Skyrail Nature Diary: May 2013

Scientists have shown that Australia was once part of a large landmass they called Gondwanaland, after an area in Africa. Africa, South America, India, Australia (including New Guinea), Antarctica, New Caledonia and New Zealand made up what used to be one supercontinent. Most of the rest of the world was united in another more northerly supercontinent known as Laurasia. A huge ocean known as the Tethys Sea separated the two. This meant that the plants and animals developed in different directions on the two supercontinents. Most significantly is the fact that there is mounting evidence that the first flowers originated in Gondwanaland. The oldest pollen fossils are found in South America and Australia. This immediately explains why the most ancient living flowering plants come from this region. The oldest flowering plant in the world is Zygogynum, found in New Caledonia. The next most ancient flowering plant is the Winterwood. Interestingly the two plants are closely related. This is because New Caledonia used to be  part of Australia. At a very early stage, a large landmass containing New Caledonia and New Zealand separated from Australia and developed in a unique direction on their own. This continent eventually split up and a large chunk of it was submerged. New Caledonia, New Zealand and a few other islands are all that is left.

India was the next landmass to separate from Gondwanaland, drifting north before crashing into Asia, creating the Himalayas. Africa separated shortly after, joining Laurasia near the Middle East. This means that India and Africa were affected early on by intermingling with Laurasian flora and fauna. This is probably also how flowers first entered Laurasia.

Next to split was South America which like Australia, stayed separate from the rest of the world. In the end a series of islands north of the continent were uplifted as a single landmass, Central America, joining South America to North America. This had drastic consequences. Like Australia, South America was at the time dominated by unique marsupials different to those in Australia. Many of these were exterminated when placental mammals from the north invaded. Only three groups of opossums survived. Finally Antarctica separated from Australia, causing Antarctica to freeze over and lose all of its original flora and fauna while Australia slowly dried out.

Gondwanaland has a number of unique plant groups. First up are the tree ferns, with four families still in existence. In Barron Gorge we have two species, the Scaly Tree Fern and Black Tree Fern. The largest of the tree ferns are in the pantropical genus Cyathea, containing 470 species. The tree ferns do not have real trunks. The "trunk" is actually the remains of the compressed leaf bases. These ferns are very old. Our Black Tree Fern only forms "trees" when growing in the shade. When growing in full sun, the Black Tree Fern tends to coppice sideways a clump of what seems like several tree ferns being only one specimen!

Southern Pines, families Araucariaceae and Podocarpaceae, are probably the most primitive conifers in the world today. As the name suggests, they are limited to the areas that used to be a part of Gondwanaland. Unlike the more familiar Northern Pines, these conifers only rarely grow in large stands. Often specimens are separated by significant distances. Only on New Zealand and New Caledonia do they approach dense populations. New Caledonia has the strangest of the lot. The only aquatic as well as the only parasitic conifers live there. The Kauris are well known thanks to their excellent timber. There are 21 species found mainly in the Australasian region (1 in New Zealand, 3 in Queensland, 1 in Fiji, 2 in New Guinea, 5 in New Caledonia and 9 in Southeast Asia). The Araucarias are well known ornamental trees, the most famous being the Norfolk Pine, Hoop Pine, Bunya Pine and the Monkey Puzzle Tree. The Podocarps are unusual in being some of the few conifers in the world apart from the Yew to have fleshy cones. The way to tell these cones apart from a fruit is that the seeds poke out the bottom of the flesh. Some also have excellent timber.

Laurels are an ancient group of flowering plants that originated in Gondwanaland, only two species of Sassafras having made it north into Laurasia. Typically laurels have glossy leaves with very obvious wax on the undersides and drip tips. These trees are adapted to heavy rainfall. The flowers are all unusual for dicots (the biggest group of flowering plants) in having petals organised in groups of threes, similar to monocots. The flowers are also very tiny and are often overlooked. Flowering is generally irregular. The biggest laurel flowers in Australia belong to the Northern Rose Walnut. There are some laurels that flower en masse in a way that can be easily observed from above. In Barron Gorge these include Mackinnon's Walnut, Brown Walnut and Yellow Walnut. Several laurels are considered to have excellent timber. In the Wet Tropics the laurels make up one of the four largest families of trees in the rainforest. All four groups are unique to Gondwanaland.

by Tore Lien Linde