Rhadinacanthus longispinus ( Agassiz, 1844 )

Burrow, Carole, Newman, Michael & Davidson, Robert, 2016, The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland, Palaeontologia Electronica 19, pp. 1-83 : 35-52

Rhadinacanthus longispinus ( Agassiz, 1844)

Figure 24 , Figure 25 , Figure 26 , Figure 27 , Figure 28 , Figure 29 , Figure 30 , Figure 31 , Figure 32 ,

Figure 33 , Figure 34 , Figure 35 , Figure 36 , Figure 37

1841 ichthyolite; Miller, pl. 8.1.

1844 Diplacanthus longispinus ; Agassiz, p. 34, 42; pl. 14.8, 9.

non 1844 Diplacanthus longispinus ; Agassiz, pl. 13.5.

1848 Diplacanthus perarmatus M'Coy ; M'Coy, p. 301.

1855 Diplacanthus perarmatus ; M'Coy, p. 585, pl. 2B.3.

1888 Rhadinacanthus longispinus ( Agassiz 1844) , in part; Traquair, p. 512.

1890 Rhadinacanthus longispinus (Ag.) ; Traquair, p. 482.

1891 Diplacanthus longispinus ; Woodward, p. 26, 27; pl. 3.1.

1904 Rhadinacanthus longispinus ; Goodchild, p. 595.

1940 Diplacanthus longispinus ; Gross, p. 28, fig. 3E, F.

1947 Rhadinacanthus (Diplacanthus) longispinus (Agassiz) ; Gross, p. 126, textfig. 14, pl. 6.1, 2.

1954 Diplacanthus longispinus Agassiz ; Waterston, p. 11.

1973 Rhadinacanthus balticus Gross ; Gross, p. 71, 72, figs. 8J, K, 9A-D, pl. 28.13-15.

1975 Rhadinacanthus longispinus ; Saxon, p. 15.

1976 Rhadinacanthus longispinus (Agassiz 1845) ; Paton, p. 12, 13.

1979 Diplacanthus longispinus Agassiz 1844 ; Denison, p. 32, fig. 21D-F.

1979 D. balticus (Gross) 1973 [ Rhadinacanthus ]; Denison, p. 32, figs. 9D, 10F.

non 1984 Diplacanthus longispinus ; Poltnig, pl. 2.1-7.

1985 Rhadinacanthus balticus? Gross ; Valiukevičius, p. 32, figs. 2, 4. 5, 6, 7.

1988a Rhadinacanthus balticus ; Valiukevičius, p. 603, table 2.

1988a Rhadinacanthus multisulcatus ; Valiukevičius, p. 603, 604, table 2.

1988b Rhadinacanthus balticus ; Valiukevičius, p. 78.

1988b Rhadinacanthus multisulcatus Valiukevičius ; Valiukevičius, p. 78, 79, fig. 2, pl. 8.9-13.

1982 Diplacanthus longispinus ; Andrews, p. 44.

1991 Rhadinacanthus longispinus Traquair ; Frickhinger, p. 243.

1994 Rhadinacanthus balticus Gross ; Valiukevičius, figs. 5, 7.

1994 Rhadinacanthus multisulcatus Gross ; Valiukevičius, p. 119, fig. 7.

1995 Rhadinacanthus longispinus ; Young, p. 67, figs. 1, 6.

1997 Rhadinacanthus longispinus ; Young, p. 48.

1999 Rhadinacanthus longispinus (Agassiz 1845) ; Trewin and Davidson, p. 543.

1999 ‘ Rhadinacanthus’ Diplacanthus longispinus Agassiz ; Dineley, fig. 16.12G.

2000 Rhadinacanthus balticus Gross ; Valiukevičius and Kruchek, figs. 1, 4, 5, 6.

2000 Rhadinacanthus multisulcatus Valiuk ; Valiukevičius and Kruchek, p. 278, figs. 1, 4, 5.

2001 Rhadinacanthus longispinus ; Hanke, Davis, and Wilson, p. 751.

2002 Rhadinacanthus multisulcatus ; Valiukevičius, p. 216.

2005 Diplacanthus longispinus (Agassiz) ; Newman and Dean; p. 3, 4.

2008 Rhadinacanthus multisulcatus ; Märss, Kleesment, and Niit, table 1.

2010 Diplacanthus longispinus Agassiz, 1845 ; Newman, p. 10-11, figs. 15-17.

2011 Rhadinacanthus multisulcatus ; Upeniece, p. 18, table 1.2.

2012 Diplacanthus longispinus ; Newman, Davidson, den Blaauwen and Burrow, p. 742, 743, fig. 2C.

2014 Rhadinacanthus multisulcatus ; Plax and Kruchek, p. 32, 36, pl. 3.17.

Holotype. Articulated fish lacking the head, NMS G. 1953.4.3 (counterpart NMS G.1859.33.4) from Cromarty.

