Diplacanthus crassisimus ( Duff, 1842 )

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 : 6-17

Diplacanthus crassisimus ( Duff, 1842)

Figures 1.1 , Figure 3 , Figure 4 , Figure 5 , Figure 6 ,

Figure 7 , Figure 8, Figure 9, Figure 10 , Figure 11 ,

Figure 12

1841 ichthyolite; Miller, pl. 8, fig. 2.

1842 Diplacanthus crassisimus Duff, 1842 ; Duff, p. 71, pl. 10, fig. 2; pl. 11, fig. 3.

1844 Diplacanthus striatus Agassiz ; Agassiz, p. 34, 41, pl. 14, figs. 1, 2.

1844 Diplacanthus striatulus Agassiz ; Agassiz, p. 34, 42, pl. 8, figs. 3, 4.

1844 Diplacanthus crassispinus Agassiz ; Agassiz, p. 34, 43, pl. 8, figs. 1, 2, pl. 14, figs. 6, 7.

1844 Diplacanthus longispinus Agassiz , in part; Agassiz, pl. 13, fig. 5.

1844 Hybodus gracilis Eichwald ; p. 827.

1845 Homacanthus gracilis (Eichwald) ; Agassiz, p. 153.

1845 Homacanthus arcuatus Agassiz ; Agassiz, p. 113, 114, pl. 33, figs. 1-5.

1846 Hybodus gracilis ; Eichwald, p. 279, pl. 1, figs. 12, 13.

1847 Diplacanthus striatus ; Miller, pl. 8, figs. 2, 4.

1848 Diplacanthus gibbus M'Coy ; p. 301.

1855 Diplacanthus gibbus ; M'Coy, p. 584, pl. 2B, fig. 4.

1859 Diplacanthus striatus ; Timbs, p. 142.

non 1881 D. striatus of Agassiz; Whiteaves, p. 160, 162.

1888 Diplacanthus striatus ; Traquair, p. 512.

non 1889 Homacanthus gracilis , N. sp.; Whiteaves, 1888, p. 96, pl. 10, fig. 4.

1890 Diplacanthus striatus, Ag. ; Traquair, p. 482.

1891 Diplacanthus striatus Agassiz ; Woodward, p. 24, text-fig. 3.

1892 Diplacanthus striatus (Ag.) ; Traquair, p. 37.

1894 Diplacanthus striatus Agassiz ; Traquair, p. 254, 255.

1895 Diplacanthus striatus Agassiz ; Traquair, pl. 2, fig. 1.

1902 Diplacanthus crassisimus Duff ; Hay, p. 274.

1904 Diplacanthus striatus ; Goodchild, p. 595.

non 1905 Diplacanthus striatus ; Lambe, p. 34, 40.

1907 Diplacanthus striatus ; Dean, pp. 217, 218, fig. 19.

1907 Ischnacanthus gracilis in part; Dean, p. 214, fig. 16.

non 1907 Diplacanthus striatus ; Eastman, p. 10, 16.

non 1908 Diplacanthus striatus ; Eastman, p. 280.

1910 Diplacanthus striatus ; Smith, p. 663.

1912 D. striatus Ag. ; Wundsch, p. 190.

non 1913 Diplacanthus striatus ; Clarke, p. 115.

non 1916 Diplacanthus striatus ?; Dean and Eastman, p. 622.

1929 Diplacanthus crassisimus Duff ; Hay, p. 544.

1937 Diplacanthus striatus Agassiz ; Watson, p. 88-95, text-figs. 14-16.

1940 Homacanthus gracilis (EICHWALD) , in part: Gross, p. 21-23, figs. 1B, 2F, pl. 1.3, 1.4.

1940 Diplacanthus striatus ; Gross, p. 23.

1942 Homacanthus gracilis (Eichw.) , in part; Gross, p. 377, 379, 381, table 1.

1947 Diplacanthus striatus ; Gross, p. 126, 127, pl. 6.3.

1954 Diplacanthus striatus Agassiz ; Waterston, p. 11, 12.

non 1966 Diplacanthus crassisimus Duff ; Gardiner, p. 50.

non 1966 " Homacanthus gracilis " Whiteaves ; Gardiner, p. 52.

1966 Diplacanthus striatus Agassiz ; Miles, p. 166, text-fig. 11.

1967 Diplacanthus striatus Agassiz ; Novitskaya and Obruchev, p. 276, figs. 5, 14.

? 1967 Homacanthus gracilis (Eichwald) ; Novitskaya and Obruchev, p. 278, fig. 15, pl. 1.2.

1969 Diplacanthus striatus Ag. ; Heyler, fig. 29.

1971 Diplacanthus striatus ; Moy-Thomas and Miles, fig. 4.13.

1973a Diplacanthus striatus Agassiz ; Miles, pp. 190-194, text-figs. 39-41.

