The lost rainforest of the West Cumberland Coalfield (Part 5): The ferns return – Pecopteris and the first colonisers of the drowned swamp
Jon Trevelyan (UK)
Simplified summary of Britain’s geological history from the Precambrian to the present day. The coloured timeline shows the major geological periods and the illustrations above represent some of the dominant landscapes and environments that characterised each interval. The red line(s) indicates the geological period(s) covered by this article.
When a Carboniferous swamp drowned, its towering lycopsids collapsed, its peat surface disappeared beneath mud, and the forest that had stood for decades, or at most a few generations, abruptly ceased to exist. Yet within surprisingly little time, new vegetation reclaimed the abandoned lake floor. The first wave of this recovery is written across the roof shales of the West Cumberland Coalfield district in thousands of beautifully preserved fronds, most commonly those of Pecopteris, the characteristic tree ferns of the Westphalian wetlands.
These ferns were not the understory filler of a stable swamp (Fig. 5.3). Instead, they were pioneer species, adapted for rapid colonisation of freshly exposed mudflats, levee deposits and receding pond margins. Their arrival marks the beginning of the ecological succession that followed each drowning event recorded in the coalfield.
Pecopteris: the tree fern of the coal forests
Pecopteris is a form genus encompassing the fronds of marattialean tree ferns, which grew widely throughout the Carboniferous tropics. In life, it formed a low to mid-height canopy well below the lycopsid treetops, depending on species, and possessed multiple arching fronds emerging from a stout trunk or rhizome. The fronds were bipinnate or tripinnate, with characteristic rounded, overlapping pinnules that give Pecopteris its distinctive appearance.
Key structural features include:
- thick central rachis (frond midrib) preserved as a strong central line;
- pinnae extending laterally in regular rows;
- pinnules often overlapping, with smooth margins and a robust midvein; and
- spores produced on the underside of the fronds.
These fronds, when shed, fell gently onto mud surfaces and were exquisitely preserved in the calm-water environment of early roof shales. Some West Cumberland Coalfield specimens preserve pinnules so cleanly that individual vein divisions are visible.
| What did a Pecopteris plant actually look like? |
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| Although Pecopteris is often spoken of as a “tree fern”, the fossil record preserves it primarily as detached fronds, not as complete plants. This makes whole-plant reconstruction more difficult than for lycopsids, such as Lepidodendron or Sigillaria, where trunks, roots and crowns are commonly preserved together (Part 2). Most palaeobotanists interpret the larger Pecopteris fronds of the Westphalian as belonging to marattialean tree ferns, a group related to living genera, such as Marattia and Angiopteris. In life, these plants are thought to have stood a few metres tall, with a short trunk or columnar stem surrounded by a dense mantle of adventitious roots (a structure seen fossilised as Psaronius trunks). From the top of this stem arose a crown of large, arching fronds, forming a low to mid-height canopy beneath the towering lycopsids. However, it is important to stress that no single Pecopteris frond can be tied with certainty to a specific parent plant unless it is found attached. Some Pecopteris-type foliage may have belonged to different marattialean species, while a minority may even have been produced by non-fern plants such as seed ferns. Reconstructions therefore combine direct fossil evidence with informed inference. For this reason, images of Pecopteris “trees” should be read as probable growth forms, not literal snapshots. What is beyond doubt is the ecological role these plants played – they formed dense stands of large ferns capable of rapidly colonising freshly exposed mud, creating the first green cover after a swamp drowned. |
Why tree ferns were the first to return
The dominance of Pecopteris in the early post-drowning landscape reflects a combination of biological and environmental advantages.
- Rapid reproduction. Tree ferns reproduced through spores – tiny, lightweight reproductive cells that disperse easily through air and water, giving them a major advantage in colonising freshly exposed mud surfaces.
- Tolerance of unstable substrates. Unlike lycopsids, which required permanently saturated peat, Pecopteris could root into fresh mud or silt even if the substrate was only seasonally waterlogged.
- Fast growth in high light. After a swamp collapsed, the surface was a low, open, well-lit landscape. Pecopteris exploited this light, quickly producing large fronds.
- Limited competition. Lycopsids could not recolonise until peat-like conditions re-established, and seed ferns preferred more stable ground. Pecopteris thrived in the ecological gap left between these groups.
Therefore, the earliest vegetation on the new surface was often a fern-dominated pioneer community, visually very different from the massive canopy that preceded it.
