The term Hystricomorpha has had many definitions throughout its history. In the broadest sense it refers to any rodent (except dipodoids) with a hystricomorphous zygomasseteric system. This includes the Hystricognathi, Ctenodactylidae, Anomaluridae, and Pedetidae. Molecular and morphological results suggest that the inclusion of the Anomaluridae and Pedetidae in Hystricomorpha may be suspect. Based on Carleton and Musser (2005), these two families are treated here as representing a distinct suborder Anomaluromorpha.

The modern definition of Hystricomorpha also known as Entodacrya or Ctenohystrica is a taxonomic hypothesis uniting the gundis with the hystricognath rodents (Carleton and Musser, 2005). There is considerable morphological support for this relationship and strong molecular support. If true, this hypothesis renders the traditional view of Sciurognathi invalid as it becomes a paraphyletic group.

The hystricomorph rodents, or at least members of Caviomorpha, are sometimes regarded as non-rodents (Graur et al., 1991; D'Erchia et al., 1996; Reyes et al., 2000). Most molecular and genetic research however confirms the monophyly of rodents (Cao et al., 1994; Kuma and Miyata, 1994; Sullivan and Swofford, 1997; Robinson-Rechavi et al., 2000; Lin et al., 2002; Reyes et al., 2004). Support for rodent polyphyly appears to be a product of long branch attraction (Bergsten, 2005).

Hystricomorph rodents appeared in South America in the early Oligocene (Flynn et al., 2003), and moved to a continent which previously had marsupials, xenarthrans, and meridiungulates as the only resident non-flying mammals. The same view on early migration might be true to Primates, which also appeared in South America before the Great American Interchange. All of this is still controversial, and new scientific discoveries on this subject are published regularly.


The following list of families is based on the taxonomy of Marivaux et al. (2002; 2004) who subjected a number of early fossil rodents to parsimony analysis and recovered support for the Hystricomorpha or Entodacrya hypothesis. Their results rendered the suborder Sciuravida as defined by McKenna and Bell (1997) to be polyphyletic and invalid. The symbol "†" is used to indicate groups where no living members survive.


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