The Tarsus of Ukhaatherium nessovi (Eutheria, Mammalia) from the Late Cretaceuos of Mongolia: An Appraisal of the Evolution of the Ankle in Basal Therians

The recently discovered Cretaceous asioryctithere Ukhaatherium nessovi, represented by three individuals with virtually complete postcrania, shows basal character states for Eutheria, that were previously only partially known from the fragmentary postcranium of Asioryctes nemegetensis. A new suite of ankle characters is revealed. This suite renders characters previously considered as present in the hypothetical common ancestor of eutherians, as diagnostic of a more nested group in the mammalian tree. Some possible primitive retentions preserved in one or both asioryctitheres are the absence of a pulley-shaped astragalar trochlea, fibula only slightly encasing the astragalus laterally, absence of an anterior plantar tubercle on the calcaneum, and presence of a ventrally curved tuber calcis (straight in most other eutherians). A set of characters in the ankle joints were plotted on a phylogenetic tree of basal therian taxa, including among others, the mentioned asioryctitheres, Vincelestes neuquenianus, Deltatheridium pretrituberculare, Mayulestes ferox, and Pucadelphys andinus. The states in the hypothetical ancestors of Theria, Metatheria, and Eutheria were inferred with the maximum parsimony algorithm. Derived states of the ankle of Eutheria are fewer, and the hypothetical ancestors of Metatheria and Eutheria turn out to be more similar to each other than previously suspected.