The rise of the modern fauna and flora

During the first two-thirds of mammalian history, when the world was dominated by dinosaurs, mammals were inconspicuous but not static. They continued to evolve significantly. Their hearing became more acute and, unlike their own reptilian ancestors and the contemporaneous dinosaurs, they developed teeth which did more than nip off their food. Their teeth became suitable for cutting up the food and finally masticating it as well.

Modern mammals can be divided to two groups, the egg-laying monotremes and everything else; i.e. the marsupials and placentals. In the Mesozoic, there were many other groups that do not fit neatly into either of those catagories. Among the most successful of these outsiders were the multituberculates, "many cusped tooth". These rodent-like mammals persisted longer than any other mammalian group, 140 million years. They eventually died out when the true rodents arose in the early Cainozoic, displacing them. Other experiments in mammalian evolution that occurred when the dinosaurs lived were the triconodonts, "triple-cusped tooth", and docodonts, "beam tooth". These animals did not survive the Mesozoic. As the names suggest, Mesozoic mammals are best known from their teeth and often that is all that is available. This reflects the fact that mammals from the Mesozoic are exceptionally rare fossils and thus our knowledge of them is extremely meagre.

Our ancestors when the dinosaurs were alive did not look like Fred Flintstone. Rather, they were small mammals that, if seen alive, would look somewhat like shrews. Unlike the other mammals, the teeth of these mammals could both cut and grind their food. It was this innovation that eventually proved a vital step in mammalian evolution for all living mammals except the egg-laying monotremes. All marsupials and placentals, either possess this dental pattern or are descended from ancestors that did.

Sixty-five million years ago, with the demise of the dinosaurs and the beginning of the Cainozoic Era, the pace of mammalian evolution quickened. Mammals soon evolved to fill the empty niches left behind. Within 5 million years of the death of the last dinosaur, the largest mammals had increased in size from that of a beaver to that of a rhinoceros. As the continents moved and the environments changed, new forms of mammals succeeded earlier ones in rapid succession.

During the Cainozoic, there are three major theatres of terrestrial mammalian evolution: Australia, South America, and North America + Europe + Asia + Africa. Of these three regions, Australia was the most completely isolated. Marsupials entered Australia from South America via Antarctica about the time the dinosaurs became extinct. After that interchange, Australia began drifting north, which it continues to do at the rate of 10 cm/year. Because of its isolation as it drifted away from Antarctica, placentals were not able to reach Australia for at least 50 million years after the marsupials did. Without placentals as competitors, marsupials radiated into more different forms in Australia than they were to do anywhere else. Eventually, this northward drift made it possible for one group of terrestrial placentals, the rodents, to reach Australia from southeast Asia by the Pliocene. {Note: there is now an enigmatic Early Cretaceous specimen from Victoria which may be a terrestrial placental mammal. If it is, the appearance of terrestrial placentals in Australia in the Pliocene may be the second time they reached this continent.} Monotremes dispersed between Australia and South America in the Cretaceous or Paleocene.

South America was not quite as isolated as Australia. At the end of the Cretaceous, it received mammals from North America as well as had interchange across Antarctica with Australia. Then, it, too, became isolated. This isolation lasted about 50 million years with two exceptions. During the middle of this period, rodents and primates managed to reach South America by rafting from Africa. Other than that, a mammalian fauna evolved there in isolation that was composed of a variety of marsupials, primarily carnivores, and endemic placental mammalian orders.

Five million years ago, about the same time as rodents reached Australia, the Panamanian land bridge was established between North and South America. But while only one family of rodents entered Australia, in South America the story was much different. There, a whole host of mammals entered from the north, largely displacing the endemic fauna. A few South American mammals also managed to go in the other direction.

All through the Cainozoic, there were numerous episodes of interchange of land mammals between North America, Asia, and Europe. In the Early Eocene, the route was across the North Atlantic between Greenland and Norway. After that, the route was via the Bering Straits. It was this area plus Africa that experienced the greatest radiation of placental mammals. By the Eocene, all the living orders had appeared and by the Oligocene, virtually all the modern families had evolved.

