Family Echimyidae Gray, 1825 . Ann. Philos., n.s., 10:341.

COMMENTS: This group is complex, and in need of revision. The family includes the most primitive fossil New World hystricognaths from the Early Oligocene of Patagonia. Reig (1986:418) noted that some living taxa in this family with brachyodont and pentalophodont molars ( Mesomys and Lonchothrix) are of the type expected in the ancestoral New World Hystricognathi . The family is also the most diverse of all Hystricognathi .