England Fossils Geology Palaeontology Plate Tectonics

The lost rainforest of the West Cumberland Coalfield (Part 6): The seed-fern story – Neuropteris, Odontopteris and Alethopteris

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Jon Trevelyan (UK)

Illustration depicting the geological history of Britain, showing various landscapes and environments over time, including ancient basement rock, shallow seas, volcanic mountains, coal swamps, desert landscapes, tropical seas, erosion by rivers, and ice ages.

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.

If the lycopsids built the forests and the tree ferns reclaimed the drowned ground, then the seed ferns – particularly Neuropteris, Odontopteris and Alethopteris – tell the story of how the Carboniferous landscape diversified beyond the swamp. These plants, long extinct but abundantly preserved in the roof shales in the West Cumberland Coalfield, flourished on levees, raised mudbanks and floodplain surfaces that were too dry for lycopsids, but too unstable for true forest soils. Their fossils show how the environment shifted from a drowned swamp, back towards a mixed floodplain-wetland mosaic.

Seed ferns were not ferns at all (see box: Ferns v seed ferns: similar leaves, very different plants). They produced seeds rather than spores, and their fronds resembled modern fern foliage only superficially. Their anatomy, ecology and fossil preservation give us a detailed view of the middle stages of the post-drowning vegetation cycle – those brief periods when the landscape supported a surprisingly rich flora before lycopsids seized control again.

Ferns v seed ferns: similar leaves, very different plants
At first glance, seed ferns such as Neuropteris, Odontopteris and Alethopteris resemble true ferns like Pecopteris (Part 5). Both produced large, pinnate fronds, and both shed their leaves readily into roof-shale environments. This superficial similarity long obscured their true relationships.

However, in biological terms, seed ferns were fundamentally different plants. True ferns relied on consistently moist conditions for their life cycle. Seed ferns produced seeds, allowing reproduction without free-standing water and giving them greater tolerance of fluctuating moisture levels. This single difference had major ecological consequences.

As a result, ferns were best suited to freshly drowned or persistently wet surfaces, while seed ferns flourished on slightly raised, firmer ground, such as levees and floodplains.

In the West Cumberland Coalfield succession, this distinction is expressed clearly – Pecopteris dominates early colonisation layers, while seed ferns appear only once the substrate stabilised. Pecopteris is included here not as part of the seed-fern flora, but as an ecological baseline, defining the wet, unstable conditions that preceded seed-fern colonisation.

The fern-like appearance of seed ferns is therefore misleading. Their leaves reflect shared ancestry, but their ecology marks a decisive step toward seed-based vegetation with greater tolerance of fluctuating moisture, a strategy that would later dominate many post-Carboniferous landscapes.
Diagram illustrating floodplain zonation, labeled sections include Shallow Pond, Mudflat, Levee, and Floodplain, with specific vegetation types: Pecopteris (fresh mud), Neuropteris (stabilising surfaces), Odontopteris (higher levees), and Alethopteris (firmer floodplain).
Fig. 6.1. Floodplain zonation (space). Conceptual cross-section of a lowland Carboniferous floodplain showing the relative spatial distribution of common medullosan and marattialean foliage types. The fern, Pecopteris, is most frequently associated with freshly deposited muds (a true fern), whereas the seed ferns, Neuropteris with partially stabilised substrates, Odontopteris with slightly raised levees, and Alethopteris with firmer, less frequently disturbed floodplain surfaces.
Diagram illustrating the relationship between disturbance and stability in substrates, showing four types of plants: Pecopteris (fresh mud), Neuropteris (stabilizing surfaces), Odontopteris (raised/firm ground), and Alethopteris (firm floodplain soils).
Fig. 6.2. Disturbance-stability gradient (process). Abstract representation of increasing substrate stability across a floodplain, from frequently disturbed to more persistent ground. Plant names are positioned to indicate relative tolerance of disturbance and burial, with Pecopteris biased toward unstable substrates, Alethopteris toward stable conditions, and Neuropteris and Odontopteris occupying intermediate positions. The diagram summarises ecological preference rather than succession.
Diagram illustrating event-window filtering showing tolerance levels of different genera (Pecopteris, Neuropteris, Odontopteris, Alethopteris) in response to flood events over time.
Fig. 6.3. Event-window filtering (time). Conceptual model showing how repeated flood and sedimentation events act as a filter on floodplain vegetation over time. Vertical bars represent successive flood events, while fading plant names indicate decreasing likelihood of persistence or rapid re-establishment through repeated disturbance. Pecopteris is shown as most disturbance-tolerant, with Neuropteris, Odontopteris and Alethopteris increasingly restricted to longer quiescent intervals.
Flowchart depicting the classification of various frond form-genera of seed ferns, including Alethopteris, Neuropteris, Odontopteris, and Pecopteris, along with their corresponding plant descriptions.

