The Paleocene and Eocene periods and beyond [1]
The relevant geological timelines and the major developments for these periods are as follows:
Epoch Mya Major events
Paleocene 66.5 - 55.8 Beginnings of many modern mammal lineages
Eocene 55.8 – 33.9 First whales; India collides with Asia
Oligocene 33.9 - 23 Seals and sea lions appear
Miocene 23 - 5.3 Human ancestors in Africa
Pliocene 5.3 – 1.8 Isthmus of Panama rises
Pleistocene 1.8 – 0.01 Ice ages begin
Holocene 0.01 - 0 Rise of Homo Sapiens, melting of glacial ice, sea levels rise
Anthropocene c1950 CE A speculator: see The Anthropocene era
Paleocene 66.5 - 55.8 Beginnings of many modern mammal lineages
Eocene 55.8 – 33.9 First whales; India collides with Asia
Oligocene 33.9 - 23 Seals and sea lions appear
Miocene 23 - 5.3 Human ancestors in Africa
Pliocene 5.3 – 1.8 Isthmus of Panama rises
Pleistocene 1.8 – 0.01 Ice ages begin
Holocene 0.01 - 0 Rise of Homo Sapiens, melting of glacial ice, sea levels rise
Anthropocene c1950 CE A speculator: see The Anthropocene era
Birds from dinosaurs [1.1]
On the previous page we noted in passing the fact that modern birds, with their insulating layer of feathers, may be descendants of the few species of dinosaurs that survived the catastrophe of the late Cretaceous period. Of recent times, and particularly during the 1990s, the relationship between the two species has conclusively been proved to be exist. A rich fossil record of feathered dinosaurs discovered in China and elsewhere documents in detail the dramatic transformation of behemoth terrestrial dinosaurs into small flight capable birds. The fossil record of flightless dinosaur specimens has revealed the existence of dinosaur specimens initially covered with fluff (presumably for warmth), then, in the case of that classification of dinosaur species described as the maniraptoran therapods, with longer hairlike strands branching into a much more orderly system of barbs projecting sideways from a central shaft (ergo the quill pen) - at this stage more for display to impress females and intimidate rivals than anything else - and then flight came almost by accident - “and it may have evolved many times in parallel as different maniraptorans found themselves generating lift from their wings as they leapt from the ground, scurried up trees or jumped between branches. Initially members of one of these maniraptorans got small, developed chest muscles and hyperelongated arms, and lost their long tails, becoming the birds of today”. [1.2]
These changes occurred very slowly, with organisms adapting to changing environmental circumstances over millions of years. In the process there occurred an overall “reshaping of dinosaur bodies into upright -walking, fast-running machines that could outpace and outhunt their rivals”. They developed lungs that take in oxygen not only during inhalation but also exhalation. Their bones hollowed out with air sacs which made them lighter for flight, their left and right collarbones fused into a wishbone formation, later endowing them with a source of energy as they flapped their wings, and their body size evolved through a gradual trend of reduction that began with the maniraptora and lasted more than 50 million years. “Exactly what drove this trend is unclear, but one possibility is that the ever shrinking physique of these feathery dinosaurs gave them entry to new ecological niches – trees, brush, perhaps even underground caves and burrows”, that were inaccessible to their larger giant cousins.
Birds therefore are just another kind of dinosaur, but they carved out a completely new way of life of their own, and today there are upwards of 10,000 species exhibiting a spectacular diversity of forms, from humming birds to ostriches.. Nor was this with any particular intent in mind: “Evolution has no foresight. It acts only on what is available in the moment, shaped by the never-ending but always changing pressures of environment and competition. There was no moment when a dinosaur became a bird, no big bang when a T. rex turned into a chicken. It was a journey. And the more scientists learn about other evolutionary transitions – fish evolving into tetrapods with limbs and digits, land mammals turning into whales, tree-swinging primates becoming upright-walking humans – the more we see a consistent theme in how this kind of transformation works: it is a marathon, not a sprint and there is no finish line. [1.3]
Once this process had started, the emerging birds which evolved alongside their dinosaur forebears unlocked their own evolutionary potential giving them access to new ecological niches and opportunities, enabling them to survive the destruction wrought by the meteor impact that killed off their land-based and flightless cousins and flourish in multifarious forms in a new environment.
