1. Institutional abbreviations

FMNH, Field Museum of Natural History, Chicago; GSE, British Geological Survey, Keyworth; MHNN, Museum d’Histoire Naturelle, Neuchâtel; NEWHM, Great Northern Museum: Hancock, Newcastle upon Tyne; NHMUK, Natural History Museum, London; NMS, National Museums Scotland, Edinburgh.

3. Systematic palaeontology

Remarks: Cynopodius was assigned to the Edestidae by Jordan (Reference Jordan1963), to the Petalodontida by Sepkoski (Reference Sepkoski2002), and to the Euchondrocephali by Ginter et al. (Reference Ginter, Hampe, Duffin and Schultze2010).

Type species: Cynopodius crenulatus Traquair Reference Traquair1881 from the Blackband Ironstone (Serpukhovian, Carboniferous), Edge Coal Series, Loanhead near Edinburgh. Neotype NMS G.1885.49.18 (Fig. 1a).

Figure 1 Cynopodius crenulatus specimens from Loanhead near Edinburgh, Scotland. (a–e) Neotype NMS G.1885.49.18: (a) ‘dorsal’ view, (b) lateral view, (c–e) 3D surface mesh images, presumed labial, lateral and lingual views respectively. (f) Tomographic slices showing a series of eight virtual transverse slices (1040, 1099, 1150, 1170, 1212, 1311, 1395, 1780) through the neotype, basalmost image to left. (g) NMS G1885.47.7, fractured area exposing vascular bone of shaft. (h) NMS G.1885.57.11. (i, j) NHMUK PV P14494, strongly curved element: (i) ‘dorsal’ view, (j) magnified view of ‘spatula’ showing rugose/reticulated surface. (k) NHMUK PV P11330 (Traquair 1914 coll.). (l) NMS G.1957.1.5649 (Dunlop coll.). Scale bars 10 mm.

Figure 2 Selected 19th Century illustrations of Cynopodius crenulatus. (a) Agassiz (Reference Agassiz1833–1843, plate 19, fig. 4; labelled as Ctenoptychius apicalis). (b) Actual specimen MHNN FOS166 (CC-by-SA licence, Museum of Natural History, Neuchâtel; https://commons.wikimedia.org/wiki/Category:Collection_of_fish_fossils_created_by_LouisAgassiz). (c) Drawing by Gertrude M. Woodward in Woodward (Reference Woodward1891, pl. 1, fig. 4). (d) Actual specimen NHMUK PV OR 42085, from Pitcorthie. Scale bar 5 mm in (b); 10 mm in (d).

Diagnosis: Spoon-shaped elements up to 3.0 cm long; straight or slightly curved shaft with an oval cross-section; spatulate end has a glossy surface, and eight to 12 sharp or rounded cusps which increase in size from lateral to medial; two medial denticulations are equal in size; lateral denticulations are sharp-tipped, medial denticulations are sharp-tipped or worn and rounded; one face of ‘spatula’ is convex and the other is concave. The convex side of the ‘spatula’ extends further down the root than on the concave or flatter side. Between the denticulations, grooves converge over the surface towards the lower margin of the shiny tissue. The shaft has a rough striated surface. Histological structure of the shaft is trabecular osteodentine or acellular bone, and the ‘spatula’ is osteodentine, tritoral dentine and outer hypermineralised dentine.

Figure 3 Cynopodius crenulatus specimens from Gilmerton, Pitcorthie, Straiton, and Niddrie, Midlothian, Scotland. (a) NMS G.1858 HM F472 from Gilmerton, the earliest known specimen collected. (b) NHMUK PV P4498 from Pitcorthie. (c) NMS G.1881.43.10.2, two small clusters possibly in coprolites? from Pitcorthie. (d) NMS G.1881.43.10.3, element from Pitcorthie lacking the ‘spatula’, exposing an impression of the obverse side. (e) NMS G.1881.43.10.4, element from Pitcorthie showing extreme curvature, shaft tissue exposed by longitudinal fracture. (f, g) NHMUK PV P11325 from Straiton: (f) cluster of dozens of elements, (g) magnified view of a ‘spatula’ and fractured shafts/bases. (h) NMS G.1894.165.26–28 from Straiton, cluster of elements. (i) NMS G.1898.61.3 from Niddrie. Scale bars 10 mm.

