Eocene Rodents, Pruett Formation, Southwest Texas; Their Pertinence to the Origin of the South African Caviomorpha

The rodents from three localities in the Pruett Formation of the Buck Hill Volcanic Group from the Big Bend region of Texas are described. Included are the paramyids Thishemys plicatus, Microparamys minutus, Lophiparamys sp. indet., and at least one indeterminate paramyid; new species, M. boskeyi, of the cylindro- dont genus Mysops and hystricognathous rodent of uncertain familial position, Prolapsus, new genus. Prolapsus is represented by P. sibilatoris, new species; P. junctionis, new species; and third, smaller but indeterminate species. The rodents both individually and collectively strongly support Middle Eocene (Bridgeran) age for the lower part of the Pruett Formation. Prolapsus is the first fully hystricognathous rodent to be reported from the Eocene of any part of the world. It is already too advanced in the characters of the cheek teeth to have been ancestral to the South American Caviomorpha. The incisor enamel is of the pauciserial type, characteristic of the primitive Eocene rodents, the families Paramyidae and Sciuravidae, and the mid-Eocene cylindrodont Mysops. Although the incisor enamel and the cheek teeth show that Prolapsus could not have been caviomorph, it seems probable that the Caviomorpha were derived from Middle American Eocene ancestors of the same general stock as Prolapsus. The presence of Prolapsus in North America is weighty argument against late Eocene trans-Atlantic migration of rodents. Unknown but probably related forms, from southwest Asia, were presumably ancestral to the African Phiomorpha.