Why is Riversleigh important?
Riversleigh (10,000ha) comprises the southern section of Lawn Hill National Park in north-west Queensland (18°59'-19°08'S, 138°34'-138°43'E). Riversleigh was gazetted as part of the Lawn Hill National Park under the Queensland National Park and Wildlife Act 1975 in 1984.
Fossil fauna from the Riversleigh site have altered our understanding about Australia's mid-Cainozoic vertebrate diversity. A 15 million-year-old complete skull and nearly complete dentition of the monotreme Obdurodon dicksoni (Archer et al. 1992, 1993) has provided new information about this highly distinctive group of mammals.
The recently extinct marsupial Thylacine Thylacinus cynocephalus, also known as the Tasmanian Tiger, was the largest living mammalian carnivore in Australia. Before Riversleigh's fossil record began to unfold, there was only one Tertiary species known, but different thylacines have been identified from Riversleigh's Oligo-Miocene faunas (Muirhead & Archer 1990; Muirhead 1993). This record has been used (Archer l991a, Archer et al., 1992) to demonstrate the potential conservation value of understanding the prehistory of a group. Although in this case understanding was too late to prevent the extinction of the Thylacine in the 1930s. Other ancestral marsupial forms found at Riversleigh include moles, bandicoot, marsupial 'lions', koala, wombat, kangaroo and possums. Placental mammals are represented by more than 35 bat species, and the Riversleigh fossil bat record is considered one of the richest in the world.
There are large number of visible archaeological traces of Aboriginal occupation and sites of cultural significance at Riversleigh, particularly near the rivers. The site at Riversleigh lies on the south-western boundary of the Waanyi Aboriginal clan territory (Oates and Oates 1970, Oates 1975, Tindale 1974). No Aboriginal people currently live in the site, although appropriate involvement is sought in the management of identified cultural sites.
What happened in Australia?
The last remnant of the supercontinent Gondwana finally separated into Australia and Antarctica between 30 and 40 million years ago. Isolated on an enormous northward-drifting raft the inhabitants of the Australian continent evolved and diversified over millions of years as the climate cycled through periods of warm and cool, wet and dry. In northern Queensland this remarkable history was preserved in one of the most extraordinary fossil records on this planet. From the unique and spectacular fossil sites at Riversleigh comes the story of more than 25 million years of evolution, diversity, and extinction.
A Riversleigh rainforest 20 million years ago. Through the thick undergrowth the large rainforest browser, Silvabestius escorts its joey. Munching on a worm is 'Thingadonta', Yalkaparidon, which belongs to a mysterious long- extinct marsupial group. The Riverseigh rainforest cockatoo (centre right) is Australia's oldest parrot. A goanna- like mekosuchine crocodile climbs a tree. Clutching a possum is the giant Harrier Hawk Pengana robertbolesi, known also as 'Flexiraptor' since it could swivel its legs backward reach into and extract prey from hollows and crevices. In the shallows the giant diprotodontid Neohelos munches aquatic plants. [Reconstruction: Anne Musser]
Late Oligocene (25 million years ago) to Pleistocene (40,000 BP)
Time line and position of the continents during formation of the Riversleigh fossil deposits
By the start of the fossil record at Riversleigh, Australia had been isolated from Antarctica for around ten million years. As the continent moved northward and ocean currents were able to circulate around Antarctica as they do today, Australia's climate gradually became drier. Riversleigh's fossil sites range in age from Late Oligocene (approximately 25 million years ago), through Early Miocene (about 20-15 million years ago), Mid to Late-Miocene (around 15-10 million years ago), Pliocene (approximately 5 million years ago) to Pleistocene (around 40,000 BP).
How fossils are preserved and collected
Riversleigh's Tertiary fossils are preserved in very hard limestone. A variety of techniques are used to remove fossil-bearing rocks. Some sites are so inaccessible that helicopters are used to transport the rocks to a drop-point from which they are transported to the University of New South Wales in Sydney. In the laboratory weak acetic acid is used to dissolve the limestone, leaving bones, skulls and teeth intact.
Every year expeditions return to Riversleigh to explore fossil-rich rock and discover even more fossils and sites. Because of its unrivalled richness, the expanse of time covered by its record, and the quality of the fossils it yields, Riversleigh was declared a World Heritage site in 1994.
Scientific papers about Riversleigh discoveries
From more than 250 fossil-rich sites at Riversleigh hundreds of new species have been described from thousands of well-preserved specimens.There are many yet to be described and many more fossil sites waiting to be discovered. Nowhere else in the world is there such a rich, detailed and continuous fossil record of the changes in fauna, habitat and climate at a single locality.
More than 250 fossil sites have been discovered in the rough, inaccessible terrain at Riversleigh. [Photo: Stephan Williams]
Our understanding of the origins, evolution and history of many of Australia's vertebrate groups has been greatly enhanced by the fossil record of Riversleigh's rich Oligo-Miocene habitats. These Tertiary faunas include ancestors and representatives of the kangaroos, rat-kangaroos, bandicoots, wombats, marsupial moles, thylacines, dasyurids, koalas, possums, pygmy possums, cuscuses, bats, rodents and platypuses and the now extinct diprotodontids, thylacoleonids, ilariids and wynyardiids. In addition to mammals there are crocodiles, snakes, lizards, turtles, lungfish, frogs, birds, snails, insects and other invertebrates.
The unique teeth and skull of Yalkaparidon (top) earned it the nickname 'Thingodonta'. Even though it was a herbivore, 'Fangaroo' (bottom) had huge canine teeth, which it may have used for defence in the same way as the pygmy deers of modern rainforests.
Some of the most unusual and remarkable animals in the Australian fossil record have been found at Riversleigh. These include 'Thingodonta' (Yalkaparidon), so named because it has a skull and teeth completely unlike any known marsupial and has been assigned to its own family. 'Fangaroo' was a small herbivorous kangaroo with huge canine teeth. Perhaps it used them for defence against predators such as the Giant Rat-kangaroo Ekaltadeta, a kangaroo that ate meat. Mekosuchus was a goanna-like crocodile that may even have climbed trees. Unlike the modern Platypus, the Riversleigh platypus, Obdurodon dicksoni, had teeth like its Cretaceous ancestors. Although found today only in desert sands, the ancestor of the Marsupial Mole, Notoryctes, has a predeccessor at Riversleigh that might have burrowed through the leaf litter of ancient Tertiary forests. Part Emu and part Cassowary, the Emuary, Emuarius, seems to represent
a form near the divergence of these two birds lineages.
Although Riversleigh provides a wealth of information about the evolution of the Australian fauna, many mysteries have also been uncovered and are yet to be solved.
Palaeontologists excavate the Riversleigh limestone (top) before transporting it across inacessible terrain by helicopter (bottom). [Photos Anna Gillespie]