Australia: The Land Where Time Began
Miocene Vegetation 23.7-5.3 Million years ago
There is not a lot of evidence of the vegetation of Australia during the Miocene. The climate fluctuated during this epoch, the result being the sorting out of vegetation types to suit the changing conditions in various parts of the continent, gradually leading to the plant communities of the present. Some landforms, such as the blown-sand deserts and dunefields, and the gibber deserts that have a highly specialised vegetation, had yet to develop.
The characteristic vegetation types of the Late Oligocene persisted through to the Early Miocene, which still had a wet climate. Much of southeastern Australia was still covered by temperate rainforests, extending from Victoria to Tasmania, west to coastal South Australia, and a belt that extended to coastal Queensland, including western New South Wales and the Southern Tablelands. Present in these forests there were often many Nothofagus brassii-type trees, and many gymnosperms, as well as broadleaved species such as those of the the families Lauraceae and Myrtaceae.
The composition of the forests varied with latitude and local conditions. The vegetation gradient, from Nothofagus dominance at the coast to the inland, where Myrtaceae and Araucaria (hoop and bunya pines) were more common. There was also a gradient from south to north, with more taxa of modern warm wet rainforests in the north and fewer in the south. Assuming the Nothofagus of brassii-type had the same rainfall requirements as at present, the brassii-type in the Miocene would require a rainfall of between 1500 and 1800 mm/year.
By the Early Miocene, some of the major elements of the open plains in Australia, Acacia, Casuarina, grasses, chenopods and daisies (Asteraceae) had become established in communities. As areas of seasonal drought spread, drier vegetation types replaced the forests. Eucalypts were present, but not dominant, in drier forests and woodland.
Throughout the Miocene, brown coal deposition continued in the Gippsland Basin. Gymnosperms were common - kauri, hoop pine, Dacyridium, cellery-topped pine (Phyllocladus) and plum pine (Podocarpus), as shown by the Yallourn pollen flora. In the older part of the sequence, Notofagus, mostly of bassii-type, was very common, the abundance declining towards the Late Miocene, suggesting increasing dryness. Some other plant types present were Casuarina, Ericales, Lauraceae, Myrtaceae, Olacaceae (wild olives), Protaceae (including Beaupria and Banksia), restios, Winteraceae, and Sapindaceae (Cupaniae). The forests of New Guinea has the most similar assemblage, growing between 1200 and 2100 m altitude, with rainfall of 1500 mm annually, and a mean annual temperature between 13 C and 15 C, in the highlands.
The nature of the vegetation around Murravian Gulf (the name of the shallow sea that covered much of the basin until the Middle Miocene), is shown by pollen in the Miocene part of the Greera Clay, in the Murray Basin, which was deposited at times of marine incursions. Dinoflagellates also occur in the pollen record, this allows the dating to be more accurate. Throughout the pollen sequence Nothofagus is present. There are many Myrtaceae, with dominating closed forest genera such as Syzygium and Acmena (lillypillies) and Tristania (water gum and bottle brush), but there are also the Eucalyptus genera that are common, Podocarpus, Dacrycarpus, Phyllocladus, Dacrydium and Microcachrys. In some samples, Araucaria are found in high numbers. There were also Casuarina throughout the sequence, while sedges and grasses can reach proportions of up to 10 %. The sedge family Cyperaceae, are highly diverse, and Acacia is also present, also present are Gyrostemonaceae, and the first known appearance in the pollen record of bladderworts (water plants), Utricularia, family Lentibulariaceae, and Gardenia, family Rubiaceae. There were also tree-ferns, the holly family (Ilex), Quintinia, the hibiscus family, Malvaceae and weinmannia, family Cunoniaceae.
The evergreen rainforest vegetation type had many Myrtaceae and some Nothofagus. A regular and high rainfall is required for this type of vegetation. The saltbush (chenopods), sedges, grasses and burr-rushes would have been growing on tidal flats and swamps on the foreshore.
Rainforest was no longer widespread in the Murray Basin by the Late Miocene, being replaced by wet sclerophyll forest. In the pollen record of these forest areas there is a large amount of charcoal found amongst the pollen, indicating the forests often burned, probably after dry seasons.
