Discover Queensland and Explore the Geodiversity and Fossils of Riversleigh
- Wayne Munday
- 7 minutes ago
- 5 min read
Sip back and discover Queensland in the outback of Australia and explore the geodiversity and fossils of Riversleigh part of the globally significant UNESCO designated Australian Fossil Mammal Sites World Heritage Area. Riversleigh is very much off the beaten track located within Aboriginal lands of the Waanyi people in Boodjamulla National Park - known as Rainbow Serpent Country. Regarded as one of the world's premier fossil sites its fossil record tells a continuous evolutionary story of Australia’s mammals and specifically marsupials alongside monotremes, birds, reptiles, amphibians and invertebrates during the Late Oligocene up to the Middle Miocene Epochs during the transition from the Paleogene to Neogene Periods of the Cenozoic Era between 23 - 16 million years ago. This was a time when the climate shifted from humid lowland rainforest to dry eucalypt forests and woodlands. The diversity and preservation quality of Riversleigh's fossils helps to understand the evolutionary paths taken by many animals now endemic or native to Australia rather than found living elsewhere naturally. The Riversleigh Fossil Discovery Centre is the primary museum that displays the fossils from the Riversleigh World Heritage Area or you can book a Queensland National Parks Pass and visit Riversleigh’s D Site the only publicly accessible area to the fossil site and also experience the Riversleigh fossil trail.
In 2019 and on the 25th Anniversary of Riversleigh being inscribed by UNESCO as a World Heritage Site, Sir David Attenborough described Riversleigh as, “one of the great wonders of the palaeontological world…". The fossil site at Riversleigh covers over 100 Km2 of freshwater limestone laid down in the Carpentarian Basin a significant intracratonic sedimentary basin that sits upon Proterozoic rocks that has experienced prolonged crustal subsidence and downwarping from the large volumes of sediment being deposited over many millions of years during the Jurassic and Cretaceous periods of the Mesozoic Era.
The Carpentarian Basin is situated beneath the Gulf of Carpentaria extending inland across Queensland and the Northern Territory of Australia. The Carpentaria Basin is an integral component of the Great Australian Artesian Basin one of the world’s largest confined groundwater artesian aquifers covering roughly 1.7 million Km2 or about 22% of Australia beneath the States of Queensland, New South Wales, South Australia and the Northern Territory. Apparently, the Great Australian Artesian Basin has enough water to fill Sydney Harbour over 130,000 times.
The fossils of Riversleigh are found in freshwater limestone deposits of a rainforest ecosystem and within the catchment area of the Gregory River (Waanyi language Ngumarryina). A notable feature of this area is its limestone karst landscape formed through the chemical erosion and weathering of the soft limestone by water by the process of dissolution to form an extensive underground network of caves and conduits that transport water through the karst landscape.
As vertebrate and invertebrates perished in or near these karst features they would become preserved by the calcium carbonate rich sediments. It is not uncommon for Riversleigh’s fossil assemblages to retain their three-dimensional integrity and be articulated in that the fossil has retained an almost life like pose by contrast to a disarticulated fossil where parts are scattered, jumbled or missing.
Riversleigh’s fossil record is predominantly made up of Therian mammals including marsupials (metatherians) and placental mammals (eutherians). Marsupials give birth to underdeveloped young and continue to grow in a pouch and placental mammals have a complex placenta that connects the developing foetus to the mother during gestation.
Both groups share the characteristic of giving birth to their young and having tribosphenic teeth a key evolutionary adaptation of therian mammals that allowed them to shear, grind and eat different food sources that contributed to their evolutionary success.
The marsupials fossil record at Riversleigh includes the large apex predator the marsupial lion or pouch lion called Thylacoleo crassidentatus, the Ekaltadeta a giant carnivorous rat-kangaroo and the much smaller cat-sized Microleo attenboroughi. The rainforest was also home to the largest known arboreal mammal the Nimbadon lavarackorum a wombat like marsupial that lived in the canopy. Riversleigh’s fossil site also has preserved early Dasyurids the ancestor to the Tasmanian Devil.
Numerous fossil assemblages of the distinctively short-faced kangaroo have been found known as Sthenurines. Among these are multiple species of Procoptodon including Procoptodon goliah the largest known Kangaroo. These extinct marsupials were large upright and walking herbivores that set them apart from modern Kangaroos that primarily hop.
Other sthenurines identified at the site include Simosthenurus occidentalis who had a deep muzzle, robust jaws and powerful teeth that allowed it to crush tough vegetation. Other Kangaroos include several species of Gumardee.
The Riversleigh fossil site also yields early monotremes the only group of living mammals that lay eggs rather than bear live young and also placental mammals. Among the monotreme fossils is the now extinct large spoon-billed platypus Obdurodon dicksoni. Unlike Platypuses today that have developed hardened gum pads or plates in their bills to grinding food Obdurodon had teeth. Though, young platypuses still possess small rudimentary teeth a vestige or remnant of a past that are lost as they mature.
Riversleigh also features a significant number of placental mammal fossils and in particular insectivorous bats. The Riversleigh fossil bat record is considered one of the richest in the world with over 35 species identified including the Leaf-Nosed Bat known as Brachipposideros nooraleebus that lived communally in limestone caves in large numbers alongside Australonycteris clarkae the oldest bat from the Southern Hemisphere and one of the oldest in the world.
The fossil record of birds at the Riversleigh fossil site highlights Pengana robertbolesi considered an unusual bird of prey because of its highly specialised leg bone called the tibiotarsus that allowed the backward and sideways leg movement. This unique adaptation allowed the bird to develop a distinctive predatory strategy to capture prey by reaching into narrow spaces such as holes and crevices. The Pengana robertbolesi is assigned to the family Accipitridae that includes hawks, eagles, kites, harriers and Old World vultures. Other birds include the large flightless birds of Dromornis murrayi ("Big Bird") and Barawertornis tedfordi along with a assemblage of owls, pigeons and songbirds all having adapted to a rainforest ecosystem.
The fossil record of Riversleigh’s reptile fossils include a wide array of snakes, turtles, lizards and crocodiles. Among these reptiles is the formidable Baru wickeni a five meter cleaver-headed crocodile. Endemic to Australia and the south west Pacific this was one of the largest of an extinct group of crocodiles called mekosuchines that ambushed its prey from the water holes.
The Riversleigh Fossil Site has yielded fossils from several frog genera including Crinia, Philoria, Lechriodus, Limnodynastes, and Litoria. These represent an assemblage of both tree dwelling (Crinia, Litoria) and water dwelling (Limnodynastes) species adapted to living in a humid freshwater environment.
Riversleigh’s fossil record reveals a once thriving rainforest ecosystem during the late Oligocene and early Miocene. The presence of freshwater species, tree-dwelling mammals, and canopy birds indicates a humid and stable climate. However, as Australia drifted northward and Antarctic glaciation influenced global climate the region experienced substantial aridification. This shift led would lead to the gradual retreat of rainforests and replaced by woodlands and grasslands. These environmental changes are reflected in the fossilised fauna. Rainforest adapted species declined while open-country animals evolved and thrived. Browsing marsupials were largely replaced by grazing species better suited to the drier landscapes. Riversleigh provides crucial, physical evidence of this long-term ecological transformation highlighting the interconnectedness of climatic change and evolutionary adaptation.