Rhadinichthys (Traquair Reference Traquair1877a) is a classic ‘wastebasket’ genus among Palaeozoic actinopterygians. A succession of diagnoses (Traquair Reference Traquair1877a, Reference Traquair1911; Moy-Thomas & Bradley Dyne Reference Moy-Thomas and Bradley Dyne1938; Lund & Poplin Reference Lund and Poplin1997; Elliot Reference Elliott2016; Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021) has sustained this nominal group as a repository for indeterminate early ray-finned fishes, because the formulations consist of combinations of generalised features. Currently, Rhadinichthys comprises 27 species found across North and South America, Europe, Russia, and South Africa, with temporal occurrences ranging from the Upper Devonian to the early Permian (Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021; Henderson et al. Reference Henderson, Dunne, Fasey and Giles2023, fig. 7g). Given this extraordinary temporal and spatial range, it is unsurprising to find that Rhadinichthys species display significant variation in dermal skull composition, dermal skeletal ornamentation, squamation patterns, fin structures, and other features that usually provide the basis for morphologically distinct genera. As a result of this loose taxonomic definition, Rhadinichthys is rarely included in evolutionary analyses. Predictably, when it is included, such as in Gardiner & Schaeffer's (Reference Gardiner and Schaeffer1989) exploration of early actinopterygian diversity, the disparate species emerge scattered across three distinct ‘terminal groups’: the Australichthys group (R. canobiensis), the Belichthys group (R. carinatus), and the Amblypterus group (R. ornatissimus). Moreover, despite 13 species pruned from Rhadinichthys in the last century, and further review and evaluation (e.g., Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021), the genus persists as ill-defined and in need of a more phylogenetically structured treatment.
The type species, Rhadinichthys ornatissimus, was first described by Agassiz (Reference Agassiz1835) and named Palaeoniscus ornatissimum. Subsequently, Traquair (Reference Traquair1877a) reclassified P. ornatissimum as Rhadinichthys lepturus, and designated two of the three specimens employed in Agassiz's description as syntypes: NMS G.1896.147.1 and NMS G.1878.18.7 (Fig. 1a, b). Some three decades later, Traquair (Reference Traquair1911) revised the species to Rhadinichthys ornatissimus and officially assigned it as the genus type. However, from the outset he expressed concern about the condition of the species (and genus) type specimens, noting that ‘they are in a very bad state of preservation’ (Traquair Reference Traquair1911). Significantly, Traquair's updated description figured other material (e.g., NMS G.1859.33.66, Traquair Reference Traquair1909, fig. 2, pl. XXVIII), and relied on these for much of the morphological detail. Currently, both syntypes NMS G.1896.147.1 and NMS G.1878.18.7 appear to have undergone further wear and tear, and, in agreement with other researchers (Giles et al. Reference Giles, Feilich, Warnock, Pierce and Friedman2023), two of the present authors (pers. obs., AMC and MIC) consider few of the remaining features as usable for diagnosis (see Fig. 1a, b). For this reason, a further specimen, NMS G.1891.24.11 (Fig. 1c, d) is offered as a neotype because it more clearly displays the specialised traits that might be used as distinctive characteristics at species and genus levels.
Types of Rhadinichthys ornatissimus. (a) Original syntype of Rhadinichthys ornatissimus, NMS G.1896.147.1. (b) Original syntype of Rhadinichthys ornatissimus, NMS G.1878.18.7. (c) Proposed neotype for Rhadinichthys ornatissimus, NMS G.1891.24.11. (d) Close-up on the cranium of NMS G.1891.24.11.
Although Rhadinichthys forms the focus of this study, the project started out rather differently. The initial aim was to update the description of the Lower Carboniferous taxon Woodichthys bearsdeni (Coates Reference Coates1998), adding data from μCT scans of the holotype skull table and neurocranium NMS G.1984.42.15 (Fig. 2), and data now available from a previously undocumented specimen, GLAHM 163425a and b (Figs 3, 4). First described by Coates (Reference Coates1998), Woodichthys bearsdeni is one of only a handful of Carboniferous actinopterygians with an at least partly described braincase (cf. review in Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023), and for this reason the species has been included in several phylogenetic analyses over the past 25 years (Coates Reference Coates1999; Cloutier & Arratia Reference Cloutier, Arratia, Arratia, Wilson and Cloutier2004; Giles et al. Reference Giles, Xu, Near and Friedman2017, Reference Giles, Feilich, Warnock, Pierce and Friedman2023). However, renewed comparison with Rhadinichthys ornatissimus, and especially with the proposed neotype, revealed a series of shared specialisations, thereby driving the study in an unexpected direction. With a sister-species relationship between R. ornatissimus and W. bearsdeni evident, this work now also evaluates the possibility of subsuming the monospecific genus Woodichthys within Rhadinichthys, refines the definition of Rhadinichthys, and reassesses the current membership of the genus. Through these systematic revisions, the intention of this study is to re-establish Rhadinichthys as an informative taxonomic unit that will contribute to a better understanding of early actinopterygian diversity and interrelationships.
Rhadinichthys bearsdeni, comb. nov. (formerly Woodichthys), microscope images of holotype braincase NMS G.1984.42.15c in dorsal (a) and ventral (b) views.
Rhadinichthys bearsdeni, comb. nov. (formerly Woodichthys), photographs of GLAHM 163425a and b: (a) part; (b) counterpart.
Rhadinichthys bearsdeni, comb. nov. (formerly Woodichthys), close-up on the cranial portion of GLAHM 163425a and b. See Fig. 15 for μCT 3D reconstruction of the same specimen. (a, c) GLAHM 163425a. (b, d) GLAHM 163425b, image reversed for ease of comparison between part and counterpart. (a, c) photographs of specimen. (b, d) Inked diagram. Abbreviations: an = angular; br = branchiostegal plates; cl = cleithrum, de = dentary; esc = extrascapular; fr = frontal; gu = gular; hym = hyomandibula; ih = interhyal; na = nasal; op = operculum; pa = parietal; p.l.l = posterior lateral line; pt = posttemporal; ro = rostral; scl = supracleithrum; scr = sclerotic ring; sop = suboperculum.
Terminology for cranial bones in the present work follows the standard nomenclature used in monographic studies of comparable taxa; most notably Gardiner's (Reference Gardiner1984) descriptions of Mimipiscis and Moythomasia. Other naming schemes are available, acknowledging the well-corroborated homology of actinopterygian frontals with sarcopterygian parietals, and actinopterygian parietals with sarcopterygian postparietals. For an efficient summary of alternative terminologies and equivalents see Pearson & Westoll (Reference Pearson and Westoll1979, table 1). For a lengthier discussion, including compound names for single bones where they are inferred to be the product of ancestrally distinct bones, see Schultze et al. (Reference Schultze, Mickle, Poplin, Hilton and Grande2021). Here, the aim is to provide descriptions using terms that will assist placement of Woodichthys and Rhadinichthys among the Actinopterygii.
Finally, this paper is contributed to celebrate the work of Tim Smithson. His studies of early vertebrates, and especially of early tetrapods and lungfish, built on a sustained and productive scouring of the Scottish Borders for lost and new fossil localities, have been outstanding and transformative. One of us (MIC) was fortunate enough to overlap with Tim's presence in Alec Panchen's research group at The University of Newcastle Upon Tyne. The benefits of Tim's advice and informal mentorship were more than significant. This offering is, we hope, a fitting tribute.