1973 Diplacanthus? carinatus Gross ; in part, Gross, p. 72, 73, pl. 36.9, 36.10.

? 1973 Homacanthus gracilis (Eichwald) ; Gross, p. 86, figs. 13H, 14G- 14M.

? 1975 Homacanthus gracilis sensu Gross ; Lyarskaya, p. 230.

1975 Diplacanthus striatus ; Saxon, p. 15, fig. 3.

1976 Diplacanthus striatus Agassiz 1845 ; Paton, p. 10, 11.

1979 Diplacanthus crassisimus Duff ; Denison, p. 32, figs. 4C, 19, 20, 21A-21C.

1979 Homacanthus gracilis , in part; Denison, p. 52, figs. 32I, 33F.

1985 Diplacanthus? carinatus Gross , in part; Valiukevičius, pl. 1.4, pl. 11.7, 11.8,?pl. 13.5- 13.8.

? 1988a Diplacanthus gravis , in part; Valiukevičius, p. 77, pl. 8.1.

1988b Diplacanthus carinatus , in part; Valiukevičius, p. 603, 604, table 2.

1990 Diplacanthus striatus ; Chaline, fig. 3.9.6.

1991 Diplacanthus striatus Agassiz ; Frickhinger, p. 242.

1994 Diplacanthus carinatus Gross , in part; Valiukevičius, figs. 5-7.

1994 Homacanthus gracilis (Eichwald) ; Valiukevičius, fig. 7.

1995 Diplacanthus striatus ; Young, p. 67, figs. 1, 7.

non 1995 Diplacanthus striatus ; Long, fig. on p. 94.

1996 Diplacanthus striatus ; Gagnier and Wilson, p. 151.

1997 Diplacanthus striatus ; Young, p. 47.

1998 Homacanthus gracilis (Eichwald) ; Valiukevičius, p. 20, 21, 23, 26, fig. 4.

1998 Diplacanthus carinatus Gross, 1973 , in part; Valiukevičius, p. 21, 23, figs. 4, 7, 8, 12, 13, 16, 18, 19, 21.

1999 Diplacanthus crassisimus Duff 1842 ; Trewin and Davidson, table 3, p. 543.

1999 Diplacanthus striatus ; Trewin and Davidson, table 3.

1999 Diplacanthus crassisimus Duff ; Dineley, fig. 6.12A-12D.

1999 Diplacanthus striatus Duff ; Dineley, fig. 6.12H.

2000 Diplacanthus striatus ; Warren, Currie, Burrow, and Turner, p. 239.

2000 Diplacanthus carinatus Gross , in part; Valiukevičius and Kruchek, figs. 1, 4.

2000 Homacanthus gracilis (Eichwald) ; Valiukevičius and Kruchek, fig. 1.

2001 Diplacanthus striatus ; Hanke, fig. 4.4.

2001 Diplacanthus crassisimus ; Hanke, p. 310, 311.

2001 Diplacanthus crassissimus [sic]; Hanke, Davis, and Wilson, p. 750, 751.

2002 Homacanthus gracilis ; Valiukevičius, p. 36

2003 Diplacanthus carinatus Gross, 1973 , in part; Valiukevičius, p. 173, 197, 199, 202, tables 2, 3, fig. 20K, 20L.

2003 Diplacanthus crassisimus Duff, 1842 ; Valiukevičius, p. 173.

2005 Diplacanthus crassisimus (Duff) ; Newman and Dean, p. 3, 4, 5.

2005 Diplacanthus crassisimus Duff ; Davidson and Trewin, fig. 3.

2006 "... Diplacanthus Striatus "; Knell and Taylor, p. 86.

2007 Diplacanthus striatus ; Burrow, p. 831.

2008 Diplacanthus carinatus ; Märss, Kleesment and Niit, table 1.

2010 Diplacanthus crassisimus Duff, 1842 ; Newman, p. 4-9, figs. 5-14

2012 Diplacanthus crassisimus ; Newman, Davidson, den Blaauwen and Burrow, p. 742.

2014 Homacanthus gracilis (Eichwald) ; Ivanov and Märss, p. 158.

2014 Diplacanthus carinatus Gross ; Ivanov and Märss, p. 158.

2014 Diplacanthus carinatus , in part; Plax and Kruchek, p. 32, 36, pl. 3.15.

? 2015 Homacanthus gracilis (Eichw.) ; Plax, p. 22, 31, pl. 4.9, text-fig. 3.

? 2015 Diplacanthus carinatus Gross , in part; Plax, p. 31, text-fig. 3.

