Plants of the Coal Measures:
Fossils from the first ever Tropical Rain Forest
Coals of the geological time called the Late Carboniferous consist of the
fossilised remains of plants that lived 300 million years ago. Britain was
then positions very near the equator as part of a supercontinent called
Pangea which stretched from the South pole to Northern temperate
latitudes. The Coal Measures – a series of rocks which formed during the
Late Carboniferous – contain the remains of plants that were the first ever
tropical rainforest. The major part of this forest lasted for 15 million years
and at its greatest development was dramatic in extent; at the time of the
Pangean supercontinent it measured 5000 km (3000 miles) long by 700 km
(450 miles) across. On the present-day map its remains, fragmented by
continental drift, are found from North America to China. The coal seams
of today are the result of compression, at a ratio estimated of up to 10:1,
of the original peat deposits produced in the swamps. It is also estimated
that a one metre coal seam is the result of 7000 years of plant growth and
deposition.
Many of the plants which lived at this time are now extinct or are
represented today by small herbaceous relatives of the Coal Measure
giants. There were no flowering plants then, such as oaks and roses, or
palms and grasses. The plant fossils are found in the coal seams or
compressed in associated rocks such as shales and sandstones. There are
also occasional occurrences, throughout the Coal Measures, of threedimensionally preserved plants showing cell-structure not normally
preserved in compressed fossils.
The dominant plants of this time, certainly for the first 10 million years,
were giant lycopsids (club mosses) which are characterised by their spirally
arranged leaf bases – the scars left on the trunk and branches when the
leaves were shed. The largest of the lycopsids was Lepidodendron (the
Scale Tree) the trunk of which is known to have reached a height of 36
metres (120 ft) before it branched into a crown of leafy shoots to give a
possible total height of about 45 metres (150 ft). The leafy shoots bore
needle-like leaves and pendant spore-producing cones called Flemingites
and Lepidostrobus. Another similar plant – Sigillaria (the Seal Tree) – had
strong vertical ribs bearing these scars. Pieces of bark of these plants are
sometimes mistaken by collectors for fish or reptile scales. The rooting
parts of both Sigillaria and Lepidodendron are called Stigmaria, which had
spirally-arranged circular scars which once bore rootlets; occasionally the
rootlets themselves are preserved. Present day lycopsids such as
Lycopodium and Selaginella are all small herbaceous plants and are
thought to be descendants of similar small lycopsids which grew in the
undergrowth alongside the Coal Measure giants.
Calamites suckowi
Another major group of plants was the horsetails, which are represented
today by Equisetum. Once again Coal Measure times produced a giant, in
the form of Calamites which reached heights of 15-18 m (50-60 ft). These
now-extinct plants had tapered, jointed trunks with whorls of branches
which in turn bore whorls of leaves. The trunks were hollow in life and
frequently became filled with sediment on burial; decay of the outer tissues
of the trunk left a natural cast of the internal structure, known as a pith
cast. The casts are commonly found as fossils, but because of decay on the
death of the plant, the outer surface is found less frequently. The two most
common calamite leaves are Annularia and Asterophyllites. Two examples
of the spore-producing cones of the Coal Measure horsetails are called
Calamostachys and Palaeostachya. Sphenophyllum, which had whorls of
wedge-shaped leaves, was a small and probably scrambling plant of the
same group.
Two entirely different types of plant had foliage resembling modern day
ferns. Firstly there were true ferns, some of which were ancestors of those
living today and reproduced by means of spores. Typical examples of the
foliage are Pecopteris, Asterotheca, Lobatopteris, Senftenbergia and
Polymorphopteris. Some of the ferns were tree-ferns and looked
something like the modern Marattia; Psaronius, for example, was a tree
about 10 m (33ft) in height. Secondly, there were now-extinct plants called
pteridosperms (seed-ferns). They include several groups which bore similar
foliage to the ferns proper, but which reproduced by means of seeds-
Trigonocarpus is an example of such a seed. These plants were
gymnosperms and belonged to a very different group from the true ferns.
The existence of this hitherto unknown and extinct group of plants from
the Coal Measures had been suspected in the last century, mainly because
of the lack of fern-type sporangia (spore sacs) on much of the fern-like
foliage and the discovery of strange and unknown anatomy in plants
whose cell structure was preserved. In the early years of this century,
fronds with this strange anatomy were found with seeds attached, which
confirmed the existence of pteridosperms.
Because of the similarity of the foliage of ferns and pteridosperms it has
often been difficult to distinguish between these two groups when they
are presented as compressed fossils in the rocks. Careful work by
palaeobotanists has now enabled most of the two sorts of compressed
foliage to be differentiated. Typical of pteridosperm fronds are Neuropteris,
Laveineopteris, Macroneuropteris, Alethopteris, Mariopteris, Paripteris and
Sphenopteris. Pteridosperns were a diverse group of plants occupying a
variety of habitats. The most common forms found as fronds resembled
modern tree-ferns, but some could be much larger; Neuropteris trees were
up to 10 m (33ft) high, Alethopteris fronds are known up to 7m (23ft) in
length. None of the pteridosperm groups of the Coal Measures survived
for long after the Carboniferous, but some of their relatives lasted until the
Early Cretaceous, about 120 million years ago. It is possible that one of the
Coal Measures pteridosperm groups – the Trigonocarpales – were the
ancestors of the cycads of the present day.
Other gymnosperms in the Coal Measures forests include the first conifers
and an extinct group called the cordaites, some species of which were
trees up to 30 m (100ft) tall bearing sword-shaped leaves up to 1m long.
The cones of these plants, one sort producing pollen, the other seeds, are
called Cordaitanthus. The conifers are poorly represented in Coal Measures
strata and are thought to have grown outside the main wetland areas.
At the time of the Coal Measures the forests grew on land which was lowlying and covered with swamps and lakes. Through this landscape flowed
rivers, and the raised banks (levees) of these rivers provided higher and
relatively drier ground. It was on this drier ground that the greatest variety
of plants grew, the swamps themselves being dominated by lycopsids, and
the lake margins by the horsetails. The landscape of dense vegetation
would have had certain aspects in common with swamplands of the
present day, such as the lower reaches of the Mississippi, although the
plants themselves were very different.
Geological and climatic changes, resulted in drying out of the Coal
Measures swamplands and gradually brought and end to the Coal Measure
forests. Descendents of some species were suited to the changing
conditions, but others were not. The conifers are an example of those that
survived.
Further Reading
Cleal, C. J. 1991. Plant fossils in geological investigations: the
Palaeozoic. Ellis Horwood, New York, London.
Cleal, C. J and B. A. Thomas. 1994. Plant fossils of the British Coal
Measures. Palaeontological Association Field Guides to Fossils:
No. 6
Kenrick, P. and P. Davis. 2004. Fossil Plants. The Natural History
Museum, London.
Stewart, W. N. and G. W. Rothwell. 1993. Paleobotany and the
evolution of plants. Second edition. Cambridge University Press,
Cambridge.
Taylor, T. N. and E. L. Taylor. 1993. The Biology and Evolution of fossil
plants. Prentice Hall, New Jersey.
Thomas, B. A. and C. J. Cleal. 1993. The Coal Measure forests. National
Museum of Wales, Cardiff.