Figure 4 Traquair’s vertical transverse thin section (uncovered) of Cynopodius crenulatus tooth from ‘Borough Lee’, NMS G.1985.5.24, photographed with NMS petrographic microscope. (a) Mounted and labelled slide. (b, c) ‘spatula’: (b) whole width (upper rectangle in a), (c) close-up of parallel gouges. (d) Base (lower rectangle in a). Abbreviation: vcn = vascular canal network. Scale bars 1.0 mm in (b); 0.1 mm in (c, d).

Figure 5 Thin sections of NMS G.1881.43.10.1, a Cynopodius crenulatus tooth from Loanhead: NMS G. 1881.43.10.1.1–5 are slices A–E respectively. (a) Specimen before sectioning, showing slices A and B, coronal sections of ‘spatula’ and shaft respectively, C and E, sagittal sections of ‘spatula’, and D, transverse section of shaft. (b–d) Slice C: (b) GX composite image, exposed surface to top, (c) Zeiss image with waveplate and crossed nicols, (d) Zeiss image, unpolarised, closeup of denteon-hypermineralised tissue boundary. (e–g) Slice A: (e) Zeiss image, crossed nicols with waveplate, (f) GX image, unpolarised, (g) Zeiss image of cusp tip, crossed nicols with waveplate. (h, i) Slice B: (h) GX image, whole width, (i) Zeiss image, close-up. (j) GX image, slice D, whole. Abbreviations: de = denteons; dt = dentine tubules; hd = hypermineralised dentine; ost? = osteodentine/acellular bone; vc = vascular canal. Scale bars 10 mm in (a); 0.25 mm in (b, c, e, f, h, j); 0.1 mm in (d, g, i).

Figure 6 Thin sections of NMS G.1898.49.12, a Cynopodius crenulatus tooth from Niddrie: NMS G.1898.49.12.1–6 are slices A, B, B’, C, C’, C” respectively. (a, b) Tooth removed from matrix by soaking in acetone: (a) lingual view, (b) lateral view. (c) Tooth reglued in matrix, labial face exposed, lines indicate positions of slices A, sagittal section of shaft, B, B’, transverse sections near spatula/base boundary, C, C’, C”, transverse sections through spatula. (d) Slice A, crushed osteodentine/bone. (e, f) Slice B’: (e) whole width, (f) close-up of area in box in (e). (g) Slice C, whole width. (h–j) Slice C’: (h) whole width, (i) lateral area (left rectangle in (h)), (j) medial area (right rectangle in a). All thin sections imaged with the Zeiss microscope, unpolarised. Scale bars 5 mm in (a, b); 0.5 mm in (e, g, h); 0.25 mm in (d, f, i, j).

Type material: Traquair (Reference Traquair1881) did not nominate a holotype, nor did he illustrate any specimens, thus all the specimens from Borough Lee (Burghlee) on which he based his description must be considered syntypes. However, the exact location of this material is not known. Waterston (Reference Waterston1954) listed NMS G.1950.38.53 as the holotype, but this specimen is missing from the NMS collection. We designate NMS G.1885.49.18 (Fig. 1a–f) collected by Joseph Blair from Loanhead as the neotype.

Type locality and horizon: Burghlee colliery, Loanhead (Burghlee Park GPS lat. 55.874653, long. −3.153883), near Edinburgh. Blackband Ironstone (Pendleian, Serpukhovian), Edge Coal Series.

Range: Viséan (mid/Asbian) to Serpukhovian.

Distribution: (See Smithson [Reference Smithson1985, fig. 1] for a map with most localities) Ayrshire: Dalry (shale on coal band), Straiton (Dunnet Shale, West Lothian Oil-Shale Formation, Asbian, Viséan); Edinburgh: Burdiehouse (Burdiehouse Limestone, mid-Viséan), Gilmerton (Gilmerton Ironstone, upper Viséan), Loanhead (Blackband Ironstone), Niddrie (shale above the South Parrot Coal Seam and South Parrot Coal Seam, Serpukhovian); Fife: Burntisland (Burdiehouse Limestone and Kingswood Stromatolite Bed, Viséan), Cellardyke (Anstruther Formation, lower Serpukhovian), Island of Inchkeith (Middle Oil Shales Group, Viséan), Pitcorthie (Anstruther Formation, lower Serpukhovian); West Lothian: Midhope Burn (West Lothian Oil-Shale Formation, Asbian, Viséan).

