A photographic guide to the freshwater and terrestrial fossils from the Pennine Middle Coal Measures Formation of a small outcrop of the Cumberland Coalfield.

A photographic guide to the freshwater and terrestrial fossils from the Pennine Middle Coal Measures Formation of a small outcrop of the Cumberland Coalfield.

Tom Garner; last updated October 2021.

The Cumberland Coalfield is a historic coal mining area in the northwest of Cumbria, stretching from Workington and Whitehaven on the west coast across to Aspatria and some minor outcrops on the edge of the Northern Fells. This guide contains a relatively complete description of the common freshwater and terrestrial fossils from a small outcrop of the Cumberland Coalfield from the Pennine Middle Coal Measures Formation. While a single marine band contains a varied marine fauna however, due to its restricted stratigraphical position these fossils are not included.

The only detailed published literature on the palaeontology of the rocks of the outcrop is supplied by Eastwood et al. (1968) and this forms a basis for the identifications given. However, due to the ephemeral nature of exposures, and the wealth of later study and literature on coal measure palaeontology, Eastwood et al., (1968) cannot solely be relied upon. Therefore, the work of Cleal and Thomas (1994), which gives a comprehensive guide to the plant fossils of the British Coal Measures has been used to inform new identifications.

Part 1: Plant remains.

Part 1.1: Stems or stem-like fossils with longitudinal striations or ribs but without regularly distributed leaf scars.

This group is restricted to Calamites in the area, which are the stems of large horsetails, that have a similar structure to modern horsetails. These are preserved either as three-dimensional ‘pith casts’ or more commonly as the flattened or compressed outer surface. Calamites suckowii has been recorded, however, other species are likely present.

Figure 1. Calamites cf. suckowii (Scale: 3.5cm across; Calamites fossils tend to be between 2-4cm across, however, larger cast fossils have been found with ~10cm diameter).

Part 1.2: Stems with leaf scars distributed regularly in vertical rows or spirally around the stem.

This group is restricted to Lepidodendron and Stigmaria. Lepidodendron Is a tree-like lycopod plant, also known as a scale tree and Stigmaria is the name given to the roots of lycopods such as Lepidodendron. Both of the remains are relatively rare. Lepidodendron lycopodiodes and other unidentified species have been recorded but are identified through the morphology and pattern of leaf scars and cushions so precise identification is difficult.

Figure 2. Lepidodendron sp. (Scale: all of the above specimens are relatively small at <1.5cm across, however, larger specimens can be found).

Stigmaria is found in the area, however, it is extremely rare and often poorly preserved. They are identified by small repeating circles where rootlets would have once attached.

Part 1.3: Leaves borne in whorls around narrow stems.

The majority of this group are sphenopsid foliage: these are divided into two groups. Firstly, there are Calamite leaves which are elongate, linear veins with a single longitudinal vein; both Asterophyllites and Annularia are considered to be the leaves of Calamites or other plants belonging to Equisetales.

Figure 3: Asterophyllites equisetiformis (top) and Annularia cf. galioides (bottom) (Scale: Asterophyllites a mid-size fossil ~1.5cm width at the widest point and fossils tend to be up to 4cm long; Annularia is very small with the entire specimen no bigger than 1.5x1cm).

The second group contains Sphenophyllum, which are broader, with a more complex vein system. This plant has been reconstructed as a scrambling, leaning, creeping plant. Sphenophyllum cuneifolium has been recorded in the area, however, many of the specimens resemble other species due to significant variation in their leaf morphology.

Figure 4. Sphenophyllum cuneifolium (Scale: very variable in size, between 1-2cm diameter).

Part 1.4: Large fern and pteridosperm fronds or parts of such fronds (pinnae and pinnules).

This group contains true ferns such as Sphenopteris (pro parte) and seed ferns or pteridosperms such as Neuropteris. There are numerous genera in the area, which are relatively distinctive, however, further identifying remains to species level remains a challenge. While partial pinna can be found, individual pinnules are most frequently found. Specimens with multiple taxa are found with as many as 4 species on a single hand specimen.

