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The family Melanotaeniidae (rainbowfishes) is one of the most speciose groups of freshwater fishes inhabiting the Australia-New Guinea region. 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 and New Guinea have been alternately land-linked and separated by water on a number of occasions over millions of years. Australia together with the Aru archipelago and New Guinea lie on the Sahul continental shelf and 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. It was during such periods that rainbowfishes were dispersed between Australia and New Guinea. The fish species of Cape York Peninsula 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. The rising sea also fragmented the range of many other plants and animals. Comparable environments and species assemblages persist in the Fly River region, Port Moresby and Popondetta areas of southern New Guinea, and across northern Australia.
The Sahul shelf is a structural platform of the ocean floor, extending from the north coast of Australia to the island of New Guinea. A continental shelf, it was once above sea level, and its surface still bears erosional features formed when streams crossed it to the oceans. The shelf was slowly warped downward by crustal forces. This subsidence is evidenced in coral atolls along its edge, composed of coral that grew as the land sank. The shelf's main divisions are the shallow Arafura Shelf, covered by the Arafura Sea and Gulf of Carpentaria; the Sahul Shelf under the Timor Sea; and the Rowley Shelf underlying a part of the northwest Indian Ocean extending to North West Cape, Western Australia. To the north lay the deeper Timor Trough and the volcanic Lesser Sunda Islands, separating the Sahul from the Sunda Shelf.
The southern half of the island of New Guinea is a part of the Australian tectonic plate and as such, since the split of the Australian plate from the Antarctic core of Gondwana, has been an integral part of the continent of Australia. Collision of the leading edge of the northward moving Australian plate with the Pacific plate has resulted in uplift and the development of the central range of mountains in New Guinea. That process is on-going, with some mountains having now reached 4884 metres ASL in little more than 3 million years since the beginning of the accelerated uplift.
Global sea levels are currently higher than at anytime during the last 120,000 years, separating Australia and New Guinea by sea. However, Torres Strait has been acting almost consistently as a land-bridge since the last interglacial about 118,000 years ago up until 5~6,000 years ago, when marine transgression closed the bridge. About 12,000 years ago, sea levels were low enough that the Arafura Shelf was exposed, and 20,000 years ago, sea levels were 120 metres below present levels. Between 12,000 and 55,000 years ago, the Gulf of Carpentaria was a large inland lake. Pollen studies have shown that the vegetation surrounding the lake was very much as it is today in the open savannah country. 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.
Not only did Cape York Peninsula provide a land link between New Guinea and north-east Australia, but also Lake Carpentaria would have provided a freshwater aquatic link. Cape York Peninsula provided the main land link, but a second land link between Arnhem Land and New Guinea formed at much lower sea levels. Prior to the flooding of what is now Torres Strait between 6,000 and 8,000 years ago, New Guinea was integrally linked to mainland Australia. This made possible the movement of terrestrial plants and animals so that a potential biological 'bridge' existed between the continent and sub-continent.
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 forms 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. One reminder of this ancient lake is the current fragmented distribution of rainbowfishes such as the Iriatherina werneri and Melanotaenia maccullochi in rivers of Arnhem Land, Cape York and southern New Guinea. This may also explain the different species from the Kimberley and Arnhem Land - Melanotaenia australis, M. exquisita, M. pygmaea, etc.
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| Sahul ~ 50,000 years ago! The changing shape of Australasia can be seen in an interactive digital map that mimics the rise and fall of sea levels over the past 100,000 years. The map is called Sahul Time, after the name for the ancient continent of Australia and New Guinea ~ Monash University: http://sahultime.monash.edu.au/ |
There is convincing geological evidence for the historical existence of Lake Carpentaria. Moreover, it has been suggested that the outflow of Papua New Guinea's Fly River was diverted westward into Lake Carpentaria during this period, although this hypothesis is controversial. Harris et al. (1996) found no evidence for a past westward diversion of the Fly River, and suggested that the outflow of the river in 'recent' geological time has always remained on an easterly course into the Coral Sea. However, the hypothesis that Lake Carpentaria provided habitat for, and facilitated gene flow among freshwater Macrobrachium populations during the late Pleistocene is supported by recent analyses (De Bruyn et. al. 2004).
Unfortunately, no rainbowfish fossils exist so their evolutionary history will probably remain obscure. However, there is some belief that rainbowfishes probably originated in the north of Australia, or in southern New Guinea and then spread eastward, north into New Guinea and southward down the northeast coast of Australia, differentiating into the various species we know today. Most New Guinea rainbowfish species are shared, at least in their origin, with the continent of Australia. In south-eastern Australia, the primary driving force behind current rainbowfish distributions appears to be climatic.
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.
Literature
De Bruyn, M., Wilson, J. C. and Mather, P. B. (2004) Reconciling geography and genealogy: phylogeography of giant freshwater prawns from the Lake Carpentaria region. Molecular Ecology 13:11, 3515-3526.
Harris, P. T., Pattiaratchi, C. B., Keene, J. B., Dalrymple, R. W., Gardner, J. V., Baker, E. K. (1996) Late Quaternary deltaic and carbonate sedimentation in the Gulf of Papua foreland basin: response to sea-level change. Journal of Sedimentary Research 66: 801-819.
© Copyright Adrian R. Tappin
Updated December, 2008
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