Australia: The Land Where Time Began
Dicroidium Flora - Middle to Late Triassic 245-208 Ma
At the end of the Permian and the start of the Triassic was a warm, wet time. There was a sudden appearance of new flora characterised by the first forked-frond Seed-ferns, Podocarp Conifers, Cycadophytes, ferns, Ginkgos. During this time the Glossopterids drop out of the fossil record. During the Triassic, the climate was changing to become hotter and drier, this led to the development of drought-resistant adaptations in some plants of the Middle and Late Triassic. The flora of this period is characterised by Dicroidium Flora
In the Triassic, the climate was warm to hot, and there was a strongly seasonal rainfall regime. It is believed there was probably a rapid change form the cool temperate climate that characterised most of the Permian to the warm, moist boundary between the Permian and the Triassic. Evidence for the climatic change comes from the Roof Shale Flora (Callipteroides Zone Flora) found at the top of coal sequences in the New South Wales South Coast coalmines, where a rich flora of conifers and Lycopods suddenly appeared and the Glossopterids just as suddenly disappeared.
The Narabeen Group Flora from New South Wales also has characteristics associated with a warm, moist climate, with no signs of a marked seasonality of rainfall, with the large leaf foliose expression of the component of the species. Plants of this flora lack the xenophyll adaptations present during the later Triassic.
In the Middle to Late Triassic, the Dicroidium Flora displays characteristics associated with a warm to hot dry climate. Except for areas of South Australia, Leigh Creek, Tasmania, northeastern New South Wales, and southwestern Queensland, the conditions conducive to coal formation were greatly reduced. In the inland areas the warm to hot climate with marked seasonal rainfall discouraged coal formation that had been supported during the Late Permian. The existence of Red Bed formations confirms an alternating sequence of long dry seasons and wet seasons with moderate precipitation.
The sea spread from Broome eastwards into the area between Geraldton and Carnarvon, then retreated in the Middle Triassic. In the Early Triassic, a marine transgression occurred in the North West Shelf area. This was followed by the formation of a river system and deltas near the Exmouth Plateau. At the end of the Triassic there was another transgression. In the Bowen Basin and Sydney Basin of eastern Australia, freshwater sedimentation was feature of the period. There was also volcanic activity on the continental shelf and land east of the basins.
The leaves of Dicroidium display a very wide range of pinnule form, the intermediates often grading from one type to another. This causes a lot of problems for the allocation of names to a particular species. Increasing adaptation to aridity is found among suites of forms, with decreasing lamina size, thickened cuticle, and leaves being reduced to narrow spikes. The various types were originally allocated to a number of genera, but now they are all included in the single genus Dicroidium. The species at the arid-adaptation end of the continuum may still be referred to Xylopteris, but based on the reproductive structures and cuticle, all appear to be very closely related, whether they are from the most foliose or the most reduced extreme.
The female reproductive structures are of the Umkomasia type. They are essentially single-seeded cupules on the ends of ultimate branches of pinnate structure, being the modified pinnule of a fertile frond.
The male reproductive structures are of the Pteruchus type, the heads, or catkins, of sporangia containing spores, each possessing 2 air bladders, allowing long distance wind dispersal.
The reproductive structures of both males and females are similar to those of Glossopterids. The sporangia of the males are similar to those of Eretmonia, but with the scale leaf reduced or missing. The single-seeded cupules are similar to those of Partha and Denkania (without scale leaves), or if in flower-like heads, are similar to Lidgettonia.
Dicroidium odontopteroides var. maltenensis, a variety of the odontopteroides type frond from Bennelong, New South Wales. Considerable variation of form is seen even in a single frond.
Plants of the Dicroidium Flora
In this flora Dicroidium is associated with a number of other plants - seed-fern, such as Lepidopteris, tree-ferns, Ginkgophytes, Cycadophytes, Conifers and Horsetails.
Lepidopteris (family Peltaspermae)
The fronds are very similar to those of Dicroidium, but their female reproductive structures are different. The family name refers to the shield-like structure, the peltate, to which the seeds are attached. This group of seed-ferns is characterised by the radically symmetric peltate heads with a central stalk, the seeds being attached to the underside of the head segments. This is similar to the Lidgettonia type female structure in the Glossopterids. Male structures known as Antevesia are similar to Pteruchus.
Lepidopteris stormburgensis. From the Middle Triassic, 225 Ma, at Bennelong, New South Wales. The fronds of this plant differs from those of ferns in frond structure, there are pinnules atttached to the rachis between the pinnae.
As at present, ferns were abundant in sheltered habitats. Petrified trunks of tree ferns show that they could reach large size, there were also delicate ferns growing on the ground.
These were becoming common in floras of the Triassic. The leaves of these plants could take a variety of forms. There were strap-like leaves, leaves divided into a few broad segments, or linear segments arranged like a fern.
In the Late Triassic the first cycad type leaves begin to appear in the fossil record. The reproductive structures of these earliest plants with cycad-like leaves are unknown, it is the cuticle structure of the leaves that indicates that they are related to cycads. Some are assigned to the form genus Tarniopteris. These leaves have long, narrow, parallel-sided form with parallel lateral veins that run to the margins at right angles to the midrib. Very larger leaves are assigned to Macrotaeniopteris. The name Taeniopteris is derived from the shape, thought to be long and thin like a tapeworm, Taenia. The specific name of Cycadopteris scolopendrina means centipede.
In the Triassic, several general types of conifer were common. A type that was abundant in some areas were Podocarps of the genus Rissikia. Some less common leaves from the Permian were scale-like leaves such as Walkomiella, and needle-leaves of Voltziopsis. The conifers would tend to grow on the drier hillsides away from water, so were less likely to be preserved as fossils, so were probably more common than is indicated by the known macrofossils.
These were common throughout the Triassic, especially in the Narrabeen Group sediments deposited in large deltas where they could be fossilised in situ. The have also been found in deposits such as the Beacon Hill Quarry, Brookvale, New South Wales, the Blina Shale, northeastern Canning Basin, Western Australia and the Erskine Sandstone, also in the northeastern Canning Basin.
Dicroidium is a seed fern. It dominated the Gondwanan vegetation following the decline of another seed fern, Glossopteris, which had previously dominated the flora. Among the other plants present in the Dicroidium dominated vegetation were ferns, other seed ferns, ginkgophytes, cycads, conifers and horsetails. This vegetation type had many components that were already showing adaptations to arid conditions that pre-adapted the plants that were to survive and flourish in the later aridification of the Australian continent. Some of these adaptations were thickening of the cuticle and reduction of the surface area of the leaves, some to the point of becoming needles or spikes, phyllodes.
Maey E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
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