Referred Material. Articulated specimens of Rhadinacanthus longispinus include: from Gamrie, Banffshire , NMS G.1870.14.143, NMS G.1891.92.336, 337, NMS G.1891.92.338 (part) and NHM P.4041 (counterpart), NMS G. 1892.8.10, NMS G. 1892.8.12, 13, NMS G. 1892.8.15; NMS G. 1892.8.16 (counterpart) and NMS G. 1892.8.17 (part), NHM P.11760; from Cromarty, Ross and Cromarty , NMS G.1859.33.4; from Tynet Burn, Moray, NHM OR.43276; from Cushnie Burn, Banffshire , NMS G.2002.59.142; from Cruaday Quarry, Orkney, NMS G.1891.92.334 and NMS G 1966.49.9; from Achanarras Quarry, Caithness , NMS . G.2014.4.32. Disarticulated specimens found useful for this study include: from Cromarty , NMS G.2014.44.1; from the west of East Murkle Bay, Caithness , NMS G.2014.4.33; from Clardon Haven, Caithness , NMS G.2014.15.1; from the Thurso area, Caithness , NMS G.1859.33.703; from Taldale Quarry, Caithness , NMS G. 1968.5.2; from Marwick , Orkney, NMS G.2014.15.2, NMS G.2014.33.4 and NMS G.2014.33.7; from Broad Taing, Orkney, NMS G. 2015.11.1; from North Ronaldsay , Orkney, NMS G.2014.33.2.

Distribution within the Orcadian Basin ( Figure 2 ). Many well-preserved specimens are known from the Moray Firth nodule beds including Cromarty, Eathie, Gamrie, and Lethen Bar, but not Edderton. Several articulated specimens are from Achanarras Quarry, but otherwise in Caithness only rare, disarticulated spines have been collected from some coastal localities (e.g., Castletown and the coast east of Thurso). From Orkney a few rare articulated specimens are known from the Sandwick fish bed. As in Caithness, spines are also found in the sediments exposed along the coast, especially in beds from the Upper Stromness series. Scales of Rhadinacanthus longispinus are also commonly found in coprolites from both Orkney and Caithness, often in beds where isolated spines were collected. A spine NMS.G.2014.33.2 ( Figure 27.6 ) was recently found in the Middle Rousay Formation at North Ronaldsay together with the placoderm Millerosteus minor .

Diagnosis. Diplacanthid having fin spines with a leading edge ridge and smooth flat sides; posterior dorsal fin spine longer than anterior dorsal spine; pectoral fin spine almost straight with minute posterior denticles; large ornamented cheek plate about a third the length of the head plus branchial region; one large thin circumorbital plate anterodorsal to the orbit.

Description. Head Region. ( Figures 24 , 25 ) The tectal region of the head is covered with polygonal tesserae, with smaller scales and/or tesserae over the rest of the head and branchial region. Elongate scales line the supraorbital sensory canal ( Figure 25.2 ). The only larger dermal structures are the thin circumorbital plates extending around a third of the anterior circumference of the orbit, the occlusal bones on the lower jaws, and relatively large ornamented cheek plates extending about a third the length of the head and branchial region. The cheek plates are ornamented with small flat tubercles or short ridges ( Figure 25.6 ). The length of the occlusal plates is equal to the diameter of the orbit ( Figure 25.1, 25.3 ). There is no sign of a postorbital plate, and no evidence is visible indicating the position of the branchial arches.

