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The Gondwanatheria is an enigmatic group of mammals known from the Late Cretaceous and Paleocene of Argentina and the Late Cretaceous of Madagascar and India. Although originally believed to be edentates, it has been argued recently that gondwanatheres comprise a derived clade of multituberculates. Aside from two tentatively referred, fragmentary dentaries (one edentulous and the other bearing a single tooth), each of the four named species of gondwanatheres is based only on isolated teeth. The discovery of a nearly complete dentary of the sudamericid gondwanathere Sudamerica ameghinoi provides important new anatomical information concerning the morphology of the lower jaw, the association and orientation of lower cheek-teeth, the direction of jaw movement, and the lower dental formula and thereby necessitates another reevaluation of the relationships of Gondwanatheria. Analysis of …