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. [3]

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. [4] This gives an almost horizontal resultant to the muscle contraction. [2]

Myomorphy

Gerbil skull displaying the myomorphous condition George's skull1.jpg
Gerbil skull displaying the myomorphous condition

This condition is found in the Muroidea (Myomorpha) and most Gliridae (Sciuromorpha: in the latter it is often referred to as pseudomyomorphy). [3] 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.

Myomorphs combine characteristics found in both the sciuromorphous and histricomorphous 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. [4] This type gives the greatest anteroposterior component of any rodent zygomasseteric system, which might explain the success of the cosmopolitan Muroidea. [2]

Related Research Articles

Articles related to anatomy include:

Maxilla Upper jaw bone

The maxilla in vertebrates is the upper fixed 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 maxillary bones are fused at the intermaxillary suture, forming the anterior nasal spine. This is similar to the mandible, which is also a fusion of two mandibular bones at the mandibular symphysis. The mandible is the movable part of the jaw.

Zygomatic bone Facial bone

In the human skull, the zygomatic bone is a paired irregular bone which articulates with the maxilla, the temporal bone, the sphenoid bone and the frontal bone. It is situated at the upper and lateral part of the face and forms the prominence of the cheek, part of the lateral wall and floor of the orbit, and parts of the temporal fossa and the infratemporal fossa. It presents a malar and a temporal surface; four processes, and four borders.

Zygomatic arch Cheek bone

In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone, the two being united by an oblique suture ; the tendon of the temporal muscle passes medial to the arch, to gain insertion into the coronoid process of the mandible (jawbone).

Orbit (anatomy) Cavity or socket of the skull in which the eye and its appendages are situated

In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is 30 millilitres, of which the eye occupies 6.5 ml. The orbital contents comprise the eye, the orbital and retrobulbar fascia, extraocular muscles, cranial nerves II, III, IV, V, and VI, blood vessels, fat, the lacrimal gland with its sac and duct, the eyelids, medial and lateral palpebral ligaments, check ligaments, the suspensory ligament, septum, ciliary ganglion and short ciliary nerves.

Muscles of mastication

There are four classical muscles of mastication. During mastication, three muscles of mastication are responsible for adduction of the jaw, and one helps to abduct it. All four move the jaw laterally. Other muscles, usually associated with the hyoid, such as the mylohyoid muscle, are responsible for opening the jaw in addition to the lateral pterygoid.

Masseter muscle

In human 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 most superficial and one of the strongest.

Medial pterygoid muscle Muscle involved in chewing

The medial pterygoid muscle, is a thick, quadrilateral muscle of the face. It is supplied by the mandibular branch of the trigeminal nerve (V). It is important in mastication (chewing).

Sciuromorpha Suborder of mammals

Sciuromorpha ("squirrel-like") is a rodent clade that includes several different rodent families. It includes all members of the Sciuridae as well as the mountain beaver species.

Myomorpha Suborder of rodents

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 teeth. They are characterized by their myomorphous zygomasseteric system, which means that both their medial and lateral masseter muscles are displaced forward, making them adept at gnawing. As in the hystricognathous rodents, the medial masseter muscle goes through the eye socket, a feature unique among mammals. Myomorphs are found worldwide in almost all land habitats. They are usually nocturnal seed-eaters.

Geomyoidea Superfamily of rodents

Geomyoidea is a superfamily of rodent that contains the pocket gophers (Geomyidae), the kangaroo rats and mice (Heteromyidae), and their fossil relatives.

<i>Galesaurus</i> Extinct genus of cynodonts from the Triassic of South Africa

Galesaurus is an extinct genus of carnivorous cynodont therapsid that lived between the Induan and the Olenekian stages of the Early Triassic in what is now South Africa. It was incorrectly classified as a dinosaur by Sir Richard Owen in 1859.

