Australia: The Land Where Time Began
The Glossopteris Flora - Permian, about 275-245 Ma
The climate warmed after the ice age of the Late Carboniferous and earliest Permian, and a rich flora evolved that was characterised by Glossopteris. Coal deposits were formed in extensive areas of cool-temperate swamps where plant communities thrived. For the first time Ginkgoes and Conifers make their appearance in the fossil record, colonising the drier hill sides, not requiring swamps for their survival. There were also many tree ferns.
As the continental ice sheets melted in the earliest Permian the rising sea inundated large areas of Australia. The southern part of the continent was the first to become dry land again when the seas retreated. In Western Australia the Canning and Carnarvon Basins and in Queensland the Bowen Basin remained flooded by the shallow seas until the Middle Permian.
As a result of folding and uplift in the New England region, the areas covered by the sea around the margins of the continent was reduced, on the east coast the remaining continental sea covered an area between Rockhampton in Queensland and Grafton in New South Wales.
Though the climate was gradually warming, the Permian was still a cold time, the higher areas still being covered with alpine glaciers, especially on the mountains in the southeast of the continent. Much of this mountainous country had risen during the Kanimblan Orogeny of the Carboniferous. During this orogeny, a wide belt extending from Tasmania to Cape York underwent a period of uplift and deformation. It was the most severe orogenic event known to affect the Tasman Geosyncline. Ice-rafted boulders and drop stones are present in sediment deposited in the Sydney Basin during the Permian, evidence that the area was still subject to freezing winters. Trees petrified in this period give further evidence of a strongly seasonal climate in the growth rings in their wood.
At this time the climate in the northern part of the continent, Queensland, Western Australia and the Top End of the Northern Territory, was warmer than that in the south of the continent, and strongly seasonal, the summers are believed to have been temperate. Especially in the half of the continent, large areas of the drainage basins were occupied by coal swamps, smaller ones occurring in Western Australia and South Australia.
As the seas retreated after the transgression in the North-West Shelf area of Western Australia in the Triassic deltas formed. The accumulated sediments incorporated the gas reservoirs of the area. The main origin of the gas was from plant remains in the the deltas, but also from the marine organic matter.
There was a great expansion of plant life as the climate was warming, the floral remnants that survived the ice age gave rise to the Glossopteris Flora. The coal swamps of the Australia Permian were not the steamy tropical swamp-jungles of the Northern Hemisphere, being cold swampy bogs where Horsetails flourished like rushes. A dense, low swamp vegetation composed of ferns, Seed-ferns, and possibly mosses like peat, herbaceous Lycopods, such as Selaginella, grew in dense masses. In the swamps and adjacent areas of high water table, trees, shrubs and Glossopteris grew that had specially adapted aeration roots to allow them to grow in boggy conditions. Mangroves use the same type of roots to grow along the coasts at present.
The Glossopterids were deciduous, because of the very cold winters, and the Ginkgos were probably also deciduous. Those plants that retained their leaves would have spent the winter in a dormant state.
Glossopterid leaves are common and widespread in the deposits from the Permian of the southern continents that were part of Gondwana. They are also the most difficult to classify, because of the great diversity of venation patterns and the problems of assigning leaves on morphology alone.
Some of the leaves had a midrib and a network of secondary veins similar to those of dicotyledons by the end of the Permian, and Parallel veins are found in some as in Monocotyledons and some conifers. as in Bennettitalean Cycads, some taenopteroid, and some were very small and are believed to possibly be from plants that were woody herbs. In this Order of plants all leaf types were present, the Glossopterid Flora had the genetic potential to evolve in many directions. Some may have been the base of direct lines leading to living plant groups.
In the Glossopterids, an essential character of the leaves is the presence of lateral veins connected by cross connections, that resulted in a mesh or net-venation pattern on the leaf lamina. If there is a midrib the leaf is classified as Glossopterids. If there is no midrib, but there is a median groove and there is also a mesh pattern on the lamina like that in Glossopteris, it is assigned to the form-genus Gangamopteris. If there is no midrib and few cross-connections between the lamina veins, it is assigned to Palaeovittaria.
