Dr Mikael Siversson, Curator of Palaeontology at the WA Museum, was our guest speaker for the May meeting and presented a talk on the Australian megafauna.

To set the scene Mikael began with an introduction to plate tectonics and briefly covered the geological mechanisms that create, destroy and drive the movement of plates. The distribution of the continental plates is fundamental in an understanding of the more recent historical distribution of animals and the route by which marsupials came to be in Australia. This route was lost following the breakup of Gondawana, that began in the Cretaceous, approximately 125 my ago.

The oldest marsupial (Sinodelphys sp.) was identified in 2003 from China and research indicates that the specimen, preserved with impressions of fur, is the earliest mammal known to have possessed distinctly metatherian (pouched) marsupial characteristics. The specimen (one only) is dated at about 125 my and existed together with placental animals and feathered dinosaur specimens .

The spread of the marsupials from China into Europe and North America occurred around 70-80 my, continuing through into South America at around 65 my and lastly across land-bridging Antarctica into Australia and Papua New Guinea at about 60 my. Antarctica and Australia were conjoined at this period, as was Australia and Papua. These first marsupials were likely to have been small insectivores, with larger herbivores, such as koalas and wombats, being more recently evolved. Returning to America, the Virginian opossum evolved from a lineage that moved from South America back into North America about 3 my ago and the shrew possum now represents a single group existing in the Andes of South America.

The earliest Australian mammals are long extinct and the monotremes that are extant, may have evolved here. Fossil evidence of the diprotodontians (marsupials) is plentiful and evidence from some of their bones indicates a body mass of about 2 tonnes for some individuals. At about 25 my one of the first diprotodontians, possibly of wombat type build, reverted, over time, to a carnivorous diet and became what is now referred to as the marsupial lion (Thylacoleo). Evolutionary changes from a former herbivorous diet led to less efficient dentition in this animal compared to other large predatory cat-like animals. This was demonstrated from a look at the dentition using a cast model of the skull of the marsupial lion. The animal was not ‘cat family’ and the action of its incisors when closing the jaw shows poor contact between them. The particular dental changes involved reduction of former premolars, with one set, the 3rdpremolar, having developed as effective “bolt cutters”. The animal also evolved a broad skull and powerful jaws. Killing prey was considered to be carried out more by wrestling actions that involved its weight, claw action with a semi-opposable thumb, and long forelimbs. Identification of the first Thylacoleo sp. was made by the English comparative anatomist, Richard Owen, in about 1859 and many examples are now known throughout Australia. Thylacoleo was the largest carnivorous Australian mammal.

Mikael mentioned other cat-like animals from the fossil record of other continents, namely the American lion, Pantheraatrox, , with its skull of 0.5 m and body weight of 4-500 kg. This was the largest cat family member ever. In South America there was Thylacosmilussparassodonia, with sabre-like teeth, and Smilodon, related more closely to a Lynx in both North and South America. Thylacoleo was not considered such an efficient killer as Smilodon. Around 40 my ago Antarctica became a separate continent allowing the circumpolar ocean circulation resulting in climatic changes and patterns of ice ages. In response to cooler climates from the Cretaceous to Cainozoic, animals existing at the time developed giant body size; a larger size retaining more heat. In the Pleistocene (2 my), for example, there were woolly mammoths in northern latitudes whilst in Australia there was the rise of the megafauna, a group of giant marsupials. The Australian ice age had its super predators with Thylacolea and the apex predator, Megalania, the giant monitor lizard. The fossil record for the Australian megafauna has been greatly increased by the recent discovery of examples from important cave sites, including the Tight Entrance cave in WA’s Margaret River region, and the most recent discovery in 2002 made by speleologists in the east Nullarbor plains. Animals are found as well-preserved skeletons at this latter site in 40 my limestone, where they had literally dropped into the cave. Probably for the last 500,000 years the bones have been lying openly, and because of this their condition is very fragile. Special techniques have to be utilised to prepare the fossil bones for transporting to Museum research departments. So far investigations have revealed two frog types, 69 species of vertebrates including many kangaroos, and two species of tree kangaroos, including an extinct short-faced type. The bones of these tree kangaroos establishes that they were the arboreal type, an interesting fact considering the treelessness of the Nullarbor. A Thylacoleo skeleton has been found lying on this cave floor and probably dates from about 300,000 years, together with a dingo skeleton that probably dates from 13,000 years. The aspect of these huge differences in ages is part of the unravelling that ongoing work has to consider. Mikael touched briefly on the debate concerning the demise of the megafauna and made comparisons with the situation in Africa, where there was a long period of co-existence between the wildlife and early humans. Much destruction of Australian fauna can be attributed to the arrival of the dingo, which is not indigenous, but an introduced species.

Finally Mikael made mention of Cave paintings found in the Kimberleys, thought by some to depict the marsupial lion and considered that they are most likely to depict the more recently extinct thylacine.

Susan Stocklmayer