Evolution Record in Australia

Evolution in Australia

Australia is a continent of contrasts. Its interior is brown, burnt by the sun and parched for water, but its margins are kinder to life and support most of its human population. Its plants and animals are for the most part quite different to those of the rest of the world - not surprising really, since they are the product of tens of millions of years of evolution on an island continent, isolated for that great span of time from contact with other large land masses.

Evolution in Australia followed a different course and produced very different creatures, perhaps none more so than its mammals. While the rest of the world contents itself with one, or at the most, two major kinds of mammals, Australia has three: two monotremes - the platypus and echidna, many, many marsupials (not just kangaroos and koalas), but only a few of the placental mammals so dominant in the rest of the world. Apart from the dingo, which appears to have arrived in Australia only thousands of years ago, the continent's only native placental mammals are rats and bats.

Australian vertebrate fossils

Until very recently, not much was known about the evolutionary history of Australia's unique mammal fauna. Soon after European's settlement in Australia, discoveries were made which showed that even more unusual mammals had once lived here.

These included giant plant-eating marsupials as big as a rhinoceros, giant long-armed kangaroos with short blunt faces and even leopard-sized meat-eaters, related not to modern meat-eating marsupials but to plant-eating possums.

Animals such as these became extinct as recently as the end of the last ice age, a time when Australia was at its most arid. Unfortunately the history of these animals could be traced back through the fossil record no more than a few million years. The kinds of land animals that lived in Australia earlier than that could only be imagined.

Fossil discoveries at Riversleigh

All that has changed in the last decades with new discoveries of fossil mammal remains in several sites in the Australian outback. The most spectacular of these discoveries was made at Riversleigh, an isolated cattle station in the far north-west of Queensland.

Here, Professor Michael Archer and a team of researchers from the University of New South Wales and other Australian institutions are resurrecting the bones of a vast array of vertebrate animals entombed in deposits of freshwater limestone.

Riversleigh is Australia's most important source of information about the evolutionary history of its vertebrate animals. Well over a hundred different fossil sites are now known from an area covering tens of square kilometres. In them are preserved in sometimes stunning detail, the remains of animals as diverse as lungfish, frogs, crocodiles, turtles, snakes, lizards, birds (including giant, flightless birds), marsupials, bats, rats and even insects and other arthropods.

It is not just the diversity and sheer numbers of animal remains preserved that makes Riversleigh so important to science, but the fact that different Riversleigh sites preserve glimpses of life at different ages in the past - from as long ago as about twenty-five million years through to yesterday afternoon. The Riversleigh fossil deposits are of such importance to Australia and the world that they have been given World Heritage listing.

Why so many Riversleigh fossils?

Why are so many animal remains preserved so well at Riversleigh? The answer to that lies in the underlying rocks which are also limestone, but laid down in the bed of an ancient sea more than five hundred million years ago.

As these rocks were exposed on what became land, rainwater began to dissolve them. The limestone-charged run-off flowed into pools, streams and lakes.

Water also carved out extensive cave systems, ideal roosts for bats and shelters for other animals. Sometimes holes in the cave roofs formed pit traps for unwary animals. The flesh decayed from the bodies of animals which died in or were washed into caves and pools, streams and lakes. The limestone rich waters quickly encased the bones in new layers of limestone, sealing them from view, but preserving and protecting them from damage for millions of years.

Ancient environments of Riversleigh

Studies so far reveal a staggering number of different kinds of animals preserved at Riversleigh. The diversity of animal life in its older sites (late Oligocene/early Miocene) is far greater than that known from Australia to-day, even in the tropical rainforests of northern Australia where diversity is at its highest.

The kinds of land animals in these older sites include many varieties of leaf-eating possums, lots of ground-dwelling browsing animals such as kangaroos and now extinct diprotodontids, together with many different omnivorous bandicoots.

Such a variety of animals could only have survived in a stable habitat containing a wealth of food sources supporting specialist as well as generalist feeders. Modern rainforests provide such conditions and it is thought that these ancient animals also lived in a rainforest environment. Significantly, many of these older Riversleigh animals have modern descendants which still live only in rainforests. As the accompanying photographs show, the rainforests are long gone from Riversleigh, the result of increasing aridity in Australia after about mid-Miocene times (about fifteen million years ago). Fossil remains in younger Riversleigh deposits reflect that change - species diversity decreases and some kinds of animals become extinct and are replaced by others better adapted to drier conditions.

Exposed high on the cliffs which confine the upper reaches of the Gregory River at Riversleigh, is an ancient cave floor from five million years ago. Remains of animals from here include more modern grass-eating kangaroos, demonstrating the spread of grasslands as the interior of Australia became increasingly arid. Also preserved in this deposit are remains of rats - which had at that time recently invaded Australia from Asia.

These fossil rat bones are remains of the midnight feasts of the large, carnivorous ghost bats who roosted in the cave by day and hunted rats and other small animals by night. The bats have a much longer history in Australia than the rats, occurring in their millions in many of the older Riversleigh sites. Interestingly, some of Riversleigh's leaf-nosed bats have close relationships with those known from Oligo-Miocene deposits in France.