Morphology of the Spines. ( Figures 24 , 26 ) In Rhadinacanthus longispinus , the median spines are relatively long and slender structures. The anterior dorsal spine ( Figures 24 , 26.1 ) is slightly shorter than the posterior one ( Figures 24 , 26.3 ) and both are slightly curved posteriorly. The posterior dorsal fin spine measures about a quarter the length of the fish. The anal fin spine ( Figures 24 , 26.4 ) is more strongly recurved than the two dorsal spines, and is positioned almost diametrically opposite the posterior dorsal. The anal spine is slightly shorter than the anterior dorsal spine. The pelvic and pectoral spines are of almost identical length ( Figures 24 , 26.5 ), both shorter than the anal spine, and each about half the length of the posterior dorsal. The prepelvic spines are about half the length of the pelvic spines, and the admedian spines are about a quarter the length of the pelvic spines ( Figure 24 ).

All median spines have a relatively long inserted part, showing the identical narrow, parallel ribs which typify all diplacanthiforms ( Newman et al., 2012; Figure 26.2 ). All spines have a broad evenly rounded leading edge ridge, separated by a single deep groove from the side wall. In cross section the sides are rather flat to slightly convex. Towards the base the posterior face widens, giving the cross section a roughly triangular shape. In cross section the exserted part of the anterior dorsal fin spine is rather high, with the height:width ratio ranging from c. 1.6 near the tip to about 1.3 FIGURE 30 . Rhadinacanthus longispinus median fin spines from mature fish, transverse section drawings. 1, 2, posterior dorsal spine NMS G.2014.4.33 from the slates west of East Murkle Bay, Caithness: 1, spine showing position of sections; 2, sections 1–8. 3, section of posterior dorsal spine NMS G.2014.33.2 from North Ronaldsay. 4, section of anterior dorsal spine NMS G. 2014.11.1 from Broad Taing, Orkney. Scale bars equal 1 cm in 1; 5 mm in 2; 1 mm in 3, 4.

towards the insertion/exsertion boundary. Length:maximum depth ratio is 18–21. The posterior dorsal fin spine has an almost circular cross section towards the insertion/exsertion boundary. The height:width ratio for this spine ranges from 1.4 near the tip to about 0.9 in the proximal exserted part, and it is extremely slender with a length:maximum depth ratio c. 30. The anal fin spine height:width ratio ranges from c. 1.4 about midspine to c. 1.3 in the proximal exserted part, and length:maximum depth ratio is c. 20. The dorsal and anal spines have an open main pulp canal in the proximal exserted part extending one-third to one-fifth of the total exserted length. In distal parts the main pulp canal is closed over, and often has a narrow median sulcus. In the inserted part, a deep groove, triangular in cross section, runs over the total length along the posterior face. The inserted part of the median spines has a distinctive groove near the base along the anterior side ( Figure 30 ), which leads into the single accessory pulp canal, typical of the median spines in Rhadinacanthus longispinus . This open accessory pulp canal along the deepest inserted part is closed over in the more distal, but still inserted part of the spines.

In the relatively short and almost straight pelvic spines, the anterior rib is a prominent structure, with only short sides below the large grooves. These spines lack a clearly delineated inserted part. In cross section the pelvic spines exhibit a slight asymmetry. The height:width ratio ranges from c. 1.1 distally to c. 0.9 proximally. The prepelvic spines are slightly higher than wide towards the tip, but broad and flat proximally with an open, shallow main pulp canal. The leading edge ridge is again a prominent feature, the sides having two smaller, well-rounded ridges paralleling it. In their proximal parts, the prepelvic spines are broader than high with a height:width ratio c. 0.7 or less. The admedian spines resemble the prepelvic spines, with a prominent, rounded leading edge ridge and just one well-rounded ridge on each side. Towards the base, the spine is much wider than long, with height:width ratio c. 0.3.

The pectoral spines of Rhadinacanthus longispinus are always asymmetrical, one side flatter than the other and extending further posteriorly. The minute, recurved denticles along the posterior edges ( Figure 26.6 ) are c. 0.4 mm apart towards the tip, c. 0.2 mm apart midspine, and c. 0.4 mm apart towards the base of the spine. In contrast to the median spines, the height:width ratio increases towards the base, ranging from c. 1.2 to1.5.

