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Anatomy
'Hystricognathi' is an
infraorder of
rodents. Hystricognaths are distinguished from other rodents by the
bone structure of their
skulls. The
masseter medialis (jaw muscle) passes partially through the
infraorbital foramen and connects to the bone on the opposite side. This, together with their lack of an
infraorbital plate and the relative size of the
infraorbital foramen, distinguishes hystricognaths from other rodent groups.
There are 18
families within the Hystricognathi, divided into two
infraorders, the
Phiomorpha and the
Caviomorpha. The Caviomorpha are mostly native to
South America, with a few species in
North America, while the Phiomorpha occur in the
Old World.

Skull of a
Nutria demonstrating the hystricognathous lower jaw and hystricomorphous zygomasseteric system
Behavior
Play behavior has been observed in seven hystricognath families. The caviomorphs chase each other, play-wrestle, and gallop. The longer-legged species chase more often than the shorter-legged species. They also rotate their heads and body muscles as a form of play (Fagen 1981).
Phiomorphan hystricognath familiæ
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Bathyergidae
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Hystricidae
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Petromuridae
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Thryonomyidae
Caviomorphan hystricognath familiæ
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Abrocomidae
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Agoutidae
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Capromyidae
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Caviidae
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Chinchillidae
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Ctenomyidae
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Dasyproctidae
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Dinomyidae
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Echimyidae
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Erethizontidae
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Myocastoridae
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Octodontidae
External links
http://animaldiversity.ummz.umich.edu/site/topics/mammal_anatomy/rodent_jaws.html
References
Fagen, Robert. ''Animal Play Behavior''. Oxford University Press, 1981.
See also
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Sciurognathi
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Hystricomorpha