The rise of mammals during the Paleocene
Mammals may be defined as that group on the tree of life that contains the egg-laying monotreme (primitive mammals that lays large yolky eggs and have a common opening for the urogenital and digestive systems, such as echindas and platypus), the marsupials (which raise their tiny babies in a pouch) and the placentals (nourishing their young before birth inside the bodies of females and after birth from modified sweat glands producing milk), as well as all the extinct descendants of their common ancestor[2]. Following the destruction wrought by the Creatceous-Paleogene (K/P) asteroid impact and the disappearance of the dinosaurs as a consequence, the Paleocene Period (65.5 – 35.8 mya) bore witness to the rise of many early mammal lineages.
But as we have seen, mammals first came onto the scene eons earlier. The earliest traces of stem mammaliaforms date to about 210 mya, during the Late Triassic, about the same time as the earliest dinosaurs. The fossilised mammals from this period were essentially small, shrew- to mouse-sized animals that had already developed mammalian hallmarks. These earliest creatures of the species got their start when the earth’s continents were still joined in a single land mass. Until recently, the conventional view has been that these protomammals stagnated for tens of millions of years during that part of the Mesozoic era (252-66 mya) that comprises the Triassic, Jurassic and Cretaceous periods, and that while their dinosaur overlords reigned supreme, the protomammals were relegated to an unremarkable existence as small insect eaters that lived on the ground and scurried through the undergrowth, generally at night[3] .
However, recent fossil finds reveal that mammals managed to evolve a wide range of feeding and locomotion styles while the dinosaurs still ruled[4]. These included the development of more sophisticated ears and teeth, which aided them in hunting prey and chewing. Furthermore, during the late Cretaceous, the emergence of flowering plants spurred the emergence of the therian mammals -the group that includes the pouched marsupials, which give birth to well-developed young. So mammals enjoyed significant success and development during the age of the dinosaurs, but after the latter’s disappearance, they flourished in a spectacular radiation of new species, eventually causing these small protoprimates to metamorphose into bipedal-walking, philosophising apes 60 million years or so later. “Just another chapter in the mammals’ evolutionary journey, now 200 million years and still counting”[5].
As an interesting sidelight, David Christian postulates that if the asteroid which was instrumental in wiping out the dinosaurs had been on a slightly different trajectory, say a few minutes faster or slower, mammals would have remained limited in numbers and our own species could not possibly have evolved. “The asteroid impact of the late Cretaceous period counts, therefore, as a crucial event in the prehistory of our own species… The crisis of the late Cretaceous is a reminder of the capriciousness and open-endedness of evolutionary change. Evolution has no pre-planned direction”[6].
[1] Depicted above in a warmer wetter time in the Miocene period are a number of mammal vertebrates which ruled the roost and roamed the rainforest: Big Bird, Madonna and Child (medium sized diprotodonts), Palorchestes (a large extinct marsupial not like any thing now seen alive) and a thylacine ancestor of the also now extinct Tasmanian tiger. With the exception of Palorchestes which lived about 20,000 years ago, these animals flourished some 25 million years ago. Source: Riversleigh Fossil Centre, Mount Isa. Photographs, including those below, by Elwyn Elms.
[1.1] See Stephen Brusatte, "Taking wing", Scientific American, January 2017, 42-49; also Richard Dawkins and Yan Wong, The Ancestor's Tale - A Pilgrimage to the Dawn of Life, Weidenfeld & Nicolson, London, 2016, 303,305.
