Postcranial skeleton of Ukhaatherium nessovi (Eutheria, Mammalia) from the Late Cretaceous of Mongolia

ABSTRACT

The postcranial skeleton of Ukhaatherium nessovi, a eutherian mammal from the Upper Cretaceous of the Gobi Desert, Mongolia, is described. Nearly the entire skeleton is represented by three skeletal assemblages associated with each other, found at Ukhaa Tolgod, a locality in the Nemegt Basin. Morphological comparisons of Ukhaatherium are made with several other mammals, including placentals, marsupials, and Asioryctes, another Cretaceous eutherian from Mongolia. As has been previously pointed out, Ukhaatherium and Asioryctes possess a number of characters that suggest they occupy a basal position in the eutherian evolutionary tree. Ukhaatherium has epipubic bones, an unexcavated astragalar trochlea, and a depressed calcaneum, among other primitive characters. Derived characters that link it with eutherians include an upper ankle joint which is more restricted to the parasagittal plane than in basal marsupials and other outgroups, and an astragalar neck that is superimposed on the sustentacular facet, as opposed to the condition found in metatherians and other outgroups, in which the sustentacular facet is either medial to the astragalus, or it is ventral to the astragalar neck but the latter overhangs the sustentacular facet medially. Ukhaatherium offers some intermediate conditions between placentals and its outgroups which facilitate a better understanding of character evolution at the root of Placentalia.