Fig. 6.4. Relationship between frond form-genera and inferred whole-plant affinities for the taxa illustrated in Plates Xa–c. Alethopteris (Plate 6.4a) and Neuropteris (Plate 6.4b) are frond-based form-genera interpreted as belonging to seed ferns (pteridosperms), with whole plants reconstructed from combinations of detached organs preserved in the fossil record. Pecopteris (Plate 6.4c) is likewise a frond form-genus, most commonly associated with marattialean tree ferns, although the complete plant remains are inferred rather than directly known. In all cases, the arrows indicate interpretive links between isolated organs and broader plant groups, not complete or biologically unified plants.

What were seed ferns?

Bar chart comparing Upper Carboniferous stages from the International Chronostratigraphic Scale (ICS) with UK divisions, showing ages in millions of years ago (Ma).
Fig. 6.5. A proportional comparison of Upper Carboniferous international (ICS) stages and traditional UK stages. International stages (left) are shown to scale, while the UK Westphalian (right) is subdivided into Langsettian (A), Duckmantian (B), Bolsovian (C) and Asturian (D). The diagram highlights the lack of one-to-one correspondence between UK and international stage boundaries.

Seed ferns (pteridosperms) were gymnosperm-grade plants with:

  • woody stems or small trunks;
  • fern-like leaves (fronds);
  • seed-bearing reproductive structures; and
  • a preference for drained or intermittently wet substrates.

They were not canopy trees in the West Cumberland Coalfield’s Westphalian swamps. Instead, they grew as sub-canopy plants or small trees, often occupying intermediate ground between the open fernlands and the wetter zones preferred by lycopsids. Their diversity increases notably throughout the Duckmantian and early Bolsovian (Fig. 6.5), matching the pattern seen in the West Cumberland Coalfield’s spoil heaps.

Neuropteris: the elegant generalist

Neuropteris is by far the most common seed-fern genus in the West Cumberland Coalfield area, and its pinnules show striking variation depending on where they grew on the frond.

  • Distal pinnules. Small, delicate, and sometimes almost feathery (Fig. 6.6). These grew near the frond tips and are preserved only in the calmest laminated shales.
Close-up of a fossilized fern leaf embedded in rock, showcasing detailed leaf structures.
Fig. 6.6. Probable Neuropteris pinnule from the distal part of a seed-fern frond, West Cumberland Coalfield. A small pinna fragment preserved in grey mudstone, shows discrete, lobed pinnules attached to a slender axis. The reduced pinnule size, wider spacing and lack of overlap are characteristic of the distal regions of Neuropteris fronds, contrasting with the broader, more crowded pinnules found nearer the frond base. Although venation is only weakly expressed in this specimen, the overall morphology is consistent with Neuropteris rather than Odontopteris or Alethopteris. (The pinnule is 3cm long. From the author’s collection.)
  • Mid-frond pinnules. These rectangular, slightly rounded, 1.5-2cm pinnules that tend to be found most often, formed the bulk of the frond and were shed singly during storms or episodic shedding.
  • Basal pinnules. Broader, more robust and sometimes asymmetric, these occur less frequently, but tell us about the overall size of the frond.

Neuropteris was highly successful because it tolerated moderate moisture, seasonal fluctuations in water level, and the disturbed sediments of floodplain levees. Many West Cumberland Coalfield roof shales contain hundreds of isolated pinnules, which is evidence of local populations shedding leaves onto mudflats near shallow channels.

Two fossilized footprints on sedimentary rock, one showing distinct detail and texture, alongside a ruler marked with measurements for scale.
Fig. 6.7. Two pinnules of the medullosan seed fern Neuropteris heterophylla, preserved as carbonaceous compressions in fine-grained Carboniferous mudstone from coastal cliff exposures near Whitehaven (West Cumberland Coalfield). The specimens display the characteristic ovate to subtriangular pinnule shape, a distinct midrib that fades distally, and evenly spaced lateral veins that arch gently and bifurcate towards the margin. (Scale in centimetres. From the author’s collection.)

Odontopteris: the dry(ish) ground specialist

Fossilized leaf imprint on a rock slab, with a measuring scale beside it.
Fig. 6.8. Fragment of odontopterid seed-fern foliage, referable to Odontopteris-type pteridosperms, preserved as a compression in fine-grained Carboniferous mudstone from an in situ coastal cliff exposure near Whitehaven (West Cumberland Coalfield). The specimen comprises a short rachis bearing closely attached, rounded to weakly lobed pinnules lacking distinct petiolules and a differentiated midrib, with venation spreading directly from the point of attachment -a diagnostic feature of odontopterid seed ferns rather than true ferns. (From the author’s collection).

Odontopteris appears alongside Neuropteris but in lower abundance. Its pinnules are:

  • more angular;
  • more sessile (less clearly stalked);
  • often slightly lobed or with a blunter outline; and
  • with veins that spread in a stiffer, less graceful pattern.

In the West Cumberland Coalfield, Odontopteris tends to occur in siltier, less laminated shales, horizons with signs of minor current activity, or beds containing calamite debris.

These associations suggest it preferred the upper levee surfaces, where the substrate was firmer and the water table lower. Its presence indicates a landscape transitioning away from deep, permanently wet conditions.

