Aspects of structural and functional features of molar dentitions in Mesozoic Theria are interpreted in relation to recently discovered mammalian taxa of primitive tribosphenic grade. Single-rank, oblique prevallum-postvallid shear and occlusion between the paracone and distal metacristid characteristic of Triassic and some Jurassic pretribosphenic therians are shown to be retained after the origin of a true protocone, as exemplified in the primitive therians Aegialodon (Early Cretaceous) and Potamotelses (Late Cretaceous). By contrast, double-rank, transverse, prevallum-postvallid shear and loss of contact between the paracone and distal metacristid, are first seen in Cretaceous metatherians and eutherians, including the Albian Pappotherium and Holoclemensia. Aegialodon, Potamotelses and the Cretaceous Deltatheridiidae may be members of a high level taxon of therians of primitive tribosphenic grade that included the ancestors of Metatheria and Eutheria.

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