Histology of the Spines. ( Figures 27 , 28 , 29 , 30 ) All spines have at least one accessory pulp canal ( Figures 27.1-2, 27.4 , 29 ). The pectoral and prepelvic spines have two accessory pulp canals, the second and uppermost one in the anterior ridge, although this canal can become infilled ( Figure 28.2 ). In all other spines the only accessory pulp canal is immediately above the main pulp canal. This accessory pulp canal has a small diameter compared with the main canal, and is irregular in shape ( Figures 27.1-2, 27.4 , 28.2 , 29 ). In cross section, the most conspicuous histological feature is the radial distribution of canals, visible over the whole length of spines including the inserted parts ( Figures 27 , 28.1-2 ). In the median spines the initial stage in development of the inserted part consists of trabecular tissue, enclosing vascular canals ( Figures 29 , 30 ). The trabecular tissue is separated from the main pulp canal by a thin lining of a dense tissue containing sparsely distributed cell lacunae and processes ( Figure 27.2 ).

The proximal end of the dentinous ornament ridges terminate sharply at the insertion/exsertion boundary, overlying the longitudinal vascular canals encased in narrow osteodenteons which open out on the inserted area. In midspine cross sections, where the main pulp canal is closed, older dentinal ridges remain clearly visible ( Figure 27.1-2 ). Denteons infill vascular canals to a greater extent towards the tip of the spine with the infilling decreasing in thickness proximally (i.e., in the newer spine layers). A denteon only forms in the main pulp canal when it is closed posteriorly ( Figure 28.6-7 ), initially forming along the posterior side of the main pulp canal but rapidly increasing in thickness centripetally towards the spine tip. A translucent tissue covers the dentine ridges ( Figure 28.6 ). The radial arrangement of ridges and vascular canals is found in the median and paired pelvic and pectoral spines, but is only sporadically developed in the prepelvic spines ( Figure 28.7 ). All spines are almost exclusively composed of dentine: trabecular dentine in the initial stages, followed by the radial arrangement of dentinal ridges, covered by a lining of more translucent durodentine, and finally by the development of denteons in vascular and pulp canals. Cell lacunae are only present in the infilling of the main pulp canal.

Shoulder Girdle Complex. ( Figures 31 , 32 , 33 ) The scapula shaft is high, with the tip often reaching higher than the proximal end of the anterior dorsal spine ( Figure 32 ). The shaft is also slender, flaring out only minimally towards the base ( Figures 31 , 32 ). Fusion of scales on the base of the scapula ( Figure 32.7 ) indicates that the skin was immediately overlying this endoskeletal structure. As in Diplacanthus tenuistriatus , the endoskeletal bone of the scapula, coracoid and procoracoid forms a continuous sheet, with the procoracoid presumed to be the area under the anterior base of the admedian spine ( Figure 31.3 ). Although we have labelled structures as pinnal plates, only the inner endoskeletal bones are exposed on specimens, and it seems possible/likely that scales rather than dermal plates formed their external cover, based on the impressions underlying the endoskeletal bone ( Figure 31.3 ). Presence of a small dermal anterior ventral plate associated with the complex appears variable, as only rarely can one be distinguished ( Figure 31 ).

Morphology of the Scales. ( Figures 34 , 35 ) Scale crown ornament shows little variation over the flanks of the fish; scale size decreases from the lateral line towards the dorsal and ventral surfaces and toward the tail ( Trinajstic, 2001). There is quite a sharp transition from head tesserae to body scales in front of the scapulocoracoid ( Figure 34.1 ), and from normal body scales to small rhombic scales on the fin webs ( Figure 34.2 ). The number of crown ridges varies from 20 or more on larger scales from the central and anterior parts of the body, to about 12 mid-body ( Figure 34.3-6 ), to fewer than 10 on scales from the end of the tail ( Figure 34.9 ). Scales edging the lateral line are markedly larger than the other flank scales ( Figure 34.7 -8). Scales on the fin webs decrease in size towards the trailing edge of the web ( Figure 34.10 ). In outline, the flank scales of Rhadinacanthus longispinus ( Figures 34.4-5 , 35.1 ) have a broadly rounded anterior margin and rather sharp lateral corners where the anterior margin meets the posterolateral edges at an angle of about 90°. The posterolateral edges are straight or slightly concave and usually denticulated and joining posteriorly in a single sharp point ( Figure 35.5, 35.8, 35.10 ). Scales without serrated posterior margins terminate in an acute angle. Scales are rather uniform in outline, with the crown widest forward of the centre, and width more or less equalling the length. The anterior rim of the crown surface is gently curved and smooth, with a serrated margin overlapping the anterior ends of the ornament ridges ( Figure 35.2, 35.8, 35.12 ). The main ridges often have flattened triangular ends anteriorly, some with multiple branches ( Figure 35.5 ), and sharp crests for the rest of their length, extending back for most or the whole length of the crown. Fine, short ridges are intercalated between the longer ridges in the anterior region of the crown ( Figure 35.2, 35.8, 35.9 ).