1. Materials
1.1. Image preparation
Fossil specimens were scanned at the University of Chicago X-ray μCT facility on a GE Phoenix 240/180 scanner using the 240 microfocus tube. Specimen NMS G.1984.42.15.c was scanned with 70 kv, 100 mA, no filter, 2,000 projections, 500 ms timing, and voxel size of 9.52 μm. Specimens GLAHM 163425a and b were scanned using 120 kv, 140 mA, 0.1 mm copper filter, 2,000 projections, 500 ms timing, and voxel size of 23.4 μm. Tomographic image stacks are available for download on Morphosource. Segmentation and anatomical reconstruction were accomplished using Materialise Mimics v.25 (biomedical.materialise.com/mimics; Materialise 2016, Leuven, Belgium). Imaging of digital models was completed using Blender 3.2 (Community 2018). Photography of fossil specimens was accomplished using a Leica DFC490 camera attached to a Zeiss Stemi SV6 microscope processed in Image-Pro Plus 6.2 using the enhanced depth of field function, and an iPhone 14 Pro rear camera.
1.2. Abbreviations
1.2.1. Institutional abbreviations
NMS, National Museum of Scotland, Edinburgh; GLAHM, Glasgow University, Hunterian Museum, Glasgow; MCZ, Harvard University Museum of Comparative Zoology, Cambridge, Massachusetts, USA.
1.3. Specimens and geological context
The type specimens of Rhadinichthys ornatissimus, NMS G.1896.147.1 and NMS G.1878.18.7, were examined and photographed at the National Museum of Scotland. NMS G.1896.147.1 (Fig. 1a) is from the Strathclyde Group (formerly known as the Calciferous Sandstone Series) at Burntisland, Fife (Firth of Forth north shore). The rostral part of the head is missing but body and median fin outlines are near complete. This specimen is in poor condition and of less diagnostic value than the other. NMS G.1878.18.7 (Fig. 1b) is from the Carboniferous limestone bed of Burdiehouse about four miles south of Edinburgh. The body is near complete although jack-knifed at the level of dorsal and anal fins (the same contortion evident in other Burdiehouse specimens, e.g., NMS G.1859.33.66). Exposed surfaces are partly eroded but retain surface ornament and the skull table is in fair condition. The proposed neotype of Rhadinichthys ornatissimus, NMS G.1891.24.11 (Fig. 1c, d), is from Straiton Oil Shale Works (about two miles south of Burdiehouse), from the roof of the Dunnet Shale, deposited during Asbian to Brigantian substages of the Carboniferous period, ~335.59–328.48 Ma BP (Aretz et al. Reference Aretz, Herbig, Wang, Gradstein, Agterberg and Ogg2020). NMS G.1889.115.3, a further whole-body specimen including a reasonably complete cranium, is also from the Straiton Oil Shale Works. Burdiehouse specimen NMS G.1859.33.66.1 and 2, part and counterpart, mentioned above, is a Hugh Miller specimen, with a fileted cranium and a splayed, well-preserved, pectoral fin. MCZ 10620 is from the Wardie Shales member of the Gullane Formation exposed at Granton Harbour, Edinburgh, and dates from ~331.1–335.5 Ma BP (Monoghan et al. Reference Monaghan, Browne and Barfod2014; Friedman et al. Reference Friedman, Pierce, Coates and Giles2018; Aretz et al. Reference Aretz, Herbig, Wang, Gradstein, Agterberg and Ogg2020). The specimen, part of the Thomas Stock collection, consists of the anterior third of a fish and exhibits some rostro-caudal compression; surface detail is poor.
All the specimens of Woodichthys bearsdeni here examined are from the Manse Burn Formation: fine marine shales ranging from the Top Hosie Limestone Marine Band down to the first thick sandstone and dated to the Pendleian (~330.34–327 Ma BP, substage of the Serpukhovian) El Zone of the Lower Carboniferous (Clark Reference Clark1989; Aretz et al. Reference Aretz, Herbig, Wang, Gradstein, Agterberg and Ogg2020). NMS G.1984.42.15.c includes the dorsoventrally flattened braincase of the Woodichthys bearsdeni holotype (Fig. 2), which was collected from the Bearsden exposure of the Manse Burn Formation (Wood Reference Wood1982). The first description of NMS G.1984.42.15.c included camera lucida drawings of the dorsal and the incomplete ventral surface, and a radiograph revealing a portion of the right supraorbital canal (Coates Reference Coates1998). GLAHM 163425a and b (Figs 3, 4) were collected by Mr. P. Gavin from a further exposure of the Manse Burn Formation in Faifley, Clydebank, approximately two miles southwest of the Bearsden locality. This full-body specimen contains a well-preserved hyoid arch that has not been documented for R. bearsdeni in previous literature. Both individuals are slightly pyritised and significantly compacted, and both preserve three-dimensional otoliths.
Source localities of all referred specimens are mapped in Fig. 5.
Collection localities of the redefined Rhadinichthys species: Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), (yellow: Faifley and Bearsden), Rhadinichthys ornatissimus (purple: Burntisland, Wardie, Burdiehouse, and Straiton), Rhadinichthys ferox (turquoise: Wardie), Rhadinichthys laevis (pink: Glencartholm).
2. Systematic palaeontology
Osteichthyes Huxley Reference Huxley1880
Actinopterygii Woodward Reference Woodward1891
Incertae sedis Genus Rhadinichthys Traquair Reference Traquair1877a
Synonymy. Rhadinichthys (Traquair), Moy-Thomas & Bradley Dyne (Reference Moy-Thomas and Bradley Dyne1938, p. 450).
‘Rhadinichthyids group', Lund & Poplin (Reference Lund and Poplin1997, p. 483).
Rhadinichthys (Traquair), Elliott (Reference Elliott2016, p. 356).
Rhadinichthys (Traquair), Schultze et al. (Reference Schultze, Mickle, Poplin, Hilton and Grande2021, p. 129).
Type species. Rhadinichthys ornatissimus Traquair Reference Traquair1877a.
Emended generic diagnosis. Fusiform body with total length ranging from 10 to 15 cm; skull roof, cheek and lower jaw ornament of fine, anteroposteriorly aligned, closely packed elongate ridges. Frontal more than twice parietal length; parietals less than two thirds of frontal width. Supratemporal with broad, medially expanded posterior portion and short anterior limb; thus, full width of skull table rear with near equal contribution from supratemporal and parietal; intertemporal narrow. Rostral with low apex and subrectangular broad midline plate with straight posterior margin; rostral ornament transitions from pitted enamel snout-cap to ridge-on-plate posteriorly; nasal notch preceded by ornament-free margin. Premaxillae broad and sub-pentagonal; suborbitals absent; opercular more than twice subopercular height; branchiostegals numerous. Scales arranged in 36+ precaudal rows; scales serrated posteriorly; scales close to dorsal midline and anterior flank bear fine ridged ornament; lateral line scales pierced occasionally by large slot-shaped pores; three basal fulcra, of which the largest bears a pair of slot-shaped pores, precede dorsal fin. All fins with fringing fulcra; pectoral lepidotrichia unsegmented proximally; all median fins in rear half of body; dorsal fin opposite or precedes level of anal fin.
Type species. Rhadinichthys ornatissimus (Traquair Reference Traquair1911).
Synonymy. Palaeoniscus ornatissimus Agassiz (Reference Agassiz1835, p. 92).
Rhadinichthys lepturus Traquair (Reference Traquair1877a, p. 437).
