Dipnoan diversity in the early Pennsylvanian of Scotland: new lungfish from the Lower Coal Measures of North Lanarkshire

In the past 150 years, only two lungfish have been recorded in the Scottish Coal Measures: Ctenodus (Sharp and Clack 2013) and Sagenodus (Beeby et al. 2020). They form part of a diverse vertebrate fauna recovered from blackband ironstones and roofing shales of productive coal seams in many collieries in the Scottish coalfields. Most specimens were collected in the second half of the nineteenth century, but little new material was added to museum collections in the century that followed. In the past 15 years or so, the senior author has visited many of the waste tips at redundant collieries in the Scottish Central Coalfield in the search for fossils. These visits have yielded a rich assemblage of fossil animals and plants, especially in the colliery waste from mining the Upper and Lower Drumgray Coal (Elliott 2014, 2018). Many of the new specimens represent animals that were very small and may have been overlooked by collectors in the past, but they show that the early Pennsylvanian vertebrate fauna was more diverse than previously realized. Here we describe a suite of new lungfish specimens collected from three sites in North Lanarkshire, which reveal that lungfish diversity in the Scottish Coal Measures was also much greater than previously described.

The new lungfish specimens were collected from the waste tips of three redundant collieries: Ardenrigg, Dewshill and Greenrigg. These new sites lie on the eastern side of the Central Coalfield c. 20 km east of Glasgow, and close to the other North Lanarkshire collieries, Airdrie, Carnbroe and Newarthill (Fig. 1), where lungfish fossils were found in the nineteenth century (Sharp and Clack 2013; Beeby et al. 2020). At Ardenrigg and Dewshill, the new specimens were collected from shales associated with the Drumgray Coals whereas at Greenrigg they were collected from the slightly older Slatyband Ironstone working. Ardenrigg and Dewshill lie within the Communis Chronozone, whereas Greenhill is in the Lenisulcata Chronozone (Fig. 1). All are of Langsettian age and form part of the Scottish Lower Coal Measures Formation (Browne et al. 1999; Elliott 2014, 2018). They are slightly older than the deposits at Airdrie, Carnbroe and Newarthill, which lie in the Scottish Middle Coal Measures Formation (Smithson 1985). Material from the ‘Bellsdyke Pit’, near Airdrie, is described here for the first time. Locality details are limited (Henrichsen 1972) and it is not clear from which of the five pits at Bellsdyke Colliery the specimen was collected. A number of different coals were mined at the colliery, including the Drumgray, Kiltongue and Virtuewell seams (www.scottishmining.co.uk), so it is possible that this specimen also originated from the Scottish Lower Coal Measures Formation (Fig. 1).

The new lungfish specimens described here are housed in The Hunterian Museum, in the University of Glasgow. They were collected from blocks of finely laminated carbonaceous shales, siltstones and mudstones in the waste tips of three redundant collieries: Ardenrigg, Dewshill and Greenrigg (for locality details see Elliott 2018). Sharp wood-chisels were used initially to split the blocks and a Swann Morton scalpel with a No. 25 blade was used to separate the finely laminated layers and remove unwanted matrix from specimens.

Photographic images were produced with a Sony DSC-W17 digital camera fitted with a Unilink digital camera adapter attached to a Wild stereomicroscope. Images of larger material were produced using a Kodak Z740 digital zoom camera with two ×10 macro close-up lenses connected in tandem. Camera sensitivity was set at ISO 80 with f/8 and exposure time 1 to ¼ s. Some specimens were scanned by micro-computed tomography (⁠$μ$ct) at the Cambridge Biotomography Centre, University of Cambridge, using a Nikon X-Tek micro-CT scanner with the following parameters: X-ray power 120–200 kV at 11.9 and 36.3 $μ$m. The scan data were processed using Mimics V19 and segmented using multiple slice editing techniques. Three-dimensional PDFs were created by exporting stereo lithography (STL) files from Mimics segmentation software. All figures were prepared using Microsoft Paint and Powerpoint. Abbreviations for institutions housing material used in this study are: GLAHM, The Hunterian Museum, University of Glasgow; NMS, National Museums of Scotland.

This published work and the nomenclatural act it contains have been registered in Zoobank (https://zoobank.org).

Osteichthys Huxley, 1880

Sarcopterygii Romer, 1955

Dipnomorpha Ahlberg, 1991

Dipnoi Müller, 1845

Family undesignated

Genus Braccodus nov.

Life Science Identifier (LSID). urn:lsid:zoobank.org:act:567394F7-533B-4A0A-A16B-BE5149E0DB8E

Type species. Braccodus kerri sp. nov.

