Zygomasseteric system

The zygomatic - masseter construction referred associated with alterations in the skull, relating in particular to the formation of a Jochbogenplatte and the shape and size of the infraorbital foramen at the anterior end of the zygomatic arch in rodents, the group-specific expression of the masseter muscle. Differences are the protrogomorphe, the sciuromorphe, the hystricomorphe and myomorphe type.

When protrogomorphen type of masseter muscle remains unchanged. When sciuromorphen type, its outer portion, however, moves in front to the snout region, when hystricomorphen type accesses its average share through the infraorbital foramen through stepping on the nose area and on the myomorphen type the transformations of sciuromorphen and hystricomorphen type are combined.

The sciuromorphe, the hystricomorphe and myomorphe type once served as the basis for the subdivision of rodents in submissions. However, they were repeatedly acquired independently of each other and are now used only as an important descriptive terms without systematic significance. Transitions from one expression to another is possible and not any rodent can be unambiguously assigned to one of four building types.

Designation

The naming of the masticatory muscles of rodents is very uneven. Below is between the superficial masseter muscle (musculus masseter superficialis ), the underlying outer masseter muscle (musculus masseter lateralis) and the deeper middle masseter muscle (musculus masseter medialis) distinguished. Particular attention will be given if the springing of the Jochbogenplatte front portion ( pars profundus or pars anterior ) of the external masseter muscle and the optional infra-orbital foramen passing through, forward portion ( pars anterior ) of the middle masseter muscle.

The superficial masseter muscle but ( then also masseter profundus ) also referred to as the outer portion, the outer masseter muscle as the mean percentage and the mean masseter muscle as a deep portion of the masseter muscle. In addition, the mean masseter muscle is often distinguished as muscle zygomaticomandibularis the masseter muscle and its front section, if it is very strong and infraorbital through the foramen passes, called musculus maxillomandibularis.

General Structure

As an adaptation to the degree of functional independence of gnawing teeth and Kaugebiss the masseter muscle is differentiated in rodents into three main parts, which can also be regarded as independent muscles. These are the superficial portion, retains the usual location for other mammals, as well as the outer and the middle layer portion of its depth. Originally springs from the masseter muscle at the bottom of the zygomatic arch and runs as normally the lower jaw nearly vertical or slightly oblique to the outer side of the ascending ramus. Here, the superficial masseter muscle and the external masseter muscle to run very shallow angle region of the mandible, while the middle masseter muscle attaches on top of the lower jaw.

The front Jochbogenwurzel is already at the most primitive rodents further forward than in the original mammals. By further forward displacement of its origin on the muzzle area of ​​the masseter muscle occupies a more and more horizontal course. In addition, the extension of its fiber bundles improves the shortening option and it becomes the main advancer of the mandible. Originally probably redesigned as an adjustment for a more effective gnawing, he is with purchase of the longitudinal chewing also an important opponent of the mandible retreating temporal muscle. With the middle masseter muscle also he remains the main contact of the lower jaw.

Building types

Extinct rodent groups are marked with a dagger († ).

Protrogomorphie

When protrogomorphen type, the masticatory muscles on any particular specializations and the masseter muscle arises from the Jochbogenunterseite. Here, the superficial masseter muscle arises from the anterior Jochbogenwurzel while the external masseter muscle along the zygomatic arch extends much further back on the cheekbone. Since its front portion not advancing on the face of the Jochbogenwurzel that Jochbogenplatte remains narrow. The mean masseter muscle is the smallest part of Masseterkomplexes. It rises just behind the superficial masseter muscle and extends back no further than back to Jochbogenmitte. The infraorbital foramen is small. However, it is often large enough to allow fibers of the middle masseter muscle can pass.

The protrogomorphe type is similar to the expression in most higher mammals. He is regarded as a primitive character and, as such, is now only the stump of a tail squirrel before. In addition, it is found in most † Ischyromyidae, fossil Aplodontiidae, the † Mylagaulidae, some fossil squirrel, some fossil Bilchen, the † Sciuravidae, the Cylindrodontidae † and † Tsaganomyidae again. In most sandy graves he has developed as a derived character from the hystricomorphen type. When stubby tail squirrel a fiber bundle of the middle masseter muscle passes through the infraorbital foramen through.

Sciuromorphie

When sciuromorphen or croissant -like type of the superficial masseter muscle arises from the front of the snout bottom below the infraorbital foramen. By inclination and widening of the lower edge of the front Jochbogenwurzel forms a broad, triangular Jochbogenplatte and the external masseter muscle extending to the snout area. Thus, its front portion pushes away at the Jochbogenwurzel by pulling up to the orbital rim of the upper jaw to the front - top of the Jochbogenplatte ago. The mean masseter muscle is similar to the protrogomorphen type and rises in the Jochbogenmitte. The infraorbital foramen is narrow.