Emended species diagnosis. Rhadinichthys with total length of approximately 12–15 cm; frontal with rounded posterolateral extremity; parietal subrectangular; rostral apex pitted with short grooves; pores of anterior lateral lines large on nasal and supratemporal; opercular bone covered in fine ridged ornament. Scale rows with eight or more members above and six or more below the lateral line in flank region; scale ornament with fine horizontal ridges across entire dorsal surface; scales on posterior and ventral parts of body generally smooth. Dorsal fin only slightly anterior to level of anal fin; pair of basal fulcral scales precede anal fin.
Syntypes. NMS G.1878.18.7, NMS G.1896.147.1.
Proposed neotype. NMS G.1891.24.11.
Referred specimens and localities. NMS G.1878.18.7 and NMS G.1859.33.66, part and counterpart, Carboniferous limestone, Burdiehouse, Edinburgh; MCZ 10620, Wardie Shales, Granton, Edinburgh; NMS G.1896.147.1, Strathclyde Group, Burntisland, Fife; NMS G.1891.24.11 and NMS G.1889.115.3, Dunnet Shale, Straiton, Midlothian.
Species. Rhadinichthys bearsdeni comb. nov.
Synonymy. Woodichthys bearsdeni Coates (Reference Coates1998, pp. 37–38).
Emended species diagnosis. Rhadinichthys with total length of around 11 cm; frontal with angled posterolateral extremity; parietal span narrow, meeting less than half of frontal posterior margin; parietal with narrow anterior process projecting into rear of frontal. Rostral apex ornamented with pits; nasal enclosed supraorbital canal with short posterior rami; jugal enclosed infraorbital canal with posterior rami; opercular and subopercular plate fine-ridged ornament diminishes posteriorly and ventrally, away from anterior and dorsal margins. Scale rows with ten or more members above and six or more below the lateral line in flank region; scale ornament maximal in nape region, flank scale surfaces mostly smooth, with slight ornament of parallel grooves across anterodorsal region. Dorsal fin mostly anterior to level of anal fin; single basal fulcral scale precedes caudal and anal fins.
Holotype. NMS G.1984.42.15.a, b, and c.
Referred specimens and localities. NMS G.1983.33.2, NMS G.1984.42.15.c, and NMS G.1987.7.133 from Manse Burn Formation, Bearsden, Glasgow; GLAHM 163425a and b from Manse Burn Formation, Faifley, Glasgow.
3. Results
3.1. Description of Rhadinichthys ornatissimus
3.1.1. Head
Details of the cranium are taken mostly from specimens NMS G.1891.24.11 (the neotype) (Figs 6c–8a) and NMS G.1889.115.3, with corroboration from type specimen NMS G.1878.18.7. In general, cranial morphology closely resembles that of R. bearsdeni comb. nov. (Coates Reference Coates1998). The general arrangement, proportions, and dermal ornament of posttemporal, parietal, frontal, supratemporal, and intertemporal, are shared in both species. Notably, the supratemporal (st, Fig. 8a) is unusually broad posteriorly, like that of the redescribed skull table of NMS G.1984.42.15.c (Fig. 9). In both examples a deeply recessed anteromedial accommodates the posterolateral angle of the frontal. The extrascapulars are not well preserved in R. ornatissimus. Details of this skull table arrangement are also visible in natural mould on the type specimen NMS G.1878.18.7. The large rostral (ro, Figs 6c, 8a) has a broad posterior plate ornamented with ridges aligned with the lateral margins, but these ridges curve medially and coalesce dorsal to the apex of the snout so that the apex is mostly ganoine-capped, pockmarked with pits and short curved grooves, unlike the central area of posterior plate which is mostly ornament-free. Notches for left and right anterior nostrils flank the snout apex, and these notches are preceded by a marked ornament-free margin.
Rhadinichthys ornatissimus, photograph of NMS G.1891.24.11 with close-ups of notable features. (a) Full specimen. (b) Dorsal fin and basal fulcral scales. (c) Dorsoventrally compressed cranium. (d) Lateral line scales and pores. (e) Pectoral fin with unsegmented lepidotrichia. (f) Pelvic fin. (g) Anal fin. Abbreviations: bfl = basal fulcra; dsphn = dermosphenotic; ffl = fringing fulcra; fr = frontal; it = intertemporal; lpt = lepidotrichia; mx = maxilla; na = nasal; pa = parietal; pop = preoperculum; ro = rostral; st = supratemporal.
Line drawings of Rhadinichthys ornatissimus and Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys). (a) R. ornatissimus NMS G.1891.24.11, specimen map with lateral line scales shaded grey. (b) R. bearsdeni NMS G.1983.33.2 specimen map with lateral line scales shaded grey (adapted from Coates Reference Coates1998, fig. 10a; reversed for direct comparison).
Rhadinichthys ornatissimus and Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys): associated dermal cranial bones in external view. (a) R. ornatissimus NMS G.1891.24.11. (b) R. bearsdeni. NMS G.1987.7.133 specimen map with lateral line scales shaded grey (adapted from Coates Reference Coates1988, fig. 9 and 1998, fig. 7a). Abbreviations: br = branchiostegal plate; clv = clavicle; dsphn = dermosphenotic; fr = frontal; gu = gular; it = intertemporal; ju = jugal; lpt = lepidotrichia; mx = maxilla; na = nasal; op = operculum; pa = parietal; pmx = premaxilla; pop = preoperculum; pt = post temporal; ro = rostral; scl = supracleithrum; scr = sclerotic ring; sop = suboperculum; st = supratemporal.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), NMS G.1984.42.15.c skull table. (a) μCT 3D reconstruction of the skull table, dorsal view. (b) Inked diagram illustrating dermal sutures, dorsal view. (c) Skull table made transparent to reveal the supraorbital and otic canals, ventral view. (d) Tomographic slice revealing histological structures of the dermal skull table, sagittal view. Abbreviations: fr = frontal; it = intertemporal; pa = parietal; soc = supraorbital canal; st = supratemporal; stc = supratemporal canal.
The nasal bone (na, Figs 6c, 8a) of NMS G.1891.24.11 resembles the slender proportions of that seen in Rhadinichthys bearsdeni comb. nov. (Fig. 4), but bears a series of pores overlying the sensory canal. Similarly sized pores are visible on portions of the skull table, also tracking the cranial lateral lines. Such easily identified pores are not evident in R. bearsdeni. A slender, triangular bone lateral to the skull table, projecting into the orbit roof and underlying the nasal, appears to be a displaced dermosphenotic. The maxilla is incompletely preserved in NMS G.1889.115.3 and 1891.24.11; enough of the posterior portion is preserved to indicate that the gross morphology and ornament of dorsally and anteriorly swept ridges resembles that of R. bearsdeni comb. nov. (mx, Fig. 8a, b). The anterior portion of the preopercular is preserved dorsal to the posterior part of the maxilla in NMS G.1891.24.11. Importantly, the ornament shows the distinctive change in pattern between dorsal and ventral areas of the bone (compare examples in Fig. 8a, b). Dorsally, ornament ridges are vertically oriented, anteriorly concave crescents; ventrally, ridges are horizontal, near straight, and parallel to the ventral margin. There are no notches in the preopercular margin or small isolated plates indicating the presence of suborbital bones. The opercular bones of NMS G.1891.24.11 and 1889.115.3 resemble those of R. bearsdeni, but with a more complete covering of finely ridged ornament. In both bones the ornament radiates from the anterodorsal corner of the plate.
