On a new species of Rhizodopsis from the Carboniferous of Scotland

During the Devonian, sarcopterygians were the most successful group of bony fish (Clack 2012), represented by three distinct clades, the coelacanths, dipnomorphs and tetrapodomorphs (Ahlberg 1991a). Coelacanths were not common, but dipnomorphs and tetrapodomorphs were diverse and represented by several different groups (Clack 2012). At that time, most sarcopterygians were marine and were significant casualties in the end-Devonian extinction event (Friedman and Sallan 2012). By the Carboniferous they were highly abundant relative to other species as a result of their gradual evolutionary change from aquatic to terrestrial. Among the dipnomorphs only the lungfish survived into the Carboniferous, and among the tetrapodomorphs only three groups remained: the megalichthyids, the rhizodontids and the earliest tetrapods. Whereas the tetrapods subsequently thrived, the rhizodonts survived only until the end of the Carboniferous (Friedman and Sallan 2012) and the megalichthyids did not survive beyond the early Permian (McGhee 2013).

During the Carboniferous, the megalichthyids were represented by Cladarosymblema (Fox et al. 1995; Clement et al. 2021), Askerichthys (Borgen and Nakrem 2016) and Megalichthys (Borgen and Nakrem 2016; Downs and Daeschler 2020). A fourth genus, Rhizodopsis, is sometimes included with the other megalichthyids (Friedman et al. 2007; Coates and Friedman 2010) but its anatomy is less well known. Originally named by Young (1866), Rhizodopsis was later recorded from the British Coal Measures by Traquair (1881) and Watson and Day (1916). Since then, it has been recorded from the Pennsylvanian of the USA by Thomson and Hahn (1968) and Schultze (1974) and from the Lower Permian of Germany by Schultze and Heidtke (1993). Fossil remains of the type species, Rhizodopsis sauroides Williamson, 1849, are common in the British Isles (Woodward 1891; Jackson 1952), but an investigation of these, and an examination of new material from the Midland Valley of Scotland, has revealed that a new species also populated this region during the Westphalian (early Pennsylvanian).

The purpose of this study is to name and describe the new species and to review the genus Rhizodopsis.

Samples of Carboniferous fossil material were collected from eight locations in the Midland Valley of Scotland (Fig. 1). Numbers on the map are fossil locations and the coordinate reference system used is the British National Grid with calculations based on geodetic datum WGS84.

1. Blairskaith Quarry, near Milngavie [55.952372, −4.2521918];

2. Greengairs, near Airdrie [55.908797, −3.9470100];

3. Longriggend, a few miles from Caldercruix [55.900928, −3.8937027];

4. Ardenrigg, at Wester Bracco [55.870086, −3.8777661];

5. Dewshill, near Shotts [55.854500, −3.8318231];

6. Calderhead, also near Shotts [55.834885, −3.7939589];

7. Greenrigg near Harthill [55.844269, −3.7349751];

8. River Avon between Linthaugh Bridge and Cot Castle [55.703639, −3.9891054 to 55.688417, −4.0077414].

Material recovered from both Blairskaith Quarry and the stretch of the River Avon from Linthaugh Bridge to Cot Castle is considered to be of Visean age and represents part of the Lower Limestone Formation (346–330.9 Ma) (Carruthers 1915; Hinxman et al. 1921; Cózar et al. 2010; Arkley et al. 2011; Browne et al. 2011).

Material recovered from Greengairs, Longriggend, Ardenrigg, Dewshill, Calderhead and Greenrigg is of Westphalian age and represents part of the Scottish Lower Coal Measures Formation (323.2–315.2 Ma) (Macgregor et al. 1920; Hinxman et al. 1921; Browne et al. 2011; McLean 2018). The bulk of this material is skull bones and scales of the coelacanth Rhabdoderma elegans and the tetrapodomorphs Megalichthys pygmaeus, Megalichthys hibberti and Strepsodus sauroides. It also includes two articulated but incomplete specimens of the new species of Rhizodopsis together with individual bones.

The material used in this study was from thinly laminated slabs of carbonaceous shales, siltstones and mudstones and includes groups of articulated and disarticulated bones and isolated ossified dermal elements. Most of the Rhizodopsis material was recovered from coal waste tips comprising shales associated with the Drumgray Coal. The largest specimens were found on the slatyband ironstone waste tips at Greenrigg near Harthill, and from in situ material from bands of carbonaceous shale in outcrops in the River Avon between Linthaugh Bridge and Cot Castle in the Stonehouse area.

The descriptions and illustrations in this paper are from material from waste tips of the Drumgray Coal at Wester Bracco. Fossils from these waste tips were used because of their superior preservation compared with that of material from the other locations.

Sharp wood chisels were used to carefully separate the thin layers of laminate from slabs of carbonaceous shale known to contain fossil material. A Swann Morton scalpel with a No. 25 blade was used to separate thinly laminated layers and assist in the removal of unwanted matrix from specimens.

For all close-up work throughout this study a Wild stereomicroscope was used for small specimens; photographic images were produced with a Sony DSC-W17 digital camera fitted with a Unilink digital camera adapter attached to the stereomicroscope. Images of larger specimens 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 of 1 to ¼ s.

A tentative reconstruction of the head of the new species was prepared using photographs of a 3D model of the skull. This was scaled up and constructed using acetate tracings of individual bones held together with clear sticky tape.

