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Savannas Timeline

Click here to view a chart showing the major events that have shaped Australia's tropical savannas through time.

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Australian tropical savannas through time

The savannas of northern Australia contain a diverse range of unique animals and plants that are not found anywhere else in the world. What is the reason for this high level of endemism?

For example, why don’t Australian savannas have large mammals like wildebeest, giraffes, lions and zebras that are found in African savannas? Why is it that northern Australia shares similar plants and animals to Papua New Guinea yet is quite distinct from the rest of Asia? Why is Australian mammalian fauna dominated by marsupials, and why have placental mammals such as monkeys or tigers not found their way here during times of lower sea level? The answers to these questions can be best explained through an understanding of Australia’s evolutionary past and long history of isolation from the other continents in the world.

Gondwana origins

Long before savannas had begun their evolution, Australia was part of the great southern landmass known as Gondwana. Gondwana consisted of what is now Africa, New Zealand, Australia, India, Madagascar, South America and the Antarctic. This super-continent straddled the south pole, but because of the disposition of the other land masses among other reasons, Gondwana was not ice-bound but was home to a great variety of plants and animals.

Around 165 million years ago this landmass began to break up, first with India separating followed by Africa and New Zealand. It was during this period that dinosaurs such as the Muttaburrasaurus would have been roaming in forests of conifers, cycads and ferns during long dark winters and continuously light summers.

Muttaburrasaurus: a Gondwanan a long way from home

Muttaburrasaurus_Dinosaur
A Muttaburrasaurus skeleton in the Queensland Museum. Source: en.wikipedia.org/wiki/Image:Diprotodon.jpg

Muttaburrasaurus was a plant-eating dinosaur that lived between 112 and 104 million years ago in the early Cretaceous period. It was a “bird-hipped” or ornithopod dinosaur that had four legs but could raise itself up on its hind legs like the better-known Iguanodon. Adults weighed around four tonnes and were around 9 metres long.

Fossilised bones of Muttaburrasaurus have been found in — where else — Muttaburra, a town just north-east of Longreach in Queensland’s savanna country.

However this dinosaur is hardly a 'savanna' or even an Australian dinosaur as it lived long before Australia was a separate continent and in conditions quite unlike the savannas of today.

When Muttaburrasaurus was alive, Gondwana was breaking up but what was to become Australia and Antarctica were still locked together close to the southern pole, so the climate would have been very cool, with long, dark winters and landscapes of temperate forests. These forests would have lacked flowering plants like eucalypts and wattles as (whole system of flowering plants, pollinated by birds and insects had not yet flourished) but rather would have been dominated by conifers, cycads and ferns.

The great Gondwana Break-up

Forty-five million years after the separation of Africa, South America split off from Gondwana, leaving Australia and Antarctica the last of the Gondwanan plates to be connected. Eventually, around 45 million years ago, Australia made its break from Antarctica and began its gradual drift north towards the equator.

While Australia was in the long process of separating from the Antarctic, the large dinosaurs disappeared, their demise thought to be precipitated by global changes in the climate and environment and then by an asteroid impact around 65 million years ago, which marked the end of the Cretaceous era. The main survivors of that great lineage are thought to be a group of smaller theropod dinosaurs that evolved beaks, a skin covered in feathers and, for many of them, the ability to fly — the group we know today as birds.

Mammals flourished as the dinosaurs declined, rapidly evolving into a variety of different types including a range of early marsupials which are thought to have originated in north America, reaching Australia via South America and the Antarctic before those continents separated. As Australia finally separated from the Antarctic and drifted northwards, further migration into Australia was blocked and Antarctica became isolated by a circumpolar ocean and cooled, eventually becoming covered in glaciers and ice.