3.1.2. Fins
All fins, except the pectoral, are situated at the rear half of the body. The pectoral fin inserts posterior to the cranium and comprises a minimum of 20 unsegmented lepidotrichia (Fig. 6e). A splayed pectoral fin, revealing the complete, broad, triangular outline, is preserved in the Burdiehouse specimen NMS G.1859.33.66. Although clearly depicted by Traquair (Reference Traquair1909, pl. 28, fig. 2), this illustration trims the extremities; the greatest extent of the fin (although not preserved in high resolution) is proportionally equal to the length of the maxilla. The pectoral fin is not ‘rather small’, contra Traquair (Reference Traquair1911). The proximal, elongate segments of the lepidotrichia account for just over half of the total length of the principal fin rays. Fringing fulcra are small and adhere tightly to the leading edge. The pelvic fin is positioned at the midpoint between the pectoral and anal fins, and contains minimally 14 short fin rays, the principal rays segmenting five or six times and bifurcating distally (Fig. 6f). Fringing fulcra are present.
The dorsal fin (Fig. 6b) is triangular and has 25 slender lepidotrichia which can segment up to eight times. No fringing fulcra are preserved, but they are probably lost; preservation is mostly mouldic. The anal fin (Fig. 6g) in NMS G.1891.24.11 is well preserved. Located almost dorsoventrally level with the dorsal, it consists of similar numbers of lepidotrichia. Each fin ray segments from four to eight times and bifurcates distally. At the first and subsequent bifurcations the anterior branch is consistently thinner than the posterior branch, which bifurcates again, distally (the anterior branch no longer bifurcates). This asymmetry in branching pattern appears to be present in the dorsal fin too, but evidence from the matrix mould is less clear. Both fins are broad-based, spanning eight or nine scale rows in contrast to three for the pelvic fin.
The caudal fin is not well preserved in any specimen examined. The most complete example observed is in NMS G.G. 1889.115.3, in which it resembles conditions in R. bearsdeni comb. nov.
3.1.3. Squamation
The flank scales of Rhadinichthys ornatissimus are rhomboid and serrated posteriorly (12–15 serrations, depending on scale shape). The exposed surface of each scale bears fine ridges, and with low angle lighting these can be seen to extend across much of each scale's surface. The ornament is most prominent towards the anterodorsal process, from which the ridges mostly extend. There is slight regional variation in ornamentation across the body, with smoother surfaces posteriorly and ventrally. The squamation consists of at least 36 obliquely oriented, precaudal scale rows, with each row containing eight or more scales above and six or more scales below the lateral line (Figs 6d, 7a). Towards the dorsal and ventral midlines, the scales become dorsoventrally shallower, but the scale rows show no marked subdivisions into fields of smaller units. Scale length is mostly uniform. As in Rhadinichthys bearsdeni comb. nov., the lateral line can be easily traced by specialised canal-carrying scales, occasional members of which are pierced by slot-shaped pores (Figs 6d, 7a) and distinct notches in the trailing edge. Both the dorsal and the anal fins are preceded by enlarge fulcral scales (Fig. 6b, g). Three fulcral scales are present in front of the dorsal fin, and a pair of pores is present on the largest of the series. A pair of fulcra scales lies anterior to the anal fin.
3.2. Redescription of NMS G.1984.42.15.c, Rhadinichthys bearsdeni comb. nov.
The following description refers to those parts of the material re-examined as part of the present project; further details of the skeletal anatomy are provided in the original report (Coates Reference Coates1998).
3.2.1. Skull table
The preserved skull roof (Figs 2a, 8b, 9) includes the paired frontals, parietals, intertemporals, and supratemporals. Dermal sutural patterns were determined from visible overlaps between each bone in cross-sectional views in the tomograms, and the shape of the supratemporal, especially, and its adjacent parietal were revised accordingly (compare visible morphology and μCT scan renderings in Figs 2a and 9b). The shape of the frontal (Fig. 9b) is as described in Coates (Reference Coates1998) and closely resembles that of Rhadinichthys ornatissimus. However, reassessment of the dermal sutures has decreased the lateral extent of the parietal by at least 25 %. After revision, the parietal maximum width measures approximately 60 % of the frontal in maximum width and 30 % in length, a proportion resembling the short and narrow parietal of Pteronisculus (Nielsen Reference Nielsen1942). Left and right parietals (Fig. 9b) are notably asymmetric, with the left parietal slightly larger in size and suturing in part with the right frontal. Both parietals bear a narrow, acute process, unnoticed in the original description, protruding from the anterior edge and enclosing the posteriormost portion of the supraorbital canal. Three convergent pit lines lie in the posterior centre of the right parietal. Identifying pit lines on the left side proves challenging, since it was more severely fractured by the otoliths punching through during taphonomic compression. Only one unambiguous pit line is found on the left parietal. At the rear border, a broad, unornamented flange underlies the preceding extrascapular series.
Paths of the paired supraorbital and otic canals are revealed through μCT scanning. The condition of the supraorbital canal (Fig. 9c) bears strong similarity to Phoebeannaia (Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023, figs 5a, 9b). In both taxa, it enters the dermal skull from the narrow anterolateral extremity of the frontal, then travels along the frontal midline and through the anterior process of the parietal, terminating just posterior to the process. The canal does not follow a straight course but kinks laterally around its midpoint. Of possible phylogenetic significance, this kink is absent in Devonian taxa such as Mimipiscis (Gardiner Reference Gardiner1984, fig. 80) and Raynerius (Giles et al. Reference Giles, Darras, Clément, Blieck and Friedman2015). Short, twig-like offshoots emerge sparsely along the anterior half of the canal in Rhadinichthys bearsdeni. These twigs either reach upward to the dorsal exterior or penetrate ventrally through the lowest layer of the dermal bone and into the compressed braincase. External pores marking the track of the line through the frontal are difficult to locate, and the path of the canal is not especially evident in the pattern or ‘fingerprint’ of dermal ornament. The otic extension of the main lateral line canal (Fig. 9c) runs close to the lateral edge of the supratemporal, as shown by Coates (Reference Coates1998, fig. 8b).
The intertemporal (Fig. 9b) is short and near rhombic as originally described. The revised supratemporal (Fig. 9b) is narrow where it flanks the frontal, but then broadens towards the dorsal midline following the contour of the frontal and sharply notches into the frontal-parietal series. This posterior expansion of the supratemporal occupies 50 % of the posterior rim of the frontal. The unusual shape (broad posterior plate with short and narrow anterolateral limb) is distinctively similar to that present in Rhadinichthys ornatissimus (compare examples in Figs 8a and 9b). A deep groove is visible near the junction of the supratemporal, frontal, and parietal bones, possibly representing an additional pit line.
Histological structure of the dermal bone was also captured by the μCT scan, albeit in low resolution. The dermal bone consists of three distinct layers: an uppermost layer of ganoine-covered ornament, a middle layer with a canal network, extending both dorsally and ventrally for passages of nerves and vessels, and a compact basal layer (Fig. 9d). The dermal ornament is clearly displayed in the three-dimensional rendering of the skull roof (Fig. 9a): ganoine ridges align anteroposteriorly, following the longitudinal direction of each skull bone (consistent with line illustration in Coates Reference Coates1998). This unidirectional pattern is consistent with that of Rhadinichthys ornatissimus.