The sciuromorphe type has at least three or four times independently developed and is found in some † Ischyromyidae, most squirrels, the † Eutypomyidae, the beavers, the † Eomyidae, the † Heliscomyidae, the † Florentiamyidae, the pocket mice, and gophers. The Squirrel and the beavers of the superficial masseter muscle on a small extension or a thickening of the bone arises from the side and below the infraorbital foramen. This is less than in protrogomorphen type, shifted downward and laterally compressed by the external masseter muscle. In the pocket mice of the superficial masseter muscle forms a bone extension on the upper jaw. The infraorbital foramen is moved forward to the middle of snout and not reduced, but also laterally compressed. The mean masseter muscle does not occur in the squirrel or hardly infraorbital through the foramen through. When the beavers, the pocket gophers and pocket mice, he also passes not through it.

Hystricomorphie

When hystricomorphen or porcupine -like type of the superficial masseter muscle arises from the front of the zygomatic arch. The outer and middle masseter muscle run approximately parallel to each other. The outer masseter muscle is unspecialized and springs like the type protrogomorphen almost along the entire length of the zygomatic arch. The front portion of the greatly enlarged middle masseter muscle occurs from below through the greatly expanded infraorbital foramen through it and moves on to the lateral snout area before. In extreme cases, the foramen infraorbital about the size of the eye socket.

The hystricomorphe type can be found under the sciurognathen rodents at the † Theridomyidae, the African Bilchen, the Pedetidae, the mandrel tail Squirrel, the † Protoptychidae, the † Simimyidae, the † Armintomyidae, the jumping mice, some fossil mice -like and the comb fingers again. It features the porcupine relatives and already occurs in the first porcupine, rock rats, cane rats and guinea pigs as well as relatives of fossil sand graves. However, all hystricomorphen skulls are not created equal. Thus, for the porcupine relatives the very wide, round-oval foramen infraorbital below and laterally covered by two appendages of the upper jaw, of which merges lower front exterior infraorbital in a subgrade. At the bottom of the foramen a bony ridge separates the ward informed located vascular and nerve specialist incomplete from the muscle compartment. The gerbils foramen infraorbital is also increased, but is communicated forward to this a smaller, extra foramen, which can pass through the isolated vessels and nerves from the masseter muscle.

Myomorphie

When myomorphen or mice like the type of transformations sciuromorphen and hystricomorphen type are combined. The superficial masseter muscle arises as in sciuromorphen type at a location below the infraorbital foramen. The outer masseter muscle behaves similarly to the protrogomorphen type and originates at the zygomatic arch, however, interfering with its front portion to the front - top of the most vertically standing Jochbogenplatte about which forms the lateral wall of the infraorbital canal. The mean masseter muscle pulls under the zygomatic arch into the orbit and its front portion pushes at the typical expression through the upper, expanded portion of the approximately keyhole-shaped, moderately enlarged infraorbital foramen through stepping on the facial surface of the maxilla before.

The outer masseter muscle receiving Jochbogenplatte however myomorpher rodents expanded differently in different genres and pushes the usually on the upper half of the snout limited infraorbital foramen laterally varying degrees together. Thus, the external masseter muscle extends in some representatives upwards not to the foramen and therefore has no effect on the shape of. For other it extends almost to the top of the snout and the foramen is laterally compressed and slit-like. In still other, only the lower half of the foramen is compressed by the Jochbogenplatte. Never, however, the foramen is as large as in hystricomorphen type.

The myomorphe type is repeatedly emerged in the mice -like independently from the hystricomorphen type. In most Bilchen it has developed from the protrogomorphen type and is also referred to as Pseudomyomorphie or Pseudosciuromorphie. In this form the foramen infraorbital relatively small and compressed. The transformations in the Ischyromyinae † † and the Cedromurinae also be regarded as derived from protrogomorphen type Myomorphie.

Used literature

• Sydney Anderson, J. Knox Jones Jr. (ed.): Orders and Families of Recent Mammals of the World. John Wiley & Sons, New York / Chichester / Brisbane / Toronto / Singapore, 1984, ISBN 0 - 471-08493 -X. Michael D. Carleton: Introduction to Rodents. Pp. 255-265.
• David Klingener: Gliroid and Dipodoid Rodents. Pp. 381-388.
• Charles McLaughlin: Protrogomorph, Sciuromorph, Castorimorph, Myomorph ( Geomyoid, Anomaluroid, Pedetoid, and Ctenodactyloid ) Rodents. Pp. 267-288.

• Lawrence J. Flynn, Louis L. Jacobs, Everett H. Lindsey: Problems in Muroid Phylogeny: Relationship to Other Rodents and Origin of Major Groups. Pp. 589-616.
• W. Patrick Luckett Jean-Louis Hartberger: Evolutionary Relationships among Rodents: Comments and Conclusions. Pp. 685-712.
• Monique Vianey - Liaud: Possible Evolutionary Relationships among Eocene and Lower Oligocene Rodents of Asia, Europe and North America. Pp. 277-309.
• John H. Wahlert: Cranial foramina of Rodents. Pp. 311-332.
• Albert E. Wood: The Relationships, Origin and Dispersal of the Hystricognathous Rodents. Pp. 475-513.

Comments

• Anatomy of rodents