The first genera is Neutropteris, which is characterised by a heart-shaped base and numerous secondary veins splitting from a primary vein in the middle of pinnule. Two species have been identified from the area: Neuropteris gigantea is relatively common, however, Neuropteris heterophylla has only been identified from mining spoil. Additionally, an identification of Macroneutropteris has also been made to a single pinnule remain- this taxon is twice the length of Neuropteris gigantea and comes to a point.

Figure 5. Neuropteris gigantea (top and middle) and unidentified Neuropteris sp. (bottom) (Scale: individual pinnules are relatively large extending up to 2.5cm in total length).

Figure 6. Cf. Neuropteris sp. (Scale: individual pinnule length ~1cm).

Alethopteris is another genera that has numerous long and slender pinnules, that have a strong mid-vein, and are inclined from a central ‘stem’. It is the only frond plant remain that has confluent pinnules with no stalk like Neuropteris or Sphenopteris. Two species of Alethopteris: Alethopteris decurrens and Alethopteris lonchitica have been recorded.

Figure 7. Alethopteris sp. (Scale: individual pinnules range from 1.5-4.5cm).

Sphenopteris has also been recorded from the area both to genus level and as Sphenopteris dilatata, however, it has not been subsequently confidently identified. However, Eusphenopteris cf. obtusiloba has been tentatively identified from the same strata, as well as several small specimens that are yet to be identified.

Figure 8. Eusphenopteris cf. obtusiloba (Scale: individual pinnules ~1cm).

Figure 9. Unidentified terminal remains including potential Sphenopteris sp. (Scale: ~1.5cm length).

Part 2: Freshwater bivalve shells.

Freshwater bivalves or ‘mussels’ can be abundant in certain strata where they form dense beds. Two genera dominate: Anthracosia and Carbonicola. Anthracosia can be differentiated from Carbonicola as it has a flattened top compared to Carbonicola, which also has faint radial lines. Identification is hindered as the shells are frequently flattened. At least 12 species of Anthracosia have been identified in the area and 2 Carbonicola. Anthracosphaerium  and Naiadites are also freshwater bivalves that have been identified.

Figure 10. Freshwater bivalves (unidentified) (Scale: 3-4.5cm in length).

Similar in form but significantly smaller are ostracods: several freshwater ostracods have been recorded including Carbonita humilis and Geisina arcuata.

Part 3: Fish remains.

Fish remains are relatively common, however require extensive splitting of shale. Four taxa have previously identified: Megalichthys, Platysomus, Rhabdoderma and Rhizodopsis. Various remains such as teeth and scales can be found, however scales are most diagnostic. Rhizodopsis, Megalichthys and Rhabdoderma (a coelacanth) are all lobe-finned fish, while Platysomus was a ray-finned fish.

Part 3.1: Teeth

The most common teeth are those of Megalichthys and Rhizodopsis, and they are most recognisable due to their size and 'typical' tooth shape. Platysomus have similarly conical and pointed teeth, however, they are ‘excessively minute’ and require magnification to examine them. That being said the abundance of Rhizodopsis scales and the scarcity of Megalichthys suggests that the majority of teeth are from Rhizodopsis.  

Figure 11. Fish teeth (unidentified) including bottom: cf. Rhizodopsis sp. (Scale: teeth are generally around 5mm in length; however, they can be as great as 11mm).

Part 3.2: Scales and plates.

The typical scales of the four fish taxa are relatively distinctive. Rhabdoderma has scales covered in fingerprint-like ornamentation; Megalichthys has large, thick scales with an enamel-like preservation and have a distinct diamond shape and pattern; Rhizodopsis has a tear-drop shaped scale with concentric circles and a ‘key-hole’ in the centre and Platysomus has small black scales that require a microscope to observe.

As with modern fish, in the case of Rhizodopsis, the annual growth rings deposited on the scales can be counted for a minimum age of the individual: this has been as counted to over 12 years from one scale collected.

