The lost rainforest of the West Cumberland Coalfield (Part 2): A forest of giants – Lepidodendron and Sigillaria

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 discussed throughout this series, while 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.

The most visually arresting fossils from the West Cumberland Coalfield, indeed from almost any British Carboniferous site, are the great patterned slabs of lycopsid bark that, if you are lucky, can be found. Their surfaces carry unmistakable geometric signatures: the diamond-shaped leaf cushions of Lepidodendron or the vertical rows of elongated scars typical of Sigillaria (Fig. 2.1).

Fig. 2.1. Living side by side in the Carboniferous swamp. A composite slab from the West Cumberland Coalfield preserving bark from two dominant lycopsid trees: diamond-patterned *Lepidodendron* (right) and vertically ribbed *Sigillaria* (top left). Their intimate association shows that these giants were contemporaries within the same swamp forests, growing together rather than replacing one another. The apparent blending of textures reflects deformation and compaction of adjacent bark fragments after burial, not a biological transition – offering a rare, tactile glimpse of how Carboniferous rainforests were built from multiple, coexisting tree forms. (*From the author’s collection*.)

These impressions, very occasionally preserved across slabs larger than a dinner plate, are the remnants of trees that once formed the canopy of vast equatorial swamps. To walk through these forests in life would have meant standing among the most unusual “trees” ever to dominate a terrestrial ecosystem.

Fig. 2.2. A dense, mist-filled, coal-forest landscape dominated by tall arborescent lycopsids (*Sigillaria* and *Lepidodendron*), their straight, columnar trunks rising from a continuously flooded swamp floor. In the foreground, a giant millipede (*Arthropleura*) moves across waterlogged ground between decaying stumps and shallow pools. Such animals were characteristic members of late Carboniferous wetland ecosystems, which developed on low-relief floodplains near the equator and later gave rise to the Coal Measures preserved in the West Cumberland Coalfield and its environs.

The lycopsids did not resemble modern conifers or broadleaved trees. Their trunks were essentially gigantic stems – tall, straight, minimally branched and with crowns confined to the uppermost few metres (Fig. 2.2). They were fast-growing, short-lived and relied on a distinctive biology that allowed them to flourish in saturated substrates where other plants struggled. Although these plants ultimately make up most of the coal itself and are usually unrecognisable in that form, nowhere in the West Cumberland Coalfield is their anatomy more clearly revealed than in the roof shales and nodules as in the roof shales and nodules recovered from this coalfield.

Lepidodendron: the geometric giant

Lepidodendron, the iconic “scale tree”, dominated the West Cumberland Coalfield canopy flora (Fig. 2.4). Mature individuals often exceeded 30-40m in height. The trunk was covered in a mosaic of rhomboidal leaf cushions, each recording the attachment and vascular pathways of a single long, grass-like leaf (Fig. 2.3). These cushions changed shape and orientation up the trunk, meaning that bark from different heights can look misleadingly like different species (see box: Why do Lepidodendron leaf scars change shape up the trunk?). This explains why spoil-heap material can sometimes appear more diverse than the living community actually was. In fact, the forest interior supported very low floral diversity, made up mostly by Lepidodendron trees.

Fig. 2.3. A large ***Lepidodendron*** bark from the West Cumberland Coalfield, preserved in red mudstone showing the smooth to weakly textured surface characteristic of thinner bark from higher on the stem. The inset highlights faint, elongate leaf-scar outlines, now subdued by oxidation and compaction within the red mudstone matrix. Such preservation contrasts with the more sharply defined scar patterns seen in grey roof shales and reflects burial under more aerated floodplain conditions, illustrating how both trunk position and post-depositional environment influence the appearance of *Lepidodendron* bark. (*From the author’s collection*.)

Fig. 2.4. *Lepidodendron* in a Carboniferous coal-forest wetland. A mature tree-sized lycopsid is shown growing in shallow standing water on a low-relief floodplain, with a tall, unbranched trunk covered in diamond-shaped leaf cushions and a forked crown formed by dichotomous branching (where a single stem or root tip divides into two equal, fork-like branches) at the apex. The stiff, strap-like leaves radiate from the crown axes, produce an open, asymmetrical canopy unlike that of any modern tree. Plants of this type dominated equatorial coal-forest wetlands during the late Carboniferous and were characteristic of the environments that produced the Coal Measures of western Cumberland, including the West Cumberland Coalfield.

