Styracopterus fulcratus, (TRAQUAIR, 1881)

Sallan, Lauren Cole & Coates, Michael I., 2013, Styracopterid (Actinopterygii) ontogeny and the multiple origins of post-Hangenberg deep-bodied fishes, Zoological Journal of the Linnean Society 169 (1), pp. 156-199 : 159-172

STYRACOPTERUS FULCRATUS ( TRAQUAIR, 1881)

Holurus fulcratus Traquair, 1881

Styracopterus fulcratus Moy-Thomas, 1937

Styracopterus fulcratus Moy-Thomas and Bradley Dyne, 1938

Styracopterus fulcratus Gardiner, 1985

Holotype: GSE 5673 (M146e) and counterpart GSE 5672 (M147e), British Geological Survey , Edinburgh, Scotland, UK, incomplete articulated fish in part and counterpart showing the dorsal portion of the trunk, scales, dorsal fin, and shoulder series (estimated body length, EBL: 12 cm).

Additional material: NHM P1663, incomplete articulated fish in part with trunk, skull, paired and median fins (EBL: 8 cm); GSE 8731, incomplete articulated fish in part showing anterior two-thirds of animal, including skull (EBL: 8 cm); GSE 2136, incomplete articulated fish in part showing anterior third of animal, including skull (EBL: 16 cm); GSE 5663 (part) and GSE 5664 (counterpart), nearly complete articulated fish (EBL: 16 cm); NMS 1891.53.49, National Museums of Scotland, Edinburgh, Scotland, UK, incomplete articulated fish in part showing anterior half of animal (EBL: 11+ cm); NMS 1891.53.50 (part) and NMS 1891.53.51 (counterpart), incomplete articulated fish showing anterior half of animal (EBL: 11+ cm), GM 1902.85.md (part), Kelvingrove Art Gallery and Museums, Glasgow Museums, Glasgow, Scotland, UK, incomplete articulated fish showing anterior half of trunk and dermal jaws (EBL: 9 cm).

Type locality and horizon: Tarras Waterfoot ( Traquair, 1881), River Esk, Eskdale, Dumfriesshire, Scotland, UK. Calciferous Sandstone Series, Cementstone Group, Holkerian regional substage (339–337.5 Mya), early Visean Stage, Mississippian Subsystem, Early Carboniferous.

Other localities and horizons: Glencartholm Volcanic Beds, Dumfriesshire, Scotland, UK. Calciferous Sandstone Series, Cementstone Group, Holkerian regional substage (339–337.5 Mya), early Visean Stage, Mississippian Subsystem, Early Carboniferous ( Gardiner, 1985; Dineley & Metcalfe, 1999).

Diagnosis (emended from Gardiner, 1985): As for genus.

Remarks: Moy-Thomas (1937) placed species within the genus Fouldenia into the synonymy of the older taxon Styracopterus in his redescription of the latter genus. Gardiner (1985) subsequently lumped all designated Styracopterus species into Styracopterus fulcratus . However, a re-examination of specimens has shown that the characters used by Moy-Thomas (1937) and Gardiner (1985) are diagnostic of many genera within a more inclusive clade: the Eurynotiformes, as diagnosed above. There are in fact two distinct taxa, one being a Visean S. fulcratus and the other representing Tournaisian fish (see diagnoses and discussion). Thus, the original genus Fouldenia is resurrected, encompassing all specimens assigned to this taxon by White (1927) and subsequent workers, as well as individuals previously attributed to S. fulcratus originating from the sediments at the Foulden Fish Bed ( Gardiner, 1985) and other Tournaisian-age localities around Northern England and Scotland ( Traquair, 1881, 1890; Moy-Thomas, 1937; Gardiner, 1985). The emended diagnosis and description of Styracopterus are thus based only on the remaining catalogued specimens of S. fulcratus , all from from the Visean of Scotland.

Among all the ray-finned fishes known from the Visean of Scotland, only Styracopterus and Rhadinichthys laevis Traquair, 1890 are said to reside at Tarras Waterfoot ( Moy-Thomas, 1937), a locality not known to contain any other vertebrate or invertebrate remains ( Lumsden et al., 1967). The type material of Tarrasius , the namesake fish and the only other taxon ever attributed to those deposits, was actually taken from misidentified Glencartholm sediments ( Moy-Thomas, 1933, 1934). Lumsden et al. (1967: 116) suggested that the same was true for the remaining taxa; they did not locate any evidence of a shared fauna or previous fossiliferous locality at Tarras Waterfoot. This assertion was subsequently supported by Dineley & Metcalfe (1999) in a survey of vertebrate material. This distinction might be important: recent work has suggested Tarras Waterfoot sediments may be Tournaisian in age (S.P. Wood, pers. comm.). It is notable that the type specimen of Styracopterus does not differ from Glencartholm material in either matrix or preservational mode, whereas fishes from proximate Tournaisian localities (e.g. Fouldenia ), which have different faunal compositions from Glencartholm, are distinct in taphonomy (L.C.S. pers. observ.; Appendix S1).

DESCRIPTION

Skull

The general structure of the skull in Styracopterus is largely as depicted by Moy-Thomas (1937) and Gardiner (1985). However, there are inaccuracies in those previous reconstructions, caused in part by the combination of differently sized individuals and the inclusion of Fouldenia among the source material. The snout of Styracopterus is capped by a subrectangular rostral with a rounded ventral margin above the level of the infraorbitals, contrary to the reconstruction by Gardiner (1985). The division between the rostral and the premaxilla is indistinguishable in the largest individuals, such as GSE 2136 ( Fig. 1F ), where it is obscured by a field of thick ganoine. This is punctured by large round openings that probably mark the sensory canal, as also found in Amphicentrum crassum ( Traquair, 1890) ( Bradley Dyne, 1939; Coates, 1988), Cheirodopsis (NMS 1885.54.34; L.C.S. pers. observ.), Eurynotus (NMS 1876.28.2, NMS 1957.1.5686; L.C.S. pers. observ.), and Paramesolepis (NMS 1891.53.25; L.C.S. pers. observ.; Appendix S1). The dorsal limit of the rostral in NMS 1891.53.50–51 and GSE 5663 ( Fig. 10D, G ) is marked by a patch of U-shaped ornament raised well above the bone and contacting both frontals.

The ventral portion of the nasal in Styracopterus expands to a blunt margin in contact with the lachrymal and premaxilla. The dorsal half of the nasal curves almost horizontally to contact the frontal and dermosphenotic. Contrary to the reconstruction of Gardiner (1985), there are no obvious lateral indentations for the nares. Ornamentation varies with size: there is little apparent ganoine in GSE 8731 ( Fig. 1A ), whereas a vertical stripe is found in NMS 1891.53.49 ( Fig. 1B ). This is joined posteriorly by additional patches of smooth enameloid in GSE 2136 ( Fig. 1F ). Similar ornamentation is found on the nasals in Eurynotus (NMS 1876.28.2; L.C.S. pers. observ.) and Benedenius (L.C.S. pers. observ.; Appendix S1).