[1.2] Ibid, 47. A fossil from a species called Ambopteryx longibrachium which lived about 163 million years ago being a dinosaur with fleshy batlike wings has also recently been identified: "Winged dinosaurs more than a legend", New York Times, reproduced in Sydney Morning Herald, 10 May 2019.
[1.3] Ibid, 49.
[2] Stephen Brusatte and Zhe-Xi Luo, “Ascent of the mammals”, Scientific American, June 2016, 20 at 22; see also Christian Maps of Time,125
[3] Brusatte and Luo, ibid at 23.
[4] Ibid, 24.
[5] Ibid 27
[6] Christian, op cit, 125..
Whales and the Eocene
The Eocene period (55.8-33.9 mya) witnessed the development of the first whales. Seals and sea lions appeared during the Oligocene (33.9-23 mya) and then came the first appearance of our human ancestors in Africa during the Miocene Period some 6 million years ago. The evolution of primates occurred during a period of accelerated change, and our own species evolved in a period of unusually rapid climatic and ecological change.
So far as other species are concerned, as had occurred during the late Devonian and early Carboniferous periods, the Paleocene and Eocene periods also contain instances of species which moved from water to land, and back again. Whales and dolphins breathe air, and platypuses, penguins and turtles are numbered among those species that have returned from land to water. Molecular genetic evidence shows that whales and hippopotamuses are related, and that lungfish are more closely related to us than to other fish. The fossil history of whales from land creatures shows numerous transitional fossils now documented from the Eocene beds of Africa and Pakistan, as they evolved from hippos to whales[1]. Hippos stayed partly on land, but whales returned to the sea and changed drastically. Sea turtles comprise another group of animals that returned from land to water - and then went back again.
The development of some species has revealed evolutionary signs of their passage back and forth from water to land and back again. The fish Bathylychnops had an additional eye looking downwards, perhaps designed for detecting predatory attacks in the water from below. A fossil of the land turtle Odontochelys, a half shelled fossil possibly evolving in water, disclosed a half shell below its body, maybe also because danger threatened from below, which may well have been on the way to a full shell.
Some sea turtles returned to the land becoming land turtles. Sea turtles have paddles and their land cousin the tortoise is equipped with walking legs. Some shelled tortoises returned to the sea as seals, as did whales and dugongs later in the piece. As these amphibian creatures slide back and forth they navigate multiple evolutionary U-turns and the only real signpost presently available to us is their DNA.
[1] Dawkins, Greatest Show, 171.
The Miocene and beyond: more mammals
Climates became cooler during the Miocene epoch from 23 to 5.3 million years ago. Reduced evaporation from the oceans meant that climates generally also became drier, so that forests shrank while steppe and desert region spread. These changes were partly due to the rearrangement of the earth’s continental masses as the Atlantic sea widened, and as Africa and India both drifted north to collide respectively with the western and eastern parts of the Eurasian landmass. The trend towards cooler drier climates accelerated during the Pliocene epoch, from 5.3 to 1.8 mya, and into the Pleistocene, the epoch in which our hominine ancestors evolved.
About 3.5 to 2.5 mya, ice sheets began to form in the northern hemisphere and in Antarctica, and by 900,000, there were already large sheets in the far north. The “ice ages” had arrived. The most recent ice age began about 100,000 years ago, and lasted until about 10,000 years (.01 million years ) ago. So, for the last 10,000 years, the earth has been in a warm, interglacial phase of these cycles. The significance of both the long-term cooling of the earth’s climates and of these shorter-term cycles for the study of hominine evolution is that they created unstable ecological conditions. All land organisms had to adapt to periodic changes in climate and vegetation, and that necessity undoubtedly accelerated the pace of evolutionary change. Modern humans are one product of this period of accelerated change[1].