The neck of the scales is concave all round. Two rounded swellings occur posteriorly, above the sharp ridge separating the neck from the base, one on either side of the median rib supporting the posterior part of the crown ( Figure 35.3, 35.6-7 ). The base is moderately convex. Scales on the anterior half of the body have subparallel ridges on the crown, and the neck is deeply concave below the strongly denticulated posterior margins ( Figure 35.8, 35.10 ). The base is oval in outline. In horizontal section, the neck is wider than long with the anterior and posterior faces meeting in sharp points ( Figure 35.3, 35.8 ). Some scales from the area in front of the anterior dorsal spine have very deep necks and flat bases ( Figure 35.12 ), and others have four or more swellings on the neck ( Figure 35.11 ). On the posterior half of the body, scales have only a single posterior point or sparse denticulations along the posterolateral edges. The base of these scales and sections across the neck are subrhombic in outline. Pores to the ascending vascular canals open out in the lower parts of the neck both anteriorly and posteriorly, close to the sharp rim that separates the neck from the base ( Figure 35.4 ).

Histology of the Scales. ( Figures 36 , 37 ) Vertical and horizontal thin sections of the scales show the average number of crown growth zones is four ( Figures 36.2, 36.4-6 , 37.4 ), the maximum is six or seven ( Figure 37.1 ), and each zone completely covers the older one. The zones are relatively wide. Each growth zone has its own vascular and dentinal canal system. Ascending vascular canals, with an opening to the exterior along the sharp boundary between the neck and the base, parallel to the concave sides of the neck. These canals are often oval in cross section (20 µm by 50 µm), the long axis of the cross section parallel to the growth lines ( Figure 36.4 ). Near the crown surface they are often connected by narrower horizontal canals with neighbouring ascending canals of the same growth zone ( Figure 36.4 ).

Where the ascending canals reach the crown table they turn horizontally, parallel with the crown surface, and situated underneath the grooves between the crown ridges ( Figure 36.4 ). These horizontal canals have a circular cross section c. 20 µm diameter; dentine tubules branch off in posterior, lateral, and upward directions ( Figures 36.2- 6 , 37 ). The layout of these tubules, extending into the ridges from both sides, was termed "Scheitelung" by Gross (1973) in his description of Rhadinacanthus balticus .

The horizontal canals in the grooves between the ridges are relatively long, extending the whole length of the ridges. Other horizontal canals, formed by ascending canals turning forwards from the posterior side of the scales, are much shorter and arranged more irregularly in the crown table. Dentine tubules in the ridges often anastomose, with lacunal widenings where they interconnect. The dentine of the ridges is more translucent compared with the dentinal tissues elsewhere in the scales but enameloid is not developed. Crown ridges, grooves, and horizontal canals tend to develop over equivalent structures in older growth zones ( Figures 36.6 , 37.1 ). Denteons were sometimes formed around canals of older growth zones ( Figure 37.1, 37.5 ), although the ascending canals remained open. Short canals run from the neck surface into the neck swellings ( Figure 36.5 ). As the scales grew, new ridges were added along the anterior rim and near the lateral corners.

Growth lines in the crown are continuous with those in the neck and base of the scales ( Figure 37.1, 37.3-4 ), but growth zones in the base can be obscured by secondary growth lines within each zone. Sharpey’s fibres radiate from the base apex. Canals of Williamson are densely developed in some scales ( Figure 36 ), but only sparsely present in others ( Figure 37 ), perhaps reflecting the age of the fish or the different preservation at different localities. Bone cell lacunae are absent.