Remark: It is likely that the first illustration of Cynopodius is one of the specimens (MHNN FOS166) from the Burdiehouse Limestone figured by Agassiz (1838, vol. 3, pl. 19, fig. 4) as Ctenoptychius pectinatus (now Ageleodus pectinatus). It appears to be the crown of a C. crenulatus tooth.

Other material: (All single elements unless specified) lower Carboniferous, Mississippian – Loanhead (Borough Lee): NEWHM G.51.75–79 (Taylor coll., 1883); NHMUK PV P 4498; NHMUK PV P 14494; NHMUK PV P 14498; NHMUK PV P 11328 and P11330 (Traquair coll.); NHMUK PV P76985; NHMUK PV P 77560; NMS G.1885.47.7.1-3; NMS G.1885.50.5.1-4; NMS G.1885.56.34; NMS G.1885.57.11.1-3; NMS G.1889.101.7; NMS G.1890.39.6; NMS G.1891.16.9.1-3; NMS G.1894.155.18 ; ?NMS G.1957.1.5649. Pitcorthie, Fife: NHMUK PV P 11326 (Traquair coll.); NHMUK PV OR 42085 (Woodward Reference Woodward1891; pl. 1, fig. 4; Figs 2c, d); NMS G.1881.43.10.1-17 (14 single specimens plus two with two elements and one with two small clusters of elements; thin sections made of NMS G.1881.43.10.1). Burdiehouse, Edinburgh: possibly MHNN FOS166 (Agassiz 1838, vol. 3, pl. 19, fig. 4; Fig. 2a, b). Straiton: NHMUK PV P11325 (cluster of dozens of elements); NMS G.1894.165.26-28; NMS G.1895.77.2. Niddrie (shale above the South Parrot Coal Seam): NHMUK PV P 11329 (Traquair coll.).; NMS G.1893.66.9; NMS G.1894.68.76-77; NMS G.1894.151.8-10; NMS G.1894.154.29-30; NMS G.1894.186.19-20; NMS G.1895.49.11-13; NMS G.1895.182.18; NMS G.1897.30.17; NMS G.1897.104.13-14; NMS G.1898.154.29-30; thin sections made of NMS G.1898.49.12; NMS G.1898176.1-2. Niddrie (South Parrot Coal Seam): NMS G.1893.135.22-27; NMS G.1893.136.9-10; NMS G.1893.137.12-17; NMS G.1893.139.12; NMS G.1894.63.38; NMS G.1896.35.20-22; NMS G.1897.98.9; NMS G.1897.102.8-9; NMS G.1897.109.9-10; NMS G.1898.61.3-4. Dalry: NMS G.1911.62.9141, Cellardyke: NMS G.1983.33.39. Gilmerton: NMS G.1858.33, HM F472 (Hugh Miller collection). Midhope Burn: GSE 5667, 5668.

Diagnosis: (Revised from Traquair Reference Traquair1881) Slender, slightly curved subcylindrical shaft; ‘spatula’ is a flattened, rounded subrhombic to subtriangular structure, up to 25 % of total height of element; glossy surface of ‘spatula’ extends onto the shaft for one-third to half its length on the convex side, and terminates slightly below the base of the ‘spatula’ on the opposite, flatter side. The eight to 12 denticulations extending along the lateral to apical margins of the ‘spatula’ increase in width from c. 1 mm laterally to c. 2 mm for the two medial denticulations. Shaft varies considerably in length and form, sometimes flattened on both aspects, sometimes obtusely carinated in front; shaft is narrowest centrally.

Description: The neotype specimen NMS G.1885.49.18 (Fig. 1a–f) is 25 mm long, maximum width of the ‘spatula’ is 7 mm, width of the base of the shaft is c. 3.5 mm. The slightly convex side is exposed; the shiny tissue of the ‘spatula’ extends for half the length of the whole element. The ‘spatula’ has eight denticulations, and is symmetrical about the midline, with four denticulations along each outer edge of the sub-triangular expanded portion. The two medial denticulations are notably larger than the lateral ones, and show wear facets at their tips. The shaft/base of the element has a rugose surface. Three-dimensional scan transverse images of the specimen crosscut a row of narrow canals within the ‘spatula’ (Fig. 1f). Other specimens from the type locality at Loanhead have up to 12 denticulations and show a range of morphologies, including variable lateral and longitudinal curvature and wear on the denticulation tips (Fig. 1g–l).

