Cretaceous zhelestid mammals are zalambdalestoids

Placental mammals (the clade that includes bats, primates and whales) developed highly varied molars as they diversified in the Palaeogene, indicating derived diets such as carnivory and herbivory¹. Cretaceous eutherian stem taxa to Placentalia had, however, more conservative dentitions-typically molars with high, sharp cusps for insectivory. The discovery 38 years ago of molars of Late Cretaceous zhelestid mammals² with relatively low (bunodont) cusps, convergent with those of herbivorous placental ungulates, suggested a previously unknown Cretaceous radiation of more placental-like fossils^3-5. However, as associated dental, cranial and skeletal material of a single individual zhelestid has never been found⁶, the hypothesis of what zhelestids are-physically and phylogenetically-has eluded rigorous testing. Here we describe Tamirkhan balcarceli, a new fossil from the Upper Cretaceous of Mongolia, whose type specimen represents a single individual with quadrangular, zhelestid-like molars in a skull and a cursorially adapted partial hind limb. Tamirkhan reveals that zhelestid molars co-occur with features that are historically diagnostic of the well-known Cretaceous clade Zalambdalestidae: elongate first lower incisors with restricted enamel and an open root extending under the cheek teeth; translacrimal canals; and fused distal hindlimbs with elongate metatarsals^7-9. Zhelestids are therefore a dental variant within Zalambdalestoidea with skeletal features of that clade.