Gondwana

The break-up of Pangaea

Pangaea was a supercontinent that formed about 300 million years ago and broke up about 180 million years ago. This was during the age of reptiles (including dinosaurs) and the age of ferns and cycads and conifers - the Permian and Triassic periods.

At the start of the Jurassic period, Pangaea started to break up and form two two large masses, Gondwana and Laurasia. Gondwana included most of the land masses in today's southern hemisphere, including Antarctica, South America, Africa, Madagascar and Australasia, as well as the Arabian Peninsula and the Indian subcontinent, which have now moved entirely into the northern hemisphere. Laurasia contained the rest of today's northern hemisphere continents including Europe, North America and Asia.

 

The break-up of Gondwana

About 180 million years ago Gondwana was starting to break into the separate continents we have today (see the diagrams below). By 140 million years ago, at the start of the Cretaceous period, Africa/South America split from Australasia/India/Antarctica. Dinosaurs still roamed the earth, the climate was warmer and sea levels were higher. The first flowering plants were emerging.

By 90 to 100 million years ago Africa & Madagascar  had split and India was moving north. Australia and Antarctica had just separated. Australia was developing its own unique dinosaurs.

About 65 million years ago, at the start of the Tertiary period, dinosaurs became extinct. The ratites, cassowary ancestors, evolved separately from other birds. By 50 million years ago mammals and flowering plants were dominating life on earth and rainforests were prevalent. Many dry-adapted flowering plants such as eucalypts, grevilleas and banksias evolved.

By 40 million years ago most of the continents were close to their present day positions. India had collided with Asia and formed the Himalayan mountains. At 33 million years ago  southernmost part of Australia (modern Tasmania) finally separated from Antarctica, letting ocean currents flow between the two continents for the first time. Antarctica became cooler and Australia became drier because ocean currents circling Antarctica were no longer directed around northern Australia into the subtropics.

About 50 million years ago the first songbirds evolved and spread throughout the world. About 55 million years ago the marsupials evolved from possum ancestors. About 15-20 million years ago Australia/New Guinea bumped into the Asian plate and some plants and animals are able to move between continents.

 

Plant evolution

Why is the Gondwana story important? The continents that formed Gondwana nowadays share plants with common ancestors from when the continents were joined. The closest relatives of some Wet Tropics plants live as far as away as the southern tip of  South America, Madagascar and New Caledonia which are isolated from the rest of the world.

The Wet Tropics rainforests are considered to be the oldest continually surviving rainforests in the world because they contain so many plants inherited from the ancient stock of Gondwana (see plant evolution). There are some areas such as Cooper Creek and Noah Creek in the Daintree and some high mountain refugial areas like Mt Lewis & Mt Bellenden Ker where Gondwanan plants still predominate. The rest of the Wet Tropics has mixed Asian and Gondwanan origins (some very recent in geological time).

 

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