Holotype. NMS G.1891.92.333 from Tynet Burn,

Moray, half a limestone nodule with an articulated fish showing the pectoral region and head exposed in ventral view, and the posterior body and tail flattened laterally.

Referred Material. Many hundreds of articulated or partially articulated specimens have been identified in public museums and private collections by the authors. The largest collections are in the NHM

and NMS, all of which were examined and photographed. Specimens used in this study include the syntypes designated by Paton (1976); NMS

G.1859.33.1, NMS G. 1953.4.4 (part) and NMS

G.1859.33.3 (counterpart) from Cromarty. Other specimens used include: from Achanarras Quarry,

Caithness, NHM P.22198; from Edderton, Ross and Cromarty, NMS G. 2014.4.29, NMS

G.2014.15.7, NMS G.2014.44.3 and NMS

G.2014.44.4; from Cruaday Quarry, Orkney, NMS

G.2014.15.21 and NMS G. 2014.4.30; from Sandside Bay , Caithness, NMS G.2014.44.7; from Corbie Den , Banffshire, NMS G.2014.33.9, NMS

G.2014.33.10 and NMS G.2014.44.2; from East

Murkle Bay, Caithness, NMS G.2014.33.11; from west of Castletown Harbour, Caithness, NMS

G.2014.33.8.1-6; from Springpark, Caithness, NMS

G.1878.5.349; from Tynet Burn, Moray, NMS

G.1891.99.10, NHM OR.36582, NMS G.Canon

Kyle no. 2 and NHM P1357 a; from Gamrie, Banffshire, NMS G.1882.60.17, NMS G. 1892.8.5 and

NMS G. 2015.11.2 , NMS G. 2015.11.3 .

Distribution within the Orcadian Basin ( Figure 2 ). The earliest appearance of the species in the Orcadian Basin is an isolated spine and a few scales found on the west side of Sandside Bay, Caithness in the Lybster Flagstone Formation ( Figure 2.2 ). The species becomes much more common in the nodule localities around the Moray Firth, such as, Edderton, Gamrie, Lethen Bar, Tynet Burn, Eathie, and Cromarty. Specimens have also been collected further north in the Cadboll Point area north of Balintore. Many small specimens have been collected from Achanarras Quarry in Caithness. Disarticulated remains have been found in the Mey Formation (e.g., Murkle Bay and Castletown), but are generally quite rare above the Achanarras/Sandwick horizon. Articulated specimens were reported by Hamilton (1987) from Ness of Litter and Lythmore, but no specimens were found by the authors. Both localities have large Mesacanthus and rare Cheiracanthus specimens present and so the record is probably a case of mistaken identity. In Orkney the species is only found in the Sandwick fish bed. A fin spine impression ( Figure 5.5 ) identified as Diplacanthus crassisimus by Mr. D. Leather (but not collected) on Westray (east side Bay of Tafts, coast before Stancro) in beds with Osteolepis panderi is the only find identified above the Sandwick fishbed.

Revised Diagnosis. Diplacanthus with maximum depth to length ratio c. 0.22–0.25; pectoral spine and posterior dorsal spine of equal length; admedian spine three-quarters the length of the pectoral spine; very short prepelvic spine positioned halfway between pelvic and pectoral spines; pelvic spine more curved than other spines, and half the length of the pectoral spine; pectoral spine with paired rows of posterior denticulations on distal half; scale crowns ornamented with transverse, serrated ridges running over a medial longitudinal ridge; orientation of transverse crown ridges changes from concave forward anteriorly to concave backwards posteriorly.