In the West Cumberland Coalfield shales, Lepidodendron bark is commonly preserved as:

broad slabs with crisp diamond patterning (Fig. 2.5);
detached sheets that once wrapped the outer trunk;
cortex fragments from higher up the tree; and
in the nodules from the shoreline, internal casts created when the trunk’s central cavity filled with siderite-rich sediment (Fig. 10.5).

These internal casts are especially striking. They record the cylindrical interior of the trunk, often showing how the outer tissues decayed while the hollow stem filled with iron carbonate. The West Cumberland Coalfield coast is one of the few British localities where such casts occur in abundance.

Lepidodendron – anatomy of a Carboniferous scale tree
Despite its tree-like appearance, Lepidodendron was not a tree in the modern sense. It belonged to the lycopsids, a lineage now represented only by small clubmosses, and achieved its enormous size using a growth strategy unlike that of any living forest tree. The trunk of Lepidodendron was dominated by a thick outer cortex rather than by woody tissue. Secondary growth was limited, and most of the trunk’s diameter was established early in life. As the tree grew taller, the cortex expanded and stretched, distorting the surface pattern of leaf cushions and giving bark from different heights a markedly different appearance. This explains why isolated slabs from spoil heaps can appear taxonomically diverse, even when derived from a single species. Each diamond-shaped leaf cushion marks the attachment of a long, narrow microphyll leaf (a small, simple leaf with a single vein) and records the position of vascular tissues and gas-exchange structures. When the leaf was shed, the cushion remained as a permanent feature of the bark, creating the characteristic geometric pattern so familiar from West Cumberland Coalfield slabs. Internally, the trunk contained a large central pith cavity. As the tree aged, inner tissues commonly decayed, leaving a hollow cylinder surrounded by living cortex. In the waterlogged conditions of the coal swamps, these cavities were prone to rapid infilling by fine sediment or siderite, producing the spectacular internal casts found along the Cumbrian coast. Reproduction occurred through large cones borne near branch forks or at the top of the trunk. Having grown rapidly and reproduced early, Lepidodendron was short-lived by arboreal standards. Entire stands could collapse within a few decades when water levels or substrate stability changed, shedding bark and trunks into the quiet waters that now preserve them.

Lepidodendron – anatomy of a Carboniferous scale tree

Despite its tree-like appearance, Lepidodendron was not a tree in the modern sense. It belonged to the lycopsids, a lineage now represented only by small clubmosses, and achieved its enormous size using a growth strategy unlike that of any living forest tree.

The trunk of Lepidodendron was dominated by a thick outer cortex rather than by woody tissue. Secondary growth was limited, and most of the trunk’s diameter was established early in life. As the tree grew taller, the cortex expanded and stretched, distorting the surface pattern of leaf cushions and giving bark from different heights a markedly different appearance. This explains why isolated slabs from spoil heaps can appear taxonomically diverse, even when derived from a single species.

Each diamond-shaped leaf cushion marks the attachment of a long, narrow microphyll leaf (a small, simple leaf with a single vein) and records the position of vascular tissues and gas-exchange structures. When the leaf was shed, the cushion remained as a permanent feature of the bark, creating the characteristic geometric pattern so familiar from West Cumberland Coalfield slabs.

Internally, the trunk contained a large central pith cavity. As the tree aged, inner tissues commonly decayed, leaving a hollow cylinder surrounded by living cortex. In the waterlogged conditions of the coal swamps, these cavities were prone to rapid infilling by fine sediment or siderite, producing the spectacular internal casts found along the Cumbrian coast.

Reproduction occurred through large cones borne near branch forks or at the top of the trunk. Having grown rapidly and reproduced early, Lepidodendron was short-lived by arboreal standards. Entire stands could collapse within a few decades when water levels or substrate stability changed, shedding bark and trunks into the quiet waters that now preserve them.

