193:. 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.
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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
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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
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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,
149:. 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.
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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
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Aplodontid, Scuirid, Castorid, Zapodid and
Geomyoid rodents of the Rodent Hill locality, Cypress Hills Formation, Southwest Saskatchewan, Master of Science thesis, Geological Sciences
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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.
447:"Evolution of the zygomasseteric construction in Rodentia, as revealed by a geometric morphometric analysis of the mandible of Graphiurus (Rodentia, Gliridae)"
133:) are considered secondarily protrogomorphous since their zygomatic condition is clearly derived from a hystricomorphous ancestor. The
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291:: in the latter it is often referred to as pseudomyomorphy). suggest that the infraorbital foramen of the extinct
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any rodent zygomasseteric system, which might explain the success of the cosmopolitan
Muroidea.
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demonstrating the hystricognathous lower jaw and hystricomorphous zygomasseteric system.
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is small. The superficial masseter originates on the lateral surface of the anterior
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Hautier, Lionel; Michaux, Jaques; Marivaux, Laurent; Vianey-Liaud, Monique (2008).
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Bones & muscles in rodents' heads that enable them to chew front-to-back
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and inserts along the ventral margin of the angular process of the
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Korth, William W.; Emry, Robert J. (1991). "The skull of
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This condition is found in most members of the family
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may be too technical for most readers to understand
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213:This condition is found throughout the suborders
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327:. Saskatoon: University of Saskatchewan
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97:is the anatomical arrangement of the
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357:"Grades and Clades Among Rodents"
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401:Journal of Paleontology
163:oriental giant squirrel
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329:. Retrieved
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297:Cedromurinae
289:Sciuromorpha
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250:Sciuromorpha
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179:Sciuromorpha
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153:Sciuromorphy
131:Bathyergidae
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367:: 115–130.
331:10 February
283:) and most
205:Skull of a
479:Categories
307:References
295:subfamily
245:Graphiurus
227:Dipodoidea
191:Geomyoidea
189:, and the
183:Castoridae
177:(suborder
421:132211896
361:Evolution
281:Myomorpha
260:Myomorphy
233:(such as
223:Myomorpha
175:Sciuridae
167:zygomatic
63:July 2017
397:Cedromus
285:Gliridae
277:Muroidea
241:dormouse
231:Muroidea
187:Eomyidae
147:mandible
129:(family
127:molerats
293:sciurid
169:region.
143:maxilla
135:rostrum
125:). The
95:rodents
49:Please
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269:Gerbil
207:nutria
185:, the
490:Skull
417:S2CID
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333:2017
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