Description. Head and Branchial Region. Watson (1937) gave detailed descriptions of the head structures and sensory lines, so we will just list updated interpretations. The only large structures visible in the head region ( Figures 1.1 , 3.3-4, 4.2-3, 4.6 -9) are the long low occlusal plates of the lower jaw ( Figures 3.4 , 4.2 ), the circumorbital bone extending about one quarter the arc of the orbit ( Figures 3.3 , 4.3 ), the ornamented cheek plate ( Figure 4.6 ), and the smaller postorbital plate ( Figure 4.7 ). Each cheek plate is slightly shorter than the occlusal plate and positioned just anterior to the shoulder girdle complex. As noted by Watson (1937, p. 88, 90, plate 10.1), branchial arches, jaws and other endoskeletal elements are only preserved in rare specimens from Tynet Burn. We have recognized one specimen, NHM OR.36582 ( Figure 4.8-10 ), which shows at least four of the branchial arches displaced but articulated. The branchial region is very short, less than the arc of the longest circumorbital bone. Elements identified as pharyngobranchials, epibranchials, ceratobranchials and possibly a basibranchial are distinguished; the epibranchials and ceratobranchials form a V-shape, and the pharyngobranchials are more or less aligned with the epibranchials ( Figure 4.10 ). Jaw cartilages and the hyoid arch do not appear to have been mineralized in this specimen. Morphology of the Spines. All the spines, except the admedian and prepelvic spines, are laterally compressed, with smooth longitudinal ridges on each side and a leading edge ridge that is only slightly wider than the other ridges ( Figures 3.1-2 , 4.1 , 5.1-4 , 6 , 7.1-4 ). The dorsal, anal, and pectoral spines are long and slender. The pectoral spine is also slender, but is highest relative to length. The dorsal and anal fin spines are deeply inserted. The inserted parts, about one fifth of the total length, lack ornament ridges but show the thin closeset parallel ribbing diagnostic of diplacanthiforms. In cross section, the spines (other than the admedian and prepelvics) are markedly higher than wide ( Figures 5.7 -8, 5.10, 6), whereas the admedian and prepelvic spines are much broader, with their width mostly exceeding the height. Below the groove separating off the leading edge ridge, the sides of the spines are slightly convex, with the lateral ridges aligned parallel to the leading edge ridge and separated by concave-based narrower grooves c. 0.2–0.35 mm wide. In mature fish, the posterior lateral ridges sometimes bear tiny, sharply crested, secondary ribs, which resemble the micro-ornament on the scale crowns. These secondary ribs run parallel with the crest and sides of the lateral ridge. Larger spines have more lateral ridges than smaller spines: the anterior dorsal fin spine has up to nine per side, the smaller posterior dorsal spine has up to six. Anal and pelvic spines are straight and closely resemble each other, having three or four ridges per side; pelvic spines differ from anal spines mainly in the lack of a long insertion. The robust, sickle-shaped pectoral spine has up to 10 lateral ridges per side in mature specimens and is always more or less asymmetrical in cross section. The lower surface is slightly flatter and extends further posteriorly than the more convex, dorsally facing side. The distal half of the spine has paired rows of small, recurved denticles c. 0.2 mm apart on either side of the posterior median axis ( Figure 5.3, 5.7 ). On all fin spines, this posterior face has an open pulp canal proximally, extending the full length of the inserted parts in the dorsal and anal spines ( Figure 6 ). In the exserted parts of these and other spines, the open canal is closed over and continues distally as the main central canal. The relatively large admedian spines, which are about three-quarters the length of the pectoral spines, and the smaller prepelvic spines (about a quarter the length of the pectorals) differ in being broad and flat proximally, with a height/ width ratio from 0.5 to 0.8, thinning to equal width and height distally. On these spines, the parallel, longitudinal ridges all have sharp crests, and are separated from each other by broad, concave grooves ( Figures 6 , 7 ). The lateral ridges, just one to three on each side, bear tiny secondary ribs parallel to the crest of the ridge. The spines have a broad open pulp canal proximally, but the canal only extends to about halfway along the spine. The prepelvic spines, which lack an insertion area, were directly intercalated between normal body scale rows, and point in a posterior, slightly ventral direction. The admedian spines abut normal body scales along their medial sides, but articulate with the posterior pinnal plate laterally. This pinnal plate articulates laterally with the base of the pectoral spine. The admedian spines point in a posterior direction, slightly ventrally.

Histology of the Spines. Transverse sections ( Figures 5.7-10, 6 ) show that spines of Diplacanthus crassisimus have one to five successively smaller accessory pulp canals above the main pulp canal, with randomly distributed, narrow calibre canals interconnecting all pulp canals and small vascular canals penetrating the trabecular dentine forming the sides of the spines. Vascular canals run longitudinally through the trabecular dentine, branching towards the spine tip. In the exserted parts of the spines, each ornament ridge always contains at least one canal running longitudinally below the crest and branching distally ( Figure 5.9 ). Pore canals branch off these canals and open out in the grooves between the ridges. The canals under the ornament extend as open grooves over the inserted parts of the median spines, forming the regularly spaced longitudinal striations. Denteons round the canals are thickest towards the distal, oldest parts of the spines, and less well developed around the pulp canals, which show some infilling by lamellar layers ( Figure 5.8 ), a distinctive character found only in D. crassisimus amongst the Scottish Middle Devonian acanthodians. The admedian and prepelvic spines sometimes have small accessory pulp canals. All spines have an outer layer of orthodentine on the crests of the leading edge and lateral ridges, a thick middle layer of trabecular dentine, and a dense laminar tissue lining the main and accessory pulp canals. In the orthodentine, the tubules are widely spaced and relatively short, between about 10 and 50 μm long, and branching near the surface. The inner layer lacks bone cells and dentine tubules, and is thus difficult to distinguish as either a laminar bone or atubular dentine layer.