Fig. 2.5. Lepidodendron bark from the West Cumberland Coalfield, illustrating variation with trunk height. (Left) close-up of thick, blocky, polygonal leaf scars typical of the lower trunk, where bark was robust and long-lived. (Right) bark slab showing smaller, more rounded to weakly rhomboidal leaf scars, reflecting thinner bark higher on the stem. Together, these demonstrate how leaf-scar shape and spacing change systematically up the trunk of *Lepidodendron*, allowing isolated bark fragments to be assigned broadly to different stem levels. (*Picture credits: (left) specimen and picture, Andy Lambert; and (right) – from the author’s collection.*)

*Why do Lepidodendron* leaf scars change shape up the trunk?**
In Lepidodendron, the shape and spacing of leaf scars vary systematically with both growth stage and position on the trunk. Near the base of the tree, bark was thickened and long-lived, producing large, blocky, often hexagonal leaf scars separated by substantial interscar tissue. Higher on the trunk, bark was thinner and leaves were shed more rapidly, resulting in smaller, more elongate, diamond-shaped scars arranged in regular vertical rows. Near the crown, scars become narrow and closely spaced as the trunk transitions into leafy branches. As a result, isolated bark fragments can often be assigned broadly to lower, middle or upper parts of the trunk, even though precise height cannot be determined.

Why do Lepidodendron leaf scars change shape up the trunk?

In Lepidodendron, the shape and spacing of leaf scars vary systematically with both growth stage and position on the trunk. Near the base of the tree, bark was thickened and long-lived, producing large, blocky, often hexagonal leaf scars separated by substantial interscar tissue. Higher on the trunk, bark was thinner and leaves were shed more rapidly, resulting in smaller, more elongate, diamond-shaped scars arranged in regular vertical rows. Near the crown, scars become narrow and closely spaced as the trunk transitions into leafy branches. As a result, isolated bark fragments can often be assigned broadly to lower, middle or upper parts of the trunk, even though precise height cannot be determined.

Sigillaria: the ribbed pillar of the swamp

If Lepidodendron was the architectural framework of the forest, Sigillaria served as its columnar counterpart (see box: Sigillaria – the ribbed architecture of the swamp). Slightly shorter on average – typically 20-30m – Sigillaria possessed a vertically ribbed trunk bearing rows of elongated leaf scars arranged like rail tracks (Fig. 2.7). These trees had even fewer branches than Lepidodendron and may have borne their leafy crown in a tight cluster at the top (Fig. 2.6).

Fig. 2.6. A reconstruction of the Carboniferous lycopsid *Sigillaria*. A mature tree is shown as it likely appeared in lowland coal-swamp forests of the late Carboniferous. The tall, rigidly columnar trunk is marked by strong, continuous vertical ribs formed by aligned rows of leaf scars, a key feature distinguishing *Sigillaria* from other arborescent lycopsids. The crown is restricted to a short terminal zone bearing sparse tufts of long, narrow microphyll leaves rather than a branched canopy. The plant’s ordered, architectural form reflects a growth strategy based on a thickened cortex rather than true woody tissue. *Sigillaria* was widespread in Westphalian swamps and contributed substantially to the biomass that formed Carboniferous coal seams.

Fig. 2.7. Flattened *Sigillaria* bark from the West Cumberland Coalfield preserved as a compressed sheet, showing well-defined vertical ribbing and parallel files of elongate leaf scars. The strongly flattened appearance reflects thin outer bark combined with intense compaction within fine-grained swamp or floodplain sediments. In contrast, Fig. 2.8 preserves the same bark architecture in higher relief, illustrating how trunk orientation, bark thickness and taphonomic history strongly influence the preserved appearance of *Sigillaria*. (*From the author’s collection*.)

Fig. 2.8. A three-dimensional fragment of *Sigillaria* trunk bark displaying bold, raised longitudinal ribs separated by deep furrows. The strong relief suggests thicker bark, likely from a lower or mid-trunk position, with sediment infilling prior to compaction preserving the original topography. In contrast to the flattened specimen shown in Fig. 2.7, this slab highlights the morphological range of *Sigillaria* bark produced by differences in growth position and preservation rather than taxonomy. (*From the author’s collection*.)

