Carboniferous Coal Forests by Peter Austen Founded 1992 Reprinted from the Hastings & District Geological Society Journal Vol. 11, No. 1, Spring 2003, Pages 18–28 Carboniferous Coal Forests – Peter Austen Introduction Plants first adapted from life in water to life on land around 430 million years ago. Initially they were just small upright stems with capsules of reproductive spores at the tip and they could only survive at the waters’ edge, but over the next 130 million years they developed roots, leaves and seeds, with some plants growing to the size of trees. These advances allowed them to colonise a wide range of new habitats, so by the time we reach the Carboniferous Period, 300 million years ago, we see the appearance of the worlds first large scale forests. At that time Britain lay near the equator as part of a large landmass called Pangea. The hot, wet, tropical climate, combined with the lowland geography across the equatorial regions of this large landmass produced the worlds first tropical rainforests, stretching from North America, across Europe into China (fig.11), and the deposits formed by these forests over millions of years formed the bulk of the worlds coal deposits. However most of the plants that colonised these forests were totally different to any that we see today. The flowering plants that we are so familiar with today had not yet evolved and the coal swamps were dominated by giant clubmosses, tree-sized horsetails, and ferns, with seed plants growing in the drier parts of the forest. Although Carboniferous coal deposits and their associated plant fossils can be found in many areas of Britain, all the fossil plants in this article come from the Writhlington Geological Nature Reserve in the Bristol/Somerset coalfield in the south-west of England. The Carboniferous Plants Clubmosses As mentioned above the plants that colonised these coal swamps were totally different to any that we see today – perhaps the most unusual of these were the giant clubmosses. Today we know the clubmosses as small herbaceous plants such as Selaginella and Lycopodium growing mainly in mountainous or alpine regions, but during the Carboniferous the coal swamps were dominated by giant tree-sized clubmosses like Lepidodendron (fig.13) and Sigillaria (fig.14). Lepidodendron could grow up to 40 metres in height and was topped by a crown of branches with leaves and cones. These giants were adapted to the swamp environments and their remains formed the bulk of the coal deposits. Although they were the size of trees their structure and method of growth was totally different to the trees of today. They grew rather like expanding poles, not branching until they reached their full height, and as they grew, leaves would grow from their trunk to assist with photosynthesis (the production of food using carbon dioxide and sunlight). Present day trees strengthen their trunks by the production of wood as they grow, but the giant clubmosses strengthened their trunks by forming layers of special strengthening cells – in reality these clubmosses were no more than giant herbs. These giants dominated the more waterlogged swamp areas (fig.9), but smaller herbaceous clubmosses could also be found in the forest community – one such plant, Selaginellites (fig.7), was very similar to the present day Selaginella. Fig.4 shows a specimen of Lepidodendron showing the leaf cushions (or scars) left on the bark after the leaves had fallen away. This specimen will have come from high up the tree, because as the tree grew taller and the base became wider the leaf cushions near the base of the tree fell away. Fig.12 illustrates the coal forest during a period of low water levels, showing the clubmosses at various stages of growth. 