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The birth of Australia began soon after the dinosaurs disappeared, 65 million years ago. It was the last landmass to split away from the ancient southern super-continent Gondwana, which for the previous 100 million years had been slowly breaking up. By 60 million years ago Gondwana had already shed South America, Africa, New Zealand and India. Last to split were the final remnants, Australia and Antarctica, some 50-60 million years ago. Australia remained close to Antarctica for several million years before finally commencing its northward drift to its present position about 40-45 million years ago.
It took many millions of years for Australia and Antarctica to fully separate, with Tasmania caught in a tug of war between. But finally, about 40 million years ago, they parted. Australia dragged Tasmania north, leaving Antarctica alone at the bottom of the world. With Australia out of the way, ocean currents were free to circle the South Pole, as they still do today, greatly influencing the world's climate.
New Guinea began to form then, along the northern edge of the Australian continental plate, developing in two parts. One part was the northern rim of the Australian plate itself and the other a string of islands off the north-east coast, away from Laurasia. The islands and mainland only came together towards the end of the Tertiary, throwing up the mighty central cordilleras there in Plio-Pleistocene times and giving New Guinea its present form.
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This map shows how sea levels were once lower around Australia. The blue sections of the map indicate dry land with a sea level drop of some 200 metres. It was during such periods that rainbowfishes were dispersed between Australia and New Guinea.
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Australia together with the Aru archipelago and New Guinea have been joined as a single landmass throughout much of their geological history. The water barrier, which is now the Arafura Sea, Gulf of Carpentaria, and Torres Strait, which separates Australia and New Guinea, represents a recent development, having resulted from rising sea levels. All of these seas are extremely shallow, with average depths ranging from about 15 to 60 metres.
This recent separation is evidenced by the distribution range of a number of rainbowfish species, which occur in both New Guinea and Australia. A similar pattern is found in other freshwater fishes, as well as some animal and plant groups. There are several plant and animal species, which only occur on Cape York Peninsula and in New Guinea. Plants, birds, reptiles, and mammals with this distribution are largely found in the northern half of the Peninsula and reach their greatest diversity in the mid-Peninsula rainforests. The fish species of the mid-Peninsula rainforests also have a strong affinity with New Guinea. The Olive and Jardine Rivers show some of the strongest relationship, with 81% and 63% of the fish species found in these rivers being common between the two countries.
Between 12,000 and 55,000 years ago, the Gulf of Carpentaria was a large inland lake. The lake would have been fresh or brackish for much of its existence. Evidence from deep core drilling reveals a pattern of establishment and marine inundation of Lake Carpentaria that appears to have been repeated. It was a freshwater lake in the Jurassic then inundated by a marine transgression (in limestone deposits), and there was a further freshwater episode in the Miocene, followed by another marine transgression.
The current distribution of a number of northern Australian and southern New Guinea rainbowfish species can be explained by the opportunities the lake and the exposed Arafura shelf provided. The Arafura shelf that defined the western boundary of Lake Carpentaria would also have provided a land-bridge to New Guinea presumably with drainages flowing west to the Timor Sea. This would have allowed potential interchange of aquatic life between West Papua, Arnhem Land and the Kimberley via coastal rivers and associated habitat quite different from that provided by Lake Carpentaria. It would also have isolated the rainbowfish fauna from these western and west-central rivers from those flowing into the eastern seaboard of Australia and south-eastern New Guinea. This may explain the different species found in the Kimberley and western Arnhem Land.
Unfortunately, no rainbowfish fossils exist so their evolutionary history will probably remain obscure. However, there is some belief that rainbowfishes originated in Western Australia and then spread eastward, north into New Guinea and southward down the northeast coast of Australia, differentiating into the various species we know today.
Many factors affect the distribution of rainbowfishes but one of the most important is biogeographical boundaries. As far as rainbowfishes are concerned, the most important biogeographical features are the drainage division boundaries. It is important to note that biogeographical boundaries do not necessarily correspond with governmental boundaries. The western half of New Guinea is the Indonesian province of West Papua. However, Indonesia is part of the Asian continental plate and was, until 20 million years ago, well separated from Australia and New Guinea. The island of Bougainville is part of Papua New Guinea, but is biogeographically most similar to the Solomon Islands. Similar biogeographical and governmental boundaries exist across the Torres Strait between the southern part of Papua New Guinea and the northern tip of Queensland, Australia.