This technique was especially valuable in the reconstruction of the ethmoid region of the head, as specimens of Rhizodopsis sauroides are invariably preserved ventral side uppermost, and with the rostrodorsal part of the head crushed. In consequence, damage to the ethmoid region of the head in type material of the holotype made R. sauroides unsatisfactory for use as a model for comparative study.

Illustrations are in dorsal, left lateral and ventral views and are shown in a horizontal orientation with the rostral region of the head directed to the left (Fig. 2a–c).

Institutional abbreviations. GLAHM, Hunterian Museum, Glasgow; NMS, National Museums Scotland; MM, Manchester Museum.

Diagnosis. Emended diagnosis after Downs and Daeschler (2020) and Clement et al. (2021). A clade of tetrapodomorph fish possessing small semi-circular lateral rostral and posterior tectals forming the external nostril dorso-ventrally; a tusk on the premaxillary lingual to the premaxillary marginal tooth row; a dentary fang lingual to a complete dentary marginal tooth row; absence of labial lamina and marginal dentition on all three coronoids; contact between the subopercular and the first submandibular; and a distinct supratemporal bone with a rostrolateral process.

Remarks. A more extensive diagnosis was provided by Borgen and Nakrem (2016) but many of the characters that they included were later shown by Downs and Daeschler (2020) to have either a wider distribution among tetrapodomorphs or were not present in some taxa included in the family. Rhizodopsis did not form part of either analysis. For many years it was placed in its own family, the Rhizodopsidae Berg, 1940 (see Schultze and Heidtke 1993; Cloutier and Ahlberg 1996) but more recently it has been shown to be either immediately related to Megalichthys (Friedman et al. 2007) or to have formed an unresolved trichotomy with other megalichthyids Cladarosymblema and Ectosteorhachis (Coates and Friedman 2010).

Diagnosis. A megalichthyid lacking cosmine cover over dermal bones and scales; surface of dermal bones heavily pitted with minute pores and lacking the larger sensory pores found in other megalichthyids: squamosal almost excluded from contact with the maxilla; anal fin opposite and slightly behind second dorsal fin; pectoral fins broad and rounded; scales ovoid with elongate rostral–caudal axis and concentric rings over much of the surface.

Diagnosis. A species of Rhizodopsis distinguished from previously described species by the presence of a postparietal shield with a deep embayment between the supratemporal and tabular bones and two to five ridges on the dermal surface either side of the mid-sagittal suture; squamosal triangular; posterior region of the opercular lobed and extended caudally; enlarged lateral extrascapulars; second dorsal fin larger than the first dorsal.

Etymology. In honour of Dr Daniel Reid Rankin (1805–82) for his dedication and contribution to the palaeontology of the Carluke area (North Lanarkshire, Scotland).

Holotype: GLAHM 163318. Ventral aspect of the rostral portion of a partly articulated individual; several larger bones of the head are present, although dissociated (Fig. 3a).

Paratype: GLAHM 163319. Part and counterpart showing the posterior region of an articulated individual (Fig. 3b); the paratype was chosen by virtue of the dorsal fins preserved within the material.

Type locality. The type and paratype of the new species were collected from the Ardenrigg Colliery coal waste tip at Wester Bracco, South Lanarkshire [55.870086, −3.8777661] from the Carboniferous rocks of the Midland Valley of Scotland.

GLAHM 163320. Parietal, left, dermal (Fig. 4a).

GLAHM 163321. Parietal, left, visceral (Fig. 4b).

GLAHM 163322. Posterior supraorbital, right, visceral (Fig. 4c).

GLAHM 163323. Premaxilla, left, visceral (Fig. 4d).

GLAHM 163324. Postparietal shield, ventral (Fig. 5a).

GLAHM 163325. Postparietal, right, visceral (Fig. 5c).

GLAHM 163330. Slab containing disarticulated juvenile specimen (Fig. 6).

GLAHM 163326. Maxilla, right, visceral (Fig. 7a).

GLAHM 163327. Maxilla, right, showing choanal process (Fig. 7b).

GLAHM 163328. Squamosal, right, dermal (Fig. 7c).

GLAHM 163329. Squamosal, left, visceral (Fig. 7d).

GLAHM 163331. Preopercular, right, visceral (Fig. 7f).

GLAHM 163332. Quadratojugal, right, visceral (Fig. 7g).

GLAHM 163333. Mandible, left, labial (Fig. 8a).

GLAHM 163334. Dentary, left, labial (Fig. 8b).

GLAHM 163335. Opercular, right, dermal (Fig. 9a).

GLAHM 163336. Opercular, left, visceral (Fig. 9b).

GLAHM 163337. Subopercular, left, dermal (Fig. 9c).

GLAHM 163338. Subopercular, right, visceral (Fig. 9d).

GLAHM 163339. Lateral gular, right, dermal (Fig. 9e).

GLAHM 163340. Submandibulo-branchiostegal, left, dermal (Fig. 9f).

GLAHM 163341. Palatoquadrate, left, lingual (Fig. 10a).

GLAHM 163342. Palatoquadrate, right, labial (Fig. 10b).

GLAHM 163343. Palatoquadrate, left, ventrolingual (Fig. 10c).

GLAHM 163344. Vomer, left, anterior view (Fig. 11a).

GLAHM 163345. Cleithrum, right, visceral (Fig. 12a).

GLAHM 163346. Clavicle, right, dermal (Fig. 12c).

GLAHM 163347. Vertebra, mid torso, rostrolateral view (Fig. 13b).

GLAHM 163348. Vertebra, more posterior, lateral view (Fig. 13c).

GLAHM 163349. Scale, dermal surface (Fig. 14a).

