The Voyage of Continent Australiaancestor of the first Australian ground-dwelling marsupials has been rediscovered as
a living animal.
The astonishing fossil finds at Riversleigh, a cattle station north-west of Mount Isa
in Queensland, demonstrate the life of the rainforest from 5–25 million years ago. The
fossil remains of ancient mammals, birds and reptiles are found in a soft freshwater
limestone which has not been compressed and means the animals’ remains retain their
three-dimensional structure. So far, more than 200 mammals have been found buried
in sediments that accumulated in pools surrounded by a forest rich in Antarctic beech,
and these include more than half the mammals and marsupials found in the Australian
rainforest today. This amazing find reveals mammalian evolution across a time span
of more than 20 million years, when the surrounding ecosystem was changing from a
rich rainforest to a semi-arid grassland community.
The Australian lyrebird was scratching away at leaf litter in the early to mid Miocene
at Riversleigh. This greatest of songbirds can sing for hours while trying to impress
female birds, and they are ranked as the best in the world for their ‘elaborate, complex
and beautiful song’. Australia is called the land of the marsupials, but it is also the
land of the songbirds since the Corvidae (crows, ravens, magpies, currawongs, birds
of paradise and others) originated in Australia about 55–60 million years ago and it is
probable that the world’s songbirds originated in the southern hemisphere and some
have since spread to the rest of the world.
Only the southern continents have large flightless birds such as the African ostrich,
the South American rhea, the Australian emu and the now extinct New Zealand moa,
which reflect an origin in Gondwanaland. Australian cassowaries still live in a form
very like that of their Gondwanan ancestors, although they are now restricted to the
rainforests of far north Queensland and New Guinea. They hunt for fallen fruits on the
forest floor in individual territories which they aggressively defend against intruders.
Their descendants are the large emus that have adapted to life on the grassy plains and
which share a common ancestry with the other large flightless birds of Gondwanan
origin. Unusual among birds are the male emus, which incubate the eggs lain by the
females; they rarely leave the nest in the eight weeks that it takes to incubate the eggs
and the young stay with their father for another year while learning where to find the
best feeding areas.
It had been assumed that many of Australia’s other birds originated elsewhere and
reached the continent by flight. However, modern DNA analysis has revealed that
Australia’s songbirds are more closely related to one another than to similar species
elsewhere in the world, suggesting the existence of early native ancestors living on
Gondwanaland. From the discovery of fossil feathers, birds are known to have been
present in Australia 120 million years ago when it was still part of Gondwanaland and
even before the first flowering plants appeared. The colourful parrots, so characteristic
of Australia, were early occupants and their beaks evolved to break open banksia
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