Fig. 2. Phylogeny of some American marsupials, based on Palma and Spotorno (1999) and Nowak (1999). Although scansorial and arboreal locomotor adaptations evolved more than once in the family Didelphidae, it is likely that the common ancestor was a terrestrial form. Furthermore, Nowak (1999) and Cartmill (1972) suggest that the terrestrial Monodelphis genus retains the primitive condition to the greatest degree.