The fact of climate cooling and drying during the Miocene epoch from 23 to 5.3 million years ago and the happenings which subsequently occurred during the Pliocene and Pleistocene periods are well illustrated by fossil finds in the Riversleigh World Heritage Fossil Site about 250 kilometres north west of Mount Isa in Queensland, Australia. The general significance of Riversleigh will be examined shortly. However, using it for the moment purely as a barometer of climate change, most of the fossils found there lived between 24 million and 15 million years ago and afford an almost continuous and detailed 25 million year transcript of Australia’s mammal groups, including the ancestors of koalas, kangaroos and wombats.
However, there is a gap in the record between about 15 million and 5 million years ago, at which period of time (the late Miocene) the rainforests that dominated northern Australia started to dry out. New finds have recently been found adjacent to Riversleigh filling in this gap including a never before seen bat specimen, a couple of large diprotodontids[2], small primitive marsupials and an ancient ring-tailed possum, whose dentures suggest these new creatures were younger and ate a diet containing dust and grit more attuned to the drier environment[3].
[1] The foregoing is an abbreviated version of Christian 130-133.
[2] Any of the extinct marsupials belonging to the genus Diprotodon, characterized by having two incisor teeth in the bottom jaw: Collins English Dictionary.
[3] Nicky Phillips, “Fossil hunters uncover a vast treasure trove at isolated site”, SMH, 3-4 August 2014.
Some sea turtles returned to the land becoming land turtles. Sea turtles have paddles and their land cousin the tortoise is equipped with walking legs. Some shelled tortoises returned to the sea as seals, as did whales and dugongs later in the piece. As these amphibian creatures slide back and forth they navigate multiple evolutionary U-turns and the only real signpost presently available to us is their DNA.
[1] Dawkins, Greatest Show, 171.
The Miocene and beyond: more mammals
Climates became cooler during the Miocene epoch from 23 to 5.3 million years ago. Reduced evaporation from the oceans meant that climates generally also became drier, so that forests shrank while steppe and desert region spread. These changes were partly due to the rearrangement of the earth’s continental masses as the Atlantic sea widened, and as Africa and India both drifted north to collide respectively with the western and eastern parts of the Eurasian landmass. The trend towards cooler drier climates accelerated during the Pliocene epoch, from 5.3 to 1.8 mya, and into the Pleistocene, the epoch in which our hominine ancestors evolved.
About 3.5 to 2.5 mya, ice sheets began to form in the northern hemisphere and in Antarctica, and by 900,000, there were already large sheets in the far north. The “ice ages” had arrived. The most recent ice age began about 100,000 years ago, and lasted until about 10,000 years (.01 million years ) ago. So, for the last 10,000 years, the earth has been in a warm, interglacial phase of these cycles. The significance of both the long-term cooling of the earth’s climates and of these shorter-term cycles for the study of hominine evolution is that they created unstable ecological conditions. All land organisms had to adapt to periodic changes in climate and vegetation, and that necessity undoubtedly accelerated the pace of evolutionary change. Modern humans are one product of this period of accelerated change[1].
The fact of climate cooling and drying during the Miocene epoch from 23 to 5.3 million years ago and the happenings which subsequently occurred during the Pliocene and Pleistocene periods are well illustrated by fossil finds in the Riversleigh World Heritage Fossil Site about 250 kilometres north west of Mount Isa in Queensland, Australia. The general significance of Riversleigh will be examined shortly. However, using it for the moment purely as a barometer of climate change, most of the fossils found there lived between 24 million and 15 million years ago and afford an almost continuous and detailed 25 million year transcript of Australia’s mammal groups, including the ancestors of koalas, kangaroos and wombats.
However, there is a gap in the record between about 15 million and 5 million years ago, at which period of time (the late Miocene) the rainforests that dominated northern Australia started to dry out. New finds have recently been found adjacent to Riversleigh filling in this gap including a never before seen bat specimen, a couple of large diprotodontids[2], small primitive marsupials and an ancient ring-tailed possum, whose dentures suggest these new creatures were younger and ate a diet containing dust and grit more attuned to the drier environment[3].
