Zygomatic arch masseter construction of rodents

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The zygomatic arch masseter design in rodents describes the group-specific expression of the masseter muscle combined with redesigns on the skull, which particularly affect the formation of a zygomatic arch plate and the shape and size of the infraorbital foramen at the front end of the zygomatic arch . A distinction is made between the protrogomorphic, sciuromorphic, hystricomorphic and myomorphic types.

In the protrogomorphic type, the masseter muscle remains unchanged. In the sciuromorphic type, on the other hand, its outer part extends to the muzzle area, in the hystricomorphic type its middle part extends through the infraorbital foramen to the muzzle area and in the myomorphic type, the transformations of the sciuromorphic and hystricomorphic types are combined with one another.

The sciuromorphic, hystricomorphic and myomorphic types once served as the basis for dividing rodents into suborders. However, they were acquired several times independently of one another and are only used today as important descriptive terms with no systematic meaning. Transitions from one form to another are possible and not every rodent can be clearly assigned to one of the four types.

designation

The naming of the masticatory muscles of rodents is very inconsistent. In the following, a distinction is made between the superficial masseter muscle ( muscle masseter superficialis ), the underlying outer masseter muscle ( muscle masseter lateralis ) and the lower lying middle masseter muscle ( muscle masseter medialis ). Particular attention is paid to the anterior section ( deep pars or anterior pars ) of the outer masseter muscle, which may arise from the zygomatic arch plate, and the anterior section ( anterior pars ) of the middle masseter muscle that may pass through the infraorbital foramen .

The superficial masseter muscle is, however, also referred to as the outer part, the outer masseter muscle as the middle part and the middle masseter muscle as the deep part (then also musculus masseter profundus ) of the masseter muscle. In addition, the average masseter muscle often than is muscle zygomaticomandibularis the masseter muscle distinction and its front section, if it is very strong and through the infraorbital foramen passes, muscle maxillomandibularis called.

general structure

As an adaptation to the largely functional independence of gnawing teeth and chewing teeth, the masseter muscle in rodents is differentiated into three main parts, which can also be understood as independent muscles. These are the superficial part, which retains the usual position for other mammals, as well as the outer and middle layers of its deep part. Originally the masseter muscle originates on the underside of the zygomatic arch and, as the closer of the lower jaw, runs almost vertically or slightly obliquely to the outside of the ascending branch of the lower jaw. The superficial masseter muscle and the outer masseter muscle run very flat to the angular area of ​​the lower jaw, while the middle masseter muscle attaches further up on the lower jaw.

Even in the most primitive rodents, the anterior zygomatic arch root is further forward than in the original mammals. By shifting its origin further forward to the area of ​​the snout, the masseter muscle takes on a more and more horizontal course. In addition, the lengthening of its fiber bundles improves the possibility of shortening it and it becomes the most important puller of the lower jaw. Originally presumably redesigned as an adaptation for more effective gnawing, with the acquisition of longitudinal chewing, it also becomes an important antagonist of the temporal muscle that retracts the lower jaw . With the middle masseter muscle, it also remains the most important closer of the lower jaw.

Building types

Extinct groups of rodents are marked with a cross (†).

Protrogomorphism

In the protrogomorphic type, the masticatory muscles do not show any particular specializations and the masseter muscle arises on the underside of the zygomatic arch. The superficial masseter muscle arises at the anterior zygomatic arch root, while the outer masseter muscle extends along the zygomatic arch to much further back on the zygomatic bone. Since its anterior section does not advance onto the facial part of the zygomatic arch root, the zygomatic arch plate remains narrow. The middle masseter muscle forms the smallest part of the masseter complex. It arises just behind the superficial masseter muscle and does not reach back any further than the middle of the zygomatic arch. The infraorbital foramen remains small. However, it is often large enough to allow fibers of the middle masseter muscle to pass through it.

The protrogomorphic type resembles the expression in most higher mammals . It is considered to be an original feature and as such only occurs today in the stump-tailed squirrel . He also can be found in most † Ischyromyidae , fossil Aplodontiidae , the † Mylagaulidae , some fossil croissants , some fossil Bilchen , the † Sciuravidae , the † Cylindrodontidae and † Tsaganomyidae again. In most sand graves , it has developed as a derivative of the hystricomorphic type. In the stubby-tailed squirrel, a fiber bundle from the middle masseter muscle passes through the infraorbital foramen .

