Cheiracanthus grandispinus McCoy, 1848

Burrow, Carole & Newman, Michael, 2020, A redescription of the three longest-known species of the acanthodian Cheiracanthus from the Middle Devonian of Scotland, Palaeontologia Electronica (a 15) 23 (1), pp. 1-43 : 19-28

Cheiracanthus grandispinus McCoy, 1848

Figure 3 , Figure 6.3-6.6 , Figures 15-23

1848 Cheiracanthus grandispinus (M’Coy) ; M’Coy, p.

300.

1855 Cheiracanthus grandispinus (M’Coy) ; M’Coy, p.

582, pl. 2B, fig. 1.

1860 Cheiracanthus grandispinus ; Egerton, p. 123.

1861 Cheiracanthus grandispinus ; Egerton, p. 73.

1888 Ch. grandispinus, M’Coy ; Traquair, p. 512.

1890 Cheiracanthus grandispinus ; Woodward and

Sherborn, p. 29.

1891 Cheiracanthus grandispinus, M’Coy : Woodward, p. 20.

1976 Cheiracanthus spp. in part; Paton, p. 18.

1979 C. grandispinus M’COY 1848 C; Denison, p. 47.

? 1985 Diplacanthus ? carinatus in part; Valiukevičius,

pl. 13.8. 1997 C. grandispinus ; Young, p. 48. 2005 Cheiracanthus grandispinus ; Newman and

Dean, p. 3, 4. 2005 C. grandispinus ; Newman and Dean, p. 4. Holotype. SM H4423 from Orkney. Material examined. From Marwick, Orkney: NMS G. 2019.9.11. From Cruaday Hill quarry, Orkney: NMS G. 2019.9.14; NMS G. 2019.9.15; NMS G. 2019.9.16. Orkney: NHMUK PV P.1363. From Achanarras : NMS G.1889.110.3; NMS G.1890.91.12; NMS G.1892.95.1; NMS G.1893.107.9; NMS G.1893.107.10; NMS G.1893.107.11; NMS G.1893.145.4; NMS G.1894.163.26; NMS G.1903.130.19; NMS G.1965.50.1; NMS G.1993.32.1; NMS G.2002.26.1481; NMS G. 2019.9.2; NMS G. 2019.9.4; NMS G. 2019.9.5; NMS G. 2019.9.6; NMS G. 2019.9.7; NMS G. 2019.9.9; NMS G. 2019.9.12; NMS G.2019.9.35; NMS G.FR1603; NMS G.FR1810; NMS G.FR1938; USCP F00130 . From Clune : NMS G.1896.24.58. From Lethen Bar : NMS G.1891.92.317; NMS G. 1864.7.12; NMS G.1966.40.24; NMS G.1973.12.105; NMS G.1973.12.106. From Gamrie : NMS G. 2019.9.3; NMS G. 2019.9.4. From Jessie Port , Tarbat Ness Peninsula : NMS G. 2019.9.8. From Cromarty : NMS G. 2019.9.10; NMS G. 2019.9.13. From Edderton : USCP F00115 b. From the Moray Firth area : NMS G.1896.24.57.

Distribution. Sandwick Fish Bed Member to middle Upper Stromness Flagstone Formation (Eifelian), Orkney; Achanarras Fish Bed Member to upper Spital Flagstone Formation (Eifelian), Caithness; Nodule beds stratigraphically equivalent to the Achanarras Fish Bed Member, Moray Firth; Kernavė Regional Substage, eastern Baltic (upper Eifelian).

Diagnosis. Cheiracanthus with Meckel’s cartilage very deep at the posterior, maximum depth c. onequarter length; scapula narrowest midshaft and widening dorsally and ventrally; branchiostegal rays broad, long, and ornamented with longitudinal ridges, with thinner rays dorsally, bending down at their posterior end; endoskeletal gill bars with short lateral projections; scale crown with marked smooth rim anteriorly, fan-shaped arrangement of ridges extending the length of crown, with narrow ridges curving away from a deep central groove posteriorly.

Description. General features. Presumed adult specimens of C. grandispinus are over 200 mm long, with the largest estimated to be up to 350 mm long (e.g., NMS G.FR1938A).