GLAHM 163350. Scale, visceral surface (Fig. 14b).

Parietal shield. The length of the rostral–caudal axis of the parietal shield of Rhizodopsis rankini is half that of the postparietal shield, taking into account the curvature of the premax-rostro-naso complex. This ratio is like that found in Megalichthys pygmaeus (personal observation) and close to that in Megalichthys hibberti (GLAHM 149415; GLAHM V30313). In Cladarosymblema narrienense (Fox et al. 1995, figs 5, 14 and 16a) and Megalichthys mullisoni (Downs and Daeschler 2020, fig. 2a–f) these proportions are similar, whereas in Askerichthys heintzi (Borgen and Nakrem 2016, fig. 14) the length of the rostral–caudal axis of the parietal shield is greater than that of the postparietal shield. No pineal foramen or pineal fenestra is observed externally, and no laminae are identified at the caudal margin of the shield.

The largest bone of this portion of the skull is the parietal (Fig. 4a and b). The rostral margin of the parietal is sagittate with two diagonal sides. These exhibit extended laminae for overlap by the posterior nasal and median postrostral bones. This bone is twice as long as it is wide, proportions typical of megalichthyids (Downs and Daeschler 2020), and extends to the level of the rostral margin of the orbit. The dermal surface lacks the large sensory pores found in other megalichthyids but is sculptured with coarse reticulate ridges, strongest over the rostral half of the bone.

The parietals of R. rankini have a rostromedial projection; the rostrolateral border is at an angle to the axis, whereas the interparietal suture is almost straight. These features also compare with those of other megalichthyids. However, in R. rankini the width of the parietals increases towards the caudal margin, in contrast to those found in C. narrienense (Fox et al. 1995, figs 7a–d and 14), M. mullisoni (Downs and Daeschler 2020, figs 2a–c and 3) and A. heintzi (Borgen and Nakrem 2016, figs 8c, d, 9c, d, 10c and 14) where the bone tapers caudally, but also where the width remains constant along its length as in M. hibberti (Borgen and Nakrem 2016, fig. 42e; GLAHM 149415; GLAHM V3013) and Ectosteorhachis (Thomson 1975, fig. 8).

Figure 4c illustrates what is interpreted as a posterior supraorbital in visceral view, like that found in C. narrienense (Fox et al. 1995, fig. 14). Extended laminae on the caudal margin of this bone would overlap onto the intertemporal.

Although the exact shape of the intertemporal is not known, the space remaining after reconstruction of the dermal head bones suggests it was a triangular element. The premaxilla (Fig. 4d) has a triangular dorsal lamina where the bone tapers caudally, but also where the width remains wider horizontal than vertical. In some specimens the ventral margin is concave. The bone exhibits six to 10 small, relatively well-spaced conical teeth of equal size, with one large conical premaxillary fang twice the size of the smaller teeth. Comparison of an example of the premaxilla of Rhizodopsis sauroides (NMS 1897.111.51), where the bone appears to be similar in shape to the premaxilla of R. rankini, shows that the latter is a much lower and wider bone and has additional marginal teeth. The premaxilla of Megalichthys hibberti (GLAHM 149415; GLAHM V3013) resembles the same bone in Rhizodopsis.

The fenestra exonasalis in R. rankini is comparatively small, round, or oval and positioned relatively far posteriorly on the snout, as in M. hibberti (GLAHM 149415; GLAHM V3013) and M. pygmaeus (GLAHM 163232; GLAHM 163233).

Postparietal shield. The postparietal shield (Fig. 5a) widens caudally and has a distinct ‘waisted’ appearance reflecting a deep embayment between the extended lateral margins of the supratemporals and the lateral corners of the tabulars. This feature is similar to although considerably more pronounced than that found in Askerichthys heintzi (Borgen and Nakrem 2016, figs 14–19), Megalichthys laticeps (Bjerring 1972) and Megalichthys pygmaeus (GLAHM 163235), although barely discernible in specimens of Megalichthys hibberti (GLAHM V3013; GLAHM 149415). Figure 5a depicts the postparietal shield of R. rankini in ventral view and Figure 5b is an interpretation of the same showing the main features of the unit.

The supratemporal of Rhizodopsis rankini has a distinct, convex, lateral margin, like that in M. laticeps and in Askerichthys heintzi (Borgen and Nakrem 2016, figs 14–19). Moreover, in R. rankini the supratemporal is shorter rostral–caudally than the tabular bone, whereas in species of Askerichthys and Megalichthys it is either similar in length to the tabular or longer than it.

The postparietal bone of R. rankini (Fig. 5c) is spatulate, but the distal portion of the bone tapers much more sharply than in other megalichthyids.

The dermal surface of the postparietal shield of R. rankini has two to five fine ridges on each side of the mid-sagittal suture of the postparietal bones. These commence at the rostral margin of the postparietal shield and are directed caudally. They may be short or reach as far back as the midlength of the bone. This feature does not seem to have been observed in other tetrapodomorphs but is clearly visible in GLAHM V2560.

Disarticulated remains of a juvenile Rhizodopsis rankini. A slab of shale containing the disarticulated remains of a juvenile R. rankini is shown in Figure 6. This figure may be used as a reference to the dermal head bones where the description and image are taken from the slab of material GLAHM 163330.

