The lower molars of tribosphenic mammals (marsupials, placentals and their extinct allies) are marked, primitively, by a basined heel (talonid) acting as the mortar to the pestle of a large inner cusp (protocone) on the opposing upper teeth. Here we report the earliest tribosphenic mammal found so far, three lower teeth in a jaw fragment from Middle Jurassic (Bathonian, ~167 +/- 2Myr) sediments of northwest Madagascar. This specimen extends the stratigraphic range of the Tribosphenida by some 25 million years, more than doubling the age of the oldest mammal known from Madagascar, and representing only the second pre-Plio/Pleistocene terrestrial mammal known from the island. Although it indicates a more ancient diversification of the Triposphenida than previously thought, this find fails to confirm molecular-clock-based models proposing a Middle Jurassic divergence of marsupials and placentals. In addition, it offers a glimpse of mammal evolution on the southern continents during the Middle through Late Jurassic, countering the prevailing view of a northern origin for tribosphenic mammals.