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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.
An
illustration of an extant cycad species called Encephalartos
voiensis and a close-up of the cones of another
species of Encephalartos.
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).
A
Bennettalean fossil.
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.
A
photograph of Ginkgo biloba, the only remaining
species of the once large group of Ginkgoales
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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