Derivation of name. From Wester Bracco Farm, the site of the Ardenrigg Colliery waste tip and the Greek odoύs tooth.

Diagnosis. Small, triangular-shaped tooth plates with length to width ratio 2:1. Four radiating tooth ridges. Teeth conical, those on tooth ridge 1 larger than on other tooth ridges and show marked increasing size labially. Tooth formula 5443. Pterygoid tooth ridge angle 80°. Pterygoid tooth plates widely separated.

Braccodus kerri sp. nov. Figure 2 

Life Science Identifier (LSID). urn:lsid:zoobank.org:act:049F32FD-9F97-4369-AC6F-22635699F613

Derivation of name. In honour of Ramsay and William Kerr, owners of Wester Bracco Farm, for allowing access to the Ardenrigg Colliery waste tip.

Diagnosis. As for genus.

Type material. GLAHM 163367

Type locality and horizon. Shale overlying the Drumgray Coals, Ardenrigg Colliery waste tip, Wester Bracco Farm, North Lanarkshire, Scotland, grid reference [NS 827657].

Description. The new taxon is represented by a small tooth plate attached to a left pterygoid. The specimen is preserved in part and counterpart, in the plane of the pterygoid (Fig. 2a), and the tooth plate has been crushed slightly and pushed dorsally. In the image prepared from the $μ$ct scan of the part specimen (Fig. 2b), the tooth plate has been oriented in life position, thus reducing the exposure of the pterygoid.

The tooth plate is c. 3 mm long and c. 1.5 mm wide, with a length to width ratio of c. 2.0 and a tooth ridge angle of 80°. The lingual edge of the tooth plate is aligned almost parallel to the pterygoid midline suture. It has four tooth ridges, which radiate from a point approximately three-quarters along the length of the lingual margin. The teeth are conical and attached directly to the base of the tooth plate rather than supported on bony ridges as in Sagenodus (see below). All the teeth are preserved, although the last on tooth ridge 1 is slightly crushed along the lingual edge. The tooth formula is 5443. In each tooth row the teeth increase in size labially, particularly on tooth ridge 1. Some of the individual teeth are not blunt cones, as they appear in the $μ$ct scan (Fig. 2b), but those at the lingual end of tooth ridges 1 and 2 have cristae (cr) running down the side of each tooth (Fig. 2c). These appear to be bands of petrodentine that have been exposed as the enamel has worn away.

The left pterygoid (Fig. 2a, b and d) is c. 8.5 mm long. It is preserved as a mixture of bone and impression on the part and counterpart. The anterior portion is c. 4 mm long and bears the tooth plate and the midline suture. The posterior portion is c. 4.5 mm long and would have contacted the parasphenoid along its medial edge. It includes a quadrate ramus, which extends posterolaterally as a broad plate of bone, from the midline suture and posterior end of the tooth plate. The midline suture is c. 3 mm long. There is a gap of c. 1.5 mm between the midline suture and the lingual edge of the tooth plate, indicating that the pterygoid tooth plates were fairly widely spaced (Fig. 2d), unlike those in Ctenodus (Sharp and Clack 2013) and Sagenodus (Beeby et al. 2020) where the tooth plates almost meet along the midline (see also Watson and Gill 1923, figs 10 and 24). The ventral surface of the pterygoid has a mesh-like texture, particularly behind the midline suture and down the medial edge where it probably formed an overlap area with the parasphenoid (Fig. 2a).

Remarks. The tooth plate is unlike any recently described from the Carboniferous of Scotland (Sharp and Clack 2013; Challands et al. 2015; Smithson et al. 2015, 2019; Beeby et al. 2020). It is also different from those of either earlier Devonian taxa or later forms. Notably, the increase in size of the teeth along each tooth ridge is much greater than previously observed, particularly on tooth ridge 1, producing large, almost bulbous teeth at the labial end of each ridge.

Family undesignated

Lanarkodus gen. nov.

Life Science Identifier (LSID). urn:lsid:zoobank.org:act:ED6EEE10-171E-4582-BF0F-7D1CE9448DFA

Type species. Lanarkodus clarki sp. nov.

Derivation of name. From North Lanarkshire, the area in Scotland where the new lungfish have been found, and the Greek odoύs tooth.

Diagnosis. Small, triangular-shaped tooth plates with length to width ratio 2.3:1. Seven radiating tooth ridges. Teeth on tooth ridge 1 laterally compressed, those on remaining tooth ridges conical. Tooth ridge angle 110°.