18 Horsetails Horsetails also grew as part of the forest community, however these did not grow in the waterlogged swamp areas, but on the slightly drier swamp margins and on the banks of lakes and rivers (fig.10). Today we know the horsetail from the sole surviving genus Equisetum, and these reproduce by releasing spores from cones. They also spread by sending out underground shoots or runners, which give off new growth away from the parent plant (this is why they are so difficult to control if you get them in your garden). The Carboniferous horsetails spread using a similar method. The main horsetail in the coal forests was Calamites (fig.15) and this could grow up to around 10 metres in height. Its leaves, known as Annularia (fig.1) and Asterophyllites (fig.3), were arranged in whorls around the branches, which in turn were arranged in whorls around the main stem. When the plant died the central pith cavity of the main stem would rapidly decay and the remaining hollow would fill with sediment. Over time this sediment would solidify leaving a cast of the central pith cavity of the stem. (fig.2) There were also smaller scrambling types of horsetail in the forest community such as Sphenophyllum (fig.16). Fig.2 shows the cast of the central pith cavity of the stem of Calamites. The specimen illustrated (fig.2) shows an aerial shoot tapering towards the base where it would have grown from the horizontal rhizome (root). Figs.1 and 3 show the radial leaves of Calamites, known as Annularia and Asterophyllites respectively. Ferns A wide variety of ferns grew in the coal swamp forests, some were tree-sized, but most were smaller herbaceous types, and like the horsetails they grew on the slightly drier swamp margins and on the banks of lakes and rivers. Ferns still thrive today, although the dominance of the flowering plants today means that they do not form as important a part of the flora as they did in the Carboniferous Period 300 million years ago. Although most of the ferns in the coal forests are now extinct, a few of them did give rise to the ferns of today – the tree fern Psaronius (fig.18), which could grow up to 10 metres in height belonged to the Marattiaceae family of ferns, species of which can be found in the tropics today. The coal forest ferns reproduced by means of spores on the underside of their leaves, similar to todays ferns. Fig.6 shows the leaves of the “true fern” Polymorphopteris. Seed Plants Another type of plant that thrived in the coal forests were the seed plants. These were able to grow in the drier parts of the forest – unlike the clubmosses, horsetails and ferns which reproduced by means of spores, the seed plants used seeds for reproduction. This meant that they were not dependant on water for reproduction and so could survive and reproduce away from the main swamp area. There were two main groups of seed plants in the coal forests – the most diverse group were the pteridosperms or “seed ferns”. Their leaves were very similar to those of the true ferns and up until early in the 20th century it was thought that these leaves actually belonged to the ferns. However, the discovery of seeds attached to the leaves in some specimens, led scientists to describe them as a totally new group of plants – the pteridosperms or “seed ferns”. The similarity of leaf shape in some species does sometimes lead to a problem of identification, and often it is only the presence of spores on the underside of a leaf that allows it to be attributed to the family of true ferns, rather than that of the “seed ferns”. As with the other main groups some of the “seed ferns” grew to the size of trees such as Medullosa (fig.17), but most were smaller herbaceous types. 19 The other group of seed plants were the tree-sized Cordaites (fig.19), the most common remains of which are long strap shaped leaves which could be up to a metre in length. It is thought that the Cordaites were ancestral to the conifers of today. The fossil record is very biased towards plants that lived (and died) near water, and there were undoubtedly many more seed plants living in forest areas on higher ground away from the waters edge that have not been found in the fossil record. Fig.5 shows the leaves of the “seed fern”, Neuropteris, and fig.8 the leaves of the most commonly found “seed-fern” in the Bristol/Somerset coalfield, Alethopteris. Demise of the Coal Forests The large landmass, Pangea, which had provided ideal conditions for the growth of the coal forests, had been formed by the collision of the southern continent of Gondwana with the northern continent of Laurasia. These two continents continued to collide throughout the Carboniferous Period, eventually leading to increased