Hystricomorphs

Hystricomorphs, or hystricognath rodents, are a major clade within Rodentia. They are distinguished by a distinctive jaw musculature, notably a large infraorbital foramen through which part of the deep masseter muscle passes to the rostrum, a pattern called hystricomorphous. This arrangement is associated with a robust skull and various dental adaptations in many members.

Living hystricomorphs include the Old World porcupines (Hystricidae) and New World porcupines (Erethizontidae), as well as a diverse array of caviomorphs such as guinea pigs, capybaras, chinchillas, viscachas, agoutis, acouchis, and pacas. Together, these groups occupy a wide range of habitats and display substantial ecological diversity, from arboreal species to large, semi-aquatic herbivores.

Habitats span forests, savannas, deserts, and wetlands across Africa, Asia, Europe, and the Americas. They are generally herbivorous, though some species take small amounts of animal matter. The nutria (Myocastor coypus) is a notable introduced species in several regions, illustrating the ecological impacts some hystricomorphs can have outside their native ranges.

Evolutionarily, hystricomorphs are one of the three major rodent lineages, diverging from other rodents in the Paleogene. They likely originated in Africa, with caviomorphs reaching South America by transoceanic dispersal, producing a rich radiation in the New World. Today, hystricomorphs are widespread in many biomes of Africa, Asia, Europe, and the Americas, while Australia and Antarctica lack native representatives.

Conservation concerns affect several species due to habitat loss, hunting, and competition with humans. Hystricomorphs remain important for understanding rodent evolution, systematics, and ecological roles in many ecosystems.