Hystricognathi

Hystricognathi is an infraorder within the order Rodentia that unites a diverse group of hystricomorph rodents. The clade includes the familiar Old World porcupines (Hystricidae) and New World porcupines (Erethizontidae), as well as a large radiation of South American rodents known as caviomorphs. Among the latter are capybaras (Hydrochoeridae), guinea pigs and their relatives (Caviidae), chinchillas and viscachas (Chinchillidae), agoutis and acouchis (Dasyproctidae), degus and allies (Octodontidae), spiny rats (Echimyidae), tuco-tucos (Ctenomyidae), and the nutria (Myocastoridae), as well as other families such as Cuniculidae (pacas) and related groups. This broad assemblage is distributed across Africa, Asia, Europe and the Americas, reflecting a long and complex evolutionary history.

A defining feature of Hystricognathi is the hystricognathous jaw, a distinctive structural arrangement of the lower jaw in which the angular process and the masseter muscles are configured differently from other rodents. This dental and jaw morphology is used to differentiate Hystricognathi from other rodent groups in many classifications and has been linked to their diverse feeding strategies and ecological roles.

Evolutionarily, Hystricognathi originated in Africa during the Paleogene, with the caviomorph branch dispersing to South America and diversifying after transoceanic dispersal during early epochs. Today, the group occupies a wide range of habitats, from tropical forests to deserts and grasslands. Ecologically, many hystricognaths are herbivorous, with some lineages adapted to fossorial or semi-aquatic lifestyles, illustrating the group's extensive morphological and ecological diversity.