3.2.2. Parasphenoid
The parasphenoid (Figs 2b, 10–12a–c) tapers rostrally and possesses an anteriorly directed dermal basipterygoid process as well as a broad, proximodistally short ascending process. Morphologically, the parasphenoid resembles that of Kansasiella (Poplin Reference Poplin1974, Reference Poplin1975; Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023). Posteriorly, the parasphenoid reaches the ventral otic fissure but does not extend across it. A buccohypophyseal canal pierces the external (ventral) surface level with the anterior margin of the basipterygoid process. The canal extends posterodorsally at a 20-degree angle (Fig. 11a) and terminates at the internal (dorsal) surface level with the posterior of the basipterygoid process.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), NMS G.1984.42.15.c parasphenoid in dorsal (a) and ventral (b) views. Abbreviations: asp = ascending process; bhc = buccohypophyseal canal; bhc.f = foramen for buccohypophyseal canal; bpt = basipterygoid process; g.pal.a.,n = groove for the palatine artery and nerve.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), NMS G.1984.42.15.c, additional details of parasphenoid anatomy. (a) Microtomographic cross-section showing the path of the buccohypophyseal canal, sagittal view. (b) Close-up of the ventral exposed surface of the parasphenoid from a specimen photo. The denticle field, as well as thin striations lateral to the denticle field, are visible from the photograph. Abbreviation: bhc = buccohypophyseal canal.
A denticle field, visible on the actual specimen and in specimen photographs (Figs 2b, 11b) but barely discernible in μCT slices, occupies the central, buccal, parasphenoid surface, including the posterior expanded region and the spiracular grooves of the ascending process. In the denticle-free peripheral regions, thin and compact striations ornamented the exposed areas. The ascending process is broad but short; its lateral extent does not exceed that of the basipterygoid process (although slightly longer than reconstructed by Coates Reference Coates1998). Thus, unlike Carboniferous actinopterygians such as Kansasiella (Poplin Reference Poplin1974) and Lawrenciella (Hamel & Poplin Reference Hamel and Poplin2008; Pradel et al. Reference Pradel, Maisey, Mapes and Kruta2016), the ascending process terminates far short of the spiracular canal.
3.2.3. Neurocranium
The neurocranium (Figs 12, 13) has a maximum length of 15 mm from the broken autopalatine articulation to the posterior of the occiput, and a maximum width of 12 mm across the postorbital processes. Most of its dorsoventral height was lost in postmortem compression. The flattened remains are preserved under the complete skull roof. The ethmoid region is incomplete because of delicate ossification and loss during preparation (Coates Reference Coates1998).
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), neurocranium reconstruction from μCT data of NMS G.1984.42.15.c and comparison with Phoebeannaia mossae. (a) Ventral view. (b) Ventral view with the dermatocranium and the otoliths removed, tilted to reveal the vestibular fontanelle and more three-dimensional details of the endocranium. (c) Dorsal view with the dermatocranium removed. (d) Neurocranium of Phoebeannaia mossae, adapted from Caron et al. (Reference Caron, Venkataraman, Tietjen and Coates2023). Abbreviations: aoc = aortic canal; art. 1st.i = articulation for infrapharyngobranchial of first gill arch; art. 1st.s = articulation for suprapharyngobranchial of first gill arch; bhc.f = foramen for buccohypophyseal canal; bpt = basipterygoid process; csp = craniospinal process; ebra = canal or foramen for the efferent branchial artery; ehya = canal or foramen for the efferent hyoid artery; ep.a = canal or foramen for the efferent pseudobranchial arteries; eth.p.a = ethmoid articulation for the palatoquadrate; fm = foramen magnum; fr = frontal; g.pal.a,n = groove for the palatine artery and nerve; hm.fa = facet for the hyomandibula; int.car = canal or foramen for the internal carotid; jc = jugal canal; lat.com = lateral commissure; lat.d.aoc = lateral dorsal aortic canal; ot = otolith; otcf = oticooccipital fissure; ppr = postorbital process; psphn = parasphenoid; spic.br = spiracular bridge; spic.f = foramen for spiracular canal; st = supratemporal; stc = supratemporal canal; st.f = subtemporal fossa; VII.pal,o.a = canal or foramen for the palatine nerve and/or orbital artery; vf = ventral fissure; X = canal or foramen for the vagus nerve.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), axial view tomographic slices revealing internal anatomy of the braincase. Abbreviations: aoc = aortic canal; art. 1st.s = articulation for suprapharyngobranchial of first gill arch; bpt = basipterygoid process; csp = craniospinal process; ehya = canal or foramen for the efferent hyoid artery; fm = foramen magnum; mtx vfon = matrix infill of vestibular fontanelle; ot = otolith; otcf = oticooccipital fissure; otw vfon = otic wall and vestibular fontanelle; ppr = postorbital process; soc = supraorbital canal; spic = spiracular canal; stc = supratemporal canal.
Details of the orbit are largely obscured, so conditions of the postorbital wall, the trigeminofacialis chamber, and myodomes, if present, remain unknown. The original description (Coates Reference Coates1998) reported an ossified interorbital septum on the specimen but μCT scans show no strong evidence of its presence, and the previously identified material is the orbit endoskeletal roof. Similarly, a foramen for the efferent pseudobranchial artery was identified at the base of the basipterygoid process (Coates Reference Coates1998), but successive rounds of three-dimensional reconstruction show no such feature. Frustratingly, some details evident in the original description and on the surface of the actual fossil are barely discernible in the rendering of the μCT data (Fig. 12a). However, slight rotation of the rendering (Fig. 12b) with the otoliths removed reveals significantly more detail, and these data can be augmented by reference to slices from the μCT stack (Fig. 13).
The general proportions and distribution of features in ventral view are comparable to that of Phoebeannaia (Fig. 12d; Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023). The most obvious differences are the broad otic region and wide occipital plate in R. bearsdeni comb. nov., as well as the absence of any significant posteriorly projecting central unit of the occiput. In R. bearsdeni the aortic canal, notochordal cotylus, and foramen magnum are anteroposteriorly short.
Proceeding from anterior to posterior, with the parasphenoid removed virtually, the ventral surface of the basisphenoid is shown to have a well-marked midline groove passing posteriorly to the hypophyseal foramen, just as in Phoebeannaia. Likewise, in both taxa the hypophyseal foramen is level with the rear of the basipterygoid processes. There is no laterally prominent postorbital process in R. bearsdeni comb. nov. Instead, the process appears to have projected ventrally and now protrudes through the ascending rim of the broad lateral commissure (on the right side of the specimen). μCT data has revealed the presence of spiracular canal and a foramen for the efferent hyoid artery piercing the posterior rim of the spiracular groove (Figs 12b, 13). The facet for the hyomandibula is situated on the rear of the lateral commissure, dorsal to the posterior exit of the jugular canal.
The roof of the subtemporal fossa is broad; notably broader posteriorly than in Phoebeannaia (compare Fig. 12a and d). The ventral extremity of the otic wall is marked most clearly by the prominence of the articular process for the first suprapharyngobranchial. This process projects from the dorsal rim of the vestibular fontanelle, as in Phoebeannaia and Kansasiella, but is barely evident in Devonian taxa: Mimipiscis and Raynerius (Gardiner Reference Gardiner1984; Giles et al. Reference Giles, Darras, Clément, Blieck and Friedman2015). The full extent of the vestibular fontanelle is not immediately clear because a large otolith has punched through the anterior portion of this region on both sides of the braincase. Virtual removal of the otoliths provides an improved picture (Fig. 12b). The location of the otic-occipital fissure and the incomplete outline of the vestibular fontanelle is discernible in slices from the μCT stack (Fig. 13), confirming the previous radiograph-based interpretation (Coates Reference Coates1998). The occipital plate is broad and terminates laterally with well-formed left- and right-side craniospinal processes. The midline structures of the plate are flanked by shallow concavities.
