The faunas found in the mountains of central Irian Jaya have experienced dramatic changes through the late Quaternary. Remains of two previously unknown species of large marsupial, Maokopia ronaldi and Protemnodon hopei, have been recovered from unrelated cave and fluvial deposits which today occur in dense upper montane forest. Direct dating of the finds has not as yet been possible, but stratigraphic, sedimentologic, and palynologic evidence indicates that these species lived near a climatic treeline in subalpine grassland in the late Pleistocene. At higher altitudes a rockshelter provided the second known mid-Holocene record of Thylogale christenseni and Thylogale sp. cf. brunii, apparently extinct grassland wallabies. The two largest remaining subalpine mammal species are being locally exterminated by hunting, leaving only a large murid, Mallomys gunung, which weighs less than 2.0 kg. The area thus records the disappearance of a grassland-adapted fauna. The possum Pseudocheirops cupreus dominates in modem hunting returns, although this species is totally absent from the local fossil records. It may thus be in the process of invading a vacated and disturbed niche from the upper montane forest.