The skull roof in Styracopterus is similar in composition to previous reconstructions by Moy-Thomas (1937) and Gardiner (1985), but the shapes of the bones differ. The frontals are longer than the parietals, extending from the level of the preoperculum to the midpoint of the lachrymal in the 8-cm fish GSE 8731 ( Figs 1A , 4A ), and to the level of its anterior margin in larger specimens such as GSE 2136 ( Figs 1G , 4C ). The frontals narrow anteriorly to form a V-shaped contact with the snout series ( Fig. 1 ). The widest portion of the frontal is marked by a rounded point in front of the dermosphenotic in GSE 5663 ( Figs 1G , 4 ). A posterolateral process that grows larger with size forms a convex (GSE 8731; Fig. 1G ) or V-shaped (NMS 1891.53.49, NMS 1891.53.50–51, GSE 5663, GSE 2136; Fig. 1 ) contact with the parietals. This is contrary to the simple linear suture illustrated by Moy-Thomas (1937) and the convex suture reconstructed by Gardiner (1985), but is similar to Benedenius ( Traquair, 1878; Fraipont, 1890; L.C.S. pers. observ.; Appendix S1).

As originally described by Moy-Thomas (1937), the ornamentation of the frontal in Styracopterus varies by size. The frontal in the small fish GSE 8731 ( Fig. 1A ) is covered by intercalating ganoine ridges, which largely mirror the lateral margins. Similar ornament is found in the midsized NMS 1891.53.50 ( Fig. 1D ), but the striations are broken and joined by small dots medial to the impression of the supraorbital canal. In the equivalently sized NMS 1891.53.49 ( Fig. 10B ), more posterior ornament is covered by a large stripe of smooth ganoine. In the largest specimen of Styracopterus, GSE 5663 ( Fig. 1G ), this is joined by two bands running along the lateral margins, which are similar to the frontal ornamentation in Benedenius ( Fraipont, 1890; L.C.S. pers. observ.; Appendix S1). Yet, in the equivalently sized individual GSE 2136 ( Fig. 4F ), finer ornament is exposed in the same areas ( Fig. 1F ). Moy-Thomas (1937) suggested that the large ganoine fields grew directly over the older, smaller ornament, and that appears to be the best explanation for the differences observed within size classes. Similar overgrowth has been observed on the dermal bones of some specimens of Eurynotus , where small striations emerge from under blotched enameloid cover (NMS 1876.28.2; L.C.S. pers. observ.; Appendix S1) Likewise, Dietze (1999) reported a similar phenomenon in Paramblypterus , where smooth ganoine increased in extent with body size to obscure or replace previous structures.

The parietals in Styracopterus are rhomboidal or even right triangular in appearance, not rectangular as reconstructed previously ( Moy-Thomas, 1937; Gardiner, 1985). The lateral margin is rounded in small and midrange specimens, and is sigmoidal in GSE 5663 ( Fig. 1 ), extending past the lateral limit of the frontal. The posterior margin contacts the extrascapular at a suture line that appears straight in GSE 8731 ( Fig. 1 ), and slightly inclined towards the midline in the larger GSE 5663 and GSE 2136 ( Fig. 1F, G ). The fine linear ornament found in GSE 8731 ( Fig. 1A ), NMS 1891.53.50 ( Fig. 1D ), and GSE 2136 ( Fig. 1F ) radiates posterolaterally, disrupted at the midpoint by a curved vertical ridge described by Gardiner (1985). An elongated band of smooth ganoine covers this ornament in midsize specimen NMS 1891.53.49 ( Fig. 1B ). In the 16-cm GSE 5663 ( Fig. 1G ), this is displaced from the midline by smaller dotted ornamentation, and is joined laterally by two or three seed-shaped ganoine splotches. NMS 1891.53.50 ( Fig. 1D ) shows a deep furrow for the supraorbital canal along the lateral midline, which curves towards the posterior margin.

The dermopterotic gives the superficial impression of a thick, shallow arc stretching back from the lateral frontal ( Fig. 4 ), as has been reconstructed for Strepheoschema ( Gardiner, 1985) as well as Cheirodopsis and Canobius elegantulus Traquair, 1881 ( Moy-Thomas & Bradley Dyne, 1938). The dermopterotic stretches from a tapered anterior limit, sitting at the midpoint of the frontal in GSE 8731 ( Fig. 1A ), and positioned behind the posterolateral process in GSE 5663 ( Fig. 1G ), to a diagonal posterior contact with the extrascapular. A fine ornament of short lateral ridges, exposed in GSE 8731 ( Fig. 1A ) and GSE 2136 ( Fig. 1F ), is almost completely covered by a wide ganoine field running along the midline in 11-cm NMS 1891.53.49 ( Fig. 1B ) and 16-cm GSE 5663 ( Fig. 1G ).

Styracopterus possesses one pair of extrascapulars that are rectangular in GSE 8721 ( Fig. 10A ), but are laterally expanded, with a curved, ‘back swept’ appearance in NMS 1891.53.49 ( Fig. 10B ) and GSE 5663 ( Fig. 1G ), matching the morphology in Phanerosteon ovensi ( White, 1927; Appendix S1; Fig. 18D ) and Eurynotus (NMS 1957.1.5686, L.C.S., pers. observ.). Short, striated ornamentation in NMS 1891.53.50 ( Fig. 1D ) is covered by distinct midline and lateral ganoine splotches in NMS 1891.53.49 ( Fig. 1B ). In GSE 5663 ( Fig. 1G ), these fields are fused into a single patch of smooth ornament with four posterior extensions, covering the lateral half of the bone.

The general morphology of the post-temporal in Styracopterus , similar to that described by Moy-Thomas (1937) and Gardiner (1985) in Fouldenia , the remaining Foulden fishes, and in many other early actinopterygians ( Gardiner & Schaeffer, 1989; L.C.S., pers. observ.; Appendix S1), is rhomboid in form ( Fig. 1A, D, F ). The posterolateral angle is more pointed, and medial margin short, in the small- est specimen (GSE 8731; Fig. 1A ) relative to the largest (GSE 2136, Fig. 1G ). A primary ornament of thin, posterolaterally diagonal ridges, as shown in GSE 8731 and GSE 2136 ( Fig. 1A, F ), is obscured by a half-oval patch of smooth ganoine along the midline NMS 1891.52.49 ( Fig. 1B ). In GSE 5663 ( Fig. 1G ), this field covers the entire anteromedial portion of the bone, and bears multiple posterior branches. Additional smaller bands and patches of enameloid are found on the posterior half of those post-temporals.