Kiandra, New South Wales
An Early Miocene flora was found at Kiandra in New South Wales, dating from between 18 and 22 million years ago. The fossil record at this site contains an abundance of Nothofagus pollen of the brassii-type, Podocarpus, Microcachrys, Dacrydium and Araucaria. There were also leaves from Lauraceae and Myrtaceae that are commonly found at this site. The presence of mosses and epiphyllous fungi indicate the climate in the locality was very warm and humid.
Chalk Mountain, Bugaldi, New South Wales
This diatomite deposit resulted from basalt damming the highland streams. In the freshwater lakes that subsequently formed, sediments accumulated concentrations of freshwater diatoms. The basalt of the rocks damming the streams and of the surrounding rocks are very resistant to erosion, with the result that the accumulating siliceous diatom shells formed a pure white diatomite in the clean water of the lakes, without sediment from eroding rock to dilute it.
The diatomite at this site has been dated by the basalts of the Warrumbungle Volcano to the Middle Miocene, between 14 and 17 million years ago. There are Eucalyptus leaves and gumnuts and flowers of Ceratopetalum (New South Wales Christmas Bush, Cunoniaceae). There were many gymnosperms and Myrtaceae (Tristania, Angophora, Baeckia, Backhousia, as well as others in the forests. There appears to have been patches of open forest, as indicated by the presence of pollen from Chenopods, Amaranthaceae, mistletoes, epacrids, restios, Goodenaceae, Rutaceae (Boronia family, and grasses demonstrate the presence of a diverse herbaceous understorey.
Ti Tree, near Alice Springs
Lignites several metres thick have been found at Ti Tree near Alice Springs in the Northern Territory, that date from the Middle Miocene. This shows that at least some rainforests existed near Alice Springs during the Miocene. Carbonaceous clays in the Etadunna Formation near Lake Eyre and the Namba Formation near Lake Frome, also indicate the presence of rainforest in the area during the Miocene. There is a high proportion of pollen from grasses in the Brassii-beech-podocarp assemblage in these inland deposits, unlike those closer to the coast. The pollen record suggests that during the Miocene there were gallery forests along the major watercourses, but grasslands between the rivers. This suggests a seasonal climate, the Namba Formation geology confirms the implication of arid times. In the Namba Formation and the pollen horizon is overlain by dolomitic and lime-rich beds, this indicates the occurrence of protracted episodes of aridity that were superimposed on a normally warm wet climate. This sort of situation is believed to have resulted from the increased occurrence of westerly winds during the Miocene.
Robe River, Western Australia
At this site, below a layer of pisolitic ferricrete, pollen assemblages indicates the presence of rainforest containing podocarps with some Nothofagus. This shows that rainforests were still present in northern Western Australian during the Miocene.
Southeastern Mainland Australia
Some taxa presently confined to lower latitude rainforests in mainland southeastern Australia disappear from these Miocene forests in the Late Miocene. Among them are Ilex, Brassii-type Nothofagus, Austrobuxus and Dissiliaria (Euphorbiaceae). Cupaniae (Sapindaceae). In central Queensland the same situation occurs, the same forms being last seen in the Middle Miocene. Brassii-type beech is an indicator of non-seasonal rainforest, began declining in the central areas of the continent, the decline gradually spreading to the coastal margins until New Guinea is now the only place where they still occur in the rainforests. The Miocene is the time when the Nothofagus pollen first appears in the pollen record of New Guinea. There is no known taxon living in the Miocene that has become completely extinct, though some went extinct locally, while many others underwent a reduction of range. The changing water regime across the contentment were sorting out the taxa according to their climatic preferences or requirements, as the vegetation types arose composed of the various assemblages suited to particular areas as the rainfall and rainforest was disappearing from much of the continent.
As the continent began to dry during the Middle Miocene, vegetation types changed, and new one arose, as the vegetation of many areas changed to drier sclerophyll, open woodland or grassland. This change also ushered in a new era of fire, especially as the eucalypts that were spreading widely in the drier conditions encouraged fire by their habit of dropping bark and branches that accumulated around their bases, as well as the flammable nature of their leaves because of the high oil content. This inhibited the establishment of vegetation that might otherwise compete with them. This would have added fire-tolerance to the conditions to which plants needed to develop a coping mechanism if they were to survive in the new conditions, modifying the post-rainforest flora even more.