Cheek and orbital region. The maxilla (Fig. 7a and b) is triangular with the same deep posterior lamina as found in Rhizodopsis sauroides (Traquair 1881, p. 172, fig. 2), Cladarosymblema narrienense (Fox et al. 1995, p. 22, fig. 11), Megalichthys hibberti (Jarvik 1966, fig. 15a; V2857; V3678), Megalichthys pygmaeus (GLAHM 163239; GLAHM 163240) and Askerichthys heintzi (Borgen and Nakrem 2016, fig. 21). The dorsal border has a small marginal overlap area for attachment of the lachrymal and jugal bones, and a small rostrally directed articular process associated with a choanal aperture is shown in Figure 7b. On the lingual surface there is a deep trough close to the ventral margin of the bone, extending from the region of the articular process and terminating at the caudal margin. Ornamentation of the dermal surface consists mainly of strong reticulate pits.

The squamosal (Fig. 7c and d), as with many other bones of R. rankini, is subject to some variation. It is triangular and unique when compared with the squamosals of other megalichthyids. The dermal surface is heavily punctate with minute pits and low ridges. An irregular ridge marks the position of the jugal sensory canal, which traverses the bone from the rostral to the caudal margin, where it connects with the rostrodorsal margin of the preopercular. The rostroventral corner of the squamosal also connects with the dorsal edge of the maxilla, although only along a short length of the margin.

The lachrymal (Fig. 7a) and a possible lateral rostral are observed dorsal to the rostrodorsal margin of the maxilla. The lachrymal appears to be triangular, and similar in shape to that found in M. hibberti (Moy-Thomas 1935, fig. 1). The bone was reconstructed from the right maxilla shown in Figure 7a that has much of the lachrimal bone still attached. The missing dorsal areas of the bone are inferred from the curvature of the preserved orbital margin. Also employed to reconstruct the bone was the extrapolation of the partially preserved jugal margin and the maxilla of R. rankini.

The form of the jugal was deduced from the shape of the surrounding bone margins after reconstruction of the completed dermal head shown in Figure 2b.

The postorbital bone (Fig. 7e) is represented by an isolated element in the specimen GLAHM 163330. The surface of this shows a few medial pores indicating the position of the underlying infraorbital sensory canal system.

The preopercular of R. rankini (Fig. 7f) is wide rostral–caudally, like those in R. sauroides (Traquair 1881, fig. 2; Watson and Day 1916, fig. 3; Thomson and Hahn 1968, fig. 15), Askerichthys (Borgen and Nakrem 2016, fig. 5), M. hibberti (Moy-Thomas 1935, fig. 1) and Megalichthys pygmaeus (GLAHM 163254). A large, oval, overlap area for the squamosal and quadratojugal is present rostrally, whereas the remainder of the dermal surface is relatively smooth. The internal surface of the preopercular has a sensory line, and like that on the squamosal it continues as a broad curved ridge traversing the bone from the rostrodorsal to the rostroventral margin, diminishing in resolution medially. Small sensory pores are found along this ridge and branches of smaller canals emerge from openings on the caudal side, dispersing in a mostly caudal direction.

The quadratojugal of R. rankini (Fig. 7g) follows the general shape shown in other megalichthyids. However, the dorso-ventral length of the bone in R. rankini is twice as long as it is broad. This ratio is like that in M. mullisoni (Downs and Daeschler 2020, fig. 7b). In R. sauroides the bone illustrated is either twice as long as it is broad (Watson and Day 1916, fig. 3) or has length equal to width (Traquair 1881, fig. 2). Figure 7g illustrates the visceral aspect of a right quadratojugal. The preopercular sensory canal traverses the bone from the mid caudal margin to an approximate medial position on the rostroventral border. Two features visible on the visceral surface are a long, thin area that overlaps the subopercular and a small V-shaped area for connection with the caudal region of the mandibular ramus.

Lower jaw. The lower jaw of R. rankini (Fig. 8a–c) is particularly well ossified, and the dentary is one of the most frequently found remains. The details in this account of the lower jaw are based mainly on three well-preserved specimens: a ramus and two dentaries. The labial surface of the lower jaw (Fig. 8a) has a long, deep, trough close to the dorsal margin; this indicates the position of the dentary articulation with the splenial, angular and surangular. The trough extends from the symphysial to the distal articulation of the jaw. Oblique sutures, running caudoventrally across the surface of the jaw, mark the boundaries between the four infradentaries. The dentary (Fig. 8b) is three-quarters of the length of the lower jaw and is armed with a large symphysial fang and 25–30 small teeth. The small teeth lie in cavities along the lingual margin with a few positioned on protuberances labial to the large fang. Ornamentation of the labial surface of the lower jaw in R. rankini forms a strong reticulate pattern.

The lingual surface of the lower jaw is present in an isolated dentary of a juvenile R. rankini (Fig. 8c). In this specimen, the bone also has a large symphysial fang accompanied by smaller teeth, with all teeth located in sockets along the dentary margin. Below this margin there is a longitudinal groove running the mesial–distal length of the bone, with a lingual shelf extending from the rostral margin slightly dorsal to this groove. However, as only the rostral portion of the inner surface of the bone is exposed, the caudal termination of this groove cannot be determined. There are no accessory teeth and no coronoid teeth or sockets observed in the specimen.

A notable feature of the lower jaw of R. rankini is that the rostral margin is arcuate and not protuberant as in Rhizodopsis sauroides (Barkas 1876, fig. 84; NMS 1898.162.17).

Operculogular bones. The opercular series of R. rankini follows a similar arrangement and pattern to those in other megalichthyids, including Askerichthys heintzi (Borgen and Nakrem 2016, figs 33–35), Megalichthys hibberti (Moy-Thomas 1935, fig. 1; GLAHM V3013; GLAHM 149415), Megalichthys pygmaeus (GLAHM 163243; GLAHM 163244) and also Megalichthys mullisoni (Downs and Daeschler 2020, fig. 9).

