Although intimately connected with the cuscuses and phalangers by means of the musk-kangaroo, the kangaroos and wallabies, together with the rat-kangaroos, are easily distinguishable from other diprotodont marsupials by their general conformation, and by peculiarities in the structure of their limbs, teeth and other organs.
Huxley in 1880 briefly suggested the arboreal origin, or primordial treehabitat of all the marsupials, a suggestion abundantly confirmed by the detailed studies of Dollo and of Bensley, according to which we may imagine the marsupials to have passed through (r) a former terrestrial phase, followed by (2) a primary arboreal phase - illustrated in the tree phalangers - followed by (3) a secondary terrestrial phase - illustrated in the kangaroos and wallabies - followed by (4) a secondary arboreal phase - illustrated in the tree kangaroos.
Cuscuses and phalangers form a numerous group, all the members of which are arboreal, and some of which are provided with lateral expansions of skin enabling them to glide from tree to tree like flying-squirrels.
Numerous types more or less nearly allied to the phalangers, such as Burramys and Triclis have also been described, as well as a flying form, Palaeopetaurus.
Of more interest is the imperfectly known Wynyardia, from older Tertiary beds in Tasmania, which apparently presents points of affinity both to phalangers and dasyures.