Australia: The Land Where Time Began
The Riversleigh Fossil Site Some of the animals found at this site
Riversleigh is situated in north-western Queensland, about 200 km north of Mt Isa, and a similar distance south of the Gulf of Carpentaria. The complex of fossil bearing sites covers an area of more than 40 km2, at least 100 of which are extinct rainforest communities. The main reason for the exceptional quality of the fossils is the high concentration of calcium carbonate in the ground water. As a result of this calcium-rich water, all water bodies such as springs, pools and rivers originating from the groundwater are conducive to fossilisation. In the present day modern animal bones can be found coated in precipitated limestone. Bones deposited on the bed of the water body can be coated with calcium after a short time, and in as little as a few months the bones can be cemented to the limestone being deposited on the bed of the water body. Once the bones are fixed within the limestone they can be mineralised over time to form precise, perfectly preserved fossils.
The area where the Riversleigh Fossil Site was discovered has hills of 3 colours, off-white, dark grey and lighter grey. The off-white hills are what remains of an ancient sea bed that covered the area in the Precambrian, about 1600 Ma, that was thrust up then eroded down to the preset state. Life at this time consisted of bacteria and the organisms that formed stromatolites.
The dark grey hills are the marine limestone deposited on bed of a shallow sea that covered the area about 530 Ma, where the material from the eroding Precambrian mountains were deposited, this time mixed with the remains of trilobites, sponges and shells of molluscs.
The least frequent are the paler grey limestones from 25 - 15 Ma. These were formed in freshwater pools, lakes and possibly streams on the floor of the rainforest. In this rainforest, the hills from the Precambrian and the later 530 Ma hills would have been present, but larger than now. The hills from the Early Miocene are a complex series of limestone deposits. The earliest limestone deposits are believed to have been deposited in large lakes. Among the fossils in this deposit are unusual crocodiles and giant flightless birds. These deposits were later covered by at least 2 deposits of limestone that probably laid down in shallower pools. Groundwater subsequently dissolved caves in this limestone where later sequences of fossils of bats and other forest creatures were deposited. The caves cover a time period of at least 20 million years. These cave deposits give clues to the drying of the Australian continent that saw the end of the rainforest over much of the now mostly arid continent.
This fossil area, more than 250 fossil sites, is the only known place in the world that has such a combination of richness, detail and continuity. The evolution of lines can be traced from 25 million years ago to 40,000 BP, as well as the changes in climate, environment, fauna and flora of a single area. At Riversleigh the evolutionary history of crocodiles, snakes, lizards, turtles, lungfish, frogs, birds, snails, insects and other invertebrates can be followed to almost the present day. With more than 40 species of bats represented at Riversleigh, it is one of the richest fossil bat sites in the world.
New types of marsupials found here include Yalkaparidon, nicknamed 'Thingodonta', whose skull and teeth are unlike any other known marsupial. Another has been nicknamed 'Fangaroo' because it is a small kangaroo with huge canine teeth. But it was a herbivore - it may have used its fangs as a defence against predators. There was also a carnivorous rat-kangaroo, Ekaltadeta ima.
One of the fossils uncovered at Riversleigh tells the story of the last days of a diprotodont mother and her young in the pouch. It seems she fell into a sink hole that had lime-rich water in it. Because of the nature of the clear-cut rims of sink holes, with little if any slope, she wouldn't have been able to get out, so some time later she drowned. Apparently her last thoughts were for the baby in her pouch because when discovered her skeleton was found to be nose to nose with the skeleton of her baby. The high levels of lime in the water provided ideal conditions for fossilisation.
Some of the discoveries at Riversleigh occurred only when the limestone encasing the known fossils was eroded away in the acid bath used to release the fossils from the limestone. One of the most unexpected was the inclusion of fossil insects and other small invertebrates in the limestone, unseen until they were exposed by the acid. Unlike the insect fossils from other sites in Australia these fossils weren't flattened in the process of fossilisation, being perfectly mineralised replicas of the original insect. The limestone had not only preserved the outer surface of the insects, but had apparently got inside the bodies so the cuticle had been preserved from both sides, the cuticle apparently being mineralised like the other, larger fossils from the site. In the acid baths have been found insects -fleas, cicadas, beetles, fly larvae, slaters and millipedes
The Riversleigh Local Fauna - Site D
This was the first deposit to be found and was given its name before it was realised there were actually hundreds of local faunas covering the area of the Riversleigh Fossil Site. The Low Lion Local Fauna (LF) and Dirk's Towers Local Fauna are possibly older then the Riversleigh Local Fauna. The Riversleigh Local Fauna contains a less diverse fauna than would be expected to accumulate from a lush rainforest. The reason is that it is believed to have been situated some distance from the rainforest, the accumulation of animals resulting from the deposition of remains originating over a huge drainage basin, the remains of most of the animals in the rainforest would have been deposited in the rainforest, or closer to it. The fossils in this local fauna were mostly of large animals such as crocodiles and turtles, and a large birds such as dromornithids, that probably foraged around the edges of the water.
