Eight species rank taxa arc described from Eocene sites in the Gobi Desert, Mongolia. Irdinolophus gen. now is erected for Desmalotherium mongoliense Osborn and for Irdinolophus? tuiensis sp. nov. The genus is placed at the stem of the family Depcrctellidae. Intraspecific variation in lekolophus magnus is documented for the first time; Pachylophus is synonymized with leleoloplms; and Deperetella cf.birmanica is recorded far north of its previously known range in Burma. The first known lower teeth of Colodon inceptus support referral ofDesmalotherium Jissum to Colodon. The stratigraphic range of the hyracodontid Ardynia is extended back from the late Eocene to the early middle Eocene. A range of primitive tapiroids and rhinocerotoids arc analysed cladistically to resolve the problematic affinities of the extinct families Dc-peretellidae and Rhodopagidae. The Depcrctellidae are shown to belong to a monophyletic superfamily Tapiroidca, whilst the Rhodopagidae belongs to the Rhinocerotoidea, but is distinct from the Hyracodontidae. Veragromovia, previously synonymized with the tapiroid Helaleles, is resurrected as a valid genus within the Rhodopagidae. Tsagan Khutcl and other sites in the Kholbolchi formation may be older than Irdinmanhan faunas in Asia, equaling with those from the Arshanto Formation of China of probable early Lutetian or latest Ypresian age. The Irdinmanhan Mergen fauna may be slightly older than that from Irdin Manha, because of the primitive aspect of its Lophialetes.