Figure 12: Rhabdoderma elegans (top; scale: 6mm across), Megalichthys sp. (middle-top; scale: 2cm length, although this specimen is unusual with other scales being significantly smaller), Rhizodopsis sp. (middle-bottom; scale: 1cm length, although specimens can be as great as 2cm in length) and cf. Platysomus sp. (bottom; scale: <1mm length).

In addition to these ‘typical’ scales, modified scales and plates are also present, although these are more difficult to attribute.

Figure 13. Plates (top and top-middle) and articulated scales from cf. Platysomus (bottom) (Scale: plates tend to be around 5mm in size, comparable to most ‘typical’ scales, but do vary significantly).

Part 3.3. Fin impressions.

A single fin impression has been recovered, thought to be a caudal (tail) fin of cf. Platysomus, which is deeply cleft and symmetrical. The morphology of the impression looks very similar to invertebrate wings, however, careful observation of the vein pattern, as well as the associated fish remains, discount this.

Figure 14. Cf. Platysomus sp. (Scale: 3cm length).

Part 3.4. Spines and other associated remains.

In addition to scales, plates and fins, other fish remains such as spines and jaw plates have been found, similarly dis-articulated.

Figure 15. Unidentified spines (top; scale: ~1cm length) and a possible fragment of the lower jaw of Rhabdoderma elegans (bottom; scale: 2mm width).

It is also worth noting that identifications of fish are based upon prior identification of remains found in the area. While the ‘typical’ scales are more confidently attributed (apart from Platysomus) many other identifications cannot securely be made based upon fragmentary remains.

In addition to fish remains, fish coprolites can, although rarely, be found. These coprolites are black, elongate ‘lumps’ that are often flattened, but are notably thicker than other fish remains. Coprolites when split appear black and glassy, although other remains such as dis-articulated groups of scales may also be coprolites.

Figure 16. Fish coprolites: black and glassy type (top; scale: 1.8cm length) and dis-articulated fish remains type (bottom; scale: 6mm length).

Part 4: List of taxa as given by Eastwood et al., (1968).

Plantae

Alethopteris decurrens

Alethopeteris lonchitica

Neuropteris gigantea

Neuropteris heterophylla

Neuropteris sp.

Sphenopteris cf. dilatata

Sphenopteris sp.

Lepidodendron lycopodioides

Lepidodendron sp.

Lycopod megaspores

Annularia galioides

Calamites suckowi

Calamites sp.

Sphenophyllum cuneifolium

Pinnularia capillacea

Animalia

Spirorbis sp.

? Anthraconia librata

Anthracosia aff. accutella

Anthracosia cf. accutella

Anthracosia aff. aquilina

Anthracosia atra

Anthracosia cf. atra

Anthracosia cf. beaniana

Anthracosia cf. cocinna

Cf. Anthracosia nitida

Anthracosia ovum

Anthracosia phrygiana

Anthracosia regularis

Anthracosia cf. retrotracta

Anthracosia simulans

Anthracosia sp.

Anthracosia sp. nov.

Cf. Anthracosphaerium cycloquadratum

Anthracospaerium turgidum

Anthracospaerium sp.

Carbonicola cf. cristagalli

Carbonicola oslancis

Naiadites cf. angustus

Naiadites productus

Naiadites quadratus

Naiadites aff. quadratus

Naiadites cf. subtruncatus

Naiadites sp.

Carbonita humilis

Carbonita sp.

Cypridina sp.?

Geisina arcuata

Megalichthys sp.

Palysomid indet.

Rhabdoderma sp.

Rhizodopsis sp.

References

Cleal CJ, Thomas BA. 1994. Plant Fossils of the British Coal Measures (Palaeontological Association field guides to fossils: Number 6). The Palaeontological Association: London.

Eastwood T, Hollingworth SE, Rose WCC et al. 1968. Geology of the Country around Cockermouth and Caldbeck (Explanation of One-inch Geological Sheet 23, New Series). Her Majesty’s Stationary Office: London.