Shoulder Girdle Complex. ( Figures 7 , 8, 9) Determining the layout of this structure is best interpreted by examination of isolated complexes ( Figure 7.5-6 ). The endoskeletal plus dermal bone elements almost completely encircled the body ( Figure 7.7 ), reminiscent of the thoracic armour in placoderms. On the scapulocoracoid, the postbranchial flange is only developed close to the base of the cylindrical shaft, and the L- or wide Vshaped procoracoid appears to have abutted the anterior edge of the scapulocoracoid. Lack of the procoracoid on isolated complexes and only rare preservation in situ on articulated specimens (e.g., Figure 7.7 , and Figure 3.5 for displaced procoracoid) indicate that the procoracoid was not fused to the scapulocoracoid. The posterior flange of the scapula reaches almost to the dorsal tip of the shaft, expanding in length ventrally towards the coracoid, which angles inwards ( Figure 7.7 ). No differentiation is visible between the internal bone surface of the scapula and the coracoid, with the latter extending back under the base of the admedian spine. The dermal armour comprises a pinnal plate articulating with or fused to the scapulocoracoid and the admedian spine, and a ventral plate fused to the base of the procoracoid towards its medial end, so that the ventral plates of each side are almost touching. The base of the pectoral fin spine is rigidly constricted or fused into the dermal and endoskeletal structures, clearly shown in serial sections of the shoulder girdle complex (Figures 8.1-2, 9). Our reconstructions ( Figure 10 ) show lateral and ventral views of the complexes.

Morphology of the Scales. ( Figure 11 ) As originally described by Gross (1947), the scales have delicate ribs arranged in semicircles with the open side in an anterior direction, at least in the anterior part of the crown. More posteriorly the delicate ribs become almost straight in a transverse direction and, without interruption, cross over a broad central ridge. The scale base is oval in outline and wider than long ( Figure 11.8-11, 11.13 -15). The base is positioned below the anterior half of the crown ( Figure 11.9 -15), with the relatively thin, posterior half of the crown extending far behind the base. The neck is moderately constricted on all sides and is also oval in cross section, but with the lateral ends of the oval often sharply pointed. Along the anterior and lateral sides, small vertical buttresses rise from the middle of the neck and support the underside of the crown. Relatively large scales near the lateral line have their crown surface nearly parallel with the base. In smaller scales such as those from the tail region, the surface of the crown is inclined strongly antero-posteriorly. The surface of the crown is more or less rhombic in shape with rounded anterior and lateral corners. The posterior corner is more sharply pointed, and the latero-posterior edges are denticulated ( Figure 11.14 ). Midflank in the posterior parts of the body, scale length is about equal to the width. In more anterior parts of the body, width of midflank scales usually exceeds the length. All scales have a broad central ridge, extending the length of the crown surface, gradually diminishing in height and width posteriorly and reaching almost to the posterior corner ( Figure 11.7 -9, 11.13-15). A relatively deep pit is sometimes present in front of the central ridge, almost immediately behind the anterior corner. Surrounding this pit, the crown surface is slightly concave, but in the central and posterior parts the crown surface is almost flat or slightly convex. Thin, sharply crested transverse ridges perpendicular to the broad central ridge cover the whole surface of the crown ( Figure 11.2 -9, 11.12- 15). These tiny ridges, 10–20 μm apart and 15–20 μm high in the anterior half, cross the broad central ridge without interruption; they are arranged in a U- or horseshoe-shape with the concave side facing anteriorly. The ridges become straighter more posteriorly. These ridges are mostly inclined antero-posteriorly, although ridges on crowns with the anterior pit can be inclined anteriorly or towards the pit. The crests of the ridges are serrated, with the sharp denticulations c. 10 μm apart.

Histology of the Scales. ( Figure 12.1-3 ) Scales of Diplacanthus crassisimus have only a few crown growth zones, with a maximum of five or six in the largest scales. Ascending canals rise up to the anterior crown from slit-like openings in the middle of the neck, widening and uniting into a canal c. 10–15 μm in diameter running parallel to the anterior rim ( Figure 12.3 ). Each growth zone has only one system of ascending canals and one canal parallel to the anterior rim and corners, with smaller canals, each c. 5 μm diameter, branching back sharply and slightly downward into the slightly concave anterior part of the crown table. These canals follow the bottom of the grooves between the narrow ridges on the surface ( Figure 12.1-3, 12.8 ). Extremely thin dentine tubules parallel the smaller canals and then turn upwards in the direction of the serrated crest of the thin transverse ridges ( Figure 12.6 ). New crown growth zones completely superpose older zones; they are thin anteriorly and wider laterally and posteriorly, with the lateral and posterior areas extending out beyond the base and neck. In the primordial growth zone, there are no canals running under the grooves between the ridges; these canals develop only with the next growth zone and are then enclosed by dentinal tissue. Only the canal system of the youngest growth zone is still open. The crests of the tiny ribs are slightly more translucent than the rest of the scales, indicating they are formed only of durodentine ( Figure 12.2, 12.7 ).