Unlike other Carboniferous neurocranial examples, such as Lawrenciella, Kansasiella, Kentuckia, and Phoebeannaia (summarised in Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023), the ventral otic fissure (Fig. 12a, b) cuts straight across the ventral surface; there is no convexity to the rear margin of the prechordal portion of the basicranium. Within the otic regions, the semicircular canals are not preserved in either specimen, but the otoliths are remarkably intact (see otolith section for details). Just anterior to the otolith is a small projection that might be the articular surface for the first infrapharyngobranchial (Fig. 12a, b). The articulation for the first suprapharyngobranchial (Fig. 11a, b) is more clearly resolved, and located posterior and lateral to the otolith.
The aortic canal (aoc, Fig. 12a, b) barely extends beyond the posterior boundary of the braincase. μCT scans reveal no hint of an internal septum, but it appears likely that the dorsal aorta divided into the lateral aortae within the canal, similar to Raynerius, Phoebeannaia, and Coccocephalus (Poplin & Véran Reference Poplin and Véran1996; Figueroa et al. Reference Figueroa, Goodvin, Kolmann, Coates, Caron, Friedman and Giles2023). A large foramen for the efferent branchial arteries (ebra, Fig. 12a, b) pierces the aortic canal near its caudal end, matching a similar opening in Phoebeannaia. The foramen magnum (fm, Fig. 12c) is identifiable as a shallow indentation close to the dorsal surface. Inspections of μCT scans show that the notochordal canal was obliterated as a result of postmortem compression; the anterior reach of the notochord is unknown.
3.3.3. Otoliths
The previous description reported a single pair of large, pear-shaped otoliths in NMS G.1984.42.15.c. Here, μCT scanning of the specimen has revealed a much smaller additional otolith, resting by the anterodorsal apex of the larger one (Fig 14a–c, e, f). GLAHM 163425a yielded left and right large otoliths of the same shape, also including a single accessory small otolith on the right side (Fig 14c–f). These small otoliths are ellipsoidal in shape, with a major axis spanning no more than 25 % of the maximum length of the larger otoliths. In anterior view, the left and right large otoliths of NMS G.1984.42.15.c lean away from each other at a gentle slope (Fig. 14a). From comparison with polypterid examples, Coates (Reference Coates1998) interpreted the large otolith as the asteriscus from the lagenar portion of the vestibular chamber. A central sulcus is clearly visible on the notional asteriscus from the left side. The smaller otolith is likely to be a lapillus from the utricular chamber, but in the absence of labyrinth infilling, identification remains conjectural.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), three-dimensional renderings of the otoliths. (a) Left and right otoliths preserved in NMS G.1984.42.15.c, front view. (b) NMS G.1984.42.15.c left and right otoliths, ventral view. (c) GLAHM 163425a otoliths, left tipping out of the page. (d) GLAHM 163425a, left otolith. (e) GLAHM 163425a, right otoliths, lateral view. (f) GLAHM 163425a, right otoliths, mesial view.
3.3.4. Hyoid arch
A well-preserved left hyomandibula and interhyal are exposed on the surface of GLAHM 163425a, both of which appear to be completely ossified (Figs 4, 15, 16). The right hyomandibula is revealed by μCT data but provides no more than corroboration of details exposed on the left side of the hyoid arch. The dorsal and ventral limbs of the hyomandibula are of approximately equal length. The dorsal limb expands proximally, while the ventral limb is uniformly narrow with no hint of the distal expansion present in outgroups and seemingly primitive examples (Giles et al. Reference Giles, Darras, Clément, Blieck and Friedman2015, Reference Giles, Feilich, Warnock, Pierce and Friedman2023). The shape (an asymmetric boomerang) resembles that of Pteronisculus as depicted by Véran (Reference Véran1988). The angle between dorsal and ventral limbs is of around 150 degrees. There is no distinct or peg-like opercular process, but rather an elongate crest that probably articulated with the anteromedial rim of the opercular. This is much clearer in the photograph (Fig. 16a) than in the scan rendering (Fig. 15). The hyomandibula is imperforate, and the dermohyal is fused to the dorsal limb, represented by a few subparallel ornament ridges.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), μCT 3D reconstruction for GLAHM 163425a and b. Abbreviations: an = angular; clv = clavicle; fr = frontal; hym = hyomandibula; ih = interhyal; ih.fa = articular facet of the interhyal; na = nasal; ot = otolith; pq = palatoquadrate; ro = rostral; scl = supracleithrum; scr = sclerotic ring.
Rhadinichthys bearsdeni comb. nov. (formerly Woodichthys), photographs of the left hyoid arch from GLAHM 163425a. (a) The hyomandibula and the interhyal. (b) Close-up of the interhyal, articular facet facing left. Abbreviations: art.cr = articular crest; dhy = dermohyal; ih = interhyal; ih.fa = articular facet of the interhyal.
The interhyal is short and subcylindrical (Fig. 16b). This interhyal is the same as the hyoid arch bone identified as the symplectic by Véran (Reference Véran1988) for Boreosomus, Pygopterus, Pteronisculus, and others. The proximal articular facet matches the distal width of the hyomandibula. The distal facet is broader and oriented anteroventrally. The brief shaft of the interhyal is waisted. Whether the anterodistal facet articulated with the mandible and/or a proximal ceratohyal or other intermediate hyoid bone is unknown.
4. Discussion and conclusion
Rhadinichthys persists as one of the most long-standing and geographically widespread genera of Palaeozoic ray-finned fishes. However, attempts to place Rhadinichthys within a well-supported phylogeny or locate it within a broader palaeobiological context have been handicapped by a succession of ill-defined diagnoses. These various definitions have allowed the accumulation of around 27 species (Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021) whose characteristics sometimes even contradict bounds of the enclosing generic diagnosis (Henderson et al. Reference Henderson, Dunne, Fasey and Giles2023). The solution offered here is to reduce genus membership to a definable core assembled from a data set consisting of the original type specimens supplemented with a more completely preserved neotype.
Regrettably, this action leads to the loss of ‘Woodichthys’ (Coates Reference Coates1998) as a distinct genus: a taxon named in honour of the late Stan Wood, whose extraordinary career in Palaeozoic fossil discovery was founded on collecting precisely these fishes, alongside many others, from the Lower Carboniferous of the Scottish Midland Valley. Woodichthys (Coates Reference Coates1998) was established as a discrete taxonomic entity because the author (MIC), in the course of that project, failed to recognise similarities other than general resemblances shared with the type material of Rhadinichthys. In the present study, re-examination of Woodichthys uncovered new derived features which, in turn, led to a thorough re-evaluation of dermal skeletal details in the type species of both genera. In particular, μCT re-examination of the Woodichthys skull table revealed sutures defining the true shape of the supratemporal. This could be described as ‘cleaver-shaped’ or shaped like the state of Oklahoma (USA), with a distinct ‘handle’ that wraps around the flank of the frontal. Importantly, this unusual supratemporal shape is also evident in Rhadinichthys ornatissimus. A further synapomorphy uniting Woodichthys and Rhadinichthys is evident in the ornament and shape of the rostral bone, the characteristic details of which are discussed below. Further to these features, additional shared specialisations, perhaps with more homoplastic distributions, include the elongate slat-like proximal lepidotrichia of the pectoral fins, the ornamentation of the preoperculum (divided into dorsal and ventral zones), the irregular distribution of the pores in the lateral line scales, and the numbers of fulcral scales preceding midline fins.
