Fossil Evidence of a Mesonychid Mammal from the Upper Eocene Ergilin Dzo Formation, Mongolia

The upper Eocene Ergilin Dzo Formation of southeastern Mongolia yields many terrestrial vertebrate fossils (e.g. Yanovskaya et al., 1977; Russell and Zhai, 1987; Dashzeveg, 1993). Its mammalian fauna is the type fauna of the Ergilian Asian land mammal ‘age’ (ALMA) (Russell and Zhai, 1987). In the Ergilin Dzo Formation, the Mesonychia (= Acreodi), which are archaic Paleogene carnivorous mammals, were cited in the faunal lists by previous researchers: as Mesonychidae indeterminate by Russell and Zhai (1987); and as Mongolestes hadrodens (Mesonychidae) and Metahapalodectes sp. (Hapalodectidae) by Dashzeveg (1993). However, the existence of the mesonychians in the formation has not been documented yet because they have been