Africa stands somewhat apart from the North America + Asia + Europe combination, although it certainly had more frequent interchanges with those other continents than Australia and South America did. However, the isolation was sufficient that a distinctly African fauna can be seen through the entire Cainozoic. The partial isolation of Africa was great enough that the Tethytheres arose in isolation there. This is the group of mammals which includes among other thinngs elephants, hyraxes, and the aquatic sirenians.

Bats, being capable of flight, crossed the water barriers that caused the division of terrestrial mammals into three or four distinct geographic realms during the Cainozoic. Although there is provincialism among bats, it is no where near as great as that seen among mammals forced to walk between various land masses. During Australia's long isolation in the Cainozoic, bats passed between this continent and Europe a number of times.

The ancestors of the whales re-entered the sea in the Eocene, their ancestors being primitive hoofed mammals. An excellent record of the intermediate forms now exists from rocks in Pakistan. The earliest whales were not as streamlined as modern forms and lacked the ability to echolocate. But by the Late Oligocene, essentially modern whales had appeared. Carnivores re-entered the sea long after the whales, the oldest seals, sea lions and walruses being Miocene in age. The herbivorous sirenians were part of the African-based tethyere radiation. Finally, there is an extinct group of herbivorous or molluscivorous marine mammals with no known antecedents and no obvious close relatives: the circum-Northern Pacific desmostylans.

The evolution of mammals through the Cainozoic was very rapid. While molluscan genera typically persisted for 5-10 million years in that time and plants for as much as 50 million years, mammalian genera commonly lasted for only 1-3 million years.

Cainozoic invertebrate faunas

The marine fauna of southern Australia can be considered to be composed of four main elements. These were:

1. Tethyan Indo-Pacific tropical genera having their major distribution in the Indo-Pacific region and representing warm-water or tropical influences.
2. Australian-New Zealand genera which may have originated in either New Zealand or Australia but are not known outside this region. These are generally temperate to cool water genera, some of which are related to forms from southern South America and even Antartica.
3. Endemic genera (i.e. not found anywhere else in the world). The number of these genera in southern Australia increased throughout the Cainozoic. These are temperate to warm-water groups, but not tropical.
4. Cosmopolitan genera (i.e. having a world-wide or nearly world-wide distribution).

The proportions of these four different elements fluctuated with time throughout the Cainozoic Era, and their relative abundances can give some general indication of the climate throughout the time interval. Thus at the time when the Indo-Pacific element was strongest, it is inferred that the sea temperatures were somewhat warmer than at present.

The response of the molluscan fauna to environmental changes, such as water temperature and ocean currents, is probably typical of the response of other groups. The earliest known molluscan fauna in southern Australia occurs in late Paleocene rocks (about 60 million years old) in south-western Victoria. This fauna contains about 90 species of molluscs of shallow-water origin, most of them assigned to cosmopolitan genera; very few of the genera are restricted to Australia. The composition of the fauna suggests that it lived in cool, temperate water. The fauna is not very similar to others of the same age found elsewhere in the southern hemisphere, such as in New Zealand and South America. This fauna lived in a seaway that opened up between Antarctica and southern Australia and which seems only to have been open from the west, thus preventing true oceanic circulation.

Molluscs are very poorly represented and little known in Paleocene rocks in other areas of Australia, particularly Western Australia. What little evidence there is suggests that the Paleocene fauna of north-western Australia may have lived in warmer water than that of south-eastern Australia.