The dorsal margin of the opercular (Fig. 9a and b) is embayed, fitting the shape of the lateral margin of the lateral extrascapulars. A smaller embayment rostrally accommodates the posterior margin of the extratemporal bone. In addition, instead of having a curved ventral margin as in most operculars, there may be a small caudal protuberance, especially in larger specimens. The visceral surface of the opercular (Fig. 9b) may also show a small, disrupted, groove or pitted area close to the rostrodorsal corner. This may have been for the attachment of the adductor operculi muscle (Long et al. 1997, fig. 62), or for hyomandibular articulation (Lebedev 1995, fig. 12). Dermal ornamentation of the opercular consists of longitudinal striae that radiate from the centre of ossification and terminate predominantly along the caudal margin. The striae combine with ridges traversing the bone; the ridges are highest near the caudal margin and become lower rostrally until replaced by a reticulated sculpture.

The subopercular (Fig. 9c and d) is two-thirds the width of the opercular and half its height. The dorsal region of the bone has a large area for opercular overlap, with the dorsorostral and dorsocaudal corners of the bone distended. A concentric ring of small pores is present on the visceral surface and proximal to the rostral edge (Fig. 9d). Mesial to the ring of pores is a circular area that may represent the scar for attachment of the interhyal ligament (Long et al. 1997, fig. 62). Ornamentation on the subopercular is like that of the opercular.

A right lateral gular (Fig. 9e) shows a narrow overlap area extending the length of the lateral margin of the bone for articulation with the submandibulars. Ornamentation of the gulars resembles that of the opercular and subopercular; however, as in many of the dermal bones the form and general features of the gulars are not constant.

The submandibulo-branchiostegal bone (Fig. 9f) is oval with the caudal margin tapering to a point. One-third of the dorsal region of the dermal surface of the bone forms an area for overlap by the subopercular, and there is a smaller region rostrally for partial overlap with the most caudal submandibular. This is similar to features in the non-megalichthyids Gogonasus andrewsae (Long et al. 1997) and Medoevia lata (Lebedev 1995).

In R. rankini the lateral extrascapular (Fig. 9g) is similar in size and dermal ornamentation to the opercular, although the general shape of the extrascapular is ovate. As a result of the enlargement of the lateral extrascapulars, it may be inferred that these bones would have had a close medial contact and this would lead to the much reduced size of the medial extrascapular that would have been wedge-shaped and narrow.

Palatoquadrate and associated dermal palatal elements. The palate of Rhizodopsis rankini (Fig. 10a–c) is represented by well-preserved dermal dermopalatine, ectopterygoid and entopterygoid bones. However, endoskeletal elements that originated from a cartilaginous palatoquadrate (Arratia and Schultze 1991) are less well preserved. The area of the pars quadrata is not preserved, and the dorsal lamina of the pars metapterygoidea is only partially preserved in a few specimens. The pars autopalatinus is preserved only in mature specimens.

The lingual or buccal surface of the palate is relatively smooth except for a long, hollow, diagonal ridge extending from the ectopterygoid region across the bone at an angle of about 35°, terminating at the dorsocaudal corner (Fig. 10a). This feature is present in both R. rankini and Rhizodopsis sauroides, but in R. rankini (Fig. 10a–c) it becomes wider before termination, whereas in R. sauroides (Fig. 10d) it remains the same width throughout its length. Similar ridges are present in many basal osteichthyans but in some species are restricted to the lingual surface of the palatoquadrate. They are thought to represent the caudal extension of the passageway connecting the spiracle to the buccal chamber (Jessen 1967; Brazeau and Ahlberg 2006).

The labial surface (Fig. 10b) is also smooth, except for a long diagonal ridge. It is heavily punctate with minute pits concentrated around the area of the pars autopalatina. A sizeable foramen is present close to the ventral margin. As with many bones of R. rankini, fine striae run from the centre of ossification to the caudal margin. The entopterygoid is finely tuberculated mesially. The tubercles develop into small denticulations as they approach the ventral border of the bone, in the region of the ectopterygoid and dermopalatine. Striae like those on the labial surface are also present on this side, although not as pronounced.

The anterior portion of the palatoquadrate in R. rankini and R. sauroides extends rostrally with the autopalatine forming a long, tapered projection, like that in Eusthenopteron foordi (Jarvik 1954). This spatulate feature is visible in ventrolingual aspect (Fig. 10c). The palatoquadrate of R. sauroides (Fig. 10d) follows the shape of that of R. rankini except that the processus ascendens has a more protuberant outline and the diagonal ridge is a different shape. No large fangs or denticulations has been observed on the ectopterygoid and dermopalatine bones.

Vomer. The vomer (Fig. 11a and b) is a laterally compressed, vertical, toothed bone with a large tusk with high sides and small teeth arranged along one edge. This structure is like those found in Megalichthys hibberti (Jarvik 1966, fig. 17; GLAHM V3043) and Cladarosymblema narrienense (Fox et al. 1995, fig. 19). Although the vomers of R. rankini are illustrated, their exact positions and orientation on the palate are unknown. However, the curvature and position of the fang, and the location of accompanying marginal teeth, suggest that the vomers were placed close to the anterior palatal fenestrae and the walls of the premaxillae. The sides of the vomer are higher laterally than medially and carry widely spaced denticles. The bone also has a medial extension, presumed to incorporate a contact or near-contact surface for the other vomer (Fox et al. 1995, p. 132). The visceral surface has an irregular flange for attachment to the endocranium. A large, finely plicate fang is present close to the high lateral sides of the bone, together with a row of much smaller marginal teeth. No replacement tusk pit has been observed in any specimens.