The base of larger scales shows four or five sets of cross-layered (laminated) bone layers. The low apex of the base cone is posterior to the low point of the scale crown. The primordial growth zone has a hollow, concave base but with successive growth zones the base becomes slightly convex. Growth lines in the base are continuous with growth zones in the neck and crown ( Figure 12.10 ). Sharpey’s fibres in the base are poorly developed or absent, and canals of Williamson are rare.

Remarks. Past workers regarded the long thin plates in the mouth region as mandibular bones supporting the lower jaw cartilages, but these have been reinterpreted as occlusal plates on the lower jaws ( Newman et al., 2012). The cheek plate probably functioned as an operculum for the branchial region, given its position extending between the mouth and the shoulder girdle. The V-shaped layout of elements in the branchial region is comparable with that observed in the Permian acanthodid Acanthodes ( Miles, 1973b) and recently in the Carboniferous shark Ozarcus mapesae Pradel et al., 2014 , deemed by the latter authors to represent the ancestral gnathostome condition ( Pradel et al., 2014, figure 3).

Middle Devonian isolated spines which Agassiz (1844 –45) assigned to Homacanthus gracilis (Eichwald), 1844 (synonym: Homacanthus arcuatus Agassiz, 1845 ) are identical to pectoral spines of Diplacanthus crassisimus . The spines are curved, laterally compressed, and have a prominent ridge along the leading edge, a few smooth longitudinal ribs on the flanks, and a double row of recurved denticles along the distal half of the posterior face of the spines ( Denison, 1979, figure 22I). Other spines from the Baltic with posterior denticles extending the length of the exserted part of the spine, which have previously been assigned to H. gracilis , are pectoral spines of D. tenuistriatus . Gross (1973) described the histology of the double row of recurved denticles in juvenile spines from erratic blocks in the Middle Devonian Baltic region, and his description accords with our observations on the histology of the recurved denticles in the pectoral spine of D. crassisimus (as well as D. tenuistriatus ). Spines assigned to H. gracilis are also recorded from the Narova, Aruküla, and Burtnieki beds (Eifelian-Givetian) in the Baltic ( Lyarskaya, 1975; Denison, 1979; Valiukevičius, 1998, table on p. 7) and Stolin beds (middle Givetian) in Belarus ( Plax, 2015).

Watson (1937, figure 16) and Miles (1973a, text-figure 39) both described and illustrated the arrangement of elements forming the shoulder girdle, but neither of their reconstructions is totally accurate. Interpretation of the structure has been hampered by the ventrodorsal ( Figure 7.1-3 ) or lateral ( Figure 7.4 ) flattening of the components, and is best interpreted by examination of isolated complexes ( Figure 7.5, 7.6 ).

As originally described by Gross (1947), the scales of Diplacanthus crassisimus ( D. striatus ) differ markedly from most other acanthodian scales, particularly in their crown ornament ( Figure 11.2 -9, 11.12-15). Gross (1947, p. 126) mentioned that the crown does not have the typical acanthodian sculpture of radial ribs, but instead possesses delicate ribs (“feine Leisten”), arranged in semicircles with the open side in an anterior direction, at least in the anterior part of the crown. Comparison of the original description and figured specimens of Diplacanthus? carinatus Gross (1973, p.73 , plate 36.8-10) – a species based on scales from erratic blocks in northern Germany transported from the Eifelian Narova Beds of the Baltic region – with the scales of D. crassisimus , indicates that D.? carinatus is also a synonym of D. crassisimus . Valiukevičius (1985, 2003) assigned scales from the Baltic Narva Regional Stage and the Eifelian? Vstrechnaya Formation, Severnaya Zemlya to D. carinatus . Plax and Kruchek (2014) and Plax (2015) assigned scales from the Givetian Goryn, Stolin and Moroch beds in Belarus to D. carinatus . Based on morphological and histological similarities and contemporaneous occurrences, Homacanthus gracilis , D. FIGURE 8. Diplacanthus crassisimus shoulder girdle thin section images. 1, 2, NMS G.2014.33.8 from west of Castletown Harbour, Caithness: 1, section through pectoral spine, pinnal plate,?procoracoid, and scapulocoracoid; 2, transverse section through base of pectoral fin spine enclosed by pinnal plate and remnants of the endoskeletal coracoid plate; 3–5, section through admedian spine NMS G. 2015.11.2 from Gamrie, Banffshire: 3, proximal end of admedian spine, inner bone base layer is the coracoid; 4, line drawing of reconstructed section; 5, scale-like ornament towards edge of admedian spine (area box in 4). Scale bars equal 1 mm in 1, 2, 4; 0.1 mm in 5.

carinatus (in part), and D.? carinatus (in part) are here all considered junior synonyms of D. crassisimus .