The site dates from the Late Oligocene to Early Miocene, about 25 million years old. The assemblage here is believed to be the oldest of the Riversleigh local faunas. At this time the Riversleigh area was covered by dense rainforest with a high diversity of animals. The region had regular rainfall and spring-fed lakes. These lakes were inhabited by large turtles and other reptiles, as well as the usual fish and snails. Living in the forest were a number of animals that were partly familiar, partly unfamiliar. A terrestrial (at least partly) crocodile, deep-headed dagger-toothed crocodiles apparently hunted on the forest floor. There were also pythons and dromornithids, thunder birds, and now extinct species of kangaroos.
Upper Site Local Fauna
This site on the side of Godthelp Hill is an erosional remnant of a higher level above the surrounding slope. It was a diverse site compared with similar habitat types of today, having almost double the number of families of mammals and double the number of marsupial species presently found in any ecosystem from Australia or New Guinea.
This fossil site dates from then Early Miocene, about 20 million years ago. At the time the Riversleigh area was a lowland rainforest, and, as rainforests of the present, it was home to a prolific, diverse fauna. At this time marsupials were very diverse. On the forest floor were marsupials that ranged from the size of small wallabies to Diprotodontids the size of cattle. Some were familiar, such as the musky rat-kangaroo, while others were distinctly unfamiliar, exemplified by 'Thingodonta'. Among the carnivores were marsupial lions, carnivorous kangaroos and semi-terrestrial crocodiles. Below the forest floor were creatures such and ancestral marsupial moles and blind burrowing snakes. Bats were also common and diverse in this forest.
On the floor of this forest were lime-rich pools that were often encrusted with carbonate, that formed effective pit traps when they were sufficiently deep. Because they lacked sloping sides, if an animal fell into them they would have difficulty getting out and drown, their remains being preserved in remarkable detail in the lime-rich environment, the bones being cemented in place as they settled to the bed of the pool.
Neville's Garden Local Fauna
About 20 million years ago this seems to have been a part of the Riversleigh rainforest on the edge of a vast wetland. The first discoveries here were teeth of an extinct platypuses and lungfish. The limestone containing the teeth is believed to have been deposited in a pool. There were also bones of many forest animals that seem to have accumulated as into the water body, and at least in one part of the deposit, it seems probably the deposits were laid down in a cave, indicated by the presence of 'straws', crystalline structures that form in caves, travertine rim pool ridges, formed as lime-saturated water flows over an rock surface, on the surface of a rock, and the presence of small stalagmites that develop under the overhung edges of caves. There were also 'cave pearls', rounded, laminated limestone structures. It is believed that it could have been connected to the Bitesantennary Site by tunnels. The bone assemblage, composed on many forest animals such as bandicoots, diprotodontids, marsupial lions, horned turtles, many possum bones and a diverse range of kangaroos give some indication of the complexity of the rainforest community in the area. Cambrian limestone ramparts near this site were the source of the reworked trilobite fossils found at the site.
Bitesantennary Local Fauna
This site sees to have been part of a chain of tunnels or caves leading to Neville's Garden Site. The structure of the deposit suggests it may have been a chamber open to the surface by a collapsed roof and about half full of water, making it a pit trap for the forest inhabitants on the forest floor.
15 million years ago, the climate had begun to change, it was slightly cooler and possibly slightly drier. The area was still covered with rainforest, but some of the less familiar animals had disappeared, leaving the way open for some of the more familiar groups to flourish. Gone were the creatures like ilariids and wynyardiids, and the dasyurids diversified, and the possums exploded, just in the forest around the GAG Site pool there were 18 species of possum, 9 of them being leaf-eating ringtails. The vegetation also had a high level of diversity, even more than at the present in Australian forests. There were also many kangaroos, both carnivorous and browsers. Bats were a bit less abundant than at other times, but were still plentiful. Overall, at the GAG Site the Dwornamor forest around this site was a vibrant place. But the winds of change had begun to blow, soon to be literally, as the world entered another glacial period. When the rainforest covered the whole area and the climate was stable, life was good for the many plants and animals in the rainforest. Under these conditions biodiversity was high, but a little change can be good, as the climate begun to change there would have been more diversity in the vegetation, and this would have encouraged even more diversity in the inhabitants of the more varied environment. But as the cooling and drying continued the good times were definitely over, life got harder and the inhabitants of the once endless forest had to adapt to a much drier world. Many could not make the change, but those that did, did it brilliantly, evolving physiological and behavioural adaptations that suited them to what was becoming the driest inhabited continent on Earth.
Rackham's Roost Local Fauna
This site is almost certainly a cave deposit from the Pleistocene, about 5 million years ago. The remnants of the cave walls and floor are still in place. One side of the cave opened on a cliff facing the Gregory River. Cracks and crevices leading out to the cliff face have a rich mass of fossil bones that accumulated on the cave floor. The inner end of the cave apparently opened to a flat amphitheatre covered by grassy woodland, and surrounded by steep slopes up to limestone rubble above which were cliffs. The cave floor, rich with fossils, abuts the limestone cave wall from the Cambrian. The roof was long ago eroded away by the weak carbonic acid in rainwater.