Alethopteris: the robust floodplain element

Fossilized impression of a leaf on a rock surface, showcasing detailed leaf structure and veins.
Fig. 6.9. A seed-fern frond fragment, cf. Alethopteris sp. from the roof-shale flora of the West Cumberland Coalfield area. The narrow, elongate pinnules and their broadly attached (slightly decurrent) bases onto the rachis favour assignment to Alethopteris rather than Neuropteris, highlighting the abundance of seed ferns alongside lycopsid trees in the West Cumberland coal-swamp vegetation. (From the author’s collection.)

Alethopteris is the “oak leaf” of the seed-fern world – broad pinnules, thick midribs, and a more robust appearance than Neuropteris or Odontopteris. In the West Cumberland Coalfield, it is less common than Neuropteris, but typically present in mixed floodplain assemblages, especially where shales show small ripple marks or woody debris.

Alethopteris tended to occupy slightly older, more stable surfaces – mudbanks that had dried enough to develop firmer substrates, but were still humid and periodically flooded. Its arrival marks a maturing floodplain.

A fossilized stone showing detailed impressions of fern leaves.
Fig. 6.10. Seed-fern frond fragments, cf. Alethopteris sp. from the roof-shale flora from the West Cumberland Coalfield area. Multiple overlapping pinnae are preserved on a single slab, showing narrow, elongate pinnules with broadly attached to slightly decurrent bases and relatively parallel margins. These characters favour Alethopteris rather than Neuropteris, illustrating the abundance and structural diversity of seed ferns growing alongside lycopsid trees in the West Cumberland coal-swamp forests. (From the author’s collection.)

What seed ferns tell us about the roof-shale environment

Seed-fern assemblages in the West Cumberland Coalfield mark the middle phase of each swamp–floodplain cycle. Their distribution reveals several key environmental transitions.

  1. Water level decline. Seed ferns appear only after the drowning pond has begun to fill or recede. Their earliest occurrences sit above the Pecopteris-dominated colonisation layers.
  2. Increased substrate stability. A firm mud surface is required for rooting. Their appearance implies that the surface was no longer truly lacustrine.
  3. Development of levees and splays. Seed ferns flourish on slight elevations. The presence of Neuropteris and Odontopteris in particular indicates the formation of small levees and overbank splays at the margins of shallow channels.
  4. Possible weak or intermittent seasonality. These plants could tolerate short periods of reduced moisture. Their abundance increases in the Duckmantian-Bolsovian transition, a time when slight climatic fluctuations are inferred elsewhere in Britain.

In short, seed-fern beds show that the West Cumberland Coalfield landscape did not remain a stable swamp. It oscillated between open water, fernlands, mixed floodplain vegetation, and eventually, in some cycles, back into swamp forest.

How the seed ferns fit into the full ecological succession

The post-drowning succession can be simplified as follows.

  1. Forest collapse and open water. Laminated shales with bark slabs, logs and delicate fronds.
  2. Tree-fern colonisation. Pecopteris spreads across exposed mudflats (Part 5).
  3. Seed-fern expansion. Neuropteris, Odontopteris and Alethopteris take hold on stabilising surfaces (Part 6).
  4. Calamite and cordaite development. Channels migrate, levees form and horsetail woods appear (Part 7).
  5. Return to swamp. If water tables rise again, lycopsids reclaim the landscape.

Seed ferns therefore represent the turning point – the moment when the drowned swamp is transitioning into a more complex floodplain (see also Part 4).

Why the West Cumberland Coalfield seed-fern assemblage is important

Seed ferns from the West Cumberland Coalfield area provide three forms of scientific value.

  1. Palaeoenvironmental indicators. Their relative proportions track changes in water level and soil stability.
  2. Palaeoclimatic signals. The modest rise in seed-fern abundance correlates with slight increases in seasonality across Europe during the Duckmantian-Bolsovian interval (Fig. 6.5).
  3. Ecological reconstruction. Together with Pecopteris and Calamites, seed ferns complete the picture of the mixed floodplain vegetation.

Few British localities preserve the full gradient as clearly as the West Cumberland Coalfield – the same spoil heap can yield Neuropteris from quiet-water grey shales and Odontopteris from siltier channel-margin beds.

Seed ferns were the bridge flora of the Carboniferous – resilient enough to weather unstable conditions, diverse enough to carve out ecological niches, and short-lived enough in any one place to record the rapid transformation of a landscape caught between drowning swamps and dynamic floodplains. In the West Cumberland Coalfield, their fossils bring this transitional world vividly to life.

Further reading

Cleal, C.J. (1993). Plant Fossils of the British Coal Measures.

DiMichele, W.A. & Phillips, T.L. (1994). Palaeobotanical and palaeoecological constraints on the origin of coal forests.

Taylor, T.N., Taylor, E.L. and Krings, M. Paleobotany (seed-fern chapters well illustrated).

Other parts in this series
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
The lost rainforest of the West Cumberland Coalfield (Part 12): A Carboniferous floodplain, fossils from a quarry and the decline of the coal forests