Postcranial Descriptions of Ilaria and Ngapakaldia (Vombatiformes, Marsupialia) and the Phylogeny of the Vombatiforms Based on Postcranial Morphology

The postcrania of the vombatiform marsupial Ilaria illumidens from medial Miocene strata of South Australia are described and compared to those of other vombatiforms, with the observation that Ilaria shares a similar morphology of the manus and pes with living wombats. While this indicates a certain degree of fossorial activity, the size and vertebral morphology of Ilaria argue against a burrowing lifestyle. Another medial Miocene vombatiform, Ngapakaldia tedfordi, is described as having a plesiomorphic vombatiform skeleton similar in many ways to that of phalangeriform possums, but with adaptations for greater size and a plantigrade, terrestrial habitus. Besides stouter and more robust limbs, these adaptations are evident in the concave dorsal surface and laterally facing fibular facet of the astragalus that creates a less flexible upper ankle joint. For this study, a cladistic analysis was made using the postcrania of all the families in the Vombatiformes and several species representing outgroups, in order to establish synapomorphies uniting the group and to evaluate the position of these two genera within it. The results indicate that the ilariids and vombatids probably share a common ancestor, based on the similarity of the metapodials and phalanges, especially the uniquely identical morphology of the proximal metapodial facets. Ngapakaldia's similarity in form to phalangeriform possums reflects the arboreal ancestry of the vombatiform clade and indicates the plesiomorphic state from which the postcrania of other, more specialized vombatiform families (i.e., fossorial wombats and ilariids) are derived.