The midrib of Glossopteris is not a single vein, but is composed of a number of a number of veins that appear to be a single strand when they are close together, but they can be clearly seen when they are spaced more widely. Leaves are often grooved along the median line, some having a keel of thickened tissue for extra support. From leaves with a clearly defined midrib (Glossopteris), there is a gradation to types with well spaced strands that aren't aggregated into a midrib (Gangamopteris). If the preservation of the leaf is not good it can be difficult to decide if there is a midrib or a median groove. Leaves classified as Gangamopteris also grade to the leaf type at the other extreme, Palaeovittaria.
All the known leaf form-genera of Glossopterids have similar fructifications, so they are classified into a single order. As more complete plants can be reconstructed from the fragmentary evidence they are given a more detailed scientific classification.
As with eucalypts today, the Glossopterids dominated the vegetation of the entire landmass of Gondwana, They were an order of plants, and the similarity of their leaves , as well as their classification into genera, such as Glossopteris, is misleading. Eucalyptus is a genus with 450 species (it may possibly be reclassified as several closely related genera). Among the genera of the Myrtaceae, the family to which Eucalypts belong, some have similar leaves but different floral structure. Their classification is scientific because the structure of all their floral structures is known, so there is no need for subjective judgment when classifying them.
Glossopterid leaves are rarely found attached to a stem. Nearly all leaves in some shale deposits are the result of the trees being deciduous, leading to the accumulation of "autumnal banks" in lakes and estuaries during autumn. Glossopterid leaves come in a wide range of sizes, from those of Glossopteris ampla, with leaves up to 1 m long and of almost a similar width, to very small leaves, with all sizes between the extremes. The leaf shape varies from short, wide leaves to long, narrow leaves, and the apices of the leaves could be indented, pointed or blunt. The blade of some leaves tapered towards the stem, others bulged out into a heart-shaped base. The leaves in forms like Glossopteris duocaudata were shaped like a swallow tail. The petioles (leaf stalk) could be long, short or missing. The venation varied from fine, regular meshes to larger meshes at the midrib or large meshes covering the whole leaf blade. There was also variation of the angle between the lateral veins and the midrib and margin. Occasionally small branches are found that show that the leaves were attached in whorls or close spirals on the ends of small branches, the scar pattern visible on the branches indication where leaves had been attached in previous growing seasons.
Glossopterids were woody plants growing to the size of trees, and probably shrubs, that were presumable adapted to grow in the different habitats available at the time. They had Araucarioxylon typy wood ( a gymnospermous wood with tracheids of regular size).
The root systems were adapted for growing in swamps, having a specialised internal structure that is believed to have assisted in aeration. These roots are called Vertebraria, as their segmented cores resemble a vertebral column. Specimens of Vertebraria have been found that have the point at which they joined the trunk, showing the transition from the structure of the root to that of the trunk. The other end of Glossopterid trees are also known, the ends of smaller branches with attached leaves. An overall picture of the plant is now known, giving some idea of what they looked like in life. The structure of the shrubby forms and woody herbs can only be guessed at, but they must have been present by the Late Permian, and they probably didn't look much different from the equivalent sorts of plants living at the present.
The earliest known Glossopterid leaves, Glossopteris cyclopteroides and Gangamopteris angustifolia are found in the glacigene sedimentary rocks at Bacchus Marsh, Victoria. These deposits resulted from glacier activity during the Late Carboniferous and Early Permian ice age. The first Glossopterids known from the Sydney Basin are found in the Dalwood Group, Lochinvar Formation, Lower Marine Series, form the Early Permian.
The origin of the Glossopterids is very uncertain, but a possible ancestral plant has been proposed, an aphlebiate plant from the Carboniferous that lost the pinnate phase of its leaves.
The similarity of leaves doesn't appear to indicate a similarity of reproductive structures among the Glossopterids.
Female Reproductive Structures in Glossopterids
These are significant in the evolution of later plants from later periods. The different types of Glossopterid fruiting bodies that have been found characterise several plant groups in the Mesozoic, the origins of which are believed to probably be in the Glossopterid gene pool.