Head and branchial region: The jaw cartilages are similar to those of C. murchisoni , differing mainly in the greater relative depth of the Meckel’s cartilage posteriorly ( Figure 15.1-6 ). The maximum depth of the palatoquadrate is c. twice the maximum depth of the Meckel’s cartilage. The fossa towards the posterior end for insertion of the adductor muscle extends about half the length of the jaw. The preglenoid process is quite rounded, and the articular cotylus is a narrow embayment of the usual form in acanthodians ( Figure 15.3 ). The lateral face of the palatoquadrate has an extrapalatoquadrate ridge ( Figure 15.3-5 ) identical to that in C. murchisoni , as described by Watson (1937) and also has a vacuity in the same position as in C. murchisoni ( Figure 15.4 ). The branchiostegal rays, with c. eight below the jaw articulation and c. 15 above, are quite broad and long, extending from the ventral edge of Meckel’s cartilage to about midway up the posterior edge of the palatoquadrate ( Figure 15.1, 15.2 ). The rays are ornamented with parallel longitudinal ridges ( Figure 15.6 ). Long thin rays above the main branchiostegals are oriented obliquely, angled back and down with a downward turn at their posterior end ( Figure 15.4 ). These are much more fragile than the branchiostegal rays and are rarely preserved. Numerous short, thin gular rays are present below the jaw line ( Figure 15.1, 15.2, 15.4- 6 ).

Notable histological features revealed by the thin sections through the branchial region are stacks of short mineralised endoskeletal elements that we identify as gill bars, or branchial rays, which we surmise to have extended out from the (unmineralised) branchial arches. Many of the gill bars in C. grandispinus have projections presumed to act as gill rakers on both sides ( Figure 16.1, 16.2 ). This arrangement, with branchial rays extending out from the arches, is like that in modern chondrichthyans (cf. Gillis et al., 2009, p. 641), rather than in the well-studied acanthodiform Acanthodes , which has separate robust gill rakers attached directly on the branchial arches (see Nelson, 1968, p. 139; Zidek, 1985, p. 156), and apparently lacks branchial rays. No mineralised gill bars were seen in the sacrificed specimen of C. murchisoni . The jaws are composed of blocky and globular calcified cartilage, as in C. murchisoni ( Figure 16.3, 16.4 ).

Pectoral region and spines: The scapulocoracoid is more robust than that of C. murchisoni , with smallest diameter near the middle of the scapular shaft, and widening towards the dorsal end ( Figures 3 , 17.1-3 ). The coracoid height is equal to or greater than scapular height. A separate procoracoid was not preserved in any of the specimens examined. The spines show a similar morphology to those of C. murchisoni , but the pectoral fin spines are slightly more robust ( Figures 3 , 17.1, 17.4 ). The dorsal spine is straight, positioned slightly behind the level of the pelvic spines, and has a basal cartilage ( Figure 17.1 ).

Relative heights and widths of the leading edge ridge and body of spine vary depending on the spine position. In paired spines, the leading edge ridge is higher than wide; pectoral spines have an elongate leading edge ridge ( Figures 18.1, 18.8-10 , 19 ), and pelvic spines have a wide leading edge ridge ( Figures 18.4, 18.5 , 19 ). In median fin spines, this ridge is wider than high ( Figure 18.6, 18.7 ). All spines, like those of other Cheiracanthus species, have a thin enameloid outer layer preserved on the leading edge ridge and the ‘shoulders’ ( Figure 18.7-9 ). Spines have a wide central cavity and lack an accessory pulp canal. Osteodentine forms most of the spine, with dentine tubules mostly only visible branching out towards the surface of the spine from the outermost longitudinal canals ( Figure 18.2, 18.9 ). Longitudinal and interconnecting canals extend the length and breadth of the spine, with smaller canals leading off to open out in the sulcus between the leading edge ridge and the ‘shoulders’ of the spines ( Figure 18.5 ), as well as into the central cavity ( Figure 18.4, 18.6 ). Canals radiate out from the longitudinal canals in the leading edge ridge ( Figure 18.2-5 ). Denteons are formed around the canals in the exserted part, but not in the inserted part. The inner lamellar layer is present proximal to the closure of the central cavity ( Figure 18.6, 18.7 ), and increases in thickness distally ( Figure 18.1, 18.8, 18.10 ). Branching dentine tubules extend through each of the lamellar layers, perpendicular to the inner surface ( Figure 18.1, 18.3 ). In the Achanarras preservation type ( Figure 18.1-5 ), a clear boundary is visible between the denser tissue of the posterior face of the spine and the osteodentine and inner lamellar layers of the main body of the spine. Remnants of a separate thin layer of tissue lining part of the central cavity are preserved towards the proximal end in some of the spines ( Figure 18.4, 18.6, 18.7 ).