[1] The foregoing is an abbreviated version of Christian 130-133.
[2] Any of the extinct marsupials belonging to the genus Diprotodon, characterized by having two incisor teeth in the bottom jaw: Collins English Dictionary.
[3] Nicky Phillips, “Fossil hunters uncover a vast treasure trove at isolated site”, SMH, 3-4 August 2014.
The significance of Riversleigh as an measure of mammal development during the Miocene
Riversleigh‘s fossil vertebrate deposits are among the richest and most extensive in the world. Numerous species of mammals hitherto unknown have been discovered among these deposits, which contain the fossilised remains of all Australian mammals living today. The most common rocks in the area are 530 million year old limestones formed when shallow inland seas covered vast areas of Australia. Hiding among the hills and ridges are younger limestones formed in freshwater pools and lakes, around 10 to 25 million years ago, long after the seas receded. It is these younger rocks which contain the Riversleigh fossils.
Riversleigh‘s fossil vertebrate deposits are among the richest and most extensive in the world. Numerous species of mammals hitherto unknown have been discovered among these deposits, which contain the fossilised remains of all Australian mammals living today. The most common rocks in the area are 530 million year old limestones formed when shallow inland seas covered vast areas of Australia. Hiding among the hills and ridges are younger limestones formed in freshwater pools and lakes, around 10 to 25 million years ago, long after the seas receded. It is these younger rocks which contain the Riversleigh fossils.
The deposits at Riversleigh are found over 40-60 square km and represent three different periods of Australia’s prehistory:
- Most are in light coloured limestone 10-25 million years old that overlie much older, darker limestones.
- Another time period is represented by a large cave at Rackhams Roost where the bones of carnivorous bats and the creatures they caught and ate 3.5 million years ago have been found in the remnants of an ancient cave high above the Gregory River. This cave developed in 538 mya limestone.
- A third time period is preserved in old river terraces where the remains of animals that lived and died in and along the Gregory River some 50,000 years ago have been found.
In short, the Riversleigh deposits represent a fossilised nutshell of the Miocene, Pliocene and Pleistocene periods.
The only site accessible to the public at this stage is known as “Site D”: the first major area of fossils located in the region, originally discovered in 1901 and named “Site D” by scientists in 1967. Scientific investigation began in 1963 and continues today in adjacent areas, Site D having been largely worked out, although fossil remains are still evident there.
Site D contains a 20 million year old collection of creatures that lived in and around a large lake in the rainforest, including 5 m crocodiles and 2m lungfish lived with huge turtles, 3 m tall flightless birds, pythons up to 8m long, giant marsupials, possums of many shapes and sizes, bandicoots, an ancestor of the Tasmanian tiger and a marsupial lion. Recently, the oldest and best preserved sperm in the world from an ancient species of freshwater shrimp has been found in 17 million year old fossils unearthed at the Riversleigh. They were huge compared to the animal’s body size[1]. In addition, large colonies of bats lived in caves overlooking the lake; their tiny bones have been preserved in a tiny pocket of limestone, called the “Microsite”, just 15 m south of Site D.
How the fossils formed[2]
About 22 million years ago, caves developed as limestone dissolved away the limestone bedrock. A few million years later, erosion or the fall of the water table led to the collapse of the roof. The resultant chamber acted as a pit trap and rainwater accumulated to form a pond at the bottom. Animals which fell in were unable to escape, died and were slowly buried in the limy mud laid down in the pond. Eventually the chamber filled completely with limestone and material washed in from the forest above. Over millions of years, erosion wore down the ground surface to expose the deposit.
Under an alternative scenario, the spring fed pools and lakes in Riversleigh’s ancient forest were supersaturated with dissolved limestone. Due to surface evaporation, ice-like crusts cemented together a fragile surface of leaves and other debris that must have looked like solid ground. Sometimes animals seeking water fell through these crusts, drowned and sank to the bottom where crocodiles and scavenging turtles would eat their fill. The remains soon became cemented in limestone. As the limey muds with their entombed remains steadily accumulated, chemicals in the groundwater flowing through the sediment gradually altered the composition of the bones and teeth, literally turning them to stone.
[1] Nicky Phillips, “World’s oldest sperm found in fossil”, SMH, 15 May 2014.
[2] This material and that which follows has been drawn from http://www.mountisa.qld.gov.au/our-region. http://www.mietv.com.au/Outback-at-Isa/Riversleigh-Fossil-Centre.aspx, and sundry other sources displayed at Riversleigh Site D and the Riversleigh Interpretive Centre at Mount Isa.
About 22 million years ago, caves developed as limestone dissolved away the limestone bedrock. A few million years later, erosion or the fall of the water table led to the collapse of the roof. The resultant chamber acted as a pit trap and rainwater accumulated to form a pond at the bottom. Animals which fell in were unable to escape, died and were slowly buried in the limy mud laid down in the pond. Eventually the chamber filled completely with limestone and material washed in from the forest above. Over millions of years, erosion wore down the ground surface to expose the deposit.
Under an alternative scenario, the spring fed pools and lakes in Riversleigh’s ancient forest were supersaturated with dissolved limestone. Due to surface evaporation, ice-like crusts cemented together a fragile surface of leaves and other debris that must have looked like solid ground. Sometimes animals seeking water fell through these crusts, drowned and sank to the bottom where crocodiles and scavenging turtles would eat their fill. The remains soon became cemented in limestone. As the limey muds with their entombed remains steadily accumulated, chemicals in the groundwater flowing through the sediment gradually altered the composition of the bones and teeth, literally turning them to stone.
[1] Nicky Phillips, “World’s oldest sperm found in fossil”, SMH, 15 May 2014.
[2] This material and that which follows has been drawn from http://www.mountisa.qld.gov.au/our-region. http://www.mietv.com.au/Outback-at-Isa/Riversleigh-Fossil-Centre.aspx, and sundry other sources displayed at Riversleigh Site D and the Riversleigh Interpretive Centre at Mount Isa.
Twenty million years ago the limestone in this bluff was mud at the bottom of a large rainforest lake. Over millions of years, the limey mud became light-coloured, fossil rich limestone. Groundwater percolating through the limestone contained chemicals that changes the bones and teeth to stone. The remnants of more than 200 kinds of prehistoric animals have been found in these rocks.
The Rackham’s Roost fossils were preserved in a cave between 5 and 3 mya. Meat eating ghost bats brought in rodents and small bats, and their bones became mixed with the guano that built up on the cave floor. About 2 mya, the cave roof began to collapse and expose the cave and its floor. With more erosion, the roof and walls were completely worn away. The old cave floor, with its fossils, was exposed to form a terrace on the river bank.
Unlike many other fossil deposits, the Riversleigh fossils are not just a snapshot in time, but a window on the development of early mammal megafauna over the past 30 million years.
Unlike many other fossil deposits, the Riversleigh fossils are not just a snapshot in time, but a window on the development of early mammal megafauna over the past 30 million years.
Mode of collection and analysis of the fossils
Originally, the fossils and their surrounding material were literally chiselled out of the rock face, but this was prone to damage the specimens. These days, light explosive is used to break the rock into large but manageable blocks for transport to Mount Isa and the UNSW, Sydney. There the blocks are treated with ascetic acid, which dissolves the limestone which dissolves the limestone without attacking the fossils. Once collected, fossils are examined and compared and hypotheses are constructed about the animals and their lifestyles. Are they accurate and reasonable?
In this regard, it should be remembered, that because the original animals were prone to dismemberment by scavengers, the resultant fossils are generally not found together in one piece but are more often than not scattered across the floor in fragments along with the bones of other animals similarly dismembered which can make interpretation a challenging exercise.