Sciuromorphy

In the sciuromorphic or squirrel-like type, the superficial masseter muscle arises in front on the underside of the snout below the infraorbital foramen . By inclining and widening the lower edge of the anterior zygomatic arch root forms a wide, triangular zygomatic arch plate and the outer masseter muscle extends to the area of ​​the snout. Thus, its anterior section moves far past the zygomatic arch root to the edge of the eye socket of the upper jaw, forward and up onto the zygomatic arch plate. The middle masseter muscle runs similar to the protrogomorphic type and originates in the middle of the zygomatic arch. The infraorbital foramen is narrow.

The sciuromorphic type has developed independently at least three or four times and occurs in some † Ischyromyidae , most squirrels , † Eutypomyidae , beavers , † Eomyidae , † Heliscomyidae , † Florentiamyidae , pocket mice and pocket rats . In croissants and beavers, the superficial masseter muscle arises from a small process or a thickening of the bone laterally and below the infraorbital foramen . This is smaller than in the protrogomorphic type, shifted downwards and compressed laterally by the outer masseter muscle. In pocket mice, the superficial masseter muscle does not form a bone process on the upper jaw. The infraorbital foramen is shifted forward to the middle of the snout and not reduced in size, but also compressed laterally. The middle masseter muscle does not or hardly passes through the infraorbital foramen in the crescent . In beavers, pocket rats and pocket mice, it does not pass through it either.

Independently of the rodents, a similar type has convergedly developed in some South American ungulates , namely among others in Paedotherium and Tremacyllus from the rodent-like group of the Pachyrukhinae within Typotheria .

Hystricomorphia

In the hystricomorphic or porcupine-like type, the superficial masseter muscle arises in front of the zygomatic arch. The outer and middle masseter muscles run roughly parallel to each other. The external masseter muscle is unspecialized and, as in the protrogomorphic type, arises almost along the entire length of the zygomatic arch. The anterior section of the greatly enlarged middle masseter muscle emerges from below through the greatly expanded infraorbital foramen and pushes itself forward onto the lateral muzzle area. In extreme cases, the infraorbital foramen is about the same size as the eye socket.

The hystricomorphic type is found among the sciurognathic rodents in the † Theridomyidae , the African dormouse , the Pedetidae , the thorntail squirrel , the † Protoptychidae , the † Simimyidae , the † Armintomyidae , the jerboa , some fossil mouse species and again the crested fingers . It identifies the porcupine relatives and is found in the first porcupines , rock rats , reed rats and guinea pig relatives as well as in fossil sand graves . However, not all hystricomorphic skulls have the same structure. In porcupine relatives, for example, the very wide, round-oval foramen infraorbitale is encompassed below and laterally by two processes of the upper jaw, of which the lower front-outside merges into a planum infraorbitale . At the bottom of the foramen, a bone ridge separates the vascular and nerve compartment in the middle from the muscle compartment. The infraorbital foramen is also enlarged in jerboa, but in the middle of it there is a smaller, additional foramen that allows the vessels and nerves to pass through separately from the masseter muscle.

Myomorphism

In the myomorphic or mouse-like type, the transformations of the sciuromorphic and the hystricomorphic type are combined with one another. As in the sciuromorphic type, the superficial masseter muscle arises at a point below the infraorbital foramen . The outer masseter muscle behaves similarly to the protrogomorphic type and arises on the zygomatic arch, but its anterior section extends forwards and upwards onto the mostly vertical zygomatic arch plate, which forms the lateral wall of the infraorbital canal . The middle masseter muscle pulls under the zygomatic arch into the eye socket and, with the typical expression, its anterior section pushes through the upper, enlarged part of the approximately keyhole-shaped, moderately enlarged foramen infraorbitale onto the facial surface of the upper jaw.

The zygomatic arch plate that accommodates the external masseter muscle is, however, expanded differently in different genera of myomorphic rodents and compresses the infraorbital foramen , which is usually limited to the upper half of the snout, to varying degrees laterally. In some representatives, the external masseter muscle does not extend up to the foramen and therefore does not affect its shape. In others, it almost reaches the top of the snout and the foramen is laterally compressed and slit-like. In still others, only the lower half of the foramen is compressed by the zygomatic plate. However, the foramen is never as large as in the hystricomorphic type.

