The order Peramelemorphia /pɛrəmɛlɪˈmɔːrfiə/ includes the bandicoots and bilbies; it equates approximately to the mainstream of marsupial omnivores. All members of the order are endemic to the twin land masses of Australia-New Guinea and most have the characteristic bandicoot shape: a plump, arch-backed body with a long, delicately tapering snout, very large upright ears, relatively long, thin legs, and a thin tail. Their size varies from about 140 grams up to 4 kilograms,[1] but most species are about one kilogram, or the weight of a half-grown kitten.
The position of the Peramelemorphia within the marsupial family tree has long been puzzling and controversial. There are two morphological features in the order that appear to show a clear evolutionary link with another marsupial group: the type of foot, and the teeth. Unfortunately, these clear signposts point in opposite directions.[2]
All members of the order are polyprotodont (have several pairs of lower front teeth)—in the case of the Peramelemorphia, three pairs. This suggests that they have evolved within Dasyuromorphia (marsupial carnivores). On the other hand, they also have an unusual feature in their feet: the second and third toes are fused together. This condition is called syndactyly, and is characteristic of the Diprotodontia (the order of marsupial herbivores that includes kangaroos, wombats, possums, and many others).[3]
Attempts to resolve this puzzle include the view that the bandicoot group evolved from the carnivores, retaining the polyprotodont dentition, and independently evolving a syndactyl hind foot; the contrary view that syndactyly is so unusual that it is unlikely to have evolved twice and therefore the bandicoot group must have evolved from a possum-like diprotodont creature, and re-evolved its extra teeth. A third view suggests that the bandicoot group evolved from a primitive carnivore, developed the syndactylous hind foot as a specialisation for climbing, and the diprotodonts then split off and evolved the two-tooth jaw that gives them their name. Recent molecular level investigations do not so far appear to have resolved the puzzle, but do strongly suggest that whatever the relationship of the bandicoot group to the other marsupial orders may be,it is a distant one.[4]
Recent molecular analyses have resulted in a phylogenetic reconstruction of the members of Peramelemorphia with quite strong support. The most basal split separates Thylacomyidae (Macrotis) from all other bandicoots. Probably the next to diverge was the recently extinct Chaeropodidae (Chaeropus). The remaining taxa comprise the Peramelidae, which divides into subfamilies Peramelinae (Isoodon and Perameles) and a clade in which the Echymiperinae (Echymipera and Microperoryctes) form a sister group to Peroryctinae (Peroryctes):[4]
Many specimens of modern peramelemorphian (e.g. Perameles spp. and Isoodon spp.) have been recovered in the fossil record from Pleistocene and Holocene fossil localities.[5] However, very few fossil species have been recovered to date. The first species of fossil peramelemorphian was described by R. A. Stirton in 1955. The specimen Stirton described was a partial lower jaw from the Tirari Desert in Central Australia, Pliocene in age. The lower jaw morphology suggested a relationship with bilbies (Family Thylacomyidae), and was named Ischnodon australis.[6]