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Riversleigh Fossil Site

Source: Go to the Register of the National Estate for more information.
Identifier: 14728
Location: Riversleigh via Gregory Downs
Mount Isa City
State: QLD
Country: Australia
Statement of
The Riversleigh Fossil Site contains a record of life in northern Australia covering the last 25 million years, which provides important insights into the evolution and radiation of species in Gondwanaland. The fossil deposits include: species that pre-date the Riversleigh deposits and are still found today such as koalas (PHASCOLARCTOS CINEREUS); now-extinct species such as meiolaniid turtles, ilariids and wynyardiids (giant koala-like marsupials); representatives of taxa that have since diversified further into highly specialised modern species such as balbarine kangaroos and bandicoots; and a record of past climatic and environmental change. The Riversleigh deposits have changed evolutionary theory as a number of species (including Thylacines and elapid snakes) have been found to predate former postulated origin times by many millions of years. The fossil deposits have also been significant in allowing the correlation of deposits elsewhere in Australia, which previously had no other contemporaries.

The richness, diversity, state of preservation and completeness of Riversleigh's fossil deposits is outstanding. Over 150 distinct fossil assemblages are recorded for the area, which includes 200 new species and many unusual animals. Riversleigh has trebled the number of terrestrial mammals known from the Tertiary period and includes an equally diverse fauna of birds, amphibians and insects. The fossil deposits also show evidence of ancient and modern faunas existing side by side; they are more diverse than any Australian fossil deposit of similar age; and they represent a record of a long period of geological time at a single locality.

The area of Cambrian Thorntonia Limestone known as the 'Grotto' is an excellent example of a medium scale karst pinnacle development, and one of the best examples in Queensland. It is also a significant aesthetic landscape, and contains Aboriginal rock art.

The Quaternary fluviatile sediments and tufa along the Gregory River include fossiliferous late Pleistocene deposits. Theses are the only deposits of their kind known in the Riversleigh area and add to the geological and evolutionary understanding of the Riversleigh region and northern Australia. The adjacent Oligo-Miocene limestone deposits known as the 'Mesas' occur as isolated erosional remnants of what may have been a formerly widespread eastern extremity of the Carl Creek Limestone. Fossils are evidenced in the exposed flanks of the Mesas and in tumble down boulders on their slopes. Most of the known fossil material within the Mesas is represented within the National Park area.

Riversleigh is highly valued by the scientific community and has spawned dozens of investigations by Australian (thirty-three scientists) and overseas researchers. The majority of these investigations are centred around theories of evolutionary change and radiation of species, particularly of mammals. Riversleigh has also become an important destination for tourists and other visitors, with many of the fossil discoveries being housed at a tourist centre at Mount Isa. Riversleigh is also an important educational site, with public participation through the Riversleigh Society and associated annual field trips.

The area contains Aboriginal heritage places that are possibly of National Estate significance but these have not been identified by the appropriate Aboriginal community or documented and assessed by the Australian Heritage Commission.
Description: The Riversleigh Fossil Site occurs within the Karumba Basin of the Gulf of Carpentaria, north-west Queensland. Riversleigh is best known for its well preserved fossil assemblages which contain a vast array of terrestrial and aquatic animals. These fossil deposits were first noted in 1901 by W E Cameron. In 1963, preliminary exploration of the area yielded six major fossil sites of considerable scientific interest. Systematic exploration of the fossil deposits commenced in 1976 and continues today.

The main fossil bearing sediments are included within the Carl Creek Limestone. This is a group of sediments of variable origins, composition and textures, representing a period of deposition spanning from approximately 25 million years ago to 12 million years ago (Miocene). The Carl Creek Limestone is found within the watershed of the Gregory River, a perennial, spring fed system. The Gregory River has carved its path through the Cambrian Thorntonia Limestone to the west of Riversleigh Homestead and Pre-Cambrian quartzites to the east. The ancestral Gregory River valley formed prior to the deposition of the Carl Creek Limestones.

The earliest Carl Creek sediments were formed by precipitation from highly calcareous fresh waters issuing from springs located along the Thorntonia limestone front. Over time, these deposits, termed tufas, were broken down into rock fragments and were then deposited downstream by the river systems to form breccias (angular rock fragments), conglomerates (rounded rock fragments) and clastic sediments (fine grained). These sediments were deposited at various intervals along the river valley and in ephemeral swamps. In places the Carl Creek Limestone was laid down upon the karstic surface of the Thorntonia Limestone. Tufa is still being actively deposited along the Gregory River today.