Remarks. Reconstructions of the whole fish ( Figures 33 , 38 ) show the deeper body profile relative to the other two Scottish species. The two diplacanthids previously referred to as Diplacanthus horridus and D. ellsi from the Frasnian Escuminac Formation, Miguasha, Canada, are more closely related to Rhadinacanthus longispinus than to D. crassisimus and D. tenuistriatus . The Canadian taxa have normal body scales with a few small parallel or converging ridges. In D. ellsi , all spines except the admedian and prepelvic ones have two to four grooves on each side; two pulp cavities of which the lower is the larger; posterior face with a central, grooved crest in which, according to Gagnier (1996), the fin web is inserted. Like D. crassisimus and D. tenuistriatus , the posterior side of the pectoral spine in D. ellsi bears two rows of recurved denticles but contrary to the development in the Scottish Middle Devonian species, these denticles become larger and further apart towards the spine tip. The relatively large scales (0.25–0.43 mm) in D. ellsi , each with about six ridges on the crown, have a high narrow neck and a flat to moderately convex base. In the neck there are two to six large openings. Scales are not overlapping, as the base of the scales is as largeor larger than the crown. In D. horridus the fin spines bear only one lateral groove and shallow striae rather than ridges along the sides. The spines show a similar structure to those of D. ellsi , with pectoral spines also having the recurved denticles along the posterior side. Some spine fragments (GIT 722-46-1, 2, 3) from the Givetian Burtnieki Stage in the Vezramnieki outcrop, Latvia are identified here as being from R. longispinus .

The relatively small flank scales (0.08–0.14 mm wide) have four or five radial or parallel ribs, an antero-posteriorly constricted neck with two large foramina anteriorly. Flank scale crowns have a relatively constant morphology over the body, with scales edging the lateral line being larger than neighbouring scales ( Trinajstic, 2001), as in Rhadinacanthus longispinus . The main variation is in the extent of the crown ridges, which extend the length of the crown on the anterior flank and only about two-thirds the length of the crown on more posterior scales. Flank scales below the lateral line have a deeper neck and base than those above the lateral line, and caudal scales have a shallower neck and base ( Trinajstic, 2001).

The original description of the scales in Rhadinacanthus longispinus Agassiz (1844, p. 43) and the accompanying figures ( Agassiz, 1844 -1845, plate 14.9) are rather poor. The description of the scales given by Gross (1947) is much better, as he lists a number of important morphological and histological features. Morphologically important features are the delicate forking of the pronounced, sharply crested ribs along the rounded, anterior edge of the larger scales, the densely placed but much smaller ribs on the lateral corner of the scales near the lateral line system, and the denticulated posterolateral edges. Histologically import-

PALAEO- ELECTRONICA.ORG ant are the relatively wide ascending and horizontal canals and the arrangement of the more delicate dentine tubules, entering the larger ridges from both sides. These features are identically developed both in scales of articulated specimens of R. longispinus and in the isolated scales from erratic blocks in northern Germany transported from the Eifelian Narova Beds of the Baltic region that Gross (1973) assigned to Rhadinacanthus balticus . We recognize this species as a junior synonym of R. longispinus because of its identical scale morphology and histology.

Valiukevičius (1998) recorded two other species of Rhadinacanthus in the Lower and Middle Devonian of the East Baltic and Byelorussia. Rhadinacanthus primaris Valiukevičius, 1986 (in Valiukevičius and Karatajūtė-Talimaa, 1986), based on isolated scales from upper Emsian to lower Eifelian deposits, is morphologically closely related to scales assigned by Gross and Valiukevičius to R. balticus . Although the radial canals are relatively wide, the ascending and horizontal dentinal canals are less wide when compared to the same canals in R. longispinus . Delicate dentinal tubules forming the "Scheitelung" of Gross (1973), are apparently not developed in R. primaris . Rhadinacanthus multisulcatus Valiukevičius, 1988a encompasses scales from the Givetian and early Frasnian of the Baltic Region and Byelorussia. Plax and Kruchek (2014) also listed R. multisulcatus as present in the Goryn Beds (Givetian) in the Pripyat Trough, Belarus. However, the morphology and histology of these scales appear to fit within the range shown by R. longispinus , indicating that R. multisulcatus is also a junior synonym of R. longispinus .

Figures

FIGURE 2. 1, Site map indicating the most important localities with Middle Devonian diplacanthids on northern Scot- land; 2, biostratigraphic table of diplacanthid occurrences in the Orcadian Basin of northern Scotland.

FIGURE 24. Rhadinacanthus longispinus from Gamrie, Banffshire, general morphology. 1, NMS G.1891.92.338; 2, 3, NHM P.4041 from Gamrie, part and counterpart respectively. Scale bars equal 1 cm.