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Figure 2. Major Drainage Divisions in Australia and New Guinea
1. Northern New Guinea
2. Southern New Guinea
3. North West (Timor Sea)
4. Gulf of Carpentaria
5. Central West (Indian Ocean)
6. Western Plateau
7. Lake Eyre
8. North East Coastal
9. South East Coastal
10. Murray-Darling
11. South West
12. Bulloo River
13. South Australian Gulf
14. Tasmanian
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There are three major landforms on the mainland of Australia; the Great Dividing Range and its associated smaller ranges, the Central Eastern Lowlands (west of the Great Dividing Range) and the Great Western Plateau. These landforms influence the major drainage patterns of the mainland. The Australian Water Resources Council has defined twelve major drainage divisions - eleven on the mainland and Tasmania's drainage system is the twelfth. These divisions are groupings of major drainage basins and are shown on the map (Fig. 2).
Within major drainage basins, there are always minor drainage basins. Minor basins sometimes have two or three rivers in a system. Other minor basins have only one river, rising in a mountain range and draining away into lonely desert land. Some only flow after heavy rains, which may be years apart. The waters that soak away into the sands from these rivers are not always lost. They may drain into an artesian or sub-artesian basin. The Great Artesian Basin is the world's largest and deepest artesian basin. It covers an area of 1,082,400 km².
Artesian springs, such as those found in large numbers on the fringes of the Great Artesian Basin, are of great significance as foci for plant and animal life - and as refuges in generally arid regions. The spring water emerges as seepages, as flowing springs, or form pools of standing water. Depending on the rate of water flow, moderately large pools may form over the spring site, some feeding streams or tails several kilometres long. Many species of fish appear to be restricted to particular groups of springs such as the red-finned blue-eye (Scaturiginichthys vermeilipinnis), which is only found at Edgbaston Springs.
New Guinea, located just north of Australia, is the world's second largest island with an area of around 836,171 km². The term New Guinea refers to the entire island, consisting of both West Papua and Papua New Guinea (PNG). New Guinea is divided between the countries of Indonesia and PNG; plant and animal distributions, however, are independent of the political boundaries. Most of these species are shared, at least in their origin, with the continent of Australia.
The mainland of New Guinea and its associated archipelagos stretch across a distance of almost 3,000 km between the equator and 12° south on the south-eastern rim of the Pacific Ocean. The climate is basically equatorial and on the coastal seaboard and river basins it is hot, wet and humid. However, New Guinea has a mountainous cordillera which runs along its centre and here ranges rise to 4509m at Mount Wilhelm the highest point in PNG and to 4884m at Pucak Jaya the highest point in West Papua. These highland regions are cooler and less humid but generally equally wet.
The majority of the island, except for the south with its extensive and inaccessible swamps and mangrove forests, is mountainous. The mountains, rivers and valleys all act as biological barriers to the movement or migration of plants and animals around the island. Indeed, geologically, the island is extremely complex, comprised of many terrains that have accreted. The biogeography of the island often reflects the independent evolutionary history of these different terrains. The complexity of the province's biogeography contributes to its rich biodiversity. Habitats range from the coastal hot, wet and swampy plains of the south to the permanently snow and ice covered mountains. The climate is wet through the year with a relatively dry period between June and August.
The western half of New Guinea is called West Papua (formerly Irian Jaya). Though never colonised, West Papua was claimed by the Netherlands as part of the Dutch East Indies from the 19th century onwards, and then Indonesia since 1961 after an invasion force of Indonesian soldiers arrived. The homeland of several hundred tribes and cultures, the native Papuan people have been racially and linguistically different from the Pacific Melanesian and Asiatic people of South East Asia for over thirty thousand years.
The eastern half (Papua New Guinea), has been an independent country since 1975 and covers a land area of 462,243 km² of which some 70% is forested with tropical rainforest. Two of PNG's rivers are of world-class size, which are the Sepik River (1126 km) and the Fly River (1200 km) and there are over 5000 freshwater lakes.
© Copyright Adrian R. Tappin
Updated July, 2005
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