Faced with these shared derived features, possible taxonomic actions might be to remove ornatissimus from the genus Rhadinichthys and reassign it to Woodichthys, or, vice versa, to subsume W. bearsdeni within Rhadinichthys. On the grounds of systematic stability, because ornatissimus is the type species of Rhadinichthys, we propose the second taxonomic act: to subsume the genus Woodichthys within the redefined Rhadinichthys. A third alternative, to maintain both genera, would be the least useful (taxonomic inaction), with the two taxa separated by minor differences of dermal ornament, fin position and scale count. To all intents and purposes, Woodichthys bearsdeni and Rhadinichthys sensu stricto would then be treated as the same operational taxonomic unit. In effect, this would be a taxonomic act deferred, given that a principal aim of phylogenetic systematics is the discovery of natural supraspecific groups (e.g., Wiley Reference Wiley1981; Smith Reference Smith1994).
The assembly of these revised descriptions draws attention to a short list of features that deserve further comment. The first is the ornamentation of the rostral bone. In general, the value of dermal ornament as a signal of relatedness, especially among early actinopterygians, is limited. Hence, the pattern of sinuous ridges on all large dermal bones of Rhadinichthys ornatissimus and R. bearsdeni comb. nov., bears a general resemblance to the ornament of Cosmoptychius (Traquair Reference Traquair1877b, pl. 3, fig. 3), Mesonichthys (Traquair Reference Traquair1901, pl. 17, fig. 7; British Geological Survey Collections specimen GSL 1311, pers. obs., MIC), and even Mimipiscis (Gardiner Reference Gardiner1984, fig. 79), among many other examples. These instances have yet to be linked to any other characteristics that might define a discrete clade among the abundance of early ray-finned fishes. Rather, these repeated occurrences probably reflect functional and/or developmental biases. However, we argue that the rostral bone ornament and shape in R. ornatissimus and R. bearsdeni comb. nov. is a valid genus-level diagnostic feature (Fig. 8a, b). In particular, we draw attention to the marked switch in ornament type between cap and plate, the straight posterior rim, and the naked (ornament-free) zone anterior to the anterior nostril notch. Once more, comparison with Mimipiscis is useful: Gardiner (Reference Gardiner1984, fig. 79) depicts a rostral with ridge ornament covering posterior plate and continuous with snout cap regions. There is no clear margin at the nasal notch, and a convex posterior boundary protrudes between the frontals. A further instance of the utility of rostral ornament is demonstrated in Trawdenia (Coates & Tietjen Reference Coates and Tietjen2018). In this example the bulbous rostral ornament unites three species extracted from the genus Mesopoma (incidentally including a former species of Rhadinichthys: ‘R. planti’). We suggest that, at genus level, rostral ornament can provide a more systematically useful badge than characteristics of the general dermal ornament.
The temporal bone series presents a second dermatocranial feature deserving brief discussion. Rhadinichthys as defined here has two canal-bearing bones, a long and posteriorly broad supratemporal and a short intertemporal, connected in series to the rear of the dermosphenotic. Two-bone temporal series are widespread among Devonian actinopterygians (e.g., Gardiner Reference Gardiner1984) and probably represent a plesiomorphic condition relative to presence of a single bone, a dermopterotic, as found in many more recent taxa (Gardiner & Schaeffer Reference Gardiner and Schaeffer1989; Grande Reference Grande2010). The different arrangements, complements, and terminologies applied to these temporal bones have been discussed at length elsewhere, with contrasting arguments presented in works such as Pearson & Westoll (Reference Pearson and Westoll1979), Gardiner & Schaeffer (Reference Gardiner and Schaeffer1989), and Schultze et al. (Reference Schultze, Mickle, Poplin, Hilton and Grande2021). The relevance to the present study is that many species currently attributed to Rhadinichthys (see Table 1) have a single temporal bone: a dermopterotic. Notably, these dermopterotic-bearing species include some of the classic (i.e., often depicted) members of Rhadinichthys, such as R. canobiensis and R. fusiformis from the Glencartholm fauna (Moy-Thomas & Bradly Dyne Reference Moy-Thomas and Bradley Dyne1938). It now appears that these species, if described accurately, belong elsewhere (a point returned to later).
Preliminary assessment of existing Rhadinichthys species in light of the new generic diagnosis, with comments on their gross morphology. Observations (except for R. ornatissimus and bearsdeni comb. nov.) are based on published specimen photos and illustrations.
The neurocranium is arguably the most important addition to the Rhadinichthys data set. Although μCT scans have added only modestly to previously available information, the broader phylogenetic context has changed, and more materials are now available for comparative evaluation. The Rhadinichthys neurocranium appears very similar to those of Phoebeannaia, Kansasiella (Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023), and Coccocephalus (Figueroa et al. Reference Figueroa, Goodvin, Kolmann, Coates, Caron, Friedman and Giles2023). A modest distinction is evident in the width of the posterior portion of the otic region, the corresponding breadth of the occipital plate, and the minimal posterior projection of the occipital unit. In other respects, these crania are mostly alike with spiracular canals, aortic canals, similarly positioned cranial fissures, large vestibular fontanelles, locations of major foramina, and prominent basipterygoid processes. Rhadinichthys provides a minor variation on what appears to be a morphotype common to many fusiform Carboniferous actinopterygians. Resolving relationships among these taxa is proving difficult, and comparisons with the neurocrania of living non-neopterygians (i.e., polypterids and chondrosteans) emphasises the morphological distance between extant so-called primitive actinopterygians and Palaeozoic examples (cf. discussion in Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023).
As for the otoliths in R. bearsdeni, existing reviews and discussions (Coates Reference Coates1998; Friedman & Giles Reference Friedman, Giles, Clack, Fay and Popper2016; Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021) note that while extant actinopterygians have three (utricular, saccular, and lagenar otoliths), the record of Palaeozoic examples is extremely uneven. Most discussion revolves around whether one or two otoliths were present in the pars inferior. The small otolith newly discovered in R. bearsdeni could either be the utricular otolith from the pars superior (as identified in the description) or the second pars inferior otolith. Of uncertain significance, the most similarly shaped large otoliths in extant fishes are the lagenar otoliths of polypterids (Coates Reference Coates1998). Aside from R. bearsdeni, Coccocephalus wildi is the only other early Carboniferous taxa with three-dimensional μCT scanned otoliths, but this reveals only a single large pear-shaped otolith present in each pars inferior chamber (Figueroa et al. Reference Figueroa, Goodvin, Kolmann, Coates, Caron, Friedman and Giles2023; see also the depiction of a single otolith in latest Carboniferous–earliest Permian actinopterygian in Figueroa et al. Reference Figueroa, Weinschütz, Giles and Friedman2024).