Diplacanthus kleesmentae Valiukevičius, 1986 (in Valiukevičius and Karatajūtė-Talimaa, 1986), based on rather poorly preserved scales from the late Emsian to early Eifelian of the Baltic region, is also similar to D. crassisimus , with numerous delicate ribs arranged in horseshoe-shape around a deep pit near the anterior edge of the scale. However, D. kleesmentae scales have only a low broad, central ridge, and the delicate transverse ridges are not present in the lateral and posterior areas of the crown; also, the crown is formed of up to 10 growth zones. Another scalebased taxon Diplacanthus gravis Valiukevičius, 1988a from the Eifelian-Givetian Aruküla Regional Stage in the Baltic differs from the Scottish species in having a convex base that is almost pointed at its deepest, and the crown normally has two paired, branching ridges medially running the length of the crown. Diplacanthus poltnigi Valiukevičius, 2003 from the Lochkovian of Severnaya Zemlya and the Canadian Arctic differs in having subparallel grooves extending the length of the crown, or converging near the posterior corner. This species should probably be reassigned to Uraniacanthus based on the crown ornament. Diplacanthus horridus Woodward, 1892 and D. ellsi Gagnier, 1996 from the Frasnian Miguasha Formation, Quebec, differ in having scales with subparallel ridges, comparable to the ornament on scales of Rhadinacanthus longispinus .

Figures

FIGURE 1. Type specimens of Scottish Middle Devonian diplacanthids: 1, Diplacanthus crassisimus (Duff, 1842) holotype NMS G.1891.92.333, from Tynet Burn, Moray; 2, Rhadinacanthus longispinus (Agassiz, 1844) holotype NMS G.1953.4.3, from Cromarty, Ross and Cromarty; 3, Homacanthus borealis holotype spine NMS G.1892.91.1, from Lybster, Caithness; 4, Diplacanthus tenuistriatus (Traquair, 1894) syntype NMS G.1891.92.340, from Gamrie, Banffshire. Scale bars equal 1 cm.

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 3. Diplacanthus crassisimuss, NMS G.2014.4.29, from Edderton, Ross and Cromarty, showing surface morphology of spines: 1, complete specimen; 2, line drawing identifying various elements; 3, circumorbital bone; 4, occlusal bones; 5, procoracoid with ventral plate attached. Scale bars equal: 1 cm for 1, 2; 1 mm for 3–5. Abbreviations: a.f=anal fin web; a.fs=anal fin spine; ad.f=anterior dorsal finweb; ad.fs=anterior dorsal fin spine; adm.s=admedian spine; co.p=circumorbital plate; lt.sc=left scapula; lt.pelv.fs=left pelvic fin spine; oc.p=occlusal plate; pd.f=posterior dorsal finweb; pd.fs-=posterior dorsal fin spine; pelv.f=pelvic finweb; prepelv.s=prepelvic spine; proc=procoracoid; rt.p.fs=right pectoral fin spine; rt.pelv.fs=right pelvic fin spine; rt.sc=right scapula; v.p=ventral plate. Arrow indicates anterior.

FIGURE 4. Diplacanthus crassisimus, articulated specimens: 1–3, NMS G.2014.44.3 from Edderton, Ross and Cromarty: 1, complete specimen; 2, right occlusal plate; 3, left circumorbital and postorbital plates; 4,NMS G.2014.15.21, a ventrodorsally flattened specimen from Cruaday Quarry, Orkney, showing prepelvic spines; 5–7, NMS G.1891.99.10 from Tynet Burn, Moray: 5, head region; 6, impression of a displaced cheek, rotated 90° counterclockwise; 7, postorbital plate; 8–10, NHM OR.36582 from Tynet Burn, Moray: 8, complete specimen with head region ventrodorsally flattened, with articulated, displaced branchial region; 9, branchial region; 10, sketch of branchial region elements, rotated 90° clockwise. Scale bars equal: 1 cm for 1, 5, 8 and 5 mm for 2–4, 6, 7, 9. Abbreviations: ad.fs=anterior dorsal fin spine; bb=basibranchial; cb=ceratobranchials; ck.p=cheek plate; co.p=circumorbital plate; eb=epibranchials; oc=occlusal plate; pb=pharyngobranchials; prepelv.s=prepelvic spine; proc=procoracoid; scap=scapulocoracoid.