There appears to have been a dramatic vegetation change in southeastern Australia at the transition from the Miocene to the Pliocene, based on the limited amount of evidence available. The Terminal Miocene Event is thought to have been responsible for this change.
Rainforest was completely replaced by open woody and herbaceous vegetation in the Latrobe Valley at this time. It was apparently a very dry phase because even the pollen record is very sparse, possibly because there was not enough water to form the sedimentary beds for pollen to settle in and be preserved. The boreholes, that normally show the vegetation changes over time by the pollen in the cores show a very barren section at precisely the time which is of interest in trying to determine what actually happened at the transition. The phase following the Terminal Miocene Event (TME) was less arid, but the TME was a forerunner of the aridity to come over much of Australia, and prepared the vegetation by forcing it to adapt to the drier times to come leading up to its most severe stage during the glacial maximum.
Terminal Miocene Event
Vertebrate fossils from a number of sites give clues to the environment from the Early to Middle Miocene.
The Pinpa local fauna is in the Namba Formation, southeast of Lake Frome. The fossils here include turtles, lungfish, crocodiles, platypus and ray-finned fish. There are many water birds, pelicans, ducks, flamingos, rails and grebes. The fact the flamingos were present indicates that at this time there were already salt lakes. The nitre-rich and salt-rich waters in rift valleys and arid lands are the place flamingos are usually found. Among the marsupial fauna there were early koalas, wombats, possums, including gliders and ringtails, carnivorous marsupials related to thylacines. The fossils indicate that a high proportion of the animals inhabited the gallery forests, being mostly arboreal.
Among the animals found here are the early platypus Obdurodon, dolphins, and 4 types of cucus/possun relatives, primitive protodontids, palorchestids (Ngapakaldia), macropodids (members of the kangaroo family). The fringe of the forest and grasslands were the most likely habitat of these browsers and grazers.
The finding of dolphin fossils in the dry saltpans in central Australia in the Middle Miocene gives some indication of the degree of change that has taken place in central Australia. The only way dolphins could have reached inland lakes in the heart of Australia was up rivers that entered the sea, presumably while the Lake Frome Sub-basin still drained to the sea. The Ngapakaldi Local Fauna, contemporaneous with them, in the Etadunna Formation has no dolphins in it. This fossil site is in the Lake Eyre Basin which was inward draining, so no rivers connected it to the sea.
Kutjamarpu Local Fauna from the Wipajiri Formation, has a more diverse range of terrestrial taxa that includes bandicoots, marsupial mice and dasyurids (native cats), a koala, several different types of possum, a diprotodontid and a primitive kangaroo, an emu, a dromornithid (large flightless bird). The increased proportion of terrestrial animals compared with arboreal forms indicates that aridification was under way, the rainforest being replaced by open woodland and grasslands.
At Alcoota, 8 Ma and Bullock Creek 10 Ma, in the Northern Territory, a fauna from Late Miocene sites differs from other sites in central Australia in having a large number of terrestrial birds and mammals. The water animals from this site include a crocodile, ducks and flamingos, and on the land a palorchestid, primitive kangaroos, diprotodontids, dromornithids and a form of emu. At Bullock Creek, a diprotodontid as big as a large pig, Neohelos was very common.
With the exception of water birds from Middle Miocene sites, birds such as eagles, cranes, shore birds, gulls, curlews, nightjars and frogmouths appear similar to modern birds. Some bird fossils from the Lake Frome area are believed to be remnants of an earlier bird fauna because they don't appear to be related to any living bird families. The Miocene was characterised by the large flightless dromornithids, some species of which were larger than ostriches. There are 7 known species in 4 genera. It is believed that the appearance of grasslands as the continent continued to dry opened the niches for them to diversify into, as occurred with the kangaroos in the Pliocene. It is thought the dromornithids may have been the first large grazers of the open plains, but some, such as Bullockornis and Dromornis have such large powerful heads and beaks that it has been suggested they may have been ether hunters of scavengers or both.
Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|