Most of the bones on the cave floor were either the prey brought to the cave by ghost bats and the remains of ghost bats and other bats sharing the cave. Apart from a python and a couple of kangaroos that probably sheltered at the entrance, all the remains are of a size consistent with them being the prey of ghost bats. This means the cave deposit is not a representative sample of the life around the site, excluding all animals too big for ghost bats to tackle. It can be assumed that in the area at the time the cave was being used there would have been a variety of large marsupials such as marsupial lions, diprotodonts, and possibly koalas and possums, though their presence is only speculated at as no evidence is known. The value in the small animals in the cave is in the clues they give as to the environment of the area around the cave, indicating how the rainforest dwellers were adapting to the world outside the rich rainforests.
The closest living relatives to the animals found in the cave all live in open habitats today. Sminthopsis leucopsis and S. murina inhabit seasonally moist areas, but the other 13 living species prefer dry to desert areas. No living species of Planigale lives in closed forest. With the exception of Planigale maculatus, found in sclerophyll forests and coastal grasslands, the Planigales are restricted to the arid areas with cracking blacksoil found in central Australia.
The teeth of all the rodents found in this site indicate they were granovores, suggesting they were more likely to to be living in a grassland habitat than rainforest. Zyzomys of the present occupy dry rock piles in northern Australia.
The presence of Brachipposideros bats, the same group that dominated the Oligocene-Miocene rainforests, at first seems to contradict the implications of the other animals that it was an arid area at the time of the Rackham's Roost in the Pliocene, but as the Orange Horseshoe Bat, the only living member of the group, presently occupies hot, humid caves in the same area, perhaps a reminder of the conditions its ancestors thrived in back when the rainforest existed, shows that its presence doesn't necessarily suggest a rainforest habitat.
By this time the rainforest had most likely all gone from the area, leaving a few of its inhabitants that had adapted to the new climatic regime. Where it once stood there were now grasslands and woodlands with scattered trees. The bats that remained inhabited the caves, roosts in trees would no longer be suitable. Bats seem to have survived the loss of the rainforests more than most of the other animals, being abundant and diverse at this time. The top of the line in bats was the carnivorous ghost bats, hunting small marsupials and the recently arrived rodents, that were enabled to reach Australia as the continent arrived in Asia, close enough, probably to islands with Asian fauna. This site is the lime-rich floor of a cave that faced away from the river. Even here the life was declining.
An unexpected find at this site was a crocodile. Several suggestions have been put forward as to how a crocodile got into a cave. One is that at some time the level of the river may have been high enough, another is that it could have been a quinkanine, a semi-terrestrial crocodile. Quinkanine crocodiles are been found in Pleistocene cave deposits in other parts of the country. Or it could have been taken to the cave by a scavenger that used the cave occasionally.
Terrace Site Local Fauna
By 25 thousand years ago this was the only part of the Riversleigh area that wasn't dry was along the banks of the Gregory River, which is spring fed. It was fringed with pandanus, tall paperbarks, cluster figs and eucalypts. The vegetation away from the edge of the river was spinifex and scattered stunted trees. As usual, as the glaciers advanced in the Northern Hemisphere, deserts advanced in Australia. Glacial ages are always characterised in Australia by increased wind speed and decreased rainfall. The climate of the area was now strongly seasonal, what rain that did fall came in the wet season. There is some evidence, in the form of midden deposits containing freshwater clams, that humans were also present at this time.
Around Australia river deposits are common, but not at Riversleigh. Part of the reason for this is that the present Gregory River system is believed to be a comparatively new system. Based on the diverse deposits found at Riversleigh it seems that in the past most of the Riversleigh area was drained by subterranean channels. This is the reason the Riversleigh Terrace Site is uncommon in the area.
The fauna at this site is dominated by riverine species such as freshwater mussels, large turtles and a variety of crocodiles. As is usual for riverine deposits, it contains the remain of animals that were washed down from other parts of the drainage system, especially at time of floods, or were caught at the edge of the river by crocodiles. As at Rackham's Roost, there is a size bias towards large size, either because they were caught by crocodiles or they fell in or got bogged at the edges and drowned. Smaller animals are less likely to be unable to get out and less likely to get bogged and would be completely consumed it caught by a crocodile.
The environment suggested by this deposit seems to be very much like the Riversleigh at the present. The present-day grasslands around many of the rivers of northern Australia are dominated by Agile Wallabies (Macropus agilis). In the rivers of coastal Australia, the Water Rat (Hydromys chrysogaster) are common. Freshwater Crocodiles (Crocodylus johnstoni) are common in the freshwater parts of rivers in the north and east of the continent. Fossils of Diprotodon optatum indicate that it ranged over a vast areas, from the Warburton River near Lake Eyre up to the Kimberley in the north of Western Australia and as far south as Tasmania. Its lifestyle is thought by some to have possibly been similar to that of the living hippopotamus. But whether it lived in or near water can only be speculated about unless more evidence is found indicating which environment it lived in.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|