During this northward journey at the leisurely pace of a few centimetres a year, Australia experienced major climatic shifts that resulted in cycles of expanding and contracting rainforests. In fact, during the early Miocene (15–-20 million years ago) the majority of the Australian continent was covered in rainforest. It is thought that many of Australia’s land animal groups had their origins in this rainforest age, with many of them filling similar roles to placental mammals on other continents. During this time northern Australia had wolf-like thylacines, cow-sized herbivores known as Neohelos and marsupial lions. Other animals included platypus, and carnivorous kangaroos.

Spread of savannas

fire grass
Trees like Eucalyptus can be seen as not just being well adapted to fire, but also as promoting fire. Photo: Kathryn Thorburn

By the mid-Miocene, around 15 million years ago, Australia had drifted north to the mid latitudes, and this combined with other global changes led to gradual drying and cooling of the climate. Consequently, the rainforests began to contract as the continent dried out and fire became more frequent in the environment. As rainforests contracted they were replaced by savanna vegetation, dominated by Eucalyptus and Casuarina species. For many of the rainforest mammals of the time, this drier savanna environment proved too hostile and led to their gradual extinction. Nonetheless, many animals either managed to adapt to the savanna conditions or persist in contracting rainforest refuges. Some of these survivors can still be found today in the wet rainforests of North Eastern Queensland and Papua New Guinea.

The age of ice and giants

Diprotodon: A Diprotodon - rather like a wombat the size of a minibus. Source: http://en.wikipedia.org/wiki/Image:Diprotodon.jpg
A Diprotodon— rather like a wombat the size of a minibus. Source: en.wikipedia.org/wiki/Image:Diprotodon.jpg

From the fossil record we can ascertain that between 40,000 and 2 million years ago, Australian savannas would have been more comparable to the African savannas of today, for they contained a diverse collection of large animals known as megafauna.

Though the megafauna of this period were unlikely to have been as large as an elephant, the savannas during this time would have hosted 3 metre tall kangaroos, giant goannas reaching 7 metres long, giant land crocodiles, 3 metres high flightless birds weighing 500 kilograms, and giant marsupial herbivores known as Diprotodons.

However, all of Australia’s megafauna weighing more than 100 kilograms perished in the late Quaternary which was about 40,000 years ago. The cause of this mass extinction has been debated by scientists for more than a century, with the main arguments being climate change, over-hunting by Aboriginal people, or landscape change caused by Aboriginal fire-stick farming.

The over-hunting hypothesis is popular given the timing of events, with Aboriginal people thought to have arrived in Australia between 40,000 and 60,000 years ago. By contrast, it is thought that the megafauna of Africa such as rhinos, giraffes and lions managed to persist as they evolved in tandem with human beings.

Product of isolation

The 40 million-year period of Australia’s isolation permitted the evolution of its plants and animals without the impact of fauna arriving from other continents. However, this isolation ended 15 million years ago when Australia collided with South East Asia. This collision formed the highlands of New Guinea and during periods of low sea level connected north Australia with New Guinea permitting the dispersal of animals and plants. Also resulting from the collision were the islands of Wallacea which acted as stepping stones for plants and animals from South East Asia to disperse into Australia. However, the presence of a deep sea channel known as Wallace’s line has prevented most mammals from ever reaching Australia, with the exception of bats and rats. As a result, Australia’s mammalian fauna remains dominated by marsupials and free from the placental mammals of Asia such as monkeys and tigers.

Questions for savanna management

An understanding of our evolutionary past stimulates interesting questions about how we should manage the savannas of northern Australia. Should we introduce some of the existing megafauna from other continents, such as the Komodo dragon from Indonesia, to make up for the disappearance of our own prehistoric megafauna? Should we maintain existing populations of feral animals such as buffaloes as a replacement for a diprotodon? Should we manage our landscapes to represent savannas 70,000 years before human occupation, or 200 years before European arrival? However, the fact is that a lot of evolutionary time has passed since the extinction of megafauna and arrival of human beings. How we decide to manage our landscapes is not determined by what we may want, but limited by what we can actually do, and such a dramatic return to our ancient landscapes is likely to be unrealistic.