One specimen from the Burdiehouse Limestone that we consider to be Cynopodius was figured by Agassiz (Reference Agassiz1833–1843, vol. 3, pl. 19, fig. 4, captioned as Ctenoptychius pectinatus; Fig. 2a, b). Andrews (Reference Andrews1982, p. 40) thought the specimen was lost, but it is reposited in the MHNN. It is identified as the spatulate region with the characteristic two large medial denticulations, from which the shaft has broken off. NHMUK PV OR 42085 from Pitcorthie, the only specimen for which there is a previously published illustration (Woodward Reference Woodward1891, pl. 1, fig. 4; Fig. 2c, d) that is definitely identifiable as Cynopodius, is 33 mm high, with the glossy region 12 mm high on the slightly convex (presumed labial) side. The spatulate region closely resembles that of the neotype. As for the Loanhead specimens, variation is seen in the wear on the ‘spatula’ denticulations and in the curvature of the elements from Pitcorthie (Fig. 3a–f). All the isolated specimens observed are preserved with the convex side exposed, with the obverse side of the ‘spatula’ only seen as impressions where these areas have broken off (e.g., Fig. 3d, h). The most informative specimens regarding the anatomical identity of C. crenulatus are NHMUK PV P11325 (Fig. 3f, g) and NMS G.1894.165.26–28 (Fig. 3h) from Straiton, slabs which preserve accumulations of some dozens of separate elements. The orientation of most of the elements on the latter slab indicate that this probably represents partial dentition of the upper and lower jaw preserved in situ.

Traquair (Reference Traquair1882, pp. 541–542) also described atypical specimens ‘aberrant in their contour’, which lacked ‘the radiating grooves and marginal crenulations of the expanded portion…’ while its surface was ‘deficient in the usual glossy ganoine layer’. However, he considered all specimens to belong to the same species. We have also observed some variation in the surface of the spatulate region, with the outer surface appearing smooth and translucent on some (Figs 1a, g, h, l, 3a, b, h, i), whereas rare specimens show a ‘bumpy’ reticulated pattern on the convex side (Fig. 1l, j). We presume that the latter results from the outer smooth layer being worn off.

The X-ray microtomograph scan of the neotype (Fig. 1f) and thin sections NMS G.1985.5.24, NMS G.1881.43.10.1 and NMS G.1898.49.12 (Figs 4–6) reveal the distinctive structure of the ‘spatula’. Vertical canals extend up through the central plane of the denticulate region (Figs 1f, 6h–j), with sub-parallel denteons radiating out towards the surface (Fig. 5b–g). The denteons are separated by a hypermineralised matrix that extends to form a thin outer layer on the cusps, particularly on the labial side (Fig. 5b–d). Polarised imaging of the thin sections shows that the crystals of the outer layer are oriented perpendicular to the outer surface (Fig. 5c, g). Fine branching dentine tubules radiate out from the central canal of the denteon to the marked boundary between the denteons and the outer tissue, extending through the hypermineralised tissue to the cusp surface. The boundary region appears to have rounded cell lacunae, with at least some of the fine tubules in the outer layer extending out from these lacunae (Figs 5d, 6i, j). Osteodentine comprises the inner and basal regions of the spatula (Fig. 6e–g). The base is trabecular osteodentine or acellular bone (Figs 5h–j, 6d).

