Australia: The Land Where Time Began

A biography of the Australian continent 

Faunas of Australia's Great Southern Rift Valley

Coarser sandstones and siltstones deposited on the beds of rivers, streams or the flood deposits are more common in Early Cretaceous in southeastern Australia than the lake deposits as at the Koonwarra site. During the Early Cretaceous, about 110 million years ago, when the sediments were bring deposited, Australia was in the process of breaking away from Antarctica, and a large rift valley was forming between the last 2 continental blocks of Gondwana. Large rivers deposited vast quantities of sediment in this rift valley, and it is in these deposits that the dinosaurs of southeast Australia have been found.

The animals fossilised in the channel and flood deposits lived close to the time of the fossils at Koonwarra, and the deposits are not far from each other, but the animals at the 2 sites are quite different. In the channel and floodplain deposits reptiles are the most common type of animal. Some of the spore and pollen of the same species occurs in the both deposits.

Exposure of stream and river channel deposits are found not far from the Koonwarra lake deposits, but no fossils have been found in them. The known fossils come from these exposures, the nearest known fossils occur on the shore of Bass Strait near Inverloch 25 km away, but in the same depositional environment. Exposure of these channel deposits have been revealed by wave erosion of coastal cliffs. Fossils in channel deposits in inland areas are often dissolved by humic acid seeping down through the overlying soil. On the coast the fossils are not dissolved before they are exposed.

At Dinosaur Cove on the sides of the Otway Ranges, 200 km southwest of Melbourne, fossil stream deposits have been found containing fossils. The deposit continues into the mountain side, where the fossils are excavated by tunnelling using the methods of gold mining. Hypsilophodontid dinosaurs are the most common fossils in the Dinosaur Cove deposits, as well as in other deposits in the Otway Ranges, and the Strzelecki Ranges to the east. These herbivorous dinosaurs had a form similar to kangaroos, but without external ears, hair and different feet, with large hind limbs much larger than their forelimbs, and a ling tail. Being reptiles, the similar appearance was only superficial, a case of convergent evolution.

Among the dinosaur bones from Early Cretaceous southeast Australia the hypsilophodontids were by far the most common, at least 3, and possibly 5 species being found among the several thousand bones recovered. Not many other dinosaur bones have been found in this sequence, mostly carnivorous theropods. Where they are found elsewhere in the world hypsilophodontids are only a minor component of dinosaur faunas, even where the deposits contain thousand of times more bones than in the deposits in southeast Australia. An unusual feature of the known Australian hypsilophodontids was the optic lobes of their brains, that in the 3 individuals for which there is a braincast of the top of the brain the optic lobes are very well developed, to the point where they made an impression on the inside of the skull bone. See Leaellynasaura aminagraphica. No known hypsilophodontid outside Australia, and there are many, had such unusually developed optic lobes. All other known hypsilophodontids from other parts of the world lived in lower palaeolatitudes well outside the Antarctic and Arctic Circles.

Fulgurotherium australe, an iguanodontid first found at Lightning Ridge, was also found here.

During summer in southeast Australia in the Early Cretaceous the light would have been similar to that at lower latitudes, but the days would have been longer. But in winter the polar regions would have 3 months of darkness as still occurs in the polar regions. The annual mean temperature of the area during winter 110 million years ago has been estimated by oxygen isotope ratios to be between -5 and -6 C. Therefore the dinosaurs would have needed to have been active when temperatures were at or below freezing.

Reptiles such as the Tuatara of New Zealand are active at temperatures down to +6o C. To function at lower temperatures suggests they would need to be warm blooded.

At the time these sediments were being deposited there were 2 types of rivers entering the rift valley, rivulets a few metres wide and much larger, high energy,  rivers that could be from hundreds to thousands of metres wide. Almost the only fossils from these deposits have been found in the deposits from the smaller rivulets. It is believed that any bones washed down in these high-energy rivers were probably smashed and ground up in the fast flowing torrent that pushed along vast amounts of sediment that was coarse and gravelly.

The fossils from the rivulet deposits are all of small animals, probably because the slower moving water of the rivulets would not have been able to carry those of large animals far from the place they entered the water. The evidence of these larger animals in the area is restricted to their smaller bones, such as the ankle bone (astragalus) of an Allosaurid dinosaur, a large carnivore. This fossil was found near Inverloch, in deposits a bit older than those from the Otway Ranges. When compared with the ankle bones of 55 allosaurids from a site in North America it was found to be of the same size as the 3rd smallest of the North American Allosaurus. The ankle bone is one of the smallest bones in an Allosaurus and the smallest that can be positively identified. It is not known if the Inverloch fossil is from a juvenile or adult Allosaurus, so it doesn't indicate if the Australian Allosaurus grew as large as their American relatives.

Plesiosaurs and turtles have been found in the deposits from the Early Cretaceous rivulets, but none have been found in the lake deposits at Koonwarra. The turtles provide the 2nd most common remains in these deposits in southeast Australia, but the plesiosaurs are known only from their teeth.

Sources & Further reading

  1. Patricia Vickers-Rich, Thomas Hewitt Rich, 1993 Wildlife of Gondwana, Reed Australia
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Last Updated 01/09/2011 
 

 

Australian Dinosaurs

Amphibian Fossil Deposits
Fossil Fish Beds
Riversleigh Fossil Field
Australian Age of Dinosaurs
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading