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Mega Clubmoss Flora
 Plant Evolution Tour: Part VI of XV

By the late Devonian and early Carboniferous (385-290 Ma), plants had developed the ability to produce secondary wood through the development of a cambium layer. Hence, vascular plants were no longer restricted to heights of less than 1 metre and forests developed with trees as tall as 35 metres. An ideal climate helped the spread of the flora of this era, creating the Great Coal Forests.

There are two parts to the story of the Carboniferous: the climate at the plants.

The Climate

Between 395-290 Ma, there was significant continental drift and dramatic shifts in the world climate. By 300 Ma, the supercontinent Pangea had formed (see map).

Between c.360-290 Ma, there were four periods of southern hemisphere glaciations (ice ages).

Throughout this period, continental climates became drier except in the equatorial zone where there was increased precipitation. It is thought that the considerable coal deposits in Russia, Western Europe and North America were created by the formation of swamps in this equatorial zone during the Carboniferous. The concentration of atmospheric carbon dioxide (CO2) between 360-286 Ma was about 3600 ppm, but this reduced to 300ppm by the late Carboniferous. Increased CO2 would have promoted rapid plant growth but also created a significant "greenhouse effect".

The reasons for the decline in the atmospheric CO2 levels in the late Carboniferous are several; however it is thought that the rapid colonisation of land by plants significantly contributed to carbon reduction. Plants would have released organic acids which weather rocks in a reaction utilising CO2. This reaction (CO2 + CaSiO3 > CaCO3 + SiO2) led to the burying of carbon as carbonate (CO32-) under the ocean floor where it could no longer be released back into the atmosphere.

In addition, plants were synthesising chemicals such as lignin (the chemical that makes plant cells woody) which were more resistant to breaking down. It has been suggested that the massive rates of accumulation of biomass (plant matter) would have exceeded decomposition by fungi, and that the burial of massive amounts of plant material during the Carboniferous led to the formation of today's coal reserves.
A photo of Huperzia squarrosa, showing a growth habit typical of Lycopods.
A photo of Huperzia squarrosa, showing a growth habit typical of Lycopods.
The Plants: Modifications

During the late Devonian (390-365 Ma) there was massive diversification of plants and the creation of many new species. However this levelled and eventually started to decline at about 360 Ma at the same time as there ware major animal extinctions. During this time, there were significant "improvements" evolving in plants.

These included:
  • The development of better reproductive structures.
  • Evolution of a more effective vascular system for water and nutrient transport including the development of a stele.
  • Better supporting structures such as above-ground root mantles, thickening of tree trunks, and an expansion of supporting root systems.
  • Modifications to leaves and root systems.
For the first 40 million years of existence, plants had either no leaves or spine-like structures which functioned like modern leaves (Kenrick 2001).

True leaves first appear in the fossil record in the mid to late Devonian (390-354 Ma) and belong to one of two groups, the microphylls and megaphylls which are both seen in the modern flora.

Microphylls are small leaves which grow out directly from the stem (meaning they are sessile) and which have a single unbranched midvein. They can be seen in Lycopods such as Lycopodium and Huperzia.
Macro foliar detail of Huperzia squarrosa, a descendant of the early lycopods.
Macro foliar detail of Huperzia squarrosa, a descendant of the early lycopods.
Megaphylls have a multi-branched veination system and are more commonly seen in the modern flora (including all flowering plants). Megaphylls tend to be attached by a stem called a petiole and so are termed "petiolate". There is some uncertainty whether the microphylls are more advanced (that they are reductions of megaphylls) or whether the microphylls are more primitive (that they developed first and the branched vascular systems of the megaphylls evolved from them).

Of further significance to plant biology, it was during the late Devonian between 395-286 Ma that seeds developed (to replace spores).

This was a major evolutionary advance, because it eliminated the requirement for external sources of water for sexual reproduction of plants. It also provided better protection and a nutrient source for the developing embryo. This process of development is thought to have started with the conversion of plants from producing spores of one size (homospory) to producing spores of two sizes (heterospory). It is thought that the larger spores were precursors for ovules, and the smaller ovules precursors for pollen.

The development of heterospory and ultimately seeds in terrestrial plants is shown in the following diagram which shows spore sizes in the geological record between the Silurian and Carboniferous.

According to Traverse (1988), megaspores appeared at around the mid Devonian (c.400 Ma) and seeds appeared during the late Devonian. By the end of the Devonian, sporangia were aborting most of the spores they contained and allowing just one to develop to maturity (and to benefit from all the energy). In addition, many species had megaspores with an outer protective coating which means that ovules had evolved. Pollen evolved at this time also.
Triassic Neocalamites
A fossil of a Neocalamites sp. - a Calamites horsetail from the Carboniferous era. This fossil clearly shows the whorled nature of the leaves of horsetails around the jointed stem. Photo by FossilMuseum.net
Significant Plants of the Devonian and early Carboniferous

The key plants of the Carboniferous were:
  • Lycopsids (Lycopods/Giant Clubmosses)
  • Equitsetopsida (Horsetails)
  • Filicopsids (Ferns)
  • Progymnosperms
Lycopods

The Lycopods were a group of early trees reaching as high as 35 m, with the central genus being Lepidodendron. While lycopods existed as long ago as 410 Ma they did not exist as trees until 370 Ma. The smaller branches were clothed in narrow-linear leaves which were arranged on the stem in the same way bristles are arranged on a bottle brush - in whorls. As these dropped off with maturity of the branches, the leaf bases (called leaf cushions) remained on the trunk giving the lycopods a distinctive patterned appearance.

Lepidodendron had cones comprising of scale leaves (sporophylls) which carried sporangia. Each cone could measure up to 50 cm in length and release over 8 billion male spores per cone. The lycopods were heterosporous and the female spores which were larger were released in the hundreds.
Lepidodendron Bark Section of Clubmoss Tree
A fossil of the trunk of Lepidodendron mannabachense, a club moss. Photo by FossilMuseum.net
The lycopods were dominant during the Carboniferous, and it has been suggested that they formed more than two-thirds of the earliest forests. Modern descendants of the lycopods (of which there are few) include the Pendant Tassel Ferns (Huperzia and Lycopodium) and small Club Mosses (Selaginella).

Equisetopsida (Horsetails or Spenopsids)

The Giant Horsetails were an important group of plants in the Carboniferous, being co-dominant with (but more successful than) the lycopods. They were very important in the northern hemisphere, and while they are represented in the Australian fossil record, they never reached the same sizes. They were found growing in swampy areas, as do their modern descendants.

Horsetails have jointed stems from which whorled microphyllous leaves are produced. A significant species of horsetail was Calamites which had two types of leaves depending on the species; either linear leaves in whorls of 4-40 measuring 3 millimetres in length, or lanceolate leaves to 8 millimetres in length in clusters of up to 52. Calamites grew to 18 metres in height and spread through the use of a creeping rhizome which sent up new shoots.

In the Giant Horsetails, the sporangia were terminal and homosporous, though some very large species which occurred in the late Carboniferous were heterosporous. When the swamps which favoured the growth of horsetails eventually disappeared, smaller-sized species developed.

Today the only living horsetail genus is Equisetum which comprises about 20 species.
Multiple Annularia Leaflets from France
A fossil of Annularia stellata, a horsetail from the Carboniferus. Photo by FossilMuseum.net
Filicopsids (Ferns)

The ferns first appeared in the early Carboniferous (c.360 Ma). Many modern ferns have substantial similarities to the ferns of this era. Some of the ferns grew to as much as 10 metres in height, such as the tree fern Psaronius which was similar to modern Dicksonia. Psaronius had megaphyllous leaves which developed as fronds. The spores were homosporous and produced in fused clusters on the undersides of the fronds, called synangia.
Pteris tremula
Pteris tremula, a modern-day fern.
Progymnosperms

The progymnosperms included the Aneurophytales, Arch├Žopteridales and the Protopidyales which were the precursors to the conifers (gymnosperms) and included about 20 genera.

It is believed that all the modern seed plants evolved from this group of plants, which no longer exists. They had an anatomy simular to the conifers which evolved later, however they reproduced from spores.

The oldest progymnosperm tree which exists in the fossil record is Arch├Žopteris which grew in excess of 10 metres in height. It was deciduous with a mixture of sterile and fertile (spore-producing) fronds. The megaphyllous leaves and branches were very frond-like in their morphology.
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