Dual origin of tribosphenic mammals

Marsupials, placentals and their close therian relatives possess complex (t ribosphenic) molars that are capable of versatile occlusal functions. This functional complex is widely thought to be a key to the early diversificati on and evolutionary success of extant therians and their close relatives (t ribosphenidans). Long thought to have arisen on northern continents, tribos phenic mammals have recently been reported from southern landmasses. The gr eat age and advanced morphology of these new mammals has led to the alterna tive suggestion of a Gondwanan origin for the group. Implicit in both bioge ographic hypotheses is the assumption that tribosphenic molars evolved only once in mammalian evolutionary history. Phylogenetic and morphometric anal yses including these newly discovered taxa suggest a different interpretati on: that mammals with tribosphenic molars are not monophyletic. Tribospheni c molars evolved independently in two ancient (holotherian) mammalian group s with different geographic distributions during the Jurassic/Early Cretace ous: an australosphenidan clade endemic to Gondwanan landmasses, survived b y extant monotremes; and a boreosphenidan clade of Laurasian continents, in cluding extant marsupials, placentals and their relatives.