Hystricomorpha

Hystricomorpha is a major clade within the order Rodentia, historically treated as a suborder or infraorder. It comprises the hystricognathous rodents, a group characterized by distinctive jaw mechanics in which the masseter muscle passes through a relatively large infraorbital foramen and the dentary shows a pronounced angular process. This hystricognathous condition distinguishes many members from other rodent groups that exhibit different gnathic configurations.

The clade includes several diverse families, among them the Old World porcupines (Hystricidae), New World porcupines (Erethizontidae), capybaras and their relatives (Caviidae), chinchillas and viscachas (Chinchillidae), and other South American hystricomorphs such as agoutis and pacas (Dasyproctidae, Cuniculidae). Hystricomorphs occupy a wide range of habitats and are distributed across Africa, Asia, Europe, and the Americas, with particularly high species diversity in tropical regions of the Americas and Africa.

Fossil and molecular evidence place Hystricomorpha as a distinct rodent lineage that originated in the early Cenozoic, with an early Paleogene to late Paleocene emergence in some estimates. The group underwent substantial diversification through the Eocene and later radiations, aided by continental connections and ecological opportunities in various biomes. Hystricomorpha is viewed as monophyletic by contemporary phylogenies, though the exact relationships among its constituent families continue to be refined as new data emerge.

Ecology and human interactions vary widely within Hystricomorpha. Many species are herbivorous or omnivorous and play important roles in seed dispersal and ecosystem dynamics. Some, such as capybaras and guinea pigs, have economic or cultural significance for humans, while others face threats from habitat loss and hunting.