The fossils incorporated into the Carl Creek Limestones include a diverse array of animal groups.

Aquatic faunas include a PLATYPUS (obdurodon dicksoni), turtles (including the giant horned Meiolaniids) various fish (including the lungfish, neoceratodus gregoryi), at least eight species of tree frogs of the genus LITORIA as well as a large number of ground dwelling frog species, crocodiles (baru and quinkana species) and water dragons (physignattus). The remains of these animals accumulated in the lime rich muds at the bottom of the freshwater habitats. Some bones were also incorporated into the tufa deposits, were they would be coated by the carbonate precipitates.

Terrestrial animals found within these deposits include various reptiles, birds (including flightless dromornithids, cassowaries, emus, waders such as storks, swiftlets and song birds). The mammal fauna is extremely rich and includes; diprotodonts, kangaroos, dasyurids, bandicoots, thingodontans (yalkaparidon coheni), marsupial lions (nimbacinus dicksoni and priscileo species), an extinct family of probable marsupials (yingabalanaridae species) and bats. Terrestrial animals may have fallen into the water, been captured by predators such as crocodiles, or their remains may have been washed into the water and then been incorporated into the tufas and limey muds and other calcareous sediments. A record of invertebrate life is also incorporated within the Riversleigh fossil deposits, including both aquatic and terrestrial forms of insects such as slaters, millipedes, beetles, ants, larvae of various flies and cicadas.

A number of other fossil bearing deposits found within the Riversleigh area are associated with ancient cave systems found within the Thorntonia Limestones. The fossil deposits range in age from approximately 5 million years to less than 2 million years old. These deposits are dominated by insectivorous bat remains and guano , as well as bones of small animals such as mice and rats. The rodent fossils are thought to represent the remnants of meals of carnivorous bats and other predators. Deposits which closely resemble these cave deposits are found filling fissures which were formed upon the Thorntonia Limestone surface.

A third group of sediments which contain fossils are the alluvial deposits found along the Gregory River. The fossils contained within these sediments have been poorly preserved and are thought to represent faunas of approximately 20,000 years ago (Pleistocene). Included within these deposits are the remains of giant turtles, crocodiles, numerous mammals and shells of terrestrial and aquatic gastropods.

The outcrops of the Carl Creek Limestone are today seen as isolated mesas (small tablelands) and rubbly deposits scattered throughout the watershed of the Gregory River over an area of 25kmsq. The upper surfaces of the mesas support TRIODIA PUNGENS hummock grasslands with scattered low trees dominated by EUCALYPTUS species. At the base of the mesas are found a mixture of open shrubland and hummock and tussock grasslands. The surrounding plains are inhabited by low open woodland of EUCALYPTUS CONFERTIFLORA and open tussock grassland and open herbland.

The area south of Burenda Plain, known as the Grotto, derives its name from its distinctive karst weathering features. The Grotto is an excellent example, among the best in Queensland, of a medium-scale karst pinnacle development. Since the late Teritary or ealry Pleistocene, the limestone has become weathered and eroded by the solution opening of grikes. In the Grotto area the grikes are closely spaced and at right-angles, thus producing separated rectangular towers of limestone. These interconnected grikes create a maze of vertically sided passage ways between the pinnacles which form a spectacular topography.

Alluvial flats adjacent to the modern rivers in the area support low open woodland on clay soils with western bloodwood (EUCALYPTUS TERMINALIS) and white wood (ATALAYA HEMIGLAUCA).
Plant communities associated with the riverine environments include fringing open forest to woodland. These communities are dominated by coolabah (EUCALYPTUS MICROTHECA), TERMINALIA platyphyllus, weeping tea tree (MELALEUCA LEUCADENDRON ), LIVISTONA RIGIDA (palm) and river red gum (EUCALYPTUS CAMALDULENSIS. Within the river systems are found numerous aquatic plants such as UTRICULARIA GIBBA ssp EXOLETA.

The freshwater systems are known to contain crocodiles, fish and invertebrate animals. The terrestrial environments support a number of mammal species, including the stripe faced dunnart (SMINTHOPSIS MACROURA), PLANIGALE species, various possums, six species of kangaroos, at least fourteen species of bat, four species of rodents, echidna (TACHYGLOSSUS ACULEATUS) and dingoes.

Report produced : 29/7/2015
AHPI URL : http://www.heritage.gov.au/ahpi/search.html