Plant Science Portal 
Seed Ferns & Cordaites
 Plant Evolution Tour: Part VII of XV

The mid to late Carboniferous and early Permian saw the decline of the Giant Clubmoss Flora and the expansion and proliferation of the seed ferns. The climate was no longer so favourable, and many parts of the world experienced 'Ice Ages'.

Amongst the lycopsids, progymnosperms, filicopsids and horsetails during the late Carboniferous (354-290 Ma) grew two other significant groups of plants; the pteridosperms and cordiates.

These two groups of plants were significant because they were were seed plants which did not use spores for reproduction.
Alethopteris
A fossil of Alethopteris, a seed fern. Photo by FossilMuseum.net
Pteridosperms: The Seed Ferns

The seed ferns were a unique group of plants because they had the appearance of ferns but reproduced using seeds. Most of the seed ferns resembled modern tree ferns however some had a prostrate creeping habit. A significant and dominant species of seed fern was Medullosa noei.

Medullosa noei grew to 10 metres in height, and had megaphyllous pinnately dissected fronds which were dichotomously branched. The vascular system of this species was reasonably advanced, and the base of the trunk had adventitious roots which provided support. The remainder of the trunk was covered in leaf petioles (leaf bases) similar to in modern tree ferns.

Seed fern ovules were probably attached to the fronds, and pollen grains were released from pollen sacks which existed on the fertile fronds of the fern. Fossils for the seed ferns date from c.354-248 Ma (late Carboniferous and Permian) and no seed ferns have survived to the present day.

A significant genus of Australian seed ferns was Rhacopteris. Many of the fossils of Rhacopteris have been found in New South Wales and generally date from the Late Carboniferous. Because Australia was experiencing an extended glacial period through much of the late Carboniferous (308-290Ma; see maps), the growth and stature of the Australian Rhacopteris flora of seed ferns was much less than those from other continents. Furthermore, the appearance of seed ferns occurred later in Australia than in other locations. White (1998) provides a pictorial overview of the Rhacopteris flora.

Cordaites

Cordaites were gymnosperms which appear in the fossil record between 330-250 Ma and were a group of tree-like plants which grew up to 30 metres tall and had trunks up to 1 metre in diameter. Cordaites had a distinctive trunk and separate branches to which narrow-elliptical leaves were attached in whorls. The variable leaves had a similar morphology to modern-day Agathis species, however the leaves of the Cordaites could each be as long as 1 metre in length and 15 centimetres in width. The leaves usually had multiple parallel veins and no distinctive midrib although some species which had needle-like leaves had a more obvious midrib.

The male and female reproductive structures were separate meaning the plant was dioecious. The male structures were usually located on short stalks radiating from the leaf axis and contained a lot of scales wrapped around the shoot axis similar to a cone. The upper scales has pollen sacs. The female structures were similar but instead of pollen sacks, had ovules.

The Climate of the Late Carboniferous and Permian

It was during the mid to late Carboniferous, around 325 to 285 Ma that the world started to cool. Pangaea was beginning to split apart into Laurasia and Gondwana.

Polar ice caps were forming and a massive ice sheet covered southern Australia, most of India and the southern halves of both South America and Africa.

Like today, Antarctica was completely clothed in ice and snow. The cooling of the climate resulted in many extinctions, including the eventual total extinction of the Giant Clubmoss Flora (which had never experienced luxuriance in population) and the decline of the horsetails..
Contact

Contact Adam Dimech

To contact me, please use the

Feedback Form

or send a message via the following social media:


Facebook Flickr GooglePlus Twitter