Body scales: Scales of larger fish (200 mm or more long) are easily distinguishable from scales of other species of Cheiracanthus , with a fan-shaped arrangement of crown ridges anteriorly ( Figure 20.1-6 ), and with the ridges curving away from a deep central groove posteriorly in the midline. Even in the fin web scales ( Figure 20.7-9 ), the same fan-shaped pattern is visible. The crown is up to 1.5 mm long on flank scales ( Figure 20.6 ) and usually longer than wide. The crown ridges are narrow but vary slightly in width, apparently randomly, and extend the whole length of the crown ( Figure 20.10, 20.11 ). Particularly on worn scale crowns, the ornament more resembles grooves cut into the crown, rather than ridges developed above the crown plane ( Figure 20.13- 17 ). Broad low longitudinal swellings line the median groove. Extra ridges are developed laterally in the posterior half of the crown, usually at a slightly lower level than the central crown ( Figure 20.10, 20.11, 20.14, 20.15 ). A broad smooth rim is present in front of the ridges along the anterior edge. In scales from smaller, presumed juvenile fish, and in the older growth zones, crown ridges are more radially arranged, extra lateral ridges are lacking, and the postero-lateral edges are sometimes serrated. This morphology is visible on primordial scale crowns exposed through flaking off of overlying growth zones ( Figure 20.5 ). The surface of the crown in all scales is convex both longitudinally and side to side. The neck and base are developed similarly to those in C. murchisoni , with a neck that is as deep or deeper than the base, and a pair of protuberances posteriorly ( Figure 20.12, 20.13, 20.16-18 ).

In many aspects, the histological structure of the scales is similar to that of C. murchisoni . The embryonic zone (primordial scale) has large lacunae interconnected by short narrow canals, with other short canals extending upwards and down towards the base cone ( Figure 21.1-4 ). The crown surface of the primordial scale is concave in longitudinal and transverse sections. In younger zones the surfaces become more and more convex both longitudinally and transversely ( Figure 21.1-9 ). The deep median longitudinal groove extends through all the zones in the posterior half of the scale ( Figure 21.4 ).

As in C. murchisoni , eight regularly spaced, radial canals extend through the lower crown ( Figure 21.9, 21.10 ), with anastomizing networks of ascending canals running up in each crown growth zone ( Figure 21.4-8 ). The junction between the posterior radial and ascending canals shows a wide lacuna in the younger growth zones. Anteriorly, the ascending canals are interconnected in a ring canal consisting of short, arched tubules before turning back horizontally ( Figure 21.11, 21.12 ). In young individuals and juvenile growth stages, each groove between the more lateral ridges is usually underlain by only one horizontal canal, and the ridges appear gently rounded in cross section. With growth of the scales the younger ridges show a more flat-topped shape, and the grooves are underlain by two to four canals which sometimes intertwine ( Figure 21.11, 21.12 ). More posteriorly, regularly placed dentinal tubules run off the canals into the ridges, sometimes forming a network between adjacent ridges. Enameloid fills most of the upper plane in each crown growth zone, with longitudinal dentine tubules near the base of each zone ( Figure 21.1-9 ). Sharpey’s fibre bundles in the base are wider than those of C. murchisoni , so the isopedin-like layering of the sheets of fibres ( Figure 21.1, 21.7 ) appears coarser than in that species. Otherwise, the bases resemble those of C. murchisoni .

The number of crown growth zones increases as the size of the fish increases ( Figure 22.1-4 ). Up to seven crown growth zones were noted in scales of the smallest fish, which was about 100 mm long ( Figure 22.1 ), and up to 15 in the largest fish sampled, which were over 300 mm long ( Figure 22.4, 22.5 ).