Preservation in the West Cumberland Coalfield roof shales
The West Cumberland Coalfield’s laminated grey shales are perfect for preserving Pecopteris. They record a time when water was shallow and still, mud settled in millimetre-thin layers, and fronds drifted down intact onto the lake bottom. The result is a flora rich in:
- complete fronds with intact rachises;
- pinnules preserved in fine relief;
- overlapping clusters of foliage transported only a short distance; and
- occasional association with Neuropteris pinnules (Part 6) or fallen Calamites leaves (Part 7).
| Why do Carboniferous plants have so many names – form genera and the Pecopteris problem? |
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| Carboniferous plants are often known by different names for different parts of the same organism. This is not taxonomic confusion, but a practical response to how fossil plants are preserved. In most cases, plants do not fossilise as complete individuals. Leaves, stems, roots and reproductive organs are shed, transported and buried separately. As a result, palaeobotanists historically gave separate form names to different organs, long before their connections were understood. Pecopteris is one such form genus. It refers specifically to a type of fern frond with a distinctive pinnate architecture and venation pattern. The trunk of the same plant, if preserved, would be called Psaronius. Its roots would have another name again. Only when attached fossils are found can these pieces be confidently linked. This approach avoids forcing uncertain connections, but it means that a single living plant may appear in the fossil record under several names. In the West Cumberland Coalfield, this is particularly clear: roof shales preserve abundant Pecopteris fronds, while nearby nodules and sandstones yield trunks and roots belonging to different plants. Understanding this naming system is crucial when interpreting fossil assemblages. A layer rich in Pecopteris does not necessarily represent a forest dominated by tree ferns in the same way that a Lepidodendron-rich horizon represents a lycopsid canopy (Part 2). Instead, it records what parts of plants were shed, transported and preserved at that moment in the swamp’s history. Seen in this light, the abundance of Pecopteris in the West Cumberland Coalfield reflects not just fern biology, but the processes of disturbance, colonisation and preservation that followed each drowning event. |
Pecopteris and the swamp-to-floodplain transition
Although Pecopteris was the early coloniser, it was not the final stage of the post-drowning succession.
- Stage 1: Tree fern colonisation. Pecopteris forms dense, low to mid-height stands on fresh mud.
- Stage 2: Seed-fern invasion. The seed ferns, Neuropteris and Alethopteris (Part 6), grow and expand into the landscape, as the substrate dries or stabilises.
- Stage 3: Calamites and Cordaites establish. These indicate the development of shallow channels and levees (Part 7).
- Stage 4: Return of lycopsids. If conditions become saturated again, that is, only once peat-forming conditions re-establish do Lepidodendron and Sigillaria regain dominance (Part 2).
The West Cumberland Coalfield preserves all these transitions, often within the same spoil heap (Part 4).
What Pecopteris reveals about climate and environment
Pecopteris is a reliable ecological indicator.
- Warm, humid conditions. Tree ferns require consistent warmth and moisture. Their abundance reinforces the equatorial palaeoclimate inferred for the Westphalian.
- Weak seasonality. No evidence of prolonged dry seasons appears in Pecopteris-rich shales. Ferns are poor drought-tolerators, so their dominance suggests high rainfall.
- Shallow, stable waterbodies. Fine lamination and intact fronds confirm low-energy, standing-water environments, which are exactly the conditions expected after a swamp drowned.
- Ecological resetting. Each Pecopteris-dominated layer marks a new cycle, showing how frequently the West Cumberland Coalfield swamp complex experienced flooding, subsidence or base-level rise.
In short, Pecopteris tells us how the swamp responded to change, and how the coal forests rebounded after drowning episodes.
A vital piece of the West Cumberland Coalfield story
Without Pecopteris, the West Cumberland Coalfield coal forest would appear as a series of dead lycopsid landscapes punctuated by empty muds. Instead, the ferns reveal a dynamic system – one capable of rapid ecological resets and swift recolonisation after disturbance.
They were the “first wave” of life returning to a landscape transformed by water – pioneers that paved the way for seed ferns, Calamites and eventually lycopsids. Their fossils capture a delicate moment between catastrophe and renewal. And in the shale tips above the old coal seams, those moments are still preserved with remarkable clarity.
Further reading
Cleal, C.J. and Thomas, B.A. Introduction to Plant Fossils (excellent ferns section).
Taylor, Krings and Taylor. Paleobotany (accessible chapters on marattialean tree ferns).
| Other parts in this series |
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| The lost rainforest of the West Cumberland Coalfield (Part 1): A window into the Carboniferous tropics |
| The lost rainforest of the West Cumberland Coalfield (Part 2): A forest of giants – Lepidodendron and Sigillaria |
| The lost rainforest of the West Cumberland Coalfield (Part 3): Beneath the giants – Stigmaria and the swamp floor |
| The lost rainforest of the West Cumberland Coalfield (Part 4): After the drowning – the roof shale succession |
| The lost rainforest of the West Cumberland Coalfield (Part 5): The ferns return – Pecopteris and the first colonisers of the drowned swamp |
| The lost rainforest of the West Cumberland Coalfield (Part 6): The seed-fern story – Neuropteris, Odontopteris and Alethopteris |
| The West Cumberland Coalfield’s lost rainforest (Part 7): Rivers, reeds and dry patches – Calamites and Cordaites on the Carboniferous floodplain |
| The lost rainforest of the West Cumberland Coalfield (Part 8): From Langsettian to Bolsovian – dating the West Cumberland Coalfield flora |
| The lost rainforest of the West Cumberland Coalfield (Part 9): Life on the equator – climate and environment in Westphalian Cumbria |
| The lost rainforest of the West Cumberland Coalfield (Part 10): Why the West Cumberland Coalfield? Geology, tectonics and preservation |
| The lost rainforest of the West Cumberland Coalfield (Part 11): Two coalfields, two records: reconstructing the Carboniferous forest at Radstock and Cumberland |