Pectoral girdle. The cleithra of R. rankini (Fig. 12a and b) has dorsal and ventral parts, similar to those found in Megalichthys hibberti (GLAHM V2582; GLAHM V2643), Megalichthys pygmaeus (GLAHM 163253), Cladarosymblema narrienense (Fox et al. 1995, figs 60–63) and Megalichthys mullisoni (Downs and Daeschler 2020, fig. 10a and b). In R. rankini the cleithrum is a large bone with an inflated and rounded dorsal part expanded rostrodorsally. The dorsal part is larger than the ventral part and the whole bone has a narrower region between the two areas. However, in Ectosteorhachis nitidus (Thomson and Rackoff 1974, fig. 1, pl. 1), owing to the parallel lateral margins, no narrow region is present.

The external surface of the cleithrum (Fig. 12a) has a long overlap area rostrally, for contact with the subopercular and possibly also the submandibulo-branchiostegal. A long straight ridge extends from the ventral edge of the bone to mid length of the rostral margin, indicating the articulation of the clavicular process. Ornamentation is like that of the opercular and subopercular, although the medial portion of the bone may be sculpted with ridges and tubercles. The visceral surface (Fig. 12b) is relatively smooth with a narrow ridge along the length of the rostral margin and an overlap area for articulation with the clavicle. A row of foramina extends across the bone on this surface and a large fossa is present proximal to the midcaudal margin, the fossa perhaps forming the posterior area for attachment of the scapulocoracoid to the cleithrum. A large foramen is present within the fossa. There are, however, no signs of roughened areas for attachment with the scapulocoracoid (Andrews and Westoll 1970b).

The clavicle (Fig. 12c and d) is triangular with a long dorsal process or spine like that in the pectoral girdle of Cladarosymblema narrienense (Fox et al. 1995, figs 61 and 62). The ventral lamina of the cleithrum bears a ridge that fits into a pronounced furrow on the visceral surface of the dorsal process. The rostroventral corner is rounded, more so in young specimens, as shown in a comparison of the two figures. An overlap area, possibly for articulation with an interclavicle, forms a thin strip along the medial margin of some specimens. The dermal surface of the clavicle is sculpted with elongate tubercles (Fig. 11c) whereas the visceral surface is smooth (Fig. 12d).

Fins. In the holotype, the pectoral fin (Fig. 3a) is much longer than other paired fins and lacks the typical rounded form found in Rhizodopsis sauroides (NMS 1888.33.230; NMS 1994.73.229) and Rhizodopsis hanbuchi (Schultze and Heidtke 1993). The paratype (Fig. 3b) shows that the second dorsal has a simple curved outline rather than displaying the lobed shape found in R. sauroides (Watson and Day 1916; Thomson and Hahn 1968). It has about 13 rays and is 50% larger than the first dorsal, which only has about eight rays.

The relative positions of the fins are important. The paratype shows that the front of the anal fin insertion is slightly behind the rear of the second dorsal insertion. This agrees with the interpretations of Andrews and Westoll (1970b) in relation to the R. sauroides specimen NMS 1897.110.29 and in R. hanbuchi (Schultze and Heidtke 1993). Notably, specimen NMS 1897.110.29 was recovered from the Virtuewell Coal at Newarthill, North Lanarkshire in the same geographical region and at a similar stratigraphic level to the remains assigned to R. rankini. That being so, NMS 1897.110.29 is now deemed a specimen of R. rankini. The caudal fin is not visible in either the holotype or the paratype.

Vertebrae. Brief accounts of the vertebrae of Rhizodopsis sauroides were provided by Young (1866) and Andrews and Westoll (1970b). In the present study, a section of vertebral column described as Rhizodopsis sp. GLAHM V2540 was re-evaluated, and because it was recovered from the Drumgray Coal at Carluke, North Lanarkshire it was deemed to be from R. rankini (Fig. 13a). The slab of material contains a portion of the caudal peduncle of a specimen from the second dorsal fin to the base of the caudal fin. In this specimen the second dorsal fin has six or seven fin rays, comparable with those in the paratype of R. rankini. A large basal scute lies caudal to this fin. Body scales are present on the same slab.

Isolated vertebrae are common in the Central Coalfield and are here assigned to R. rankini. The vertebrae are narrow osseous rings, occasionally with a long smooth neural spine attached, and sometimes with a well-defined neural arch (Fig. 13b and c). The vertebrae of R. rankini resemble those of Megalichthys hibberti (Andrews and Westoll 1970b) but the bony rings are much smaller and the centra have a thinner wall than the more robust vertebrae of Megalichthys. The neural spines of R. rankini are relatively short, compared with the longer and slightly broader haemal spines that are jointed midway along their length, as in Eusthenopteron foordi (Andrews and Westoll 1970a, fig. 25, p. 281).

Squamation. The body scales of Rhizodopsis rankini (Fig. 14a and b) are like those of Rhizodopsis sauroides figured by Williamson (1837, 1849) and Young (1866), and examples of the squamation of R. sauroides in the near-complete specimens MM. L8322 and MM. L8323 from Fenton.

