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Hystricomorpha (from Greek ὕστριξ, hystrix 'porcupine' and Greek μορφή, morphē 'form') is a term referring to families and orders of rodents which has had many definitions throughout its history. In the broadest sense, it refers to any rodent (except Dipodoidea) with a hystricomorphous zygomasseteric system. This includes the Hystricognathi, Ctenodactylidae, Anomaluridae, and Pedetidae. Molecular and morphological results suggest the inclusion of the Anomaluridae and Pedetidae in Hystricomorpha may be suspect. Based on , these two families are discussed here as representing a distinct suborder Anomaluromorpha.
The hystricomorph rodents, or at least members of Caviomorpha, are sometimes not regarded as rodents.; ; Most molecular and genetic research, however, confirms the monophyly of rodents.; ; ; ; ; Support for rodent polyphyly appears to be a product of long branch attraction.
Hystricomorph rodents appeared in South America in the Eocene, a continent which previously had , , and Meridiungulata as the only resident nonflying mammals. They apparently arrived by rafting across the Atlantic Ocean from Africa. The same type of migration may have occurred with primates, which also appeared in South America in the Eocene when it was an isolated continent, long before the Great American Interchange. All of this is still controversial, and new scientific discoveries on this subject are published regularly.
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