Reconstruction: Our observations of structures in the head to pectoral region of Cheiracanthus grandispinus are shown in Figure 23 .

Figures

FIGURE 3. Cheiracanthus grandispinus holotype SM H4423 from Orkney. 1, mirrored illustration of specimen and scales from M’Coy (1855, plate 2.B, figure 1); 2, specimen in present condition. Scale bar equals 20 mm.

FIGURE 6. Specimens sacrificed for thin sectioning; numbered slices are vertical unless indicated. 1, 2, Cheiracanthus murchisoni NMS G.2019.3.6 from Gamrie; slices 36-38 horizontal. 3, 4, Cheiracanthus grandispinus, NMS G.2019.9.7 from Achanarras. 5, 6, Cheiracanthus latus NMS G.2019.3.3 from Jessie Port; slices 18, 19 horizontal. 7, 8. Cheiracanthus latus NMS G.2019.3.7 from Gamrie. Scale bars equal 20 mm. a.fs, anal fin spine; d.fs, dorsal fin spine; gr, gular rays; Mc, Meckel’s cartilage; pect.fs, pectoral fin spine; pelv.fs, pelvic fin spines; sc, scapulocoracoid; SEM, area where scales were removed for scanning electron microscopy; small numbers indicate suffix on NMS G specimen number for each thin section.

FIGURE 15. Cheiracanthus grandispinus head and branchial region, morphology. 1-2, NMS G.1966.40.24 from Lethen Bar. 3, NMS G. FR1603 from Achanarras Quarry, Caithness. 4, NMS G.2002.26.1481 from Achanarras Quarry. 5, NMS G.1903.130.19 from Achanarras Quarry, dorsoventrally flattened. 6, NHMUK PVP.1363 from Orkney. Scale bars equal 10 mm. addf, adductor muscle fossa; artc, articular cotylus; br, branchiostegal rays; cr.epq, extrapalatoquadrate ridge; gr, gular rays; Mc, Meckel’s cartilage; pdbr, posterodorsal branchiostegal rays; pq, palatoquadrate; pqv, palatoquadrate vacuity; pregl, preglenoid process. Anterior to right in 1-3, 6, to left in 4-5.

FIGURE 16. Cheiracanthus grandispinus head and branchial region, histology. 1-3, NMS G.2019.9.8: 1, 2, NMS G.2019.9.8.8 series of endoskeletal branchial rays, some showing gill raker projections; 3, NMS G.2019.9.8.14 calcified cartilage blocks; 4, NMS G.2019.9.35.1, calcified cartilage of Meckel’s cartilage? Scale bars equal 0.1 mm. gb, gill bars; gbp, gill bar projections; gcc, globular calcified cartilage; s, scale.

FIGURE 17. Cheiracanthus grandispinus pectoral region, spines morphology. 1, USCP F00130a from Achanarras, whole fish. 2, NMS G.FR1810 from Achanarras, scapulocoracoids and pectoral fin spines. 3, NMS G.2019.9.12, isolated scapulocoracoid from Achanarras. 4, disarticulated incomplete fish NMS G.2019.9.10.1 from Cromarty,?two pectoral, one median (?dorsal), one pelvic spine. Scale bars equal 50 mm in 1, 10 mm in 3, 4, 2 mm in 2. cor, coracoid; fs, indeterminate fin spine, probably a pelvic; median fs, dorsal or anal fin spine; pect.fs, pectoral fin spine; pect. fw, pectoral fin web; sc.s, scapular shaft. Anterior to right.

FIGURE 18. Cheiracanthus grandispinus spines histology. 1-5, NMS G.2019.9.7 from Achanarras: 1-3, NMS G.2019.9.7.21, thin section of distal end of pectoral spine, with closeups of the structure of the leading edge ridge and boundary between the main spine osteodentine and the areas bounding the trailing edge groove; 4, NMS G.2019.9.7.11, TS pelvic spine; 5, NMS G.2019.9.7.12, TS through both pelvic fin spines. 6, 7, NMS G.2019.9.8 from Jessie Port, median spine,?anal: 6, TS NMS G.2019.9.8.8 at proximal end of exserted part; 7, NMS G.2019.9.8.15, towards mid spine. 8-10, NMS G.2019.9.35.1-7 from Achanarras, pectoral spine: 8, 9, NMS G.2019.9.35.2, midspine; 10, NMS G.2019.9.35.3, more distal. Scale bars equal 0.5 mm in 1, 4-8, 10, 0.1 mm in 2, 3, 9. bc, basal cartilage.

FIGURE 19. Cheiracanthus grandispinus NMS G.2019.9.7, serial sections of pectoral (22-15) and pelvic (9-14, 23) spines, thin section drawings, numbered as on map in Figure 6.4.

FIGURE 20. Cheiracanthus grandispinus scale morphology. 1-3, 5-9, light microscope images; 4, 10-18, SEM images; midbody scales unless detailed otherwise. 1, NMS G.1891.92.317 from Lethen Bar. 2, NMS G.2019.9.8 from Jessie Port, Ross and Cromarty. 3, 4, USCP F00115b from Edderton. 5, NMS G.2019.9.9 from Achanarras, primordial scale revealed after abrasion/flaking of overlying crown growth zones. 6-9, NMS G.2019.9.10.1 from Cromarty: 6, mid-body scales; 7-9, pectoral fin web: spine and web; 8, distal scales; 9, scales near fin base. 10-12, NMS G.2019.9.11 from Marwick: 10, view of crown before removal of scale from matrix; 11, anterocrown view; 12, posterior view. 13, NMS G.2019.9.3.27.1 from Gamrie Den of Findon, laterocrown view. 14-18, NMS G.2019.9.13.8 from Cromarty, Eathie to Navity: 14, NMS G.2019.9.13.8.16, anterocrown view; 15, NMS G.2019.9.13.8.2 pair of scales in posterocrown view; 16, NMS G.2019.9.13.8.8, laterocrown view; 17, NMS G.2019.9.13.8.6, lateral view; 18, NMS G.2019.9.13.8.17, posterobasal view. Scale bars equal 0.5 mm in 1-9, 0.2 mm in 10-18. p, neck protuberance; ps, primordial scale. Arrows indicate anterior.

FIGURE 21. Cheiracanthus grandispinus scale histology. 1, NMS G.2019.9.3.9 from Gamrie, vertical longitudinal section through posterior protuberance. 2, 9, 12, NMS G.2019.9.8 from Jessie Port, Ross and Cromarty: 2, NMS G.2019.9.8.14, anterior crown vertical transverse section; 9, NMS G.2019.9.8.10, crown vertical transverse section; 12, NMS G.2019.9.8.13, crown horizontal section. 3, NMS G.2019.9.35.6, from Achanarras, midscale vertical transverse section. 4, 5, 10, 11, from Achanarras: 4, NMS G.2019.9.2.4, posterior half of scale, vertical transverse section; 5, NMS G.2019.9.2.2, vertical longitudinal section; 10, NMS G.1893.107.9.2, horizontal section through crown base; 11, NMS G.1893.107.9.2, horizontal section through midcrown. 6-8, NMS G.2019.9.10 from Cromarty: 6, NMS G.2019.9.10.3, off-centre vertical longitudinal section; 7, NMS G.2019.9.10.5, vertical transverse section; 8, NMS G.2019.9.10.9, oblique vertical section through side of scale. Scale bars equal 0.1 mm. Arrows indicate anterior.

FIGURE 22. Cheiracanthus grandispinus comparison of scale structure in different sized fish. 1, NMS G.2019.9.5.2, from Achanarras, small fish c. 10 cm long, vertical transverse section through scale anterior. 2, NMS G.2019.9.6.1, from Achanarras, medium sized fish less than 30 cm long, vertical transverse section through scale anterior. 3, NMS G.2019.9.8.12, from Jessie Port, medium sized fish, vertical transverse section through scale anterior. 4, NMS G.2019.9.9.14, from Achanarras, fish 30 cm or longer, vertical transverse section through scale anterior. 5, NMS G.2019.9.9.7, from Achanarras, fish 30 cm or longer, vertical longitudinal section. NMS G.2019.9.35.4, medium sized incomplete fish, vertical section of scale showing Sharpey’s fibre bundles through base. Scale bars equal 0.1 mm.

FIGURE 23. Cheiracanthus grandispinus head and branchial region reconstruction.