Typical scales of R. rankini are ovoid with the rostral–caudal axis the longest. The rostral region of the scales of the lateral line has dorsal and ventral lobes, with the lower lobe usually the larger of the two. As in R. sauroides the dermal surfaces of the scales of R. rankini (Fig. 14a) are ornate with concentric rings over much of the surface, and most pronounced on the exposed area of the scale. In this area, the rings are crossed by long striae originating from the centres of the scales and radiating caudally to terminate along the posterior margin of the free part. As in R. sauroides (Fig. 13h), the visceral surface of the scale (Fig. 14b) is generally smoother than the dermal surface, with the area below the exposed region of the scale punctate and carrying a small, pear-shaped median boss.

Close inspection of the paratype reveals three endoskeletal pelvic fin radials, each with the rostral margin of a large scute at their base. Using the first dorsal as a guide, the distal ends of the radials appear to lie slightly behind the rear insertion of the first dorsal fin. This suggests that the front insertion of the pelvic fin is slightly behind the rear insertion of the first dorsal. If that is so, then the positions of the pectoral fin, caudal to the first dorsal, and with the anal fin caudal to the second dorsal, are characteristic of R. rankini and R. sauroides (Andrews and Westoll 1970b). This configuration of the relative positions of the fins, and the fact that the second dorsal fin is 50% larger than the first dorsal fin, is similar to the tentative interpretation of the osteolepiform Callistiopterus clappi (Thomson and Hahn 1968). In addition, the basal segments of the lepidotrichia in R. rankini are very long, bifurcating distally, with large basal scutes at the bases of both dorsal and pectoral fins in the type specimens. These scutes, which are most probably also present at the bases of the remaining fins, are spatulate, with rounded rostral margins and tapered caudal extremities; the surfaces of the bones are covered with minute pits.

A comparison of the dermal bones of R. rankini, R. sauroides (Traquair 1881; Watson and Day 1916; Thomson and Hahn 1968) and R. hanbuchi (Schultze and Heidtke 1986, 1993) reveals significant differences between the three species, as follows.

The postparietal shield of R. rankini (Fig. 5a and b) has a distinct ‘waisted’ appearance, owing to the deep embayment between the lateral margins of the supratemporals and the lateral corners of the tabulars. This embayment is less pronounced in the postparietal shield of R. sauroides (Traquair 1881, fig. 1; MM. L8323), and in R. hanbuchi (Schultze and Heidtke 1993, fig. 2a) the lateral margins of the supratemporal are almost straight, and the embayment along the spiracular margin is very shallow. The distal portion of the postparietal of R. rankini narrows sharply as it nears the rostral margin (Fig. 5c), whereas in R. sauroides (Traquair 1881, fig. 1; NMS1889.77.380) and R. hanbuchi (Schultze and Heidtke 1993, fig. 2a) the bone remains the same width in this area. The shapes of other paired bones of the postparietal shield in R. hanbuchi differ from those of the other two species. Both R. sauroides and R. hanbuchi have supratemporals with longer rostral–caudal axes, shouldered along the rostral margins, whereas in R. rankini this margin follows the lateral edges of the bone as a convex curve. The irregular outline of the tabular also differs between the three species.

Among the bones of the cheek, the squamosal in R. rankini (Fig. 7c and d) is triangular, but it is quadrilateral in R. sauroides (Watson and Day 1916, contra Traquair 1881, who showed it with an oval outline, and Thomson and Hahn 1968, who illustrated the squamosal with an irregular outline), and may be irregular in R. hanbuchi (Schultze and Heidtke 1993). The quadratojugal is a short bone in R. sauroides (Traquair 1881; Thomson and Hahn 1968, contra Watson and Day 1916, who showed it to be longer rostral–caudally) but it is twice as long as that in R. rankini (Fig. 7g).

There are also differences in the form of the opercular series among the three species. The opercular of R. rankini (Fig. 9a and b) is greater horizontally than vertically as in R. hanbuchi (Schultze and Heidtke 1993, fig. 2a and b). In R. sauroides (Traquair 1881; Thomson and Hahn 1968) these distances are the same. The subopercular bones in both R. sauroides and R. rankini are narrower than the opercular, about half the vertical distance, whereas in R. hanbuchi the subopercular is of similar dimensions to the opercular. Finally, there are differences in the shapes of the submandibulo-branchiostegal. In R. sauroides and R. rankini the bone is oval with the same width as the subopercular, but in R. hanbuchi it is similar in shape to the subopercular but only about half the width of that bone.

Well-preserved fish-scales in Carboniferous rocks are of considerable value in assessing variation in fish squamation although conclusions based simply on scales are normally insufficient to validate a taxonomic assignment. The identification of fragmentary material, such as scales, needs great caution. Nevertheless, information from the above study provides a guide to the validity of Rhizodopsis taxa and permits reassessment despite this limitation.

The scales of Rhizodopsis rankini (Fig. 14a and b), discussed above, are typically ovoid with the rostral–caudal axis the longest. The dermal surface is sculpted with concentric rings crossed by long striae, producing a mesh-like pattern. The visceral surface is smoother than the dermal, and the area below the exposed region is punctate with a small, pear-shaped median boss.

The scales of Rhizodopsis robusta Woodward, 1891 (Fig. 14c) are similar to those in R. rankini, with the main differences on the free surface. In R. robusta the sculpture is of thick rounded ridges parallel to the caudal margin, together with strong concentric rings. In contrast, R. rankini has finer ridges and concentric rings.

The scale morphology in Rhizodopsis cf. robustus Roemer, 1865 (Fig. 14d) has a mesh-like surface that closely resembles those found in R. robusta, R. sauroides and R. rankini.