Comparison and remarks: Woodward (Reference Woodward1891, p. 154), in his diagnosis for Cynopodius, described the element as a ‘spine-like body’, and in his plate 1 caption, described it as a spine. Barkas (Reference Barkas1873, pl. 1, fig. 21) figured a tooth-like element from the Northumberland Coal Measures, captioned as Ctenoptychius apicalis, which bears some resemblance to the Cynopodius ‘spatula’, but it has just a single large central cusp rather than the pair of large medial cusps that characterise Cynopodius. This specimen was most probably correctly assigned by Barkas to Ctenoptychius apicalis, by comparison with the type specimen for that species (Agassiz Reference Agassiz1833–1843, vol. 3, pl. 19, fig. 1, 1a). Ginter et al. (Reference Ginter, Hampe, Duffin and Schultze2010) suggested that Cynopodius elements could be part of a pterygopodial (clasper) apparatus, and Itano (Reference Itano, Fernández, Baños-Rodriguez, Cloutier and Miyashita.2024) reiterated that idea, suggesting that the elements are holocephalan frontal claspers. Many late 19th Century fossil fish workers, including Woodward (Reference Woodward1891, p. 40) considered the frontal claspers of holocephalans to be dermal structures, and thus bony and/or dentinous ‘ichthyodorulites’ (translated from Greek as ‘fish spears’), but Reis (Reference Reis1895, p. 385) asserted they are ‘calcified fibrocartilage’ like the pelvic claspers of elasmobranchs. This view was reiterated by Patterson (Reference Patterson1965, p. 199, pl. 23) who described the frontal clasper as a ‘small rod of heavily calcified fibro-cartilage, armed with scales’ in living chimaeroids, as well as in the Carboniferous Menaspis and early Jurassic Squaloraja (Patterson Reference Patterson1965, p. 172). His interpretation of the structure of the frontal spines was based on his assessment that ‘the lack of ornament, the form of the base of the spine, and the texture of the surface… all suggest calcified fibro-cartilage rather than dentine’. Recent investigations on the development of the tenaculum (= frontal clasper) in extant chimaeroid holocephalans (Cohen et al. Reference Cohen, Coates and Fraser2025) show that the structure in adults is composed of a dense core of cartilage encased in a fibrous connective tissue sheath.

Frontal claspers are hollow (Reis Reference Reis1895, pl. 12, figs 5, 12; Patterson Reference Patterson1965, pl. 23), whereas Cynopodius elements are not. The highly vascular bone/osteodentine forming the shaft/base of the latter differs greatly from fibrocartilage in Recent animals, which is composed of collagen fibres, fibroblasts and chondrocytes, and mostly lacks blood vessels (Brelje & Sorenson Reference Brelje and Sorenson2024, ch. 5, MHO40). As noted by Patterson (Reference Patterson1965), fossil fibrocartilage differs from bone in both surface texture (e.g., Itano & Duffin Reference Itano and Duffin2023, fig. 4) and histology.

Traquair (Reference Traquair1881) was undecided as to whether the Cynopodius crenulatus specimens were teeth or dermal appendages, but he considered them to be selachian (chondrichthyan) and commented on their resemblance to Ctenoptychius pectinatus, i.e., Ageleodus pectinatus. He made thin sections of both taxa (Paton Reference Paton1997), and in discussing Cynopodius, Traquair (Reference Traquair1881, p. 35) noted that ‘The microscopic structure reminds us of Ctenoptychius pectinatus’. He did not identify a distinct enameloid layer in thin sections of either species. It seems possible his assessment of their similarity was based on misinterpretation of the scratches made in polishing the Cynopodius section (Fig. 4). It is likely that he considered these to be thick parallel tubules like those in Ageleodus (Agassiz (Reference Agassiz1833–1843, vol. 3, pl. m, figs 4, 5: tooth root vertical sections); Owen Reference Owen1867, pl. 4).

Based on the evidence we have provided here, in particular the specimens clustered together amidst pyrite (possibly representing a replacement of soft tissue by sulphate-reducing bacteria (Stig Walsh, pers. comm.)) and the shaft/base being bone or osteodentine not fibrocartilage, we are confident that the Cynopodius elements are teeth. The histological structure of C. crenulatus is distinctive. Schultze & Bolt (Reference Schultze and Bolt1996, p. 32) noted that the Cynopodius elements have a ‘crown of dense orthodentine, base of trabecular dentine’, based on a personal communication from Rainer Zangerl (then of FMNH). However, it seems possible this assessment was based on misinterpretation of the Traquair section (Fig. 4), as detailed above. The inner region of the ‘spatula’ or crown is composed of a trabecular osteodentine, which extends down and forms the tooth shaft/base. Alternatively, the base could be acellular bone; no dentine tubules or bone cells are visible (Fig. 5h–j), but there is no obvious transition from the crown osteodentine to that of the base.

