Partially preserved temnospondyl mandibles from the Late Permian-Early Triassic Buena Vista Formation of Uruguay are referred to the basal stereospondyl taxon Mastodonsauridae. These represent the earliest known members of this group for South America. In most cases, this assignment was based on the characteristic morphology of the postglenoid (= postarticular) area of the lower jaw together with the presence of a hamate process. Comparisons with basal mastodonsaurids indicate that the Uruguayan specimens are phenetically similar to Gondwanan and Laurasian Early Triassic taxa, such as Watsonisuchus, Wetlugasarus, and Parotosuchus. Nevertherless, they display some characters which have not previously been described in Mesozoic temnospondyls. The Permo-Triassic Uruguayan mastodonsaurids support a Gondwanan origin for the group, an event which probably occurred sometime during the latest Permian.

Key words: Temnospondyli, Mastodonsauridae, lower jaw, Permian, Triassic, Buena Vista Formation, Uruguay.

Graciela Pińeiro fossil@fcien.edu.uy, Departamento de Evolución de Cuencas, Sección Bioestratigrafía y Paleoecología, Facultad de Ciencias, Iguá 4225, Montevideo CP 11400, Uruguay; Claudia A. Marsicano claumar@gl.fcen.uba.ar, Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Ciudad Universitaria Pab. II, Buenos Aires C1428 EHA, Argentina; Ross Damiani rossano1973@gmail.com, Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, D-70191 Stuttgart, Germany.