Lanarkodus clarki sp. nov. Figure 3 

Life Science Identifier (LSID). urn:lsid:zoobank.org:act:30427365-B573-4A2E-A798-0C870A790A4F

Derivation of the name. In honour of Dr Neil Clark, Curator of Palaeontology, The Hunterian Museum, University of Glasgow, for his support and encouragement in the search for new vertebrate fossils in the mining waste of the Central Coalfield.

Diagnosis. As for genus.

Type material. GLAHM 163368

Type locality and horizon. Shale overlying the Drumgray Coals, Dewshill Colliery waste tip, Dewshill, North Lanarkshire, Scotland, grid reference [NS 855640].

Description. The new taxon is represented by two small tooth plates, the type specimen from Dewshill Colliery, GLAHM 163368 and another from Ardenrigg Colliery, GLAHM 163369 (Fig. 3)

The largest of the two tooth plates is from Dewshill (Fig. 3a and b). Tooth ridges 1 and 2 are incomplete but represented by impressions in the matrix. The tooth plate is c. 6 mm long and 2.6 mm wide with a length to width ratio of 2.3:1. The tooth ridge angle is 110° and it has seven tooth ridges. All the teeth on tooth ridges 3–7 are preserved. They are conical and attached directly to the base of the tooth plate rather than supported on bony ridges as in Sagenodus (see below). The teeth increase slightly in size towards the labial end of each tooth ridge, but they do not show the profound size increase seen in Braccodus. The teeth on the preserved portion of tooth ridge 1 appear to be laterally compressed, but the tooth ridge is too damaged to be certain.

The smaller tooth plate from Ardenrigg (Fig. 3c and d) has four ridges. It is c. 2.5 mm long and 1 mm wide. The tooth ridge angle is 50°. This is the same as the tooth ridge angle between tooth ridges 1 and 4 in the Dewshill specimen. The teeth on tooth ridges 2–4 are simple cones but those on tooth ridge 1 are laterally compressed. This is particularly clear in the damaged last tooth. The tooth formula is 3332.

Remarks. It is probable that the tooth plate from Ardenrigg is a juvenile of Lanarkodus from Dewshill. The tooth ridge angle between tooth ridges 1 and 4 is the same and both appear to have laterally compressed teeth on tooth ridge 1. Differences in tooth form between those on ridge 1 and other tooth ridges are common among Scottish Mississippian lungfish (Smithson et al. 2015, 2019; Challands et al. 2015) but have not been observed before among Pennsylvanian forms. This may be the first example of heterodonty in Pennsylvanian lungfish.

Family Sagenodontidae Jaekel, 1911

Genus Sagenodus Owen, 1867

Type species. Sagenodus inaequalis Owen, 1867

Sagenodus sp.

Material. Sagenodus is represented by two small pterygoid tooth plates from Ardenrigg, GLAHM 163370 and GLAHM 163371 (Fig. 4), and a larger pterygoid tooth plate from Bellsdyke, NMS G 1957.1.5743 (Fig. 4).

Description. The larger of the two specimens from Ardenrigg GLAHM 163370 (Fig. 4a and b) is 18.5 mm long and 6.5 mm wide with a length to width ratio of 2.8 and a tooth-ridge angle of 110°. The smaller specimen GLAHM 163371 (Fig. 4c and d) is 7 mm long and 2.5 mm wide with a length to width ratio of 2.8 and a tooth-ridge angle of 110°. The tooth plate from Bellsdyke NMS G 1957.1.5743 (Fig. 4e) is 43 mm long and 15.5 mm wide, with a length to width ratio of 2.8 and a tooth-ridge angle of 115°. The tooth plates from Ardenrigg have five tooth ridges. These radiate from a point approximately three-quarters along the length of the gently curved lingual margin. The Bellsdyke specimen has seven tooth ridges that radiate from a more posterior position. The laterally compressed teeth are attached to bony ridges with deep furrows between them. On the largest specimens the teeth at the lingual end of each ridge are worn to a blade. Although most of the teeth on the smallest specimen are damaged (Fig. 4c and d), there no evidence that the lingual teeth were strongly worn. In the Bellsdyke specimen, tooth ridge 1 is gently curved, but the tooth ridges of Sagenodus are typically straight.

Remarks. The tooth plates from Ardenrigg and Bellsdyke are like those of Sagenodusquinquecostatus (Beeby et al. 2020, fig. 19) from the slightly older Serpukhovian deposits of Lothian and Fife. They are both relatively long and narrow and differ from the much larger, shorter and broader plates of Sagenodusinaequalis (Beeby et al. 2020, fig. 9) found at Newarthill.