Originally, the fossils and their surrounding material were literally chiselled out of the rock face, but this was prone to damage the specimens. These days, light explosive is used to break the rock into large but manageable blocks for transport to Mount Isa and the UNSW, Sydney. There the blocks are treated with ascetic acid, which dissolves the limestone which dissolves the limestone without attacking the fossils. Once collected, fossils are examined and compared and hypotheses are constructed about the animals and their lifestyles. Are they accurate and reasonable?
In this regard, it should be remembered, that because the original animals were prone to dismemberment by scavengers, the resultant fossils are generally not found together in one piece but are more often than not scattered across the floor in fragments along with the bones of other animals similarly dismembered which can make interpretation a challenging exercise.
What do the fossils tell us?
More than 60,000 fossil specimens from more than 300 kinds of animals have been recovered from the entire Riversleigh section. These have provided palaeontologists with a history of the earth and how animals of today have evolved from their ancestors of 25 million years ago. They provide an insight into which animals have changed and how they have evolved over time, and which animals have become extinct. For instance, some ancient animals, unable to adapt to environmental pressures, disappeared while other retreated and now their descendants only exist in smaller areas. For example, the musky-rat kangaroo is found only in a few areas of Queensland’s Wet Tropics.
More than 60,000 fossil specimens from more than 300 kinds of animals have been recovered from the entire Riversleigh section. These have provided palaeontologists with a history of the earth and how animals of today have evolved from their ancestors of 25 million years ago. They provide an insight into which animals have changed and how they have evolved over time, and which animals have become extinct. For instance, some ancient animals, unable to adapt to environmental pressures, disappeared while other retreated and now their descendants only exist in smaller areas. For example, the musky-rat kangaroo is found only in a few areas of Queensland’s Wet Tropics.
A joint heritage listing
Together Riversleigh and its sister site in South Australia, Narracoote, both of which are the subject of the same World Heritage Listing, tell much about the story of Australian marsupials over the last 25 million years and are therefore of international significance. The fossils of these sites represent two key phases in the development of Australian mammals:
Riversleigh today
Over the last 20 million years, Riversleigh’s animals have developed from developed from a spectacular and diverse collection of rainforest animals to the creatures now found in Australia’s semi-arid areas. Descendants of the many native animals that once dominated the area include the big red kangaroos and agile wallabies, the mouse-sized marsupial planigales and stripe-faced dunnarts, mice, flying foxes, fishing bats, emus and galahs, and, around rivers and waterholes, frogs, goannas, pythons, long necked turtles and freshwater crocodiles. The important thing to remember is that all Australian mammals still extant have their ancestors in the fossil finds at Riversleigh.
Together Riversleigh and its sister site in South Australia, Narracoote, both of which are the subject of the same World Heritage Listing, tell much about the story of Australian marsupials over the last 25 million years and are therefore of international significance. The fossils of these sites represent two key phases in the development of Australian mammals:
- Riversleigh - The diversification phase of mammals – a period where mammals evolved into many different forms. This occurred between approximately 25-15 mya (in the Miocene period). The Riversleigh area contains remarkably rich 25 to 5 million year old fossil deposits which represent this phase of mammal evolution; and
- Narracoote - The modernising phase of Australian mammals – where we see the modern ancestors of animals today. The well preserved fossil mammals at Narracoote represent this phase, with fossils which range in age from 178,000 to 18,000 years before the present.
Riversleigh today
Over the last 20 million years, Riversleigh’s animals have developed from developed from a spectacular and diverse collection of rainforest animals to the creatures now found in Australia’s semi-arid areas. Descendants of the many native animals that once dominated the area include the big red kangaroos and agile wallabies, the mouse-sized marsupial planigales and stripe-faced dunnarts, mice, flying foxes, fishing bats, emus and galahs, and, around rivers and waterholes, frogs, goannas, pythons, long necked turtles and freshwater crocodiles. The important thing to remember is that all Australian mammals still extant have their ancestors in the fossil finds at Riversleigh.