Marsupials: The fossil record

Our knowledge of the evolutionary history of the gliding marsupials remains quite poor, with continuing debate as to whether the fossil species were able to glide. The oldest petauroid possums discovered so far include two undescribed species from Geilston Bay in Tasmania, which date back some 30–20 million years, although there is no suggestion at this stage that these species could glide.

Other fossils from approximately 20 million years ago at Riversleigh in north-western Queensland, Australia, have so far revealed two unnamed genera of petaurids; one genus containing two smaller species and a genus with a single larger species, although it is not known if these were able to glide. None of these species is particularly close to any lesser gliding possums of the genus Petaurus, with one genus labelled ‘pre-Petaurus’ because its teeth are similar to both the lesser gliders and the non-gliding Leadbeater’s Possum. It was not known if these species were able to glide. The most likely gliding marsupials come from the Hamilton Local Fauna fossil site in Victoria, Australia, which has revealed two petaurids, similar to the living Yellow-bellied Glider and Squirrel Glider, which are approximately five million years old. The most recent fossil deposits are the Pleistocene deposits in the Naracoorte Caves World Heritage Area in South Australia, where fossils most similar to Sugar Gliders have been discovered.

There are two extinct species of possums that were initially described from the early Pliocene deposits in Victoria as non-gliding ringtail possums of the genus Pseudocheirus. However it now seems likely that these species were relatives of the modern day Greater Glider. When these species were first described it was noted that the dental morphology most resembled that of the Greater Glider, but they were placed in the genus of non-gliding ringtail possums because it was not possible to demonstrate that these species could glide. More recently, an extinct Greater Glider, Petauroides ayamaruensis, has been described (with considerable reservation) from Pleistocene deposits in Papua (western New Guinea), though most recent fossils suggest this may not be a glider after all. Other fossils attributed to the modern Greater Glider have also been found from Pleistocene deposits in the Naracoorte Caves in south-eastern South Australia.

The lineage which includes the Greater Glider could have evolved within rainforest, where the related non-gliding, rainforest-dependent Lemuroid Ringtail Possum, Hemibelideus lemuroides, still persists in the rainforests of north Queensland. The Lemuroid Ringtail Possum has a vestigial flap of skin, less than 25 millimetres wide, in the groin region of the hind legs. It does not glide, but leaps from branch to branch, appearing to flatten its body somewhat. It frequently makes leaps of 2–3 metres with its legs outstretched like those of a glider, landing heavily on a cushion of foliage. This has led to conclusions that some of these long jumps have the appearance of true glides and that Lemuroid Ringtail Possums should be capable of limited gliding. Lemuroid Ringtail Possums appear to be both morphologically and functionally intermediate between the gliding and non-gliding members of the Phalangerida and may be some distance along the road to development as a true glider.

The fossil record of the Feathertail Glider remains quite poor, with the first fossils of the genus not being found until 1985 at Riversleigh. A number of teeth and a whole jaw, approximately 20 million years old, have since been found, although it has yet to be established whether this taxon is a species of feathertail glider, most closely related to the non-gliding Feathertail Possum, or a new taxon structurally between the living forms. Fossils most akin to feathertail gliders have also been discovered from the Pleistocene deposits in the Naracoorte Caves in South Australia.

It has been argued that marsupial gliders did not proliferate before the beginning of the Pliocene, five million years ago because open forests were not widespread in Australia before this time. The ability to glide in the marsupials has been considered to be a response to the opening up of the forests as the climate dried during the Pliocene. As the trees became further apart due to the change in vegetation from rainforest to open woodland, the possums had to jump further and further between trees until they had developed a gliding membrane that allowed them to travel between the spread-out trees that occur in eucalypt forests.

A Pliocene radiation of the gliding petaurids is supported by genetic studies; however, the presence of Petaurus-like animals from the Oligo—Miocene epochs implies they may have been present in rainforest before the forests dried out and became open woodlands during the late Pliocene. It is possible, however, that they did not glide until the Pleistocene, less than two million years ago.

The discovery of five-million-year-old fossils of gliding possums of the genus Petaurus from the Hamilton Local Fauna site suggests that these could have been inhabitants of Victorian rainforests, although it is difficult to conceive of a need to glide in a rainforest where all the crowns of the trees are touching. These same deposits also revealed the presence of a pre-greater glider, so the idea of rainforest gliders should not be abandoned too quickly.