Family undesignated.

Genus Conchopoma Kner, 1868

Type species. Conchopoma gadiforme Kner, 1868

Conchopoma sp.

Material. Conchopoma is represented by two basihyal tooth plates, one from Ardenrigg GLAHM 163372 preserved in dorsal view (Fig. 5a), the other from Greenrigg GLAHM 163373 preserved in ventral view (Fig. 5b).

Description. The basihyal tooth plates are flat ovoid discs, bilaterally symmetrical along the shortest, anteroposterior diameter (Fig. 5). The shortest diameter of the Ardenrigg specimen is c. 4 mm, the longest c. 5 mm. The Greenrigg specimen is slightly larger, with a shortest diameter of c. 6 mm and a longest of c. 7.5 mm. The anterior edge is gently convex, the lateral edges are almost straight, and the posterior edge has a complicated shape, being concave laterally, much straighter medially and with a midline notch (Fig. 5b). Teeth are exposed on the Ardenrigg specimen. They are small, conical and of a similar size across the entire surface of the plate. There is no evidence that the teeth on the edges are larger than those towards the centre.

Remarks. These specimens are the first record of Conchopoma in the Pennsylvanian of Europe. The shape of the basihyal tooth plates is different from those of the two Conchopoma specimens described from the slightly older Scottish Serpukhovian locality of Loanhead (Smithson et al. 2019), and those of Conchopomagadiforme from the Lower Permian of Humberg, Germany (Heidtke 1986). The new specimens are relatively much broader: those from Ardenrigg and Greenrigg have a length to width ratio of c. 1:1.25 compared with 1:1 in the specimens from Loanhead and Humberg. Furthermore, the teeth on the new specimens are much smaller than those from Loanhead. On the Loanhead specimens there are c. 9 teeth per square mm (Smithson et al. 2019, fig. 3e and f) compared with c. 25 teeth per square mm on the Ardenrigg specimen (Fig. 5a).

Family undesignated

Genus undesignated

Material. Among the lungfish specimens collected from Ardenrigg is a small parasphenoid GLAHM 163374 (Fig. 6). It is preserved in ventral (palatal) view (Fig. 6a), but details of the dorsal (visceral) surface have been revealed in a $μ$ct scan of the specimen (Fig. 6b).

Description. The parasphenoid is c. 15 mm long. It comprises two parts; the anterior corpus and the posterior stem. The corpus is 9.5 mm long and represents c. 65% of the length of the parasphenoid. The posterior stem is c. 6.5 mm long. The anterolateral edges of the corpus are slightly damaged, but otherwise the parasphenoid is well preserved.

The corpus is diamond-shaped and is c. 10 mm wide. The anterolateral edge is broken on the right side of the corpus but is largely intact on the left. The bone is very thin and formed the overlapping suture with the left pterygoid. The area of overlap on the palatal (ventral) surface is clearly visible on the scan (Fig. 6c, labelled sop) as an area of fine ridges and grooves. These extend from a shallow step on the surface of the corpus to the anterolateral edge of the parasphenoid. The margin of this shallow step on the right side is marked by the broken anterolateral edge and the area of overlap is missing. Two areas have been described on the parasphenoid of Sagenodus copeanus (Schultze and Chorn 1997) that mark regions of sutural contact with the pterygoids. The first is on the ventral surface of the anterior tip of the parasphenoid and overlapped the pterygoids; the second is on the dorsal surface of the anterior lateral margins of the corpus and was overlapped by the pterygoids. The posterolateral edges of the parasphenoid are sinuous, with a convex margin at the most lateral part of the corpus where it would have met the pterygoid, and a concave margin where it meets the posterior stem. Most of the palatal surface is flat, but the midline is marked by a pair of shallow depressions that diverge posteriorly. The dorsal surface of the corpus bears a deep midline groove that extends posteriorly from the anterior tip of the parasphenoid across the corpus and onto the posterior stem. The groove diverges posteriorly and is supported by a prominent ridge on either side. These ridges extend onto the posterior process to form its lateral edges where the process joins the corpus. This broad tapering trough probably marks the anterior continuation of the notochord beneath the braincase as seen in the extant lungfish Neoceratodus (Goodrich 1930, fig. 315b). Similar ridges have been described in Sagenodus copeanus (Schultze and Chorn 1997), but they are absent on the scan of the parasphenoid of Sagenodus quinquecostatus (Beeby et al. 2020, fig. 2b).

