Litokoala

Litokoala is an extinct genus of marsupial, and along with Nimiokoala are closely related to the modern Koala. The three genera may have diverged at an earlier date, although the drying of the continent and the expansion of Eucalyptus forests towards the late Miocene may have delayed the evolution of cranial features unique to the modern genera. This indicates that either fossil genera could be an ancestor of the modern genus, or the modern genera has a common ancestor to both. More material needs collection to improve their taxonomical relationships. The genus lived about 10–16 million years ago in the middle Miocene Riversleigh of Queensland. This area is described as a rainforest habitat at time of sediment deposition. It exhibits a different diet to the modern species, with the dental symphysis unfused, indicating a diet that was properly varied in nature, unlike the specialised nature of the genera Phascolarctos. The size is estimated to be only half of the modern genus. Cranial adaptions featured are in between the extant Common Brushtail Possum and Koala with minor divergence from either. This genus and Nimiokoala are similar in most anatomical features so far as is known, except that the genus Litokoala contains superficial messateric process while Nimiokoala has "more marked basiooccipital-basisphenoid flexion and a more extensive posterior attachment of the pterygoid" (Louys et al., 2009, p. 989), which make these features basal in their taxonomical position in relation to the Phascolarctos. Ths basiocranial (back of skull) features are similar to Phascolarctos, while anterior (facial) of this the feature exhibit similarities with the genus Trichosurus. Only partial fragments of all collection are known, with only the posterior section from the Zygomatic process known from L. kutjamarpensis's skull.

References

LOUYS et al.,CRANIAL ANATOMY OF OLIGO-MIOCENE KOALAS (DIPROTODONTIA:PHASCOLARCTIDAE): STAGES IN THE EVOLUTION OF AN EXTREME LEAF-EATING SPECIALIZATION Journal of Vertebrate Paleontology 29(4):981–992, December 2009