FIGURE 25. Rhadinacanthus longispinus head morphology. 1, 2, NMS G.1891.92.338 from Gamrie, Banffshire; 1, head region; 2, magnified image of sensory line scales; 3, NMS G.1892.8.15 from Gamrie, Banffshire, head region. 4, 5, NHM OR.43276 from Tynet Burn, Moray: 4, complete specimen, head to left; 5, internal surface of right cheek plate, long anterior circumorbital bone, polygonal tesserae on tectal region. 6, NMS G.1892.8.10 from Gamrie, head region. Scale bars equal 1 cm for 1, 3, 4, 5, 6; 1 mm for 2. Abbreviations: ck.p=cheek plate; co.p=circumorbital plate; e=eye stain; oc.p=occlusal plate; so.sl=supraorbital sensory line. Arrows indicates anterior direction.

FIGURE 26. Rhadinacanthus longispinus spine morphologies. 1, 2, isolated anterior dorsal fin spine of NMS G.2014.33.4 from Marwick, Orkney: 1, whole spine; 2, long insertion zone showing fine parallel ribs; 3, posterior dorsal fin spine of NMS G.2014.33.7 from Marwick, Orkney; 4, anal fin spine of NMS G.1859.33.703 from Thurso, Caithness; 5, pectoral fin spine of NMS G.1968.5.2 from Taldale Quarry, Caithness; 6, denticulated posterior of pectoral fin spine on specimen NMS G.1870.14.143 from Gamrie, Banffshire. Scale bars equal 1 cm. Abbreviations: p.fs=pectoral fin spine; scap=scapula.

FIGURE 27. Rhadinacanthus longispinus dorsal spine histology. 1, 2, transverse section of anterior dorsal fin spine NMS G.2015.11.1 from Broad Taing, Orkney: 1, whole section indicated by white line; 2, magnified area; 3–5, transverse sections of posterior dorsal fin spine NMS G.2014.4.33 from the slates west of East Murkle Bay, Caithness (Mey subgroup): 3, section through insertion area; 4, 5, section through exserted part of spine; 6, transverse section of posterior dorsal spine NMS G.2014.33.2 from North Ronaldsay (Rousay Beds). Scale bars equal 1 mm in 1, 3, 4, 6; 0.1 mm in 2, 5.

FIGURE 28. Rhadinacanthus longispinus paired and anal spines histology. 1–5, sections of spines on articulated fish NMS G.2014.4.32 from Achanarras; 1, transverse section of pectoral fin spine just distal to insertion area; 2, more distal transverse section of pectoral fin spine; 3, 4, longitudinal section of proximal half of anal fin spine; 5, longitudinal section of distal half of pelvic fin spine; 6, 7, NMS G.2014.44.1 from Cromarty, Ross and Cromarty, section through partial articulated specimen; 6, transverse section of pelvic fin spine; 7, transverse section of prepelvic spine. Scale bars equal 1 cm in 3; 0.5 mm in 1–2, 5–7; 0.1 mm in 4.

FIGURE 29. Rhadinacanthus longispinus, from a small bonebed (NMS G.2014.15.1 is a sample) at Clardon Haven, Caithness, spine histology. Drawings of Sollas serial sections of various spines from juvenile fish. Abbreviations: adm.s=admedian spine; ad.fs=anterior dorsal fin spine; a.fs=anal fin spine; pelv.fs=pelvic fin spine; p.fs=pectoral fin spine; prepelv.s=prepelvic spine.

FIGURE 30. Rhadinacanthus longispinus median fin spines from mature fish, transverse section drawings. 1, 2, posterior dorsal spine NMS G.2014.4.33 from the slates west of East Murkle Bay, Caithness: 1, spine showing position of sections; 2, sections 1–8. 3, section of posterior dorsal spine NMS G.2014.33.2 from North Ronaldsay. 4, section of anterior dorsal spine NMS G.2014.11.1 from Broad Taing, Orkney. Scale bars equal 1 cm in 1; 5 mm in 2; 1 mm in 3, 4.