Sigillaria often occupied the margins of peat domes, where the substrate was firm enough to support their slender trunks but still permanently moist. In the West Cumberland Coalfield, Sigillaria occurs both as:

impressions in grey roof shales, often with well-preserved leaf-scar detail; and
as ironstone casts, especially in coastal exposures and eroded spoil.

Fig. 2.9. A beach-collected ironstone nodule preserving a flattened external impression of sigillarian (*Sigillaria*) lycopsid bark, from the West Cumberland Coalfield, showing fine, closely spaced vertical ribbing characteristic of *Sigillaria* trunks. The subdued relief reflects strong compaction of thin outer bark within fine-grained swamp or floodplain sediments, a common mode of preservation in the West Cumberland coal-swamp succession. (*From the collection of Duncan White*.)

The co-occurrence of Lepidodendron and Sigillaria in particular horizons suggests that the West Cumberland Coalfield’s swamps were mature, long-lived systems rather than newly colonised landscapes (Fig. 2.5).

Sigillaria – the ribbed architecture of the swamp
At first glance, Sigillaria appears simpler than Lepidodendron, but its anatomy reflects an even more specialised growth form. The most distinctive feature of Sigillaria is its rigidly columnar trunk, marked by strong vertical ribs that run uninterrupted from base to crown. These ribs mark vertical rows of leaf scars, each scar showing where a single microphyll leaf was attached. Unlike the diamond cushions of Lepidodendron, the scars of Sigillaria are elongated and tightly organised, giving the trunk a highly ordered, almost architectural appearance. This regularity makes Sigillaria bark fragments instantly recognisable in the field. Branching was extremely limited. The trunk terminated in a short apical crown zone bearing sparse tufts of long, narrow leaves, rather than a broad canopy. This growth form produced a tall, slender silhouette, well suited to densely packed swamp forests, where vertical space was at a premium. As in Lepidodendron, the trunk was supported largely by a thick cortex rather than by wood, and a central pith cavity was commonly present. The root system, known as Stigmaria (Part 3), spread horizontally through soft mud, anchoring the tree in permanently waterlogged substrates. This made Sigillaria particularly successful along the margins of peat domes and shallow swamp basins. In the West Cumberland Coalfield sequence, the presence of well-preserved Sigillaria alongside Lepidodendron indicates long-lived, stable swamp conditions (Fig. 2.1). These trees were not early colonists but characteristic components of mature lycopsid forests.

Sigillaria – the ribbed architecture of the swamp

At first glance, Sigillaria appears simpler than Lepidodendron, but its anatomy reflects an even more specialised growth form. The most distinctive feature of Sigillaria is its rigidly columnar trunk, marked by strong vertical ribs that run uninterrupted from base to crown.

These ribs mark vertical rows of leaf scars, each scar showing where a single microphyll leaf was attached. Unlike the diamond cushions of Lepidodendron, the scars of Sigillaria are elongated and tightly organised, giving the trunk a highly ordered, almost architectural appearance. This regularity makes Sigillaria bark fragments instantly recognisable in the field.

Branching was extremely limited. The trunk terminated in a short apical crown zone bearing sparse tufts of long, narrow leaves, rather than a broad canopy. This growth form produced a tall, slender silhouette, well suited to densely packed swamp forests, where vertical space was at a premium.

As in Lepidodendron, the trunk was supported largely by a thick cortex rather than by wood, and a central pith cavity was commonly present. The root system, known as Stigmaria (Part 3), spread horizontally through soft mud, anchoring the tree in permanently waterlogged substrates. This made Sigillaria particularly successful along the margins of peat domes and shallow swamp basins.

In the West Cumberland Coalfield sequence, the presence of well-preserved Sigillaria alongside Lepidodendron indicates long-lived, stable swamp conditions (Fig. 2.1). These trees were not early colonists but characteristic components of mature lycopsid forests.

The unusual anatomy of the lycopsid canopy

Both Lepidodendron and Sigillaria shared a growth strategy that differs fundamentally from that of modern trees.