Infratemporal fossa Cavity that is part of the skull

The infratemporal fossa is an irregularly shaped cavity that is a part of the skull. It is situated below and medial to the zygomatic arch. It is not fully enclosed by bone in all directions. It contains superficial muscles, including the lower part of the temporalis muscle, the lateral pterygoid muscle, and the medial pterygoid muscle. It also contains important blood vessels such as the middle meningeal artery, the pterygoid plexus, and the retromandibular vein, and nerves such as the mandibular nerve (CN V3) and its branches.

Deep cervical fascia

The deep cervical fascia lies under cover of the platysma, and invests the muscles of the neck; it also forms sheaths for the carotid vessels, and for the structures situated in front of the vertebral column. Its attachment to the hyoid bone prevents the formation of a dewlap.

Buccal space

The buccal space is a fascial space of the head and neck. It is a potential space in the cheek, and is paired on each side. The buccal space is superficial to the buccinator muscle and deep to the platysma muscle and the skin. The buccal space is part of the subcutaneous space, which is continuous from head to toe.

Coronoid process of the mandible

In human anatomy, the mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size. Its anterior border is convex and is continuous below with the anterior border of the ramus. Its posterior border is concave and forms the anterior boundary of the mandibular notch. The lateral surface is smooth, and affords insertion to the temporalis and masseter muscles. Its medial surface gives insertion to the temporalis, and presents a ridge which begins near the apex of the process and runs downward and forward to the inner side of the last molar tooth.

The montane African climbing mouse or remarkable climbing mouse is a species of rodent in the family Nesomyidae. It is found in Democratic Republic of the Congo, Kenya, Rwanda, Tanzania, and Uganda.

Facial lymph nodes

The facial lymph nodes comprise three groups:

Zygomatic plate

In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch (cheekbone). At the back, it connects to the front (maxillary) root of the zygomatic arch, and at the top it is connected to the rest of the skull via the antorbital bridge. It is part of the maxillary bone, or upper jaw, which also contains the upper cheekteeth. Primitively, rodents have a nearly horizontal zygomatic plate. In association with specializations in zygomasseteric system, several distinct morphologies have developed across the order.

Submasseteric space

The submasseterric space is a fascial space of the head and neck. It is a potential space in the face over the angle of the jaw, and is paired on each side. It is located between the lateral aspect of the mandible and the medial aspect of the masseter muscle and its investing fascia. The term is derived from sub- meaning "under" in Latin and masseteric which refers to the masseter muscle. The submasseteric space is one of the four compartments of the masticator space. Sometimes the submasseteric space is described as a series of spaces, created because the masseter muscle has multiple insertions that cover most of the lateral surface of the ramus of the mandible.

References

  1. ↑ {* Bell, Sean D. (2004). Aplodontid, Scuirid, Castorid, Zapodid and Geomyoid rodents of the Rodent Hill locality, Cypress Hills Formation, Southwest Saskatchewan, Master of Science thesis, Geological Sciences. Saskatoon: University of Saskatchewan. Retrieved 10 February 2017.
  2. 1 2 3 4 5 Wood, Albert E. (1965). "Grades and Clades Among Rodents". Evolution. 19: 115–130. doi: 10.1111/j.1558-5646.1965.tb01696.x .
  3. 1 2 3 4 Korth, William W.; Emry, Robert J. (1991). "The skull of Cedromus and a review of the Cedromurinae (Rodentia, Sciuridae)". Journal of Paleontology. 65 (6): 984–994. doi:10.1017/S0022336000033291.
  4. 1 2 3 4 5 6 Hautier, Lionel; Michaux, Jaques; Marivaux, Laurent; Vianey-Liaud, Monique (2008). "Evolution of the zygomasseteric construction in Rodentia, as revealed by a geometric morphometric analysis of the mandible of Graphiurus (Rodentia, Gliridae)". Zoological Journal of the Linnean Society. 154 (4): 807–821. doi: 10.1111/j.1096-3642.2008.00453.x .
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