FIGURE 31. Rhadinacanthus longispinus NHM P.4041, from Gamrie, Banffshire, shoulder girdles; 1, 2, pectoral region and head on the part (anterior to right); 3, magnified view of the impression of the admedian spine; 4, 5, pectoral region and head on the counterpart (anterior to left). Scale bars equal 1 cm. Abbreviations: adm.s=admedian spine; ck.p=cheek plate; co.p=circumorbital plate; cor/adm.s=coracoid attached to base of admedian spine; lt.sc=left scapula; lt.p.fs= left pectoral fin spine; oc.p=occlusal plate; p.p=pinnal plate; rt.p.fs=right pectoral fin spine; rt.sc=right scapula; v.p=ventral plate.

FIGURE 32. Rhadinacanthus longispinus. 1-2, NMS G.1966.49.9 from Orkney, pectoral region and head; 3-4, NMS G.1891.92.334 from Orkney, pectoral region and head; 5–7, isolated scapula NMS G.2014.15.2 from Marwick: 5,?medial view; 6,?lateral view, with 7, area with remnants of pinnal plate fused to scapula base. Scale bars equal 1 cm. Abbreviations: adm.s=admedian spine; lt.sc=left scapula; lt.p.fs= left pectoral fin spine; p.p=pinnal plate; rt.p.fs=right pectoral fin spine; rt.sc=right scapula.

FIGURE 33. Reconstructions of Rhadinacanthus longispinus. 1, lateral view. 2, ventral view. Dark grey fill indicates dermal elements, light grey fill indicates endoskeletal elements.

FIGURE 34. Rhadinacanthus longispinus squamation types on different body areas. 1–3, NHM OR.43276 from Tynet Burn, Moray; 1, surrounding scapula shaft; 2, anterior dorsal fin web; 3, mid-flank. 4, NMS G.1892.8.13 from Gamrie, Banffshire, mid-flank; 5, NMS G.2002.59.142pt from Cushnie Burn, Banffshire, mid-flank; 6–10, NHM P.11760 from Gamrie, Banffshire, 6, complete specimen with A, B, C areas magnified in the following images; 7, midflank with lateral line (area A); 8, below lateral line (area A); 9, tail (area B); 10, posterior dorsal fin web (area C). Scale bars equal 1 cm in 1–2, 6, 9–10; 1 mm in 3–5, 7, 8. Abbreviations: fw sc=fin web scales; ll sc=lateral line scales; sc=body scales; scap=scapula shaft, tess=head tesserae.

FIGURE 35. Rhadinacanthus longispinus, scanning electron micrographs of scales. 1–4, scales slide NMS G.1892.8.17.1 from Gamrie, Banffshire: 1, 2, crown view of patch of articulated scales, and closeup of scale; 3, lateral view; 4, anterior view. 5–7, scales slide NMS G.1870.14.143.1 from Gamrie, Banffshire: 5, laterocrown view; 6, lateral view; 7, scales from each side of body, stuck together base to base, in posterior view. 8–12, scales slide NMS G.1892.8.15.1 from Gamrie, Banffshire: 8, crown view; 9, crown view; 10, basal view; 11, lateral view; 12, anterior view. Scale bars equal 0.3 mm. White adhesions on scale bases are pyrite crystals. Arrows indicate anterior.

FIGURE 36. Rhadinacanthus longispinus NMS G.2014.4.32 from Achanarras Quarry, Caithness, scale histology. 1, articulated squamation from midflank area; 2–6, thin sections of midflank scales: 2, oblique vertical section; 3, magnified view of central crown of the same scale; 4, horizontal section of crown; 5, oblique vertical section; 6, oblique vertical section. Scale bars equal 0.1 mm. Abbreviations: ac=ascending canal; cW=canal of Williamson; gz=growth zones; hc=horizontal canal; w=neck swelling. Arrows indicate anterior direction, where relevant.

FIGURE 37. Rhadinacanthus longispinus NMS G.2014.44.1 from Cromarty, Ross and Cromarty, scale histology. 1, vertical transverse section; 2, vertical transverse section; 3, vertical longitudinal section; 4, oblique vertical section; 5, vertical section of double scale; 6, vertical longitudinal section. Scale bars equal 0.1 mm. Arrows indicate anterior direction, where relevant.

FIGURE 38. 'Life' reconstructions of: 1, Diplacanthus crassisimus, 2, Diplacanthus tenuistriatus, and 3, Rhadinacan- thus longispinus.