The posterior stem is c. 6.5 mm long and 4.5 mm wide at its posterior end. It tapers anteriorly and is 3 mm wide where it joins the corpus. In palatal view a broad, shallow groove extends along the midline but fades out where the posterior process meets the corpus. It is probably equivalent to the median depression seen on the parasphenoid of Ctenodus cristatus (Sharp and Clack 2013), Sagenodus copeanus (Schultze and Chorn 1997) and Sagenodusinaequalis (Beeby et al. 2020), and may mark the path of the dorsal aorta (Sharp and Clack 2013).

Remarks. The parasphenoid from Ardenrigg is unlike those of other Pennsylvanian lungfish including Ctenodus (Sharp and Clack 2013), Conchopoma (Schultze 1975) and Sagenodus (Beeby et al. 2020) (Fig. 6d–g). Conchopoma has a spade-shaped corpus that is covered in denticles on the palatal surface (Schultze 1975; Heidtke 1986; Smithson et al. 2019), and the proportions of the parasphenoids of Ctenodus and Sagenodus are different, with a longer posterior stem and a shorter and narrower corpus (Watson and Gill 1923; Schultze and Chorn 1997; Sharp and Clack 2013; Beeby et al. 2020). In addition, the main area of overlap with the pterygoids is on the lateral margins of the corpus rather than at the tip as in S. copeanus (Schultze and Chorn 1997).

Until recently, only four lungfish genera had been described from the Scottish Mississippian and just two from the Pennsylvanian. New discoveries in the Scottish Borders and the Midland Valley have revealed a much more diverse fauna. At least six genera have been recognized in the Tournaisian at Burnmouth (Smithson et al. 2015; Challands et al. 2019), four in the early Visean at Gin Head (Challands et al. 2015) and six in the Namurian at Loanhead (Smithson et al. 2019). To these can now be added the four taxa described here from the early Westphalian at Ardenrigg. These new discoveries demonstrate that Carboniferous lungfish were much more diverse than previously realized and add to the growing evidence that the rate of lungfish evolution did not decline significantly after the Devonian, as originally proposed by Westoll (1949), but remained high throughout the Carboniferous.

Many of the new lungfish taxa recently described from the Scottish Carboniferous are small and represent fishes that range in size between 60 and 300 mm long. These are much smaller than previously described Carboniferous lungfish. For example, some specimens of Ctenodus are estimated to be from fishes well over a metre long (Sharp and Clack 2013, p. 199). Only Uronemus lobatus from the Burdiehouse Limestone (Traquair 1871, 1873) is in the same size range as the new taxa. Carboniferous lungfish were clearly not only taxonomically diverse but also morphologically disparate, and probably occupied a variety of different freshwater niches. The different examples of dentition found among lungfish present at the same locality, as at Ardenrigg and Loanhead (Smithson et al. 2019), suggest that they were exploiting a broad range of food sources including both soft-bodied animals and those with hard shells.

The new lungfish specimens described above were collected from finely laminated black shales, rich in bivalves and ostracods (Elliott 2014). These sediments differ from the blackband ironstones at Airdrie and Carnbroe and the roofing shales overlying the coal at Newarthill from which lungfish were collected in the past, in being more clearly laminated and containing a more diverse shelly fauna. This suggests that the new lungfish were living in a small lake environment rather than a more extensive and tree-dominated coal swamp. These finely laminated shales have so far yielded a diverse fish fauna including a number of new species of actinopterygians (Elliott 2014, 2018), and it is anticipated that further exploration of similar sediments in the waste tips of redundant collieries across the Scottish Midland Valley will yield similar results.

We thank R. and W. Kerr, owners of Wester Bracco Farm, for access to the Ardenrigg Colliery waste tip. All the new lungfish material collected from sites in North Lanarkshire has been accessioned into the collections of The Hunterian Museum, University of Glasgow, and we are grateful to N. Clark at the Museum for his support and encouragement. We thank S. Walsh, National Museums Scotland, for permission to study specimens in his care, and K. Smithson, Cambridge Biotomography Centre, University of Cambridge, for her help with the $μ$ct scanning. We are grateful to Z. Johanson, Natural History Museum, London, for encouraging us to formally name the new lungfish tooth plates. T. Smithson thanks R. Kilner, Director and J. Head, Curator of Vertebrate Palaeontology, for access to research facilities in the University Museum of Zoology, Cambridge.

FME: writing – original draft (equal); TJC: writing – original draft (equal); TRS: writing – original draft (equal)

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.