uplift of the land and the drying out of the great coal swamps. The giant clubmosses, which had dominated the swamp communities, were totally dependant on this wetland habitat and they could not survive the drying out of the swamps, although some smaller forms did persist through to the Permian Period and eventually gave rise to the quillworts of today. The large horsetails also died out at the end of the Carboniferous Period, although some smaller forms did survive to give rise to todays only horsetail Equisetum – because of the similarity in reproductive structures it is thought that Equisetum may have been related to Calamites. Unlike the clubmosses and horsetails the ferns continued to evolve until many of them were wiped out by a mass extinction event at the end of the Permian Period (245 million years ago). This extinction event wiped out 95% of all plant and animal species on Earth, and the few ferns that survived steadily increased and diversified, moving into a wider range of habitats, and eventually giving rise to the ferns of today. The pteridosperms or “seed-ferns” survived through to the Cretaceous Period, but by the end of the Cretaceous, 65 million years ago, they had become extinct, possibly outcompeted by the newer and more adaptable flowering plants. It is thought that the only descendants left by the “seed ferns” are the cycads. The other group of seed plants, the Cordaites, also became extinct in the Permian Period, possibly ousted by their more successful relatives, the conifers. Acknowledgements Figures 1-8. All of the specimens illustrated in figures 1 to 8 were collected by Peter & Joyce Austen and Ed & Biddy Jarzembowski between 1984 and 1994 from the Writhlington Geological Nature Reserve, Radstock, Avon, and are all now housed in the National Museum of Wales, Cardiff. Figures 2 and 8 are from Thomas and Cleal (1994) and figures 1, 3, 4, 5, 6 and 7 are from Cleal and Thomas (1994). Figure 9 is from Professor Ralph E. Taggart’s Michigan State University website http://taggart.glg.msu.edu/isb200/carbfor.htm and is from a diorama on display in the Field Museum of Natural History, Chicago, U.S.A. Figures 10 and 11 are from Thomas and Cleal (1993). Figure 12 is from Proctor and Jarzembowski (1995) and Austen (2001). Figures 13, 14, 15 and 16. Reconstructions are by Peter Austen using illustrations from Scott (1900), Crookall (1969), British Museum (Natural History) (1975), Chaloner and Collinson (1975), Bassett and Edwards (1982) and Stewart and Rothwell (1993). Figure 17 is from Cleal and Thomas (1994). Figure 18 is from Stewart and Rothwell (1993). Figure 19 is from Thomas and Cleal (1993). 20 References AUSTEN, P. A. 2001. The Writhlington experience. In: BASSETT, M.G., KING, A.H., LARWOOD, J.G., PARKINSON, N.A. and DEISLER, V.K. (eds). A Future for Fossils. 67-70. National Museum of Wales, Geological Series No.19, Cardiff. BASSETT, M. G. and EDWARDS, D. 1982. Fossil Plants from Wales. Geological Series No. 2, National Museum of Wales, Cardiff, 42pp. BRITISH MUSEUM (NATURAL HISTORY) 1975. British Palaeozoic Fossils. British Museum (Natural History), London, 203pp. CHALONER, W. G. and COLLINSON, M. E. 1975. An illustrated key to the commoner British Upper Carboniferous plant compression fossils. Proceedings of the Geologists’ Association, 86(1), 1-44. CLEAL, C. J. and THOMAS, B. A. 1994. Plant Fossils of the British Coal Measures. Palaeontological Association Field Guides to Fossils: Number 6. The Palaeontological Association, London, 222pp. CROOKAL, R. 1955-1976. Fossil plants of the Carboniferous rocks of Great Britain. [Second section]. Memoirs of the Geological Survey of Great Britain, Palaeontology, 4, Part 1 (1955), 1-84, Part 2 (1957), 85-216, Part 3 (1964), 217-354, Part 4 (1966), 355-572, Part 5 (1969), 573-792, Part 6 (1970), 793-840, Part 7 (1976), 841-1008. PROCTOR, C. J. 1994. Carboniferous fossil plant assemblages and palaeoecology at the Writhlington Geological Nature Reserve. In Jarzembowski, E. A. (ed.) Writhlington Special Issue. Proceedings of the Geologists’ Association, 105(4), 277-286. PROCTOR, C. J. and JARZEMBOWSKI, E. A. 1995. Habitat reconstruction in the Westphalian of Writhlington. Open University Geological Society Journal, 16(1), 11-14. SCOTT, D. H. 1900. Studies in Fossil Botany. Adam and Charles Black, London, 533pp. STEWART, W. N. and ROTHWELL, G. W. 1993. Paleobotany and the Evolution of Plants. Cambridge University Press, Cambridge, 521pp. THOMAS, B. A. and CLEAL, C. J. 1993. The Coal Measures Forests. National Museum of Wales, Cardiff, 32pp. THOMAS, B. A. and CLEAL, C. J. 1994. Plant fossils from the Writhlington Geological Nature Reserve. Proceedings of the Geologists’ Association, 105(1), 15-32. 