FIGURE 5. Diplacanthus crassisimus spine morphology and histology: 1, 2, NMS G.2014.4.29 from Blackpark, Ross and Cromarty; 1, anterior dorsal fin spine; 2, posterior dorsal fin spine; 3, pectoral spine of NMS G.1878.5.349 from Springpark, Caithness; 4, pectoral spine NMS G.2014.44.7 from west side of Sandside Bay, Caithness; 5, pectoral spine impression, Snaky Noust, Westray, Orkney (HY501.405); 6, longitudinal section of pelvic spine NMS G.2014.33.10 from Corbie Den, Banffshire; 7, transverse section of anterior dorsal fin spine NMS G.2014.33.9 from Corbie Den, Banffshire; 8, 9, transverse section of anterior dorsal fin spine NMS G.2014.33.11 from the south of East Murkle Bay, Caithness, with magnified image of same specimen; 10, transverse section NMS G.2014.44.2 of pectoral fin spine and bone of scapulocoracoid from Corbie Den, Banffshire. Scale bars equal 1 cm for 1–6 (1, 2 at same scale); 1 mm for 8, 10; 0.1 mm for 7, 9.

FIGURE 6. Drawings of serial transverse sections of the spines of Diplacanthus crassisimus. 1, sketch of NMS G.2014.15.7, from Edderton, Ross and Cromarty, showing the position of sections in 2–6; the sections run from the distal end to the base of each spine. 2, anterior dorsal fin spine (ad.fs); 3, anal fin spine (a.fs); 4, right pelvic fin spine (pelv.fs); 5, left pelvic fin spine; 6, left pectoral fin spine (p.fs). 7, Sollas sections of a prepelvic spine (prepelv.s) on NMS G.2014.44.4. Scale bars equal 1 mm.

FIGURE 7. Diplacanthus crassisimus shoulder girdles. 1, NMS G.Canon Kyle no. 2 from Tynet Burn, Moray, showing ventral view of the shoulder girdle complexes; 2, NHM P.1357a from Tynet Burn, Moray, pectoral girdle region; 3, NHM P.22198 from Achanarras Quarry, Caithness, complete specimen; 4, NMS G.2014.44.3 from the Edderton, Ross and Cromarty; 5, 6, left shoulder girdle complex of NMS G.2014.4.30 from Cruaday Quarry, Orkney; 7, NMS G.2014.44.4 from the Edderton, Ross and Cromarty, natural transverse section through articulated uncompressed shoulder girdle region (prepelvic spines subsequently Sollas sectioned). Scale bars equal 1 cm.

FIGURE 10. Reconstruction of the pectoral girdle of Diplacanthus crassisimus. 1, lateral view. 2, ventral view showing Miles' (1973a) interpretation on the left hand side of the fish and the authors’ revised version on the right hand side of the fish. Dark grey fill indicates dermal elements, light grey fill indicates endoskeletal elements.

FIGURE 11. Diplacanthus crassisimus from Gamrie, Banffshire, squamation. 1-5, NMS G.1882.60.17, articulated specimen showing scale morphologies: 1, the complete specimen, with A–C denoting areas of squamation shown in Figure parts 2–5: 2, 3, area A; 4, area B; 5, area C. 6, NMS G.1892.8.5, mid-body squamation. 7–11, scales slide NMS G.1882.60.17.1, Scanning Electron Micrographs (SEMs): 7, 8, crown and anterior views; 9, lateral view of scale pair; 10, basal view; 11, undersurface of crown (neck and base broken off). 12–15, scales slide NMS G.2015.11.3.1, SEMs: 12, 13, crown and posterior views; 14, 15, laterocrown and lateral views. Scale bars equal 1 cm for 1; 1 mm for 2–5; 0.2 mm for 6–11, 14, 15; 0.1 mm for 12, 13. Arrows directed anteriorly.

FIGURE 12. Diplacanthus crassisimus scale histology. 1–7, NMS G. 2014.33.10 from Corbie Den, Banffshire, thin sections of a patch of midflank scales: 1, group of scales sectioned in various orientations; 2, vertical longitudinal section; 3, crown horizontal section; 4, horizontal section through the base; 5, 6, vertical longitudinal sections through articulated scales; 7, crown horizontal section. 8, 9, NMS G.2015.11.3 from Gamrie, Banffshire, isolated midflank scales: 8, crown horizontal section; 9, vertical section through the base and lower neck, imaged with cross nicols. 10, 11, NMS G.2015.11.2 from Gamrie, Banffshire: 10, vertical longitudinal section; 11, vertical longitudinal section. Scale bars equal 0.1 mm. Abbreviations: cgz=crown growth zones; dt=dentine tubule; p=canal opening; pc=pore canal; spc=spiral canal. Arrows indicate anterior direction, where relevant.