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The Jurassic Period saw the complete melting of the polar ice caps, and a general warm and moist climate which extended across most of the planet. The Podocarp conifers, as well as some other conifers and cycads began to flourish and diversify.
Some important gymnosperm genera of the Jurassic (in Australia) were:
Agathis
Agathis was a genus of dominant conifers, which are sometimes referred to as "Kauri Pines".
Species of the genus had elliptical leathery (sclerophyllous) leaves and bud scales at the base of each shoot which represent a year's growth. The cones of this genus were large and round.
Agathis jurassica was a common species of the genus during the Jurassic, though there were others. Unfortunately, as Australia became drier,
Agathis retreated. Today Kauri Pines can only be seen growing wild in confined remnant rainforests.
A photograph of the foliage of Agathis
robusta.
There are approximately 21 species of Agathis today, so the group has not suffered the same fate as some other dominant flora before it, such as the
Glossopterids. However it is no longer a prominent part of the landscape. (A live specimen of
Agathis robusta planted in 1863 can be seen growing in the gardens of Burnley College, Melbourne, Australia).
Agathis may have led to the more advanced conifers which appeared in later eras.
The Podocarps or ‘Southern Conifers’
The Podocarps were derived from the Glossopterids. Like the Agathis species, this is a group that still survives today, but has lost the dominance it once had. Podocarps have long narrow-linear leaves or scales along their stems. An important and typical Podocarp genus was
Pentoxylon. It is thought that the Podocarps may have led to the Pandanales - a group of palms.
Important Podocarp genera include Podocarpus and
Dacrydium.
A fossil of Metasequoia
occidentalis, a gymnosperm from the Eocene.
[Image: FossilMuseum.net]
What are Gymnosperms?
The Coniferales (which includes Agathis, the Podocarps and other coniferous species) are classified as 'gymnosperms'. There are some important features of gymnosperms:
* Gymnosperms are mostly evergreen shrubs and trees.
* They have tracheids, but no xylem vessels.
* In the phloem, gymnosperms have sieve cells, but no companion cells - hence their flow of sap is restricted and less efficient than the Angiosperms that develop later.
* They have true megaphyllous leaves, and endospermic seed.
* They have unisexual inflorescences, with male prothallia being many-celled with motile gametes, and the female prothallus being large.
* They produce cones regardless of whether the gametes were fertilised or not.
* They have fine pollen.
The gymnosperms, which include pines and conifers, are still a common feature of many ecosystems today and many such as
Pinus radiata have economic importance too.
Foliage detail of Thuja, a
conifer species.
In the Atherton Tableland (Queensland, Australia) remain some remnant rainforests from the Jurassic era. Growing together are Kauri Pines and Podocarps, as well as an arboreous cycad called
Lepidozamia hopei; all of which give a glimpse of what the forests of the Jurassic would have looked like.
Of course, the plants in the rainforest of today are at a more evolved stage, and some 'original' Jurassic species are extinct so it is not exactly the same - but simular enough to be of value and interest.
A fossil of Ptilophyllum pecten, a
cycad from the Jurassic.
[Image: FossilMuseum.net]
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