Papua New Guinea notable feature in common for the two landmasses is the existence of several species of marsupial mammals, including some kangaroos and possums, which are not found elsewhere. Papua New Guinea is a megadiverse country. Many of the other islands within PNG territory, including New Britain, New Ireland, Bougainville, the Admiralty Islands, the Trobriand Islands, and the Louisiade Archipelago, were never linked to New Guinea by land bridges. As a consequence, they have their own flora and fauna; in particular, they lack many of the land mammals and flightless birds that are common to New Guinea and Australia. Australia and New
Fauna of New Guinea this family, 29 genera are native to New Guinea. They are believed to have migrated to New Guinea during two different periods. The first group, called the "old endemics" form part of an ancient clade including other old endemics from Australia and the Philippines and likely migrated to New Guinea during the Late Miocene or Early Pliocene. The second group includes a number of species of the genus "Rattus". These are believed to have moved to New Guinea later. Most New Guinea species form a monophyletic group with some Moluccan species, which is most closely related to the Australian group