The present redefinition of Rhadinichthys prompts further evaluation of phylogenetic placement, especially in the context of the broad systematic frameworks provided by Schultze et al. (Reference Schultze, Mickle, Poplin, Hilton and Grande2021), and Giles et al. (Reference Giles, Feilich, Warnock, Pierce and Friedman2023). Furthermore, the most recent depiction of R. ornatissimus (Gardiner & Schaeffer Reference Gardiner and Schaeffer1989, fig. 16b) is quite unlike the material presented here, with dermopterotic, suborbitals, and a different dentition. This previous version, as noted in the introduction, suggests affinities with genera such as the Upper Carboniferous genus Amblypterus. However, if ‘Woodichthys’/Rhadinichthys is employed as a guide for placement in previous phylogenetic hypotheses, then there is a general trend through successive analyses to move the genus out of the actinopterygian crown. Coates (Reference Coates1999) placed ‘Woodichthys’ on the actinopteran stem, but Cloutier & Arratia (Reference Cloutier, Arratia, Arratia, Wilson and Cloutier2004) resolved ‘Woodichthys’ as a stem actinopterygian, a result that has persisted throughout a series of subsequent phylogenies. ‘Woodichthys’ is often placed in these as sister-group to all other Carboniferous taxa (e.g., Choo et al. Reference Choo, Lu, Giles, Trinajstic and Long2019; Giles et al. Reference Giles, Feilich, Warnock, Pierce and Friedman2023). In contrast, Caron et al. (Reference Caron, Venkataraman, Tietjen and Coates2023), using a data set based solely on endocranial characters, generated trees that hint at a different solution. If Phoebeannaia is used as a proxy for Rhadinichthys (given the neurocranial similarity) then the genus probably occupies a more nested position among the Carboniferous actinopterygian radiation. However, consistent with the preceding discussion of the neurocranium, the wider phylogenetic significance of this clustering has yet to be determined. At a more parochial level of relationships, Rhadinichthys is the exemplar for a collection of genera grouped as rhadinichthyids (Lund & Poplin Reference Lund and Poplin1997; Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021). Rhadinichthyid members also include the South African genus Mentzichthys and Bear Gulch taxa such as Wendyichthys and Cyranorhis. Relative to other Carboniferous actinopterygians, none of these appears especially close to the present definition of Rhadinichthys, particularly in the composition of the dermal skull and the elaborate patterns of regionalised squamation. It is unlikely that rhadinichthyids in the currently published sense will survive further examination.
To conclude this overview, few of the 27 or so Rhadinichthys species are likely to remain in the genus. Table 1 provides a summary, in which all current members (as listed in Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021) are compared to the new genus diagnosis. Out of the total of 27 species (including R. ornatissimus and R. bearsdeni), 14 lack sufficient data to determine their status as Rhadinichthys. This is hardly surprising given the incomplete condition of many fossil actinopterygians, and it seems that ‘Rhadinichthys’ species from the extremities of the temporal range (Devonian and Permian examples) are among the worst preserved and least characterised. Ten species are excluded because they clearly contradict the genus diagnosis. Several bear a dermopterotic rather than intertemporal and supratemporal bones. Consequently, R. canobiensis, perhaps the most widely cited and depicted of Rhadinichthys species (e.g., Moy-Thomas & Miles Reference Moy-Thomas and Miles1971) belongs elsewhere. This leaves a mere four species: R. ornatissimus, R. bearsdeni, R. ferox, and R. laevis, all of which are from the uppermost part of the Middle Mississippian and the earliest Late Mississippian of Scotland.
Rhadinichthys viewed in this light assumes a more morphologically, spatially, and temporally confined identity. Rhadinichthys laevis from Tarras Water Foot, Eskdale, is a probable outlier of the classic Glencartholm fish fauna (Dineley & Metcalf Reference Dineley and Metcalf1999), characterised as inhabiting a nearshore marine environment. Rhadinichthys ornatissimus and R. ferox originate from the Oil Shales of the Scottish Midland Valley, deposited on the floor of a lagoonal environment, Lake Cadell (Loftus & Greensmith Reference Loftus and Greensmith1988) up to 50 km wide (Dineley & Metcalf Reference Dineley and Metcalf1999) and approximately coeval with Glencartholm. Rhadinichthys bearsdeni is the youngest of the set, coming from the marine deltaic environments of the Manse Burn Formation. The time span of the entire genus now ranges from approximately 335 Ma to 328 Ma, a modest seven million years, (much reduced relative to former estimates) with species ranging from marine into non-marine habitats. The quantity and geographic spread of R. ornatissimus material suggests that it thrived in the Oil Shale lagoon, with individuals present in collections from many if not all of the major Victorian oil shale works: Straiton, Broxburn, and elsewhere (Clarkson & Upton Reference Clarkson and Upton2006). Other actinopterygian genera of the biota (summarised from the Wardie ichthyofaunal list of Dineley & Metcalf Reference Dineley and Metcalf1999) include Eurynotus, Nematoptychius, Cosmoptychius, Gonatodus, Wardichthys, and Elonichthys. Sarcopterygians include Rhizodus, Megalichthys, and unidentified lungfish, while the sharks include Acanthodes, Diplodoselache, Tristychius, and Sphenacanthus. Many of these fishes approach or exceed a metre or more in length, in contrast with the ten centimetres or more of Rhadinichthys. Therefore, as one of the smallest members of the lagoonal fish biota, and alongside the abundant shrimps, it seems likely that this study provides a more informed picture of one of the lower components of the Scottish Late Mississippian aquatic food chain. Rhadinichthys might be diminutive in these respects but now, data enriched, it represents a potentially useful addition to aid resolution of the Devonian–Carboniferous actinopterygian evolutionary radiation (Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023; Giles et al. Reference Giles, Feilich, Warnock, Pierce and Friedman2023).
Finally, the current study presents an exercise in the treatment of wastebasket taxa. In many instances such groups embody a history in which an initially monophyletic (and often monospecific) genus accretes taxa until it balloons into para- or polyphyletic status, i.e., transforms from diagnosable clade to grade or, eventually, a nominal cluster with boundaries set by arbitrary convention (Smith Reference Smith1994). The original specimens, usually the type material, often lack the detail of subsequent discoveries, and these, in turn, tend to be extracted to form a hierarchy of more precisely defined groups. Meanwhile, the characteristics of the original specimens, first identified as apomorphies, are re-evaluated as plesiomorphies of a larger and more diverse entity. In this way, wastebasket taxa may be eliminated via successive taxonomic extractions. Among Palaeozoic vertebrates, Labyrinthodon (see Desmond Reference Desmond1982 for historical context) and the Labyrinthodontia (for evolving views see Romer Reference Romer1966; Carroll Reference Carroll1988; Clack Reference Clack2002) present a good example. Others, however, survive because a kernel of systematic value persists and is built upon via new discoveries and redefinition. Rhadinichthys represents one such, but it also exemplifies a smaller version of the problem with palaeoniscids (see Palaeoniscimorpha in Schultze et al. Reference Schultze, Mickle, Poplin, Hilton and Grande2021; for a history of the term in various forms see Gardiner & Schaeffer Reference Gardiner and Schaeffer1989). The major challenges are (1) to discover the appropriate stem groups to which members of this vast grade belong (Jefferies Reference Jefferies1986) and (2) to determine whether a diagnosable, monophyletic rump group exists to which this name might be applied. For the present, this remains unresolved (Giles et al. Reference Giles, Xu, Near and Friedman2017, Reference Giles, Feilich, Warnock, Pierce and Friedman2023; Caron et al. Reference Caron, Venkataraman, Tietjen and Coates2023).
5. Acknowledgements
We thank the editors for the invitation to contribute to this volume honouring the work of Dr Tim Smithson. Patrick Gavin is thanked for the discovery and donation of the new specimen of Rhadinichthys bearsdeni to the collections of the Hunterian Museum, University of Glasgow. Neil Clark is thanked for providing access to the Hunterian Museum, University of Glasgow collections. Nick Fraser and Stig Walsh are thanked for providing access to the collections of the National Museums of Scotland; likewise, Stephanie Pearce for access to the collections of the Comparative Museum of Zoology, Harvard University, USA. Mark Webster, University of Chicago, provided valuable advice to the lead author during the initial phase of this project. April Neander is thanked for assistance with use of the PaleoCT facility, University of Chicago. We are most grateful to two anonymous reviewers for their insights, suggestions, and especially for assistance with updates on the geological settings. This study was partially supported by NSF EAR 2218892 (MIC).
6. Competing interests
The authors declare none.
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