Until recently all marsupials were usually classified in the indisputably mono-phyletic order Marsupialia, but scientific certainty being what it is to taxono-mists, this tidy order has now been cleaved into a controversial array of ordinal level classifications—some of which utilize more than a half dozen orders (Aplin and Archer, 1987; Archer, 1984; Ride, 1964; Szalay, 1993, 1994). A currently popular ordinal designation for the particular marsupials under consideration in this paper is Diprotodontia—the kangaroos, wombats, koalas, and phalangeroids—which share several derived specializations including reduction of the lower incisor series to a single procumbent pair (diprotodonty), and partial fusion of the second and third toes of the hind-foot. The phalangeroids, in turn, are a diverse subgroup of diprotodonts that encompass "many unique or relict species and genera that are united only by a common adaptation to life in forested environments" (Smith, 1984b), a phrase that nicely echoes the old arboreal theory of primate origins. Phalangeroids typically have claws on all five digits of each extremity except the first digit of the hindfoot, which is clawless and opposable. The hands of most phalangeroids are capable of grasping, some with the first digit opposed to the others, others with digits 1-2 opposed to digits 3-5.
Among phalangeroids, several natural clades are easily recognizable, while other groupings are less certain (Baverstock, 1984; Figure 1). Most researchers agree that one phylogenetic outlier is the honey possum, Tarsipes
rostratus, classified in its own family, Tarsipedidae. Several miniature species of feathertail possums (Acrobates and Distoechurus) form a distinct clade, and they are now usually placed in the family Acrobatidae. Another confident clade contains the pygmy-possums of the family Burramyidae (Cercartetus and Burramys). The nocturnal, arboreal Cercartetus is particularly relevant as a parallel to primitive primates (Cartmill, 1974a). Together, the living tarsi-pedids, acrobatids, and burramyids comprise an ecologically definable guild containing tiny (5-70 g) pollen and nectar feeders that will be discussed below under the category "Miniature Flower Specialists."
The family Petauridae is a natural grouping of arboreal species containing the familiar sugar gliders and their gliding congeners (Petaurus spp.), along with a few nongliding species of the genus Dactylopsila (including its subgenus Dactylonax), and the nongliding Leadbeater's possum, Gymnobelideus leadbeateri. The petaurids range in size from about 100 to 700 g and rely on diets containing significant amounts of insects, tree exudates, fruit, and flower products. In size, arboreality, and diet, this radiation is of obvious interest to students of primate evolution. This clade will be discussed below in the section on "Small-bodied Omnivores."
The final grouping of phalangeroids contains the typical possums and cuscuses of Australasia. Recently, it has been popular to recognize two families, Phalangeridae and Pseudocheiridae, which nevertheless are more closely related to each other than to any of the lineages outlined above. These largest of the phalangeroids (0.7-5 kg) are typically nocturnal, arboreal mammals specialized to a folivorous diet, particularly to leaves of Eucalyptus. The family Phalangeridae includes the brushtail possums (Trichosurus), the best studied of all the phalangeroid marsupials; the monotypic scaly tailed possum (Wyulda); and the slow-climbing cuscuses, traditionally put in a single genus Phalanger, but now inevitably split into several, following recognition that the group is paraphyletic (= monophyletic with some weird descendents) with respect to the clade of Trichosurus and Wyulda. Genera now often used within the phalanger group are Spilocuscus, Strigocuscus, and Ailurops. The family Pseudocheiridae contains the largest of the gliding possums (Petauroides volans), and several nongliding taxa, including the lemuroid ringtail possum (Hemibelideus lemuroides) and a diversity of other ringtail possums (so named because their prehensile tails coil into a tight spiral). The ringtails were traditionally classified in one genus, Pseudocheirus, but they are now recognized as being paraphyletic with respect to Hemibelideus and Petauroides (and are therefore undergoing taxonomic revisions, usually including the use of the genera Pseudochirops and Petropseudes). The phalangerid-pseudocheirid radiation is discussed below under "Larger-bodied Folivores."
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