The scales of Rhizodopsis hanbuchi Schultze and Heidtke, 1993 (Fig. 14e) have a mesh-like surface similar to that in R. rankini, but the caudal margin of the free surface is serrated.

In Rhizodopsis mazonius Hay, 1900 (Fig. 14f) the exposed free part of the scale is ornamented with long, densely arranged ridges. This ornamentation differs from the typical mesh-like sculpture of Rhizodopsis scales, and R. mazonius is therefore excluded as a valid Rhizodopsis species.

In Rhizodopsis savenkovi Obruchev, 1955 (Fig. 14g) the scale has the typical mesh-like sculpture of Rhizodopsis, although the longitudinal ridges are more pronounced on the exposed free part of the scale.

Typical scales of Rhizodopsis sauroides (Fig. 14h) follow a similar pattern to that in R. rankini, with the dermal surface highly ornate with concentric rings. The rings are more pronounced on the exposed area of the scale and are crossed by long striae, producing a mesh-like pattern. As in R. rankini, the visceral surface is smoother than the dermal surface, with the area below the exposed region punctate and bearing a small, pear-shaped median boss.

Finally, the assignment of Rhizodopsis granulatus as a distinct species of Rhizodopsis is extremely doubtful and there is no evidence to indicate that this species existed. The original diagnosis was based on the visceral and dermal surfaces of the scales of Rhizodopsis sauroides described as Holoptychius sauroides (Williamson 1849). However, identical visceral scale surfaces were figured later under the name of Rhizodus granulatus (Salter 1861). This inconsistency was the product of nomenclatural and systematic confusion, which frequented early classification of sarcopterygians. Furthermore, Young (1866) in a description of Rhizodopsis also commented on the common error of mistaking the visceral structure of the scales for dermal surface ornamentation. This surface has the appearance of small granulations, which, I believe, was Salter's misinterpretation of his figured scales.

Thus, with the exception of R. mazonius and R. granulatus all other named species of Rhizodopsis differentiated by scale morphology are here judged to be valid.

I would like to thank S. Walsh (National Museums Scotland) and N. Clark (Hunterian Museum, Glasgow) for their continuous support and encouragement and for allowing access to the museum collections. I would also like to thank K. Sherburn and D. Gelsthorpe (Manchester Museum) for their courtesy in allowing access to the museum collection. Thanks go to W. Elliott for producing images of the paratype and holotype. Special thanks go to T. Smithson (Department of Zoology, University of Cambridge) for his invaluable help in the production of this paper in its present form.

FME: conceptualization (lead), formal analysis (lead), investigation (lead), methodology (lead), resources (lead), software (lead), visualization (lead), writing – original draft (lead), writing – review & editing (lead)

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

The author declares 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.

GLAHM: V2540. Rhizodopsis sp. Series of vertebrae.

GLAHM: V2560. Megalichthys hibberti. Postparietal shield, dorsal aspect.

GLAHM: V3013. Megalichthys hibberti. Head of large individual.

GLAHM: V3043. Megalichthys hibberti. Palate with ring vertebrae.

GLAHM: 149415. Megalichthys hibberti. Head of large individual.

GLAHM: 163232. Megalichthys pygmaeus. Parietal shield, dorsal aspect.

GLAHM: 163233. Megalichthys pygmaeus. Parietal shield, dorsal aspect.

GLAHM: 163235. Megalichthys pygmaeus. Postparietal shield.

GLAHM: 163238. Megalichthys pygmaeus. Palatoquadrate, labial aspect.

GLAHM: 163243. Megalichthys pygmaeus. Opercular, left, lateral aspect.

GLAHM: 163244. Megalichthys pygmaeus. Subopercular, left, lateral aspect.

GLAHM: 163253. Megalichthys pygmaeus. Cleithrum, right, external aspect.

GLAHM: 163254. Megalichthys pygmaeus. Preopercular, left, lateral aspect.

MM: L8322. Rhizodopsis sauroides. Near complete fish, absent caudal region, ventral aspect.

MM: L8323. Rhizodopsis sauroides. Near complete fish with absent caudal region, showing parietal shield.

MM: L10932. Rhizodopsis sauroides. Neurocranium, various parts in eight boxes.

NMS: 88.33.43. Rhizodopsis sauroides. Anterior part of fish, ventral aspect showing pectoral fin.

NMS: 95.77.48. Rhizodopsis sauroides. Palatoquadrate, labial aspect.

NMS: 95.177.4. Rhizodopsis sauroides. Palatoquadrate, labial aspect.

NMS: 98.162.17. Rhizodopsis sauroides. Lower jaw.

NMS: 1859.33.118. Rhizodopsis sauroides. Anterior part of fish, dorsal aspect.

NMS: 1888.100.1. Rhizodopsis sauroides. Anterior part of fish, ventral aspect.

NMS: 1994.73.229. Rhizodopsis sauroides. Anterior part of fish, ventral aspect showing pectoral fin.

NMS. 1895.186.13. Rhizodopsis sauroides. Ethmosphenoid.

NMS. 1897.110.29. Rhizodopsis sauroides. Posterior part of fish.

NMS: 1897.111.51. Rhizodopsis sauroides. Premaxilla.

NMS: 1897.112.12. Rhizodopsis sauroides. Anterior part of fish, dorsal aspect.

NMS: 1898.17.17. Rhizodopsis sauroides. Anterior part of fish, ventral aspect.

NMS: 1898.17.19. Rhizodopsis sauroides. Scale, internal surface.