There are 2 distinct types of female reproductive structures among the Glossopterids, and they are different enough to divide the Glossopterids into 2 separate groups. One section has massive "fruits" containing many seeds and are carried on unmodified leaves. Within this group, the evolutionary sequence leads from a condition with fruits having many seeds to fewer seeds and also a trend towards the fusion of the bract (cover leaf), part of the fruit, with the receptacle containing the seeds. For the purpose of inclusion in a new classification, a suggested designation Glossopteridales, Sect. Megafructi.
The other Section, in which the seeds are attached to modified leaves or scale-leaves. Classification of this Section then becomes Glossopteridales, Sect. Microfructi. A similar situation exists in male reproductive structures that are known to belong to Glossopterids, the sporangia being borne of modified leaves or scale-leaves.
The first attached reproductive structures, from South Africa, are all from this section. Many specimens have now been found in India and Australia, allowing the structure of some of the different types to be understood in more detail, making it possible to make reconstructions.
All the reproductive structures of this type are borne on normal leaves, being either stalked or sessile, attached to the petiole or to the leaf blade at any point along the midrib. The distinguishing of a number of genera has been enabled by where they are attached. It has been found that some names given before enough knowledge had accumulates were premature, being given to fossils that were deformed by the process of fossilisation, and even to the state of maturity of the structure, instead of diagnostic features.
It appears the basic arrangement may be fruit composed of a receptacle (core), seeds being attached all around it, and protected by a bract (cover-leaf) while it is developing, thought to have fallen off when the fruit is ripe. The receptacle was either spherical or cylindrical (or of oval cross-section by dorso-ventrally flattening).
There are several lines of evolution arising from the basic fruit with the seeds attached all around the receptacle and a cover leaf. Some lines involve fusion of the fruit parts, either with each other or with the foliage leaf they are borne on. Some have reduced seed numbers, still others show a combination of both trends.
The fruit, are either stalked or sessile, attached to the leaf base or the petiole of the foliage leaf, in Plumsteadia, Scutum and Dictyopteridium. Seeds can be wingless (or with a surrounding narrow wing), or have a pronounced wing (Indocarpus type). Plumsteadia and Dictyopteridium have the former type, while Scutum has the latter, with an appearance of a fluted ring around the compressed fruit.
Specimens described as "Lanceolatus, from South Africa, have sessile fruit attached further up the leaf, and possibly fused to it, especially in the younger stages. When specimens in the Plumstead collection where examined in Johannesburg, the state of preservation wasn't good enough to determine if the cover-leaf was fused to the seed-bearing organ. The fruit has uniform rounded seeds, but the receptacle is much flatter and probably unifacial, appearing to be of the same general type as Plumsteadia.
In Australia, a fertile structure that takes the fusion to a later stage, described as Senotheca, appearing to consist of 2 rows of seeds attached to the midrib edges. Jambadostrobus and Venustotrobus are 2 genera from India. The former has a Plumsteadia-type fruit in which 2 or 3 seeds attached to the midrib in the mid region of the lamina. In the latter, a Scutum-type fruit has a wing and a cover leaf that is attached in the middle of the leaf.
Another form of fruit from India, from where the genus was originally described, described as "Ottokaria", has a long stem, and the head appears to be flattened, with a prominent wing and a coverleaf. This fruit is of the Plumsteadia-type, the coverleaf has become a wrap-around structure. It is the Indian specimens and reconstructions that define the genus.
Austroglossa is a form in which the seed number has been reduced in the Megafucti. The fruit has become a small number of Nummulospermum type, platyspermic seeds, that attach to a small branch attached to the petiole of a Glossopteris leaf.
A few specimens display the final stage of seed reduction, the single large Samaropsis-type seed with a narrow wing being attached to the base of a Glossopteris leaf. They are a larger version of the seeds seen in Plumsteadia. Similar large seeds have been described in South Africa, in which the seeds are attached directly to the midrib of Glossopteris leaves. This arrangement is the final stage of seed reduction that began with Jambadostrobus fruit.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|