Abstract

The discovery of tribosphenic molars of primitive construction in the Upper Milk River Formation (early Campanian), Alberta, documents the occurrence of a relict middle Late Cretaceous mammalian species that shows closest dental resemblance to Lower Cretaceous therians of metatherian–eutherian grade now classified as Theria, incertae sedis. The molar crowns of This new species have a wide stylar shelf, small, low protocone, distal trigonid crest, and lack conules and a shelf-like preparaconule crista mesial to the paracone; these and other coronal features indicate a more primitive level of molar evolution than known for Cretaceous Metatheria or Eutheria. Although too late in time to be ancestral to Cretaceous higher Theria, the new Milk River species may clarify aspects of their origin.

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