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 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 North America, while the Phiomorpha occur in the Old World. 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.