The orbit in Styracopterus is bounded by the dermosphenotic, nasal, lachrymal, and jugal, but not the premaxilla, as inferred by Gardiner (1985). The dermosphenotic is unfortunately broken in all known specimens, but may be reconstructed from fragments ( Fig. 4 ). In GSE 8731 ( Fig. 1A ), the posterior portion is wide with a ventral margin that curves posterodorsally. The thick process abuts the dermopterotic, and ends on the frontal in GSE 8731, NMS 1891.53.50, and GSE 5663 ( Fig. 1A, D, G ). Overall, the morphology matches that in Fouldenia , and likewise lacks the posterior process that would give the dermosphenotic a T-shaped form (contra Gardiner & Schaeffer, 1989). Ornamental ridges are diagonal on the posterior portion in GSE 8731 ( Fig. 1A ), and are horizontal on the anterior portion in NMS 1891.53.49 ( Fig. 1B ). In GSE 5663, this latter ornament is covered by a drop-shaped splotch of smooth ganoine, just as in Eurynotus crenatus Agassiz, 1833 –1844 and Benedenius (NMS 1876.28.2, NMS 1957.28.2; Liège Benedenius , L.C.S. pers. observ.; Fraipont, 1890).

The jugal is not well preserved in any specimen of Styracopterus , but unornamented sections appear in GSE 8731, NMS 1891.53.50, and GSE 2136 ( Fig. 1A, D, F ), where it abuts the preoperculum and extends to the ventral midline of the orbit. The lachrymal is not much better preserved, limited to anterior sections ending near the premaxillae in GSE 8731, NMS 1891.53.49, and GSE 2136 ( Fig. 1A, B, F ). These appear rectangular and tall, with some evidence for an ornament of circular ganoine fields in NMS 1891.53.49 ( Fig. 1B ). There is no sign of an infraorbital canal.

Jaws and dentition

The only endoskeletal element known in Styracopterus is the parasphenoid. The dorsal aspect visible in NMS 1891.53.49 ( Fig. 1B ) is anteriorly narrow with thin lateral ridges that may mark the parabasal canals, and thus matches the morphology in Fouldenia . The parasphenoid is interrupted near its midline, obscuring the likely position of the bucco-hypophysial canal. However, another posteriorly expanded portion reveals a central ridge and paired triangular indentations, as in the parasphenoids of other early actinopterygians (e.g. Mimipiscis, Choo, 2011 ; Moythomasia, Gardiner, 1984 ; Platysomus superbus Traquair, 1881 , L.C.S., pers. observ.; Appendix S1). This is again disrupted by an impression of the hyomandibula obscuring the posterior extent of the bone. The parasphenoid is visible in lateral view in the lower part of orbit in GSE 8731 ( Fig. 1A ), where it is inclined anteroventrally and appears quite robust, thickening towards the snout.

The maxilla in Styracopterus is largely as described by Moy-Thomas (1937) and Gardiner (1985). The triangular posterior expansion exhibits a rounded peak near the vertical posterior margin of the bone in GSE 2136 ( Fig. 1F ), where both maxillae are exposed as mirror images. The dorsal margin slopes into a thick, pointed anterior ramus gradually in GSE 8731 and GM 1905.82md ( Fig. 1A, B ). The transition is steeper and more curved in GSE 2136 ( Fig. 1F ), reflecting the deeper posterior expansion in larger individuals ( Fig. 4 ). The jaw margin is sigmoidal in GSE 8731, NMS 1891.53.50-1, and GSE 2136 ( Fig. 1A, D, E, F ), and overlaps the mandible throughout its length. The margin of the maxilla curves upwards diagonally posterior to the dentary. The general form of the maxilla in Styracopterus is not far from the taller maxillae of Amphicentrum and Cheirodopsis ( Moy-Thomas & Bradley Dyne, 1938; Bradley Dyne, 1939; Coates, 1988; L.C.S., pers. observ.; Appendix S1).

The ventral edge of the maxilla in Styracopterus is lined by a wide band of smooth ganoine that covers the entire lateral surface of the anterior ramus ( Fig. 1 ). A second band begins at the level of the anterior edge of the orbit, separated from the first by a deep furrow, and splits into a V-shape posteriorly. The ventral arm runs laterally, whereas the dorsal band moves upwards along the dorsal edge of the maxilla, with both ending at the level of the back of the orbit in smaller specimens: GSE 8731, GM 1902.85md, and NMS 1891.53.50-1 ( Fig. 1A, C, D, E ). In the ~16-cm specimens GSE 5663 and GSE 2136 ( Fig. 1F, G ), these two bands continue horizontally past the midline of the posterior expansion, with the more ventral member meeting the posterior margin. Smaller blotches of enameloid are found around this ornament ( Figs 1 , 4 ). As in the skull roof, the portions of smooth ganoine farther away from the jaw margin, or only present in larger individuals, cover a finer ornamentation, exposed in GM 1902.85md, NMS 1980.53.49, and GSE 2136 ( Fig. 1B, C, F ). This consists of fine, long diagonal ridges radiating posterodorsally onto the posterior expansion of the maxilla from the anterior ramus. Fields of thick ganoine are similarly present on the ventral and anterior maxillae of the eurynotiforms Amphicentrum , Cheirodopsis , Paramesolepis , Wardichthys , Eurynotus , and Benedenius , and in all genera except the last, fine ornament is visible on the posterior expansion ( Fraipont, 1890; Coates, 1988; L.C.S., pers. observ.; Appendix S1).

The dentary of Styracopterus extends nearly the entire length of the lower jaw and appears gracile in comparison with the maxilla. The anteriormost portion forms a narrow ‘beak’ in GSE 8731 and GSE 5663 ( Fig. 1A, F ), as the symphysis curves to form an acute point with the edentulous jaw margin. As in Fouldenia and Benedenius ( Van Beneden, 1871; Fraipont, 1890; L.C.S., pers. observ.; Appendix S1), the dentary in Styracopterus is covered laterally by two thick, ganoine bands that are only partially distinct ( Fig. 1 ). The lateral midline is punctuated by a line of large, regularly spaced circular openings for the mandibular canal in GSE 2136 ( Fig. 1F ), which resemble the canal pits in the snout ornament. Such openings are also found in the smooth ganoine covering the mandibles of Eurynotus (NMS 1957.1.5686; L.C.S., pers. observ.) and Amphicentrum ( Coates, 1988; Appendix S1). As reconstructed by Gardiner (1985), the angular in Styracopterus is a narrow, half-crescent in GSE 8731, NMS 1891.53.51, and GSE 2136 ( Fig. 1A, D, F ), and is similar in general morphology to those in Cheirodopsis and Paramesolepis ( Moy-Thomas & Bradley Dyne, 1938; Appendix S1).