The outer crown tissue, with its subparallel denteons extending out from the inner osteodentine towards the surface (Fig. 5b–g), resembles that of the crown structure in dentitions of Helodus and crown holocephalans (e.g., Johanson et al. Reference Johanson, Underwood, Coates, Fernandez, Clark, Smith, Pradel, Janvier and Denton2021a, fig. 6d, f). Johanson et al. (Reference Johanson, Underwood, Coates, Fernandez, Clark, Smith, Pradel, Janvier and Denton2021a, p. 717) described this tissue in holocephalans as ‘tritoral dentine’; previous authors labelled it as tubular dentine (Patterson Reference Patterson1965), vascular pleromin (e.g., Ørvig Reference Ørvig1980), tubate dentine (Ørvig Reference Ørvig1985) and orthotrabeculine (Stahl Reference Stahl and Schultze1999). In Cynopodius as in holocephalans, this tissue extends out towards the surface of the cusps, separated by a hypermineralised matrix (sensu Stahl Reference Stahl and Schultze1999). Ørvig (Reference Ørvig1985, figs 15–17, 33) considered that the fine processes extending from the boundary between the denteons and the hypermineralised tissue in chimaerid tooth plates housed fibres, rather than dentine tubules extending through the boundary as previously interpreted. Meredith Smith et al. (Reference Meredith Smith, Underwood, Goral, Healy and Johanson2019) showed that both types of structure occurred, with the latter developed towards the outer (older) end of the denteon, and cells they characterised as ‘whitloblasts’ along the boundary towards the inner (younger) end of the denteon sending out extensive, ramifying tubules (Johanson et al. Reference Johanson, Manzanares, Underwood, Clark, Fernandez and Smith2021b, fig. 9f). In Cynopodius, it seems that dentine tubules extend through the denteon–hypermineralised tissue boundary, and processes extend out from cells along the boundary (Fig. 5d), in the same area, rather than being separated as in holocephalans.

In holocephalan dentitions, the outer layer of hypermineralised tissue is mostly worn down so that the denteonal canals open out as pores on the surface (e.g., Stahl Reference Stahl and Schultze1999, figs 20, 21). In Cynopodius, the hypermineralised tissue is not worn down to expose the denteonal canals. Johanson et al. (Reference Johanson, Underwood, Coates, Fernandez, Clark, Smith, Pradel, Janvier and Denton2021a, p. 213) recognised basic histotypes present in all holocephalan dental material they examined, with a ‘tritoral dentine tissue surrounding vascular canals showing a regular arrangement perpendicular to the tooth surface, along with a supporting trabecular dentine … and a laminar basal dentine layer (no canals)’. Cynopodius teeth have a similar crown tissue and trabecular osteodentine, but lack a laminar basal layer.

Figure 7 Type specimens of Cynopodius robustus sp. nov. from the Hiemstra Quarry near Delta, Iowa. (a–b) Holotype FMNH PF 14923: (a) concave (presumed lingual) side, (b) other side, terminal denticulations = cusps. (c–d) Paratype FMNH PF 14924: (c) convex side, (d) concave side. (e–f) Paratype FMNH PF 14922: (e) concave side, (f) magnified view of ‘spatula’. (g) Paratype FMNH PF 14925a, concave side. (h–i) Paratype FMNH PF 14925b: (h) convex side, (i) magnified view of ‘spatula’. (j–k) Paratype FMNH PF 14027: (j) detached ‘spatula’, convex side exposed, associated with Ageleodus (Ag) elements, (k) magnified view, oblique angle. Scale bars 5 mm in (a-e, g, h, j, k); 1 mm in (f, i).

LSID: http://zoobank.org;urn:lsid:zoobank.org.pub:9CAD9024-A61E-4533-ABDC-8B9A022BC86B

Etymology: Distinguishing the stouter elements of the new taxon from those of the more slender and elongate type species.

Holotype: FMNH PF 14923 (Fig. 7a, b).

Type locality and horizon: Hiemstra Quarry, near Delta, Iowa, USA; lower Waugh Member, Sainte Genevieve Formation: Lower Carboniferous, Mississippian, Viséan.

Paratypes: FMNH PF 14922, 14924, 14925 (two specimens), FMNH PF 14027 (associated with Ageleodus).

Diagnosis: Elements are up to 23 mm high, relatively straight. Spatulate expansions are c. 35 % total height of elements, with eight or ten denticulations. Shaft does not narrow centrally, and has a higher width:height ratio compared with C. crenulatus.

