The Giant Clubmoss Flora

Australia: The Land Where Time Began

The Giant Clubmoss Flora - Middle to Late Devonian and Early Carboniferous, about 385-325 Ma

Land plants had achieved 2 great evolutionary advances by the Middle Devonian in their structure and reproduction. The first land plants were limited to small size until they evolved the ability to produce secondary wood. The advance that allowed them to grow taller was the evolution of cambium, a tissue that produces xylem on the inner side of the cambium layer and phloem on the outer side of the cambium. The xylem transports water from the roots to the growing points of plants, and as new layers of it are produced the plant's diameter increases, together with the strengthening of the xylem, this allowed plants to grow to the size of trees. The phloem provided a means of transporting the sugar from photosynthesis to all parts of the plant and movement of plant hormones between different parts of the plant. The larger size attained could now help protect the plants from predation by the many herbivorous animals.

As plants reached the size of trees spore dispersing mechanisms began to be required and methods of wind dispersal were improved. The problem of the necessity for the presence of free water for the gametes to swim in was obviated by the development of heterospory, where the female egg cell is retained in the sporophyte.

By the Early Carboniferous the giant Clubmoss flora was dominant. The end of their domination occurred when Australia broke form Pangaea and moved towards the South Pole, rotating through 90^o on its way. During the Late Devonian and Early Carboniferous, there was a lot of volcanic activity on the continental shelf off the the east coast as Australia was breaking away from Pangaea. The line of volcanoes contributed large amounts of material to the sediments between them and the Queensland coast.

The primitive land plants comprising the Early Devonian flora shared close relationships with the vegetation from of the rest of the world at the time. This close relationship diminished as the continent became further separated from North America and northern European section of Pangaea, that were equatorial and tropical at the time. The main centre of the giant Clubmoss Flora was in the parts of Pangaea that Australia was separating from, and with increasing separation the Australian forests were evolving in isolation so diverged more ad more from the forests of Pangaea towards the Early Carboniferous. This flora didn't reach the same luxuriant state it reached in the northern hemisphere, and unlike the northern hemisphere, it didn't form coal deposits in Australia. At least partly, this was because as Australia moved away it headed for the South Pole, so the climate got progressively cooler.

As the Late Carboniferous approached the world climate entered a cooling phase, culminating in a glacial phase at the end of the Late Carboniferous. This global cooling was compounded by Australia's drift from the equator. During the Early Carboniferous, the floras of Australia and South America, especially Argentina, which was at about a similar latitude as Australia, were closely related.

The main groups of higher plants were already delineated by the Early Carboniferous. The Lycopods (clubmosses), Articulates (Horsetails) and ferns were reproducing by spores. The plants reproducing by seed were the Gymnosperms and Pro-Gymnosperms, and seed ferns.