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Background    Gaudeamus  is an enigmatic hystricognathous rodent that was, until recently, known solely from fragmentary material from early Oligocene sites in Egypt, Oman, and Libya.  Gaudeamus ' molars are similar to those of the extant cane rat  Thryonomys , and multiple authorities have aligned  Gaudeamus  with  Thryonomys  to the exclusion of other living and extinct African hystricognaths; recent phylogenetic analyses have, however, also suggested affinities with South American caviomorphs or Old World porcupines (Hystricidae).    Methodology/Principal Findings  Here we describe the oldest known remains of  Gaudeamus , including largely complete but crushed crania and complete upper and lower dentitions. Unlike younger  Gaudeamus  species, the primitive species described here have relatively complex occlusal patterns, and retain a number of plesiomorphic features. Unconstrained parsimony analysis nests  Gaudeamus  and  Hystrix  within the South American caviomorph radiation, implying what we consider to be an implausible back-dispersal across the Atlantic Ocean to account for  Gaudeamus ' presence in the late Eocene of Africa. An analysis that was constrained to recover the biogeographically more plausible hypothesis of caviomorph monophyly does not place  Gaudeamus  as a stem caviomorph, but rather as a sister taxon of hystricids.    Conclusions/Significance  We place  Gaudeamus  species in a new family, Gaudeamuridae, and consider it likely that the group originated, diversified, and then went extinct over a geologically brief period of time during the latest Eocene and early Oligocene in Afro-Arabia. Gaudeamurids are the only known crown hystricognaths from Afro-Arabia that are likely to be aligned with non-phiomorph members of that clade, and as such provide additional support for an Afro-Arabian origin of advanced stem and basal crown members of Hystricognathi.

Craniodental Morphology and Systematics of a New Family of Hystricognathous Rodents (Gaudeamuridae) from the Late Eocene and Early Oligocene of Egypt