Secondary growth was limited. Most of the trunk’s thickness was formed early. The trees did not build annual rings like conifers or broadleaves, making them structurally more like gigantic monocots (i.e. flowering plants characterized by seeds with one embryonic leaf).
A hollow pith chamber formed in the centre. As the tree aged, the inner tissues often rotted away, leaving a hollow cylinder – exactly the space filled by siderite in the West Cumberland Coalfield nodules (Fig. 10.5).
Rapid growth and reproduction. Lycopsids could reach full height in only a few decades. Their reproductive structures (sporangia) were located in large cones at the top of the trunk or near branch forks.
High water demands. These trees relied on saturated soils. Their roots – Stigmaria – extended horizontally through soft mud, anchoring the trunk with regular rootlets.

This biology explains both their ecological dominance and their eventual vulnerability. Once water levels shifted or substrate stability decreased, whole stands could die within a short interval, producing the mass fall of bark and trunks recorded in some West Cumberland Coalfield roof shales.

Preservation in the West Cumberland Coalfield roof shales

The way the lycopsids appear in the local fossil record reveals much about their final moments.

Detached bark slabs. When a lycopsid died, the outer bark (cortex and periderm) often separated from the underlying tissues. In still-water environments, these sheets settled gently onto the mud, producing the broad impressions found in roof shales.
Compressed trunk sections. Horizontal or oblique compressions occur where trunks collapsed during drowning events. These can be several centimetres thick even after compression.
Ironstone casts. Perhaps the most striking preservational style, these form when trunk cavities or roots filled with siderite. The West Cumberland Coalfield’s coastal exposures produce many of these, often with beautifully preserved internal textures (Fig. 10.5).
Red versus grey shales. In the West Cumberland Coalfield, grey shales represent the original quiet-water lacustrine deposits. Red shales reflect later oxidation, often along channel-like or fracture-controlled paths. Lepidodendron bark occurs in both, but the crispest detail is typically in the grey.

Ecological implications

Fig. 2.10. A flooded lowland wetland dominated by arborescent lycopsids. Straight, unbranched *Sigillaria* trunks with ribbed bark stand alongside *Lepidodendron*, identifiable by its diamond-patterned leaf cushions and dichotomously forked crown. The trees rise from a continuously waterlogged substrate with sparse fern undergrowth, reflecting the dense, equatorial swamp forests that developed on Westphalian floodplains and later gave rise to the West Cumberland Coalfield Coal Measures.

The abundance of Lepidodendron and Sigillaria in the West Cumberland Coalfield is more than a matter of preservation bias. Their distribution indicates:

ever-wet, low-oxygen soils;
extremely high water tables;
little seasonal fluctuation; and
deeply organic, peat-forming environments.

In short, the West Cumberland Coalfield forest was a classic mid-Westphalian lycopsid swamp, dominated by canopy lycopsids at the height of their ecological power. In younger Westphalian strata elsewhere, these taxa decline sharply as climate shifts toward increased seasonality and floodplain dynamism. The West Cumberland Coalfield flora therefore provides a snapshot of the phase when lycopsids were still unchallenged giants – shortly before the world began to change in ways that favoured tree ferns and seed ferns.

A canopy unlike any seen today

Imagining the living forest requires setting aside modern analogies. These were not coniferous or angiosperm forests. They resembled no extant biome. An observer walking through such a landscape would have seen elongated trunks rising like columns, the canopy far above and supported by a limited range of species. Underfoot would lie a soft, saturated substrate of decaying organic matter, laced with Stigmaria roots and periodically flooded.

It was an ecosystem both structurally simple and biologically extreme, its very stability dependent on waterlogged conditions that few plants today could tolerate. Its ultimate fate, preserved in the roof shales, was to collapse when water levels shifted even slightly.

The lost rainforest of the West Cumberland Coalfield (Part 2): A forest of giants – Lepidodendron and Sigillaria

Lepidodendron: the geometric giant

Sigillaria: the ribbed pillar of the swamp

The unusual anatomy of the lycopsid canopy

Preservation in the West Cumberland Coalfield roof shales

Ecological implications

A canopy unlike any seen today

Further reading

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Lepidodendron: the geometric giant

Sigillaria: the ribbed pillar of the swamp

The unusual anatomy of the lycopsid canopy

Preservation in the West Cumberland Coalfield roof shales

Ecological implications

A canopy unlike any seen today

Further reading

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