Moy-Thomas (1937) and Gardiner (1985) noted, but did not illustrate, ‘eight cylindrical blunt teeth’ found on the maxilla of Styracopterus specimen GM 1902.85md ( Fig. 1C ). In this individual, the anterior process of the maxilla bears a homogeneous set of large, triangular teeth with acrodin caps, tapered crowns, and robust bases. The morphology and preservation are superficially similar to a robust set of teeth along the margin of an isolated jaw of Mesolepis wardi Young, 1866 (NHM P8044; L.C.S., pers. observ.), and a maxillary tooth visible on the anterior portion of the maxilla in a specimen of Paramesolepis tuberculata ( Traquair, 1890) (NMS 1891.53.25; L.C.S., pers. observ.). Likewise, the Eurynotus crenatus specimen NMS 1859.33.F515 (L.C.S., pers. observ.) shows the impression of such an acrodin-caped fang well forward of and distinct from the denticulated tooth plates, and specimen NHM P11679 has a full row of pointed teeth on the mesial surface of the maxilla above the jaw margin ( Traquair, 1879; L.C.S., pers. observ.).

Evidence of marginal dentition remains confined to GM 1902.85md ( Fig. 10C ); all other individuals exhibit a seemingly edentulous gape covered in ganoine ornament. The marginal dentition of other Styracopterus specimens might be obscured by a ‘beak’ of dermal bone and/or ganoine plate, as the maxilla significantly overlaps the mandible in articu- lated jaws. Unfortunately, the mesial surface of the maxilla is not observed, preventing confirmation. However, Amphicentrum , another superficially edentulous fish, has a field of apparent tooth cusps on the mesial maxilla, well dorsal to the sharpened edge of the gape ( Bradley Dyne, 1939; Coates, 1988; NMS 1894.73.472; L.C.S., pers. observ.; Appendix S1). Likewise, the maxillary teeth in Eurynotus sit in an internal position, hidden from lateral view, and are thus not visible in most specimens ( Traquair, 1879; NMS 1859.33.F515; L.C.S., pers. observ.; Appendix S1). Within the Glencartholm fauna, Cheirodopsis has a set of six or more incisor-shaped teeth rooted on the mesial side of the maxilla, covered laterally by ganoine-ornamented bones (NMS 1957.1.5781; L.C.S., pers. observ.). Farther afield temporally and geographically, a similar, but reversed, arrangement is found Aeduella , where a ‘flange’ of the maxilla covers the lingual aspect of tooth bases ( Gottfried, 1987).

In NMS 1891.53.50–51 ( Fig. 1D, E ), an individual in which the dermal cheek bones are missing, two near-parallel rows of denticles lacking acrodin caps are situated medial to, and separate from, the remnants of the dentary, but show some alignment with the lower jaw. The crowns of these anteriorly ovoid and posteriorly rectangular denticles are pointed outwards from the matrix. The rows are separated by an area in line with the width of the frontals in the same individual and/or the parasphenoid in the similarly sized specimen NMS 1891.53.49 ( Fig. 1B ), which also possesses outwardly oriented denticles alongside the anterior limit of the palate. The denticles are probably associated with lower tooth plates, as in Fouldenia (see below), Mesolepis wardi (NHM P8042; L.C.S., pers. observ.), and Eurynotus crenatus (NMS 1859.33.F515; L.C.S., pers. observ.), which have highly similar denticle morphologies. These plates would probably have been constructed from the fused coronoids or other mesial mandibular elements, as described for similarly positioned elements in Amphicentrum granulosum Young, 1866 , Cheirodopsis , and Eurynotus ( Bradley Dyne, 1939; Traquair, 1879; Coates, 1988; L.C.S., pers. observ.; Appendix S1).

Specimen NMS 1891.53.50–51 also shows a second pair of denticle rows situated around the midline of the disarticulated skull ( Fig. 1D, E ), again with crowns pointed outwards. These denticles are conical with blunted apices, the largest featuring a narrow, ‘lophodont’ edge perpendicular to the row, and showing signs of wear-related loss at their crowns. The denticles are heterogenerous in both spacing and size: many of the more posterior members are an order of magnitude larger than the mandibular denticles, whereas more anterior elements are smaller and more broadly spaced. This is similar to the arrangement of the primary, ventrally pointed row of denticles on the upper toothplates of Eurynotus and Mesolepis ( Traquair, 1879; Watson, 1928; NMS 1874.3A; L.C.S., pers. observ.; Appendix S1).

As with the mandibular set, the space between the upper rows of denticles in NMS 1891.53.50–51 matches the width of the parasphenoid at the same points in NMS 1891.53.49, narrowing anteriorly ( Fig. 1B, D, E ). In NHM P1663, where a badly preserved skull is observed in nearly ventral view, small, closely packed denticle crowns distinguishable from the maxillary teeth are pushed through a gap just ventral to the mandible and away from the maxilla. The orientation of the denticles in NMS 1891.53.50–51 ( Fig. 1D, E ) and NHM P1663 suggests the existence of paired palatal tooth plates with ventrally pointed denticles, perhaps derived from the ectopterygoids, as in Amphicentrum and Cheirodopsis ( Traquair, 1879; Moy-Thomas & Bradley Dyne, 1938; Bradley Dyne, 1939; Coates, 1988; L.C.S., pers. observ.; Appendix S1).

Suspensorium

As previously described, the suspensorium in Styracopterus is nearly vertical ( Moy-Thomas, 1937; Gardiner, 1985). This is supported by the impression of a superficially narrow and cylindrical hyomandibula in NMS 1891.53.50 ( Fig. 1D ), in which a knoblike opercular process marks the transition from the erect ventral portion to a slightly inclined dorsal half. The true and relative sizes of these parts may be obscured by overprinting of the dermal bones. The head of the hyomandibula is expanded and robust in NMS 1891.53.50, GSE 8731, and possibly GSE 5663 ( Fig. 1A, C, G ). The head is covered by a triangular dermohyal with fine, linear ornament and a dropshaped ganoine patch, as noted in GSE 2136 and GSE 5663 ( Fig. 1A, G ).

The hyomandibula is bounded anteriorly by a very erect preopercular, fully preserved in Styracopterus specimen GSE 2136 ( Fig. 1F ; Gardiner, 1985), which is positioned so that the vertical anterior margin aligns with the rear of the gape. The posterior margin is bounded by a cylindrical ridge and tracks the angle of the hyomandibular, bending diagonally at the level of the horizontal pitline in GSE 2136 to form the narrow, rounded apex ( Fig. 1F ). The preopercular thus resembles those in Amphicentrum ( Coates, 1988) , Eurynotus ( Gardiner & Schaeffer, 1989) , Cheirodopsis , and Paramesolepis ( Moy-Thomas & Bradley Dyne, 1938; L.C.S., pers. observ.; Appendix S1). The ornament above the premaxilla consists of fine striations radiating from the midline, whereas a ventral process connecting with the quadratojugal is covered in smooth ganoine ( Fig. 1F, G ).

Opercular series

The operculum is small and rectangular in the specimen of Styracopterus with the best-preserved example: the large fish GSE 5663 ( Fig. 1G ). In line with Moy-Thomas’ (1937) reconstruction, The suboperculum is taller and is almost identical to that in Benedenius ( Traquair, 1878; Fraipont, 1890; L.C.S., pers. observ.; Appendix S1; Fig. 15C ). It exhibits a pointed anteroventral process angled towards the jaw joint by a diagonal ventral margin, as in Benedenius and Fouldenia (L.C.S., pers. observ.; Appendix S1; Fig. 15 ). This process might be considered diagnostic for Styracopteridae , were similar structures not present in Tarrasius , Strepheoschema , Holurus , Phanerosteon , Paramesolepis , Amphicentrum , Eurynotus , and Cheirodopsis ( Moy-Thomas & Bradley Dyne, 1938; Gardiner, 1985; Coates, 1988; Gardiner & Schaeffer, 1989; Sallan, 2012; L.C.S., pers. observ.; Appendix S1). There are differences in the relative sizes and orientation of opercular series bones between Styracopterus size classes. In 8-cm GSE 8731 ( Fig. 1A ), the operculum is inclined forwards at an almost 45° angle, and is nearly equal in length to a curved suboperculum ( Fig. 4A ). In 11-cm specimens NMS 1891.53.49 and NMS 1891.53.50–51 ( Fig. 1B, D ), the operculum is still tilted slightly anteriorly and ends just above the curved preopercular, giving way to a taller suboperculum ( Fig. 4B ).

In GSE 2136 ( Fig. 1F ), a posteroventrally diagonal line bisects the operculum ornament into an anterior field of curved lateral ridges and a posterior field of concentric lines. This is obscured in larger individuals such as NMS 1891.53.49 ( Fig. 1B ), in which a broken plate of ganoine is found near the anteroventral corner. The ornament of the suboperculum is also double layered in larger specimens of Styracopterus . Thick, wavy lines radiate posteriorly from a primary vertical stripe of ganoine in GSE 2136 ( Fig. 1F ). In contrast, the anteroventral process is covered by diagonal ridges in GSE 5663 ( Fig. 1G ); although this is obscured by smooth ganoine distally. In that same specimen, elongate enameloid bands are found on the ventral half of the suboperculum. Similar ornament is found on an articulated suboperculum from the late Tournaisian deposits of Symond’s Yat, Herefordshire, England, previously attributed to Styracopterus sp. (NHM P62956; L.C.S., pers. observ.; Appendix S1, see below), as well as the more extensive smooth ganoine of the Liege specimen of Benedenius ( Fraipont, 1890; L.C.S., pers. observ.; Appendix S1).

Gulars and branchiostegals

The branchiostegal rays are not completely preserved in Styracopterus , although individual rays are found in several specimens. The primary ray is the tallest and most distally expanded in GSE 5663 ( Fig. 1G ), and bears an ornament of two intercalated and wavy bands ( Figs 1G , 4C ). The second ray is thinner but likewise ornamented, but further members of series are covered with one or two longitudinal enameloid bands and/or fine linear ornament ( Figs 1G , 4C ). Whereas rectangular branchiostegals directly under the opercular bones are elongate, those originating ventral to jaws in GSE 8731 and NMS 1891.53.49 ( Fig. 1A, B ) are shorter and more pointed, but remain nearly horizontal in orientation. This is similar to the general orientation and form of the series in Benedenius ( Fraipont, 1890) and most other eurynotiforms (e.g. Amphicentrum , Cheirodopsis , Eurynotus ; Traquair, 1879; Moy-Thomas & Bradley Dyne, 1938; Coates, 1988; L.C.S., pers. observ.; Appendix S1), in which the ventralmost branchiostegals are relatively short and barely visible in lateral view ( Fig. 15 ).

The only evidence of the gular series consists of a badly preserved, displaced lateral gular originating just posterior to the front of the dentary in NMS 1891.53.49 ( Fig. 1B ) and similar in form to the branchiostegals. Unlike the branchiostegals, the gular is completely covered in ganoine ( Fig. 1B ). The arrangement of the preserved parts leaves space for a short median gular, as in Fouldenia , but it is not clear whether more anterior skeletal material represents this bone. Gular material in other specimens might be hidden by the mandible, as appears to have been the case in Benedenius , Amphicentrum , Eurynotus , Cheirodopsis , Paramesolepis and other early fishes ( Traquair, 1879; Fraipont, 1890; Moy-Thomas & Bradley Dyne, 1938; Coates, 1988; L.C.S., pers. observ.; Appendix S1; Figs 15–19 ).

Shoulder series

The dermal shoulder series in Styracopterus is relatively well preserved and matches the composition described by Moy-Thomas (1937). Unfortunately, the same cannot be said for the endoskeletal girdle, which is not visible in any specimen. The leaf-shaped supracleithrum extends to the midpoint of the suboperculum ( Figs 1 , 4 ). The ornament consists of prominent, densely packed, intercalating ganoine ridges running along the vertical axis and fusing towards the midline. The supracleithrum changes from a thin gracile bone with a definite ventral point in the 8-cm specimen GSE 8731 ( Fig. 1A ) to a wide, rounded form in the 16-cm individuals GSE 5663 and GSE 2136 ( Fig. 1F, G ).

The rectangular postcleithrum in Styracopterus is wide and elongate, but would have been obscured in life by the supracleithrum, except for a rounded posteroventral corner abuting the cleithrum, as shown in GSE 8731 ( Fig. 1A ). Exposed postcleithra in other specimens resemble the same bone in Amphicentrum ( Coates, 1988) . The ornament preserved in GSE 5663 ( Fig. 1G ) consists of thick diagonal striations.

The cleithrum in Styracopterus is very tall and erect, with a wide crescent-shaped dorsal process and a tall ventral body with a rounded posterior extension ( Figs 1 , 4 ). Thus, it resembles the cleithra of other eurynotiforms, particularly Fouldenia and Eurynotus ( Traquair, 1879; Gardiner & Schaeffer, 1989; L.C.S., pers. observ.; Appendix S1; Fig. 15 ). The half-ovoid dorsal process of the cleithrum runs to the midpoint of the suboperculum in GSE 8731, NMS 1891.53.50, and GSE 5663 ( Fig. 1A, D, G ). The ornament in Styracopterus specimen GSE 8731 ( Fig. 1A ) is similar to Fouldenia , and other Foulden taxa (L.C.S., pers. observ.; Appendix S1), in that it consists of robust, nearly vertical lines running from apex to base ( Fig. 13A ). This ornament is covered by wider bands of smooth ganoine in the larger specimens NMS 1891.50.49 and GSE 2136 ( Fig. 1B, F ). The ventral part of the cleithrum is tall in GSE 2136, and features an ornament of short horizontal ridges covered by smooth enameloid. Like Fouldenia , there is a short posterior extension near the ventral surface of the cleithrum.

The cleithrum contacts the clavicle at a concave anterior margin situated beneath the opercular series and jaw joint in NMS 1891.50.49 ( Fig. 1B ). The clavicle in Styracopterus runs along nearly the entire ventral length of the skull, as exposed in GSE 8731, NMS 1891.50.49, GSE 2136, and GSE 5663 ( Fig. 1A, B, F, G ), curving anteriorly to a rounded point. The clavicle of Styracopterus is shallow relative to that of Fouldenia , and is completely covered by the mandible in life ( Figs 1 , 4 ). The bone is covered with short, wavy bands in NMS 1891.53.49 and GSE 5663 ( Fig. 1B, G ), an ornament obscured by smooth ganoine patches in the latter.

Paired fins

The pectoral fins in Styracopterus originate ventrolaterally, although the endoskeletal attachment is not visible in any specimen. However, GSE 8731 and GSE 5663–4 ( Fig. 2A, E, F ) show a gap in squamation between the cleithrum and the angled bases of the lepidotrichia, which suggests a naked lobe over the missing radials. The fins in GSE 8731 ( Fig. 2A ) are elongate and curved, with pointed distal margins and short bases. This results in a scythe-shaped fin similar to those in Eurynotus , Benedenius , and Mesolepis ( Agassiz, 1833 –1844; Traquair, 1879; Boulenger, 1899; Traquair, 1907; L.C.S., pers. observ.; Appendix S1; Fig. 15 ). In this individual and others (GSE 5663–4, NMS 1891.53–50; Fig. 2C, E, F ), lepidotrichia are thin and tightly packed with enlarged ovoid bases, similar to the median fin rays in Fouldenia . Segments are rectangular, with a ridge along the anterior edge. The rays become thinner distally but there is no evidence for bifurcation. Rays supporting the anterior margin in the 16-cm specimen GSE 5663–4 ( Fig. 2E, F ) are more robust than, and may be twice as wide as, the other lepidotrichia, with an abrupt transition. However, the absolute size of the fin is invariant between specimens despite a doubling of body length, rendering it relatively larger in smaller specimens.

Traquair (1881) noted impressions of robust pectoral fringing fulcra in the holotype of Styracopterus (GSE 5672–3; Fig. 2D ), and these served as a basis for the species and eventually genus names ( Traquair, 1890). Elongate, blade-like fulcra strongly resemble their likewise paired counterparts in Eurynotus (NHM P11679, NHM P11676; L.C.S., pers. observ.; Appendix S1), and those of Benedenius in lateral view ( Traquair, 1879; Fraipont, 1890; Appendix S1). Each fulcral scale possesses a tapered distal margin and is covered by thick ganoine. More distal pairs are progressively shorter and thinner. In the smallest specimen, GSE 8731 ( Fig. 2A ), six pairs of fulcral scales sit along the primary lepidotrich, each overlapping a third of the length of their neighbour. The number of fulcra is unchanged in larger specimens NMS 1891.53.50–51 ( Fig. 2B, C ) and GSE 5663–4 ( Fig. 2E, F ), yet more distal scales appear lozengeshaped rather than elliptical. These contact each other at nearly straight margins oriented at a 45° angle relative to the fin axis.

The pelvic fins of Styracopterus sit very low on the body, separated by a distance of just two or three median ventral scales in NHM P1663 ( Fig. 2G ). The fins appear small and triangular, but may be truncated by the better-preserved anal fin. The lepidotrichia in NHM P1663 and GSE 8731 ( Fig. 2G, H ) are thin and densely packed, divided into elongate segments with no evidence of bifurcation, thus resembling the pectoral rays of Fouldenia . The similarly sized individual GSE 8731 ( Fig. 2H ) possesses fringing fulcra that are slimmer versions of their pectoral counterparts. These are similar to the proximal fringing fulcra on the pelvic fins in a very small specimen of Eurynotus (NMS 1874.3A; L.C.S., pers. observ.; Appendix S1). A furrow on the anterior midline of each fulcral scale originates from a wider groove holding the apex of its more proximal neighbour. Three semi-articulated pelvic fulcra are observed in lateral view in the larger specimen GSE 5663–4 ( Fig. 11I ), and appear relatively thicker and wider. A large, ovoid basal fulcral scale overlaps the pelvic fin in both NHM P1663 and GSE 5663–4 ( Fig. 2G, I ). This is ornamented with three thick longitudinal bands of ganoine that fuse near the apex of each scale. Unfortunately, the endoskeletal components of the pelvic complex are not visible in any specimen.

Median fins

An exact reconstruction of the dorsal fin in Styracopterus was not possible because of incomplete preservation. However, GSE 8731 ( Fig. 3A ) suggests a long yet sloped anterior margin, whereas GSE 5663–4 ( Fig. 3C, D ) shows the posterior portion is quite short and rounded. The fin base sits along the diagonally oriented dorsal margin of the body, originating near the point of maximum depth in larger specimens GSE 5672–3 and GSE 5663–4 ( Fig. 3B, C, D ). In GSE 8731, GSE 5672–3, and GSE 5663–4 ( Fig. 3A, B, C, D ), the primary lepidotrichia are unjointed, have ovoid bases, and are fused to fringing fulcra like those of the paired fins, just as the leading fin rays of the dorsal fins in Fouldenia , Amphicentrum , Cheirodopsis , Eurynotus (NMS 1876.28.2 and NHM P11676), Paramesolepis , and Wardichthys , among other eurynotiforms (L.C.S., pers. observ.; Appendix S1). These have been refered to elsewhere as ‘horns’ ( Weems & Windolph, 1986). Other lepidotrichia supporting the anterior margin of the dorsal fin in GSE 5672 and GSE 5663–4 ( Fig. 3B, C, D ) are proximally double the width of those in the posterior half of the fin, which is another commonality with the Eurynotiformes mentioned above (L.C.S., pers. observ.; Appendix S1). Although rays in the anterior portion of the dorsal fin taper to a point, bifurcation is observed in more posterior lepidotrichia, with extent increasing towards the posterior. This biased distribution of dichotomization is also found in Amphicentrum , Eurynotus , and Aesopichthys ( Coates, 1988; Poplin & Lund, 2000; L.C.S., pers. observ.; Appendix S1). It seems to allow more posterior flexibility, as indicated by the non-uniform orientations in which these lepidotrichia are preserved in Styracopterus . The angle of fin-ray origination decreases gradually along the dorsal fin. This is aligned with a loss in height following a peak in the anterior half of the fin. Other styracopterid and eurynotiform fishes also have such posteriorized peaks relative to Platysomus , Aesopichthys , and other deep-bodied ray-finned fishes (L.C.S., pers. observ.; Appendix S1). Lozenge-shaped, paired fringing fulcra on the dorsal fins of Styracopterus resemble the distalmost fulcra on the pectoral fins ( Fig. 3A, B, C ). Eurynotus also features paired scales on its dorsal fin (NMS 1876.28.2; L.C.S., pers. observ.; Appendix S1), yet fringing fulcra are usually singular on the median fins of other early actinopterygians (e.g. Platysomus and Aesopichthys ; L.C.S., pers. observ.; Appendix S1).

The anal fin is exhibited in lateroventral view in NHM P1663, and the larger specimen GSE 5663–4 ( Fig. 3E, F, G ) displays a nearly complete fin in part and counterpart. The anal fin of Styracopterus is an acute triangle with slightly curved margins rather than the sickle shape of Fouldenia ( Figs 4 , 13 ). The anal fin in Styracopterus has a short base originating in the posterior third of the trunk. It bears lepidotrichia resembling those of the dorsal fin in terms of morphology, as well as changes in dichotomization and height along the fin. Likewise, there is an increase in the relative thickness of the primary anal lepidotrichia with body size in Styracopterus , such that fewer rays appear to make up the anterior margin in GSE 5663 than NHM P1663 ( Fig. 3E, F ). In GSE 5663–4 ( Fig. 3F, G ), these lepidotrichia appear to bifurcate into curved terminal segments, conforming to the fringing fulcra. It is not clear whether this is a preservational artifact, a trait specific to the anal fin, or a general characteristic of Styracopterus obscured in other fins. In this same individual, proximal fulcral scales are like their pectoral fin equivalents, but more distal fulcra are squat and trowel-shaped, with much greater overlap between successive pairs. These approximate the fringing fulcra morphology in Cheirodopsis and Eurynotus (NMS 1874.3A, P42077, NMS 1876.28.2, NMS 1893. 20; Moy-Thomas & Bradley Dyne, 1938; L.C.S., pers. observ.; Appendix S1). In Eurynotus , the fulcra form a thick and almost solid anterior margin for the anal fin, just as in Styracopterus (L.C.S., pers. observ.; Appendix S1).

In NHM P1663 and GSE 5663–4 ( Fig. 3E, F, G ), erect basal fulcra precede the anal fin, with the last two doubling the height of their anterior neighbours to overlap the fringing fulcra. An expanded base supports an elongated distal process, superficially divided into two rami. The proximal portions also resemble the successively taller, spine-like anal basals found in Amphicentrum , Cheirodopsis , and Eurynotus (L.C.S., pers. observ.; Appendix S1).

Tail and caudal fin

The posteriormost portion of Styracopterus is only preserved in GSE 5663–4 ( Figs 3H, I , 12J ), which possesses a nearly complete tail and caudal fin. Styracopterus has an epichordal fin, which appears to be diamond-shaped and lacks fulcra ( Fig. 3H, I, B ). It originates dorsally and ventrally after a two- or threescale-wide gap following the posterior margin of the caudal fin and the apex of the last axial fulcral scale. The fine, well-separated lepidotrichia emerge diagonal to the body wall, have short segments, and taper distally without bifurcation. Although epichordal fins and lobes are ancestral for osteichthyans, and possibly gnathostomes, the loss of this fin was once thought to define the actinopteran crown (see discussion in Patterson 1982 and Gardiner & Schaeffer, 1989). This was because such a fin was associated primarily with the Devonian stem-taxon Cheirolepis among Palaeozoic fishes, and with Polypterus among living forms ( Pearson, 1982; Long, 1988; Arratia & Cloutier, 1996). However, Watson (1925) found the same structure in the Permian taxon Palaeoniscum , whereas Patterson (1982) noted a wider but incomplete distribution, involving derived taxa such as Bourbonnella ( Gardiner & Schaeffer, 1989) . In fact, Fouldenia ( Figs 11 , 12 ) and other Eurynotiformes such as Paramesolepis (NHM P20425–6; L.C.S., pers. observ.) and Cheirodopsis (NHM P20222; L.C.S., pers. observ.) also bear distinct epichordal fins, as do most examined Palaeozoic actinopterygians with elongate axial lobes and completely preserved tails (L.C.S., pers. observ.; Appendix S1). This suggests the presence of an epichordal fin could be the base state for all Palaeozoic actinopterygians, and will require further investigation.

Whereas the tail of Styracopterus is superficially inequilobate in GSE 5663–4 ( Fig. 3H ), the caudal fin itself is nearly homocercal, with a shallow median cleft. The ventral lobe is large and rounded. Lepidotrichia are similar in segment morphology to those of the median fins, and start tapering at around threequarters of their length. While the majority of lepidotrichia are missing their tips, those in the ventral lobe tend to end in a pointed segment, whereas more dorsal fin rays bifurcate with no apparent regularity. Bifurcation is also observed in lepidotrichia underlying the fringing fulcra and ventral/anterior margin ( Fig. 3H, J ), as in the anal fin. Although the lepidotrichia in the dorsal lobe are finer than those near the anterior margin, the transition is not as abrupt as in the median and paired fins.

Caudal basal fulcra form a continuous series between the anal and caudal fins ( Fig. 3G, H, J ). The basals resemble the anal set: more anterior basals are squat with rounded apices and linear ornament oriented parallel with the body axis ( Fig. 3G ). They transition into tall, erect scales with wide bases and long posterior processes that expand distally, giving them a slightly sigmoid appearance. These abut the primary caudal lepidotrichia. The distal processes consist of two triangular rami joined at the midline around a central groove and furrow. These have a deltoid shape in lateral view. Fringing fulcra are prominent along the ventral margin of the caudal fin ( Figs 3H, J , 4 ), and resemble the set on the anal fin of the same individual and in Eurynotus (L.C.S., pers. observ.; Appendix S1). The first caudal lepidotrich in Styracopterus has very thick, almost fulcra-like proximal segments, and appears to blend into this set.

Squamation

GSE 5663–4 ( Fig. 4C ), the largest and most complete specimen of Styracopterus , possesses 74 sigmoidal trunk scale rows. These rows curve such that the dorsalmost scales have their posterior margins directed near vertically. The laterally compressed GSE 5663–4 ( Fig. 4C ) has 25 scales per row over most of the flank, despite posterior decreases in body depth. The smaller, more roundly fusiform GSE 8731 ( Fig. 4A ) has just 18 scales per row at maximum depth, indicating that scale counts increase during ontogeny yet scale height is ultimately related to body depth. A prominent ‘hinge line’ is found on the caudal peduncle of GSE 5663–4 ( Figs 3H , 4C ), marking the start of ~61 additional caudal scale rows along the peduncle and axial lobe.

Scales on the lateral trunk are tall and largely rhomboid, with smooth margins and rounded posteroventral corners. Imprints of the mesial side of these scales in NMS 1891.53.51 ( Figs 2B , 4A ) show tall, robust pointed pegs originating from the middle of the dorsal margin and sitting within a similarly shaped central divot on the next member of the row. Fraipont (1890) and Boulenger (1899) illustrated nearly identical flank scales in Benedenius , and the same general morphology and articulation is found in Amphicentrum , Eurynotus , Cheirodopsis , and other eurynotiforms ( Agassiz, 1833 –1844; Traquair, 1879; Bradley Dyne, 1939; Coates, 1988; L.C.S., pers. observ.; Appendix S1). Lateral line scales in Styracopterus are marked by a horizontal, cylindrical ridge running along the horizontal midline. These scales otherwise match their neighbours in size and ornamentation.

The ornament, shape, and relative heights of flank scales vary between size classes and positions on the trunk. In the ~8-cm, largely fusiform individuals GSE 8731 and NHM P1663 ( Fig. 4A ), the tallest, most rectangular scales are found ventral to the lateral line on the anterior flank. These bear up to six lateral bands of ganoine. The top three of these are dorsoventrally directed and form a leaf-shaped, nested pattern similar to the scale ornament in Fouldenia . The bottom three bands are more horizontal, tapering and fusing posteriorly. Shorter scales near the midpoint of the body have truncated versions of the same ornament, with the reduction in height sucessively eliminating more ventral stripes. Even shorter scales near the dorsal and ventral margins in the anterior half of the body, and all scales on the posterior half, are simply ornamented with four horizontal, rectangular bands that fuse posteriorly ( Fig. 4A ). Such scales have a greater anterodorsal slope and a more prominent posterodorsal margin than their counterparts along the anterior flank, giving them a leaf-like form similar to the trunk squamation of smaller specimens of Fouldenia ( Fig. 13 ). This also describes nearly the entire flank squamation in the smallest two Styracopterus specimens, GSE 8731 and NHM P1663 ( Fig. 4A ), which is far more homogeneous than large individuals. In the midsize individuals GSE 5672–3, NMS 1891.53.50–51, and NMS 1891.53.49 ( Fig. 4B ), the nested ornament increases in size to contain up to seven concentric bands, resembling the anterior flank scales of the largest specimens of Fouldenia ( Fig. 13D ). Scales near the ventral margins of the anterior half of the body in midsize Styracopterus are ornamented entirely by nested ridges, rather than the simple striations found in the smaller GSE 8731 ( Fig. 4A, B ). Scales on the dorsal surface and posterior half of midsize fish are taller than their counterparts in GSE 8731 ( Fig. 4A, B ), and bear an additional horizontal ganoine band. A distinct field of ovoid scales is found near the dorsal fin in midsize Styracopterus , such as GSE 5672–3 ( Fig. 3B ), with posterior margins oriented towards the lepidotrichial bases. These are covered with three or four flat ganoine lines that fuse at the apex, resembling the ornament of the pelvic basal fulcra in miniature.

In the largest individuals, GSE 5663–4 and GSE 2136 ( Fig. 4C ), anterior flank scales are elongate and rectangular, with an ornament of nine or ten diagonal striations over three horizontal bands. The nested pattern is broken; most of the ridges run off the posterior edge or fuse with a marginal line of ganoine. The more dorsal and ventral scales remain rhomboid, adding yet another horizontal line of ganoine to bring the total to six per scale ( Fig. 4C ). Scales on or near the caudal peduncle in GSE 5663–4 do not differ greatly from the posterior flank scales of smaller specimens, and become shorter and more elongately rhomboid near the caudal fin base ( Figs 3H, J , 4A, C ).

There is no evidence of a distinct field of squamation along the anteroventral surface of the trunk in Styracopterus , unlike Fouldenia (see below). Nor is there a trace of ventral ridge scales in the smallest specimen NHM P1663 ( Fig. 4A ). However, two slightly enlarged, deltoid scales are visible on the ventral margin around the trunk midline in the midsize fish NMS 1891.53.49 ( Fig. 4B ). These bear an ornament of fine longitudinal bands. In the largest specimen, GSE 5663–4 ( Figs 2E, F , 4C ), a series of four squat, trapezoidal scales is preserved in medial view at the ventral margin of the body, just anterior to the pelvic basal fulcra. These ventral ridge scales cover a third of the distance between the shoulder girdle and pelvic fins. However, anterior members of the series could be obscured by the intact pectoral fins. The ridge scales are not much larger than the adjacent trunk squamation, and thus their overall state is similar to that in Benedenius ( Traquair, 1878; Fraipont, 1890; Boulenger, 1899; L.C.S., pers. observ.; Appendix S1).

Prominent ridge scales cover the entire dorsal midline from nape to fin in Styracopterus , as originally illustrated for the holotype by Traquair (1881, 1890; Moy-Thomas, 1937; Gardiner, 1985). Scales in the anterior half of the series are smaller, more rounded, and horizontally oriented than the erect ridge scales near the dorsal fin ( Figs 3 , 4 ); however, ontogenetic differences are apparent. In the 8-cm fishes GSE 8731 and NHM P1663 ( Figs 3A , 4A ), horizontal ridge scales along the anteriormost dorsum have rounded margins but are otherwise indistinguishable from the proximate trunk squamation, in contrast to the more erect, spine-like members in the posterior half of the series. These ridge scales have a rounded proximal portion joined to a curved, pointed apex, and an ornament of thick vertical striations. In the midsize fish NMS 1891.53.50–51, NMS 1891.53.49, and GSE 5672–3 ( Fig. 4B ), the dorsal ridge scales are enlarged and the anteriomost members are distinguishable by their acuminate shape and central grooves. The longitudinal ornament found in smaller fish is replaced by thick horizontal bands in more posterior ridge scales ( Fig. 4B ). Although the posterior half of the dorsal ridge series is unknown in the largest specimens (GSE 5663–4, GSE 2136; Fig. 4C ), the anteriormost ridge scales have the same morphology as the most posterior scales of the smallest specimens ( Fig. 4C ). Pointed ridge scales overlap each other at 45° angles and bear a diagonal banded ornament.

Caudal squamation is only observed in GSE5663–4, but exhibits considerable variation along the length of the peduncle and axial lobe ( Figs 3H, J , 4C ). Peduncle squamation above the hinge line consists of rhomboid scales with rounded margins and thick ganoine cover; however, scales situated along the lepidotrichial bases are small and seed shaped, with no discernable ornament pattern. Elongate, diamond-shaped scales cover the base of the axial lobe and feature a single furrow in their otherwise solid ornament. These transition to smaller, seed-shaped scales at the horizontal midpoint of the axial lobe, yet the ornament pattern is retained ( Fig. 4C ). Smaller versions of these scales are found in the portion bearing the epichordal fin.

The basal and fringing fulcra on the peduncle and axial lobe are very similar in morphology to those in Fouldenia and Benedenius ( Traquair, 1878; Fraipont, 1890; L.C.S., pers. observ.; Appendix S1; Figs 3H , 4B, C , 12A, B , 13 ). The scales in Styracopterus decrease in size and length posteriorly, so that the last few resemble the paired fulcra of the median fins ( Fig. 3H, I ). As in Fouldenia and Benedenius ( Fraipont, 1890; L.C.S., pers. observ.; Appendix S1), The distalmost fulcral pair in Styracopterus occurs at the level of the last caudal fin ray base ( Fig. 3H, I ).