21 Fig.1. Annularia stellata. Writhlington Geological Nature Reserve, Radstock, Avon, x 0.85. Fig.2. Calamites suckowii. Writhlington Geological Nature Reserve, Radstock, Avon, x 1. Fig.3. Asterophyllites equisetiformis. Writhlington Geological Nature Reserve, Radstock, Avon, x 1. 22 Fig.5. Neuropteris flexuosa. Writhlington Geological Nature Reserve, Radstock, Avon, x 1. Fig.4. Lepidodendron aculeatum. Writhlington Geological Nature Reserve, Radstock, Avon, x 0.85. Fig.6. Polymorphopteris polymorpha. Writhlington Geological Nature Reserve, Radstock, Avon, x 0.6. Fig.7. Selaginellites gutbieri. Writhlington Geological Nature Reserve, Radstock, Avon, x 0.9. Fig.8. Alethopteris serlii. Writhlington Geological Nature Reserve, Radstock, Avon, x 1. 23 Fig.9. Reconstruction of an Upper Carboniferous Coal Measure Swamp Forest (from a diorama on display in the Field Museum of Natural History, Chicago, U.S.A.) Fig.10. Reconstruction of the margins of an Upper Carboniferous Coal Measure Swamp Forest (from “The Coal Measures Forests” by B.A.Thomas and C.J.Cleal, National Museum of Wales, 1993) Fig.11. Map of the super-continent Pangea during the Upper Carboniferous Period (300 million years ago) showing the coal forest areas (in blue) stretching from North America, through Europe into China (from “The Coal Measures Forests” by B.A.Thomas and C.J.Cleal, National Museum of Wales, 1993) 24 Fig.12. Reconstruction of an Upper Carboniferous forest community. The forest was dominated by the clubmoss Lepidodendron (shown at various stages of growth). The swamp is depicted at low water stage, but the forest floor would have been flooded some of the time (reconstruction by Chris Proctor from Proctor & Jarzembowski (1995)). 25 26 Stigmaria - root axis Stigmaria dichotomizing rootlets Knorria - decorticated bark of Lepidodendron Lepidodendron - bark showing diamond shaped leaf scars Lepidodendron - leafy shoots Stigmaria - root axis with rootlets attached Drawings not to scale L - lamina S - sporangium P - pedical Lepidostrobophyllum cone scales (sporophylls) (side view) S P L Lepidostrobus - cone Lepidostrobophyllum - cone segment (cross-section of cone Lepidostrobus) Fig.13. Reconstruction of clubmoss Lepidodendron (height - 40m.) S.mamillaris Stigmaria - root axis with rootlets attached S.elongata S.elegans S.cumulata S.boblayi S.ovata Drawings not to scale Stigmaria - root axis Stigmaria - the shallow rooting systems of Lepidodendron and Sigillaria are so similar that the same genus, Stigmaria, is applied to both. Sigillaria - various leaf scar patterns S.brardi S.laevigata Bark of Sigillaria showing various stages of decortication S.rugosa S.scutellata S.tessellata Stigmaria dichotomizing rootlets Sigillariostrobus cone Lepidophylloides cone scale Cyperites - leaves of Sigillaria and possibly Lepidodendron Fig.14. Reconstruction of clubmoss Sigillaria (height - 30m.) 27 Asterophyllites leaves of Calamites Annularia - leaves of Calamites Myriophyllites - rooting system Palaeostachya cone Drawings not to scale Calamites - pith cast of stem Calamostachys cone Fig.15. Reconstruction of horsetail Calamites (height - 10m.) Sphenophyllum part of stem Sphenophyllostachys cone Generalised reconstruction of Sphenophyllum Drawings not to scale Detail of stems S.oblongifolium S.saxifragaefolium S.emarginatum Sphenophyllum various leaf shapes S.majus S.cuneifolium S.myriophyllum S.trichomatosum Sphenophyllum - stem bearing whorls of wedge shaped leaves Fig.16. Reconstruction of horsetail Sphenophyllum (height - up to 1m.) 28 Fig.17. Reconstruction of ‘seed fern’ Medullosa (height - 10m.) Fig.18. Reconstruction of ‘true fern’ Psaronius (height - 10m.) Fig.19. Reconstruction of ‘seed plant’ Cordaites (height - 30m.)