Description: The type specimen FMNH PF 14923 (Fig. 7a) is 22 mm high, maximum width c. 3.3 mm, minimum width 3.2 mm at the margin between the enameloid layer and the shaft, maximum width 4.5 mm near base. The slightly concave side of the ‘spatula’ is exposed. Along the latero-apical edges, the spatulate end bears five denticulations of each side, which increase in size latero-medially (Fig. 7a, b). The outer denticulations are pointed, but the two medial denticulations are rounded off, showing wear surfaces on one side where the thin outer layer is missing. The ‘spatula’ height is 35 % total height. FMNH PF 14924 (Fig. 7c, d) is a smaller specimen (12.5 mm high, 3.0 mm width, base possibly broken off; ‘spatula’ height is 36 % total estimated height) with broken medial denticulations. It has been prepared out from the matrix, showing the different extent of the dentinous region of each side (Fig. 7c, d). The smallest specimen FMNH PF 14922 (Fig. 7e) is c. 10 mm high, estimated minimum shaft width 2.1 mm, maximum width 2.5 mm near base, with eight denticulations, mostly rounded, on the ‘spatula’ (Fig. 7f), for which the height is 33 % of total height. Two elements are preserved on FMNH 14925 (Fig. 7g–i). One element with eight denticulations (Fig. 7g) has the slightly concave side exposed, and the other element (Fig. 7h, i), possibly with ten denticulations, shows the slightly convex side. For both elements the ‘spatula’ height is 36 % of total height. The detached spatulate region of another element is exposed on FMNH PF 14027 (Fig. 7j, k), associated with three Ageleodus teeth. This Cynopodius element has ten denticulations, with the medial four all rounded off, and the lateral three on each side with sharp points.

Remarks: Ginter et al. (Reference Ginter, Hampe, Duffin and Schultze2010) listed West Virginia as a region in which Cynopodius is found, namely at Greer in the marine lower Carboniferous (Serpukhovian) Bluefield Formation (Schultze Reference Schultze2013). Garcia & Miller (Reference Garcia and Miller2014) mentioned its presence in the Mississippian (Namurian) of Kentucky as well. Previous authors, as listed in the synonymy, noted its occurrence in Iowa, but our systematic description here of Cynopodius robustus represents the first detailed publication on Cynopodius from outside Scotland. The main differences between C. robustus and the type species are the degree of longitudinal curvature and relative robustness of the elements. C. robustus elements are relatively straight compared with those of C. crenulatus, and whereas the ‘spatula’ in C. robustus comprises c. 35 % of the total height of the tooth, in C. crenulatus it is c. 23%, and the shaft/root is slenderer.

4. Functional interpretation

If Cynopodius elements are chondrichthyan teeth, as we and others have proposed, a comparison with similar teeth in Recent fishes could indicate the feeding mode of the shark in which the Cynopodius elements formed the dentition. The closest comparison is with long-toothed coral reef fishes, in particular with an acanthurid teleost Ctenochaetus striatus, the striated surgeonfish. This fish has close-set teeth with an elongate base and an expanded crown with four or six denticulations (Bellwood et al. Reference Bellwood, Hoey, Bellwood and Goatley2014, fig. 2a; Fig. 8; see also Purcell & Bellwood (Reference Purcell and Bellwood1993). It feeds on a surface film of blue-green algae and diatoms on coral or rock, as well as on small invertebrates (Froese & Pauly Reference Froese and Pauly2025). These and other Recent long-toothed marine and freshwater fish have developed, via convergent evolution, long flexible teeth with which they can remove fine detrital particles from algal turfs (Bellwood et al. Reference Bellwood, Hoey, Bellwood and Goatley2014); i.e., they are specialist detritovores. It seems unlikely that Cynopodius teeth were flexible, as their base is bone, but the striking similarity between the denticulated crowns on these teeth and those of the teleost Ctenochaetus striatus suggests that Cynopodius also fed on algae. This feeding strategy has also previously been proposed tentatively for Ageleodus (Turner Reference Turner2013), which has similar denticulations on the tooth crown but has a much shorter base/root. Algal limestones at the base of the Burdiehouse Limestone have long been known (Kennedy & Pringle Reference Kennedy and Pringle1946) and Ageleodus teeth have been found associated with non-calcified dasycladalean alga in siderite nodules from the lower Carboniferous lower Strathclyde Group (Mississippian, Viséan, previously the Lower Oil Shale Group of the Calciferous Sandstone Measures) in a shallow marine deposit (Anderson Reference Anderson2009, fig. 5a). This association supports the possibility of both the Ageleodus and Cynopodius fishes being algal browsers.

Figure 8 SEM image of teeth of the Recent coral reef acanthurid fish Ctenochaetus striatus, the striated surgeonfish (Bellwood et al. Reference Bellwood, Hoey, Bellwood and Goatley2014, fig. 2a). Scale bar 0.5 mm.