By late Eocene time (about 40 million years ago), when the next youngest molluscan fauna of any significance lived, the basic elements of the modern molluscan fauna had been established. At about this time the rift between Australia and Antarctica had widened sufficiently to allow marine oceanic circulation to be established, so allowing marine larvae to be carried to southern Australia from elsewhere. Australian endemic genera form a significant part of the fauna, but there was also an influx of immigrants from tropical regions to the north and from New Zealand, probably as a result of increased oceanic circulation south of Australia. In addition, cosmopolitan elements are still significantly represented. This fauna lived in warmer water than that of the Paleocene, but not as warm as that of the more recent Oligocene and Miocene faunas. It occurs in south-western Western Australia, South Australia, and western Victoria, and is essentially uniform in composition across the continent.

The opening up of the Drake Passage between South America and Antarctica permitted the Antarctic Circumpolar Current to become established in late Oligocene time (about 23 million years ago). This meant that marine larvae could be carried eastwards from South America to Australia and New Zealand, and that larvae from Australia and New Zealand could be carried to South America.

The Oligocene and Early to Middle Miocene faunas (about 30 to 10 million years old) provide evidence of significant warming across southern Australia, with maximum temperatures in the Middle Miocene indicated by the presence of several Indo-Pacific genera that are not found in younger rocks. This warming was not uniform across southern Australia, temperatures in Western Australia being higher that in the south-east, but nowhere could conditions at this time be regarded as tropical. The temperature differences between east and west persisted through the Miocene into the Pliocene, but a gradual cooling took place during the late Miocene and Pliocene. By latest Pliocene time (1 million years ago) there seems to have been more uniformity in temperature across southern Australia, and at this time the fauna was essentially modern in its composition.

Quaternary megafauna and its extinction

In other parts of the globe at the same time, there evolved similar gigantic animals that dwarfed their modern descendants or nearest relatives. In South America lived the giant ground sloth Megatherium, in Eurasia and North America there was the Mastadon and Mammoth, while in India a giant tortoise appropriately named Colossochelys roamed the land. In New Zealand, until less than 400 years ago, there was a variety of ground birds called moas, some being gigantic forms such as Dinornis , and others being of more modest size such as Euryapteryx.

These forms all represented evolutionary lineages that had become progressively larger during the Late Cainozoic and then suddenly became extinct in the last 50,000 years. For over a century a similar phenomenon has been recognised on many other continents as well except, remarkably, Africa. Why did it occur? The two causes generally offered have been severe climatic change and the appearance of humans. That the fauna of Africa interacted with humans as they slowly acquired weapon-making ability and gradually increased their impact on the environment, is given as the reason that the fauna there coped better with humans than on other land masses.

When one looks at the evidence on a place-by-place basis, the case for humans causing mass extinctions of the megafauna versus climatic change being the trigger varies. New Zealand is a good case in point. About 10,000 years ago, long before the Maoris arrived, there was a major climatic shift there. This resulted in the widespread development of forests at the expense of grasslands and tundra. The fossil evidence suggests that the moa fauna declined considerably at that time. When the Maoris did arrive about 1,000 years ago, there were still a large number of living moas. Archaeological evidence clearly shows that they were hunted intensely and within about 500 years, were extinct. Thus in at least one place, both climate and human activity took their toll on the megafauna.

Extinction of the megafauna in Australia is more difficult to document than in New Zealand for the simple reason that it happened much further back in time, at least 15,000 years ago and perhaps as long ago as 100,000 years. Thus, much of the evidence of the type available in New Zealand has simply disappeared owing to the operation of normal geological processes. No one has ever found an undoubted kill site with the butchered remains of Diprotodon or the large, flightless bird Genyornis, for example. If humans did cause the extinction of the megafauna here, it could well be that the event occurred for reasons other than actual hunting. With the arrival of humans, the practice of burning off the country to make movement through it by people easier as well as to improve the fodder for grazing marsupials began. Whether such changes to the environment could have brought about a breakdown of a previously existing precarious natural balance is not known. The introduction of the dog was not a factor because they did not appear in Australia until about 3000 years ago, long after the megafaunal extinction events had taken place. But this is not to say that humans may not have altered the environment in other ways such as the inadvertent introduction of diseases and parasites.

