Australia: The Land Where Time Began

A biography of the Australian continent 

Angiosperms

There was a sudden cooling, possibly with polar glaciation, after the warm, wet conditions of most of the Jurassic. It has been suggested that the eustatic sea level rise in the Early Cretaceous may have resulted from the melting ice caps, and possibly seafloor spreading that was occurring as Gondwana was breaking up. The faunal evidence from the southeast coast of Australia indicates that the sea was cool to cold. By the Late Cretaceous the oceans had warmed. At the time of high sealevels the marine transgression that covered much of central Australia has left sediments containing dropstones and ice-rafted boulders from melting icebergs. They indicate either mountain glaciers or rivers that froze in winter.

The changing climate of the Cretaceous had a big influence on the evolution of the Australian vegetation. The forests of conifers, cycads and ferns were struggling under the new climatic conditions. All continents experienced the inundation of vast areas of land, that resulted in the denudation of these areas as the seas retreated, exposing all this bare land for colonisation. The combination of climatic change and the availability of new land created the conditions for a burst of diversification. The old flora appears to not have had the flexibility to take advantage of the new opportunities, the Angiosperms diversified greatly to fill the new niches.

Palynology, the study of pollen, has allowed the discovery of the history of the early stages of monocots and dicots, the 2 main branches of the flowering plants, by studying the fossil record. The oldest known pollen, a "generalist" pollen about 124 Ma, has been found in the Barremian division of the Cretaceous from locations in Laurasia and west Gondwana. Pollen has been found from shortly after this time of several different types, Magnolioid, Lauralian, and Liliod. Their discovery indicates that radiation had already begun from the more general pollen type to more specialised types such as the 3 mentioned above. Western Gondwana appears to have been a centre of diversification, with early stages of evolution of Angiosperms at about 120 million years ago, as a concentration of early types of pollen from this time has been found. It has been suggested that they may have first appeared in western Gondwana from where they spread to other parts of the Southern and Northern Hemisphere, having diversified rapidly, as many families of flowering plants appeared over a very short period of time. The area of western Gondwana where the centre of diversity was situated was at the time a vast rift valley forming between northern Africa and northern South America.

In such an environment there would have been a mixture of habitats that changed with time, with swamps forming, arms of the sea spreading increasingly and high sedimentation rates as water courses carried the eroded material from the higher ground around the margins of the deepening and widening depression to depocentres in the rift valley, then all being gradually drowned as the valley became part of the sea. Many of the flowering plants would have been adapted to various parts of the changing ecosystem, such changing conditions tending to push the evolution of species better adapted to the various niches as they became available, then spreading before the area was completely covered by the sea.

Research has been showing that the earliest Angiosperms tended to be in areas of seasonal aridity. It is believed that they originated in riverine and fringe environments, then becoming shrubby weeds and spindly trees that infiltrated the established vegetation.

As can be seen in the rift valley of modern Africa, they have a microclimate that is different from that of the surrounding areas. The vegetation of the African rift valley has been described as having a "prehistoric look", with its giant groundsels and Lobelia in the Mountains of the Moon. It is thought that the appearance of the modern rift valley gives a glimpse of what the rift valleys forming between the continents as Gondwana fragmented would have looked like. The supercontinents had a network of rift valleys as separation got under way, just the time when the flowering plants were evolving. It is believed that this network of isolated environments not only led to the evolution of the flowering plants, by the presence of new niches under changing and often difficult conditions, but their sudden diversity early in their evolution can be explained by the diversity of habitats in the rift valleys, that were at least partially isolated from the surrounding areas.

There are a number of reasons that have been suggested for the rise of the Angiosperms at the expense of the previous flora. By the Late Jurassic, the ancient vegetation was declining in diversity. The new Angiosperms had enough genetic variability to adapt quickly to changed conditions. The older vegetation was slow growing, slow maturing and had a less efficient reproductive mechanism. The seeds of Angiosperms contained a bigger food supply for the embryo, so had a better chance of geminating and becoming established. They also evolved more efficient means of pollination, developing partnerships with insects, mammals and birds, where the animals carried the pollen of a plant to the stigma of another plant of the same species. The new efficiency of their getting the pollen to other plants helped with a more rapid adaptation to changing conditions. The Angiosperms arose at the start of the break-up of the supercontinent. This allowed the major families to spread to all parts of the world before the continents were isolated from each other.

Angiosperms and the Demise of the Dinosaurs

It has been suggested that dinosaurs allowed the Angiosperms to flourish because the huge herbivorous species that had arisen by the Jurassic had overgrazed the vegetation because they biggest could browse on the tops of trees up to 15 m above the ground, so they would have eaten many of the growing points of these slow-growing plants. The plants growing in higher latitudes would have been dormant or growing more slowly in winter but the browsing would have continued unabated, so they eventually reached the point where they couldn't regenerate fast enough, being replaced by the faster growing, more adaptable Angiosperms.

It has also been suggested that the rise of the Angiosperms lead to the demise of the Dinosaurs because they were unable to adapt to eating the new type of plant that often had toxic chemicals in its tissues such as alkaloids, where they plants could have caused a problem seen in cattle that eat pastures of lush, immature, fast-growing legumes such as clover and lucerne. This condition can be fatal if untreated in cattle.

Sources & Further reading

Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994