The myomorphic type arose several times independently of one another from the hystricomorphic type in the mice . In most of the dormice it has developed from the protrogomorphic type and is also referred to as pseudomyomorphism or pseudosciuromorphism. In this form, the infraorbital foramen is relatively small and compressed. The remodeling of the † Ischyromyinae and the † Cedromurinae are also understood as myomorphism derived from the protrogomorphic type.

literature

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Individual evidence

  1. ↑ a b c d e f g h Storch, 1988 (p. 7)
  2. ↑ a b Carroll, 1988 (p. 490)
  3. ↑ a b Vaughan and coworkers, 2000 (p. 297)
  4. ↑ a b c Pflumm, 1996 (pp. 462–463)
  5. ↑ a b c d e f g Korth, 1994 (p. 9)
  6. ↑ a b c d e f g h Starck, 1995 (p. 600)
  7. ↑ a b Carleton, in Anderson and Jones, 1984 (p. 257)
  8. ↑ Müller, 1989 (Fig. 727A, p. 665)
  9. ↑ a b c Starck, 1995 (p. 599)
  10. ↑ a b c d e f g h i j Starck, 1995 (p. 610)
  11. ↑ Müller, 1989 (p. 666)
  12. ↑ a b c Wood, in Luckett and Hartenberger, 1985 (p. 477)
  13. ↑ a b c Maier, 2004 (p. 534)
  14. ↑ a b Korth, 1994 (p. 10)
  15. ↑ a b McLaughlin, in Anderson and Jones, 1984 (p. 267)
  16. ↑ Korth, 1994 (p. 37)
  17. ↑ Korth, 1994 (p. 85)
  18. ↑ Korth, 1994 (p. 99)
  19. ↑ Korth, 1994 (p. 122)
  20. ↑ a b Vianey-Liaud, in Luckett and Hartenberger, 1985 (p. 293)
  21. ↑ Korth, 1994 (p. 55)
  22. ↑ Korth, 1994 (p. 67)
  23. ^ Wood, in Luckett and Hartenberger, 1985 (p. 492)
  24. ↑ Luckett and Hartenberger, in Luckett and Hartenberger, 1985 (pp. 692–693)
  25. ↑ a b c d Korth, 1994 (pp. 10–11)
  26. ↑ a b Pflumm, 1996 (p. 464)
  27. ^ Wahlert, in Luckett and Hartenberger, 1985 (p. 315)
  28. ↑ a b Korth, 1994 (p. 111)
  29. ↑ Korth, 1994 (p. 125)
  30. ↑ Korth, 1994 (p. 135)
  31. ↑ Korth, 1994 (p. 149)
  32. ↑ Korth, 1994 (p. 163)
  33. ↑ Korth, 1994 (p. 189)
  34. ↑ Korth, 1994 (p. 173)
  35. ↑ Korth, 1994 (p. 199)
  36. ↑ McLaughlin, in Anderson and Jones, 1984 (p. 271)
  37. ↑ Ellerman, 1940 ( p. 44 )
  38. ↑ Marcos D. Ercoli, Alicia Álvarez and Adriana M. Candela: Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates. Communications Biology 2, 2019, p. 202, doi: 10.1038 / s42003-019-0423-5
  39. ↑ a b c d Korth, 1994 (pp. 11–12)
  40. ↑ Pflumm, 1996 (p. 466)
  41. ^ Wood, in Luckett and Hartenberger, 1985 (p. 494)
  42. ↑ Klingener, in Anderson and Jones, 1984 (p. 381)
  43. ↑ a b c d Vianey-Liaud, in Luckett and Hartenberger, 1985 (p. 289)
  44. ↑ Korth, 1994 (p. 77)
  45. ↑ Korth, 1994 (p. 249)
  46. ↑ Korth, 1994 (p. 252)
  47. ↑ Flynn and co-workers, in Luckett and Hartenberger, 1985 (pp. 592–593)
  48. ^ Wood, in Luckett and Hartenberger, 1985 (pp. 479-480)
  49. ↑ a b c Korth, 1994 (pp. 12-13)
  50. ↑ Pflumm, 1996 (p. 465)
  51. ↑ Vianey-Liaud, in Luckett and hard Berger, 1985 (pp 296-297)
  52. ↑ a b Vianey-Liaud, in Luckett and Hartenberger, 1985 (p. 305)
  53. ↑ Holden, 2005 ( Gliridae , p. 819)
  54. ↑ Korth, 1994 (p. 43)