The mandible of Asfaltomylos patagonicus from the late Middle Jurassic (Callovian) of Cerro Condor (Chubut, Argentina) combines a derived dentition (?i, ?c, ?5p, 3m) with tribosphenic molars together with plesiomorphic mandibular characters such as the presence of a postdentary trough and a mandibular foramen in a very anterior position. Talonids on m2 and m3 are wider than long which represents an important synapomorphy with Gondwanan australosphenidans. Talonids have an inflated hypoconid and bulging hypoconulid. As in other australosphenidans and toothed monotremes, talonid wear in Asfaltomylos is apical and no wear facets occur within the talonid basin. On m3, a common wear facet covers the apices of hypoconid and hypconulid, and the hypocristid. Crompton's (1971) standard wear facets 5 and 6 of tribosphenic boreosphenidans are not developed; therefore, it is uncertain whether a functional protocone, with its facets 5 and 6, would be present in the upper molars. The highly specialized apical wear on the hypoconid, hypoconulid, and the hypocristid in Asfaltomylos and other australosphenidans may correspond to some upper molar structures, such as crests extending from the metacone to grind against hypoconid and hypoconulid. In contrast to other australosphenidans, in Asfaltomylos the trigonid of the ultimate premolar is not triangulated, a plesiomorphic character. A PAUP 4.0 analysis based on 277 cranial and postcranial characters has corroborated the position of Asfaltomylos as the most basal member of the Australosphenida, and suggests that australosphenidans are independent from boreosphenidans.