Australia: The Land Where Time Began

A biography of the Australian continent 

Roof Shale Flora - Permian-Basal Triassic Transition Zone

Dicroidium callipteroides, the first of the forked-frond Seed-ferns, appeared in the fossil record, only briefly, in the boundary between the Permian and the Triassic. Unlike Dicroidium species which appeared in the Triassic, where the fronds fork once, the fronds fork repeatedly. It is believed it may be ancestral to Dicroiodium on the one hand and on the other, Lepidopteris and the Peltasperm line of Seed-ferns. Its ancestry is unclear.

Glossopterids are the Seed-ferns that preceded Dicroidium callipteroides. It has been suggested that as a result of the sudden climate change, there may have been a "throw back" to the pinnate form, and the forked-frond Dicroidium complex could have arisen from the "aphlebial" Glossopterids.

The Lycopods of this flora and that of the Narrabeen Group from the Early Triassic include an Australian genus Cylomeia, the appearance of which differs from that of Lepidodendron model of Lycopods that were characteristic of the Giant Clubmoss Flora. Each plant is like a small palm, and does not have bottle-brushes of leaves on the smaller branches. It had a single woody trunk about 1 m high with a crown of ribbon-like leaves that were flattened into a disc. In the coal mines where they are found the miners refer to the fossils of separate leaf-whorled crowns as "chrysanthemums". They had root buttresses (rhizomes) that were lobed and covered with stigmarian rootlets.

Cylomeia had well organised cones of the Cyclostrobus type and reproduced by spores. Species, such as Cylomeia capillamentum, that produced large cones appear to have carried them at the stem apex in the centre of the leaf whorl. Species that produce smaller cones are thought to have had their cones hanging out of the crown on a thin stem. This is not certain as the small cones that have been found attached to a stem or attachment, the few known with slim stems in attachment seem to be so thin that they would not be expected to be strong enough to support the weight of the cone. All Cyclostrobus cones have large female spores (megaspores) on the sporophylls lower down on the cone, and microspores on the distal sporophylls.

Small herbaceous Cylomeia-type Lycopods have been found in the Narrabeen Group deposit in the Capertree Valley in New South Wales. The bases of their cones are about  cm wide and contain mature megaspores in the centre of the leafy discs. This indicates that they at their full size.

It is significant that in the Roof Shale Flora Podocarp-type conifers suddenly appear. The modern distribution supports the suggestion that they arose in Gondwana and that their evolved from Glossopterids.

Sources & Further reading

Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994

Floras of Ancioent Australia

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading