Hystricognathi

The Hystricognathi are an infraorder of rodents, distinguished from other rodents by the bone structure of their skulls. The masseter medialis (a jaw muscle) passes partially through a hole below each eye socket (called 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.

The 18 families within the Hystricognathi are divided into two parvorders, the Phiomorpha and the Caviomorpha. The Caviomorpha are mostly native to South America, with a few species in the Caribbean and North America, while the Phiomorpha occur in the Old World.

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.

Phiomorphan hystricognath families

 * Bathyergidae (African mole rats)
 * Heterocephalidae (naked mole rats, monotypic taxon)
 * Hystricidae (Old World porcupines)
 * Petromuridae (dassie rat, monotypic taxon)
 * Thryonomyidae (cane rats)

Caviomorphan hystricognath families

 * Abrocomidae (chinchilla rats)
 * Capromyidae (hutias)
 * Caviidae (guinea pigs, wild cavies, and capybaras)
 * Chinchillidae (chinchillas and viscachas)
 * Ctenomyidae (tuco-tucos)
 * Cuniculidae (pacas)
 * Dasyproctidae (agoutis and acouchis)
 * Dinomyidae (pacaranas and their fossil relatives, including some of megafaunal size)
 * Echimyidae (spiny rats)
 * Erethizontidae (New World porcupines)
 * Myocastoridae (coypu)
 * Octodontidae (13 species including the degus)