The 'blitzkrieg' model of human-caused extinction of the megafauna was first proposed for North America. It posits a brief period when humans intensely hunt the megafauna, causing its extinction. What is thought to have happened is that humans arrived in Alaska 11,000 years ago after having crossed the Bering Landbridge from Siberia. What they found was a mammalian fauna that had no experience of being hunted by humans. In this veritable Garden of Eden, there was a human population explosion as the hunting society thrived for a generation in any one area until the animals of the megafauna became locally extinct. As that happened, the humans moved on, ever southward until they reached Tierra del Fuego at the southern tip of South America. Whether such an extreme model of human-caused extinction of the megafauna is appropriate for Australia is moot. Unlike North America where butchered wooly mammoth bones have been found, there is no firm evidence for the megafauna having actually been hunted in Australia.

During the Quaternary, the climatic trends towards more arid conditions which had prevailed in Australia during the last half of the Tertiary continued. Added to this was the climatic shifts brought about as the glaciers advanced and retreated in the northern hemisphere. During times of glacial advance, the Australian climate became exceptionally harsh with dune fields developing in what are today grasslands and even forests in Tasmania and Victoria. Because the glaciers advanced and retreated several times during the Quaternary and the megafauna extinction is only coincident with the last maximum, (if it in fact is), the question naturally arises why these extinctions did not take place earlier?

Thus the weight of evidence at the present time would favour a human cause for the extinction of the Australian megafauna, be it as a direct effect of hunting or indirectly through habitat alteration or possibly a combination of both. However, all the evidence cannot be said to be in and future research may change the current consensus.

Development of the modern flora

Within Australia, the same floral suite that occurs in Eocene rocks at the Alcoa Anglesea coal mine can be found today forming the rainforest along the Daintree River north of Port Douglas, Queensland. This shift reflects the changing climatic conditions within the continent over the past 50 million years. Likewise the rise and spread of Eucalyptus, bluebush and saltbush during the Oligocene corresponds with the progressive general dessication of the continent during the Cainozoic as it moved northward out of the polar latitudes and into the subtropical ones. This uniquely Australian flora adapted to the steadily declining general quality of the soil as the continent was widely leached upon reaching low latitudes and did not undergo the geological renewal processes of extensive glaciation or mountain building.

Some elements of the living Australian flora interchanged with those of New Zealand, New Guinea and South America during the Early Cainozoic just before land connections across the southern end of the world were severed. Prominent among these is the southern beech or Nothofagus which occurs on these landmasses today in cool temperate regions. Fossils of Nothofagus have also been found in Antarctica. To walk through a forest of Nothofagus is to experience the type of environment that the marsupials which reached Australia from South America via Antarctica would have passed through.

What happened with Australia's flora during the Cainozoic mirrors what occurred elsewhere as the Earth's climate gradually became cooler. Ranges of plants generally shifted towards the equator as the era progressed. But there were interesting exceptions to this generality which go to show that each species adapts to the particular circumstances it finds itself in rather than follows a mysteriously set general trend. One of these exceptions is the movement of plants into the Arctic Circle as the temperatures there declined and their subsequent withdrawal again from that region as the mean annual temperatures became even colder. This curious circumstance came about because during the Early Cainozoic when the temperatures at high northern latitudes were relatively high, the plants in question could not have survived the annual period of continuous darkness in the Winter. At those temperatures, they would have continued to metabolise in the dark when they could not obtain any energy input from the sun. As the temperatures became colder, a threshold was reached where they would shut down rather than metabolise. In those conditions, they could survive the Winter darkness and so the area inside the Arctic Circle became accessible to them. But as the temperature became even colder, these same plants could no longer tolerate the conditions and so their ranges shifted in the opposite direction, towards the equator, then following the general Cainozoic trend.

References:

Stanley, S. M. 1986. Earth and life through time. W. H. Freeman & Co., New York.

White, M. E. 1993. The nature of hidden worlds. Reed, Chatswood, NSW.