showing the enlarged
infraorbital canal present in most members of the Hystricomorpha: This condition is termed
hystricomorphy. The modern definition of Hystricomorpha, also known as
Entodacrya or
Ctenohystrica, is a taxonomic hypothesis uniting the
gundis with the
hystricognath rodents. Considerable
morphological and strong
molecular support exists for this relationship. 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 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
metatherians,
xenarthrans, and
meridiungulates as the only resident nonflying mammals. They apparently arrived by
rafting across the
Atlantic 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. ==Families==