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 Plant Evolution Tour: Part VIII of XV

During the Permian (280-235 Ma), the ice age ended and the world warmed up. There was a rapid evolution of a range of plants including the Ginkgoales and Bennettites, as well as the dominant flora of the era; the Glossopterids. For the first time, seed plants became the dominant reproductive type.

The continental drifting of the Carboniferous era led to the formation of the super-continent Pangea by the early-Permian (300 Ma).

This led to a relatively cool climate with much glaciation, but by the mid Permian the climate was warming. As the continental ice sheets contracted, the waters rose and many parts of Australia and other continents became flooded with shallow seas.

That said, the climate was still cool and many areas of high altitude remained covered in snow. Cool-temperate swamps became a common feature of the landscape (particularly in Australia) and aside from that produced in the Carboniferous, a lot of coal was also produced in the Permian.

Some significant groups of plants evolved during the Permian in the climatic conditions which became progressively drier; these being the Bennettites, Cycads, Ginkgos and Glossopterids.

Cycadales: The Cycads

The cycads first appear in the fossil record dating back to 280 Ma whereafter their fossils (especially compression fossils) become very common. The early cycads grew to as much as 15 metres or as little as three metres which is similar to modern cycads. In fact there is remarkable similarity between extant cycad species and many of the fossilised specimens which have been discovered.
A typical cycad. Photo by Steve Williams, used under a Creative Commons licence.
The cycads were all dioecious (male and female plants) and had a distinctive trunk upon which grew 'fronds' composed of pinnate leaves. The trunk was sometimes branched and is composed of leaf bases. The female reproductive structures of cycads are called megasporophylls which are modified leaves and contain the ovules. The male reproductive structures are also megasporophylls which contain pollen sacks. These features have been well-preserved in fossils.

The origins of cycads are not entirely clear, however it is suspected that they evolved from the Medulosaceae; a family of Seed Ferns.

Bennettitales (Bennettites)

The Bennettites are very similar to Cycads, and were widely spread across the planet between the early Triassic to late Cretaceous (248-140 Ma).

It was once considered that the Bennettales might have been a precursor to flowering plants, however this now appears not to be the case. This conclusion was drawn from the Bennettite Williamsoniella coronata and W. gigas which has reproductive structures which appear to be very "flower-like" however there are other morphological features which differentiate them, such as the presence of one instead of two integuments surrounding the ovule (Willis & McElwain 2002).

Like Cycads, the Bennettites had long 'frond-like' pinnate leaves which radiated from a central trunk. Unlike Cycads, it appears that the Bennettites were deciduous.

Ginkgoales (Ginkgos)

The Ginkgoales first appear in the early Permian fossil record (c.280 Ma) and are limited today to a single species; Ginkgo biloba which has become popular in recent times for its herbal qualities. G. biloba is a native of China and has been cultivated for centuries.
Ginkgo biloba
Ginkgo bioloba.
At their peak, the Ginkgoales were represented by approximately 16 genera which were distributed throughout many parts of the world. The leaf morphology of the Ginkgoales differed significantly between species; from almost entire leaves to heavily lobed specimens. However ginkgoalean leaves are often quite recognisable for their dichotomous venation and their distinctive shape which resembles the leaves of the maidenhar fern and is the origin of their common name "maidenhair tree".

Many fossilised leaves of Ginkgoales have been found. It is widely thought that the Ginkgoes were deciduous (which is consistent with high rates of leaf fossilisation) and also because G. biloba is winter-deciduous. G. biloba is also dioecious (male and female trees) and so were its ancestors. The male trees produce catkins which release pollen, and female trees produce ovules which are arranged on a similar structure. (For more information on Ginkgoes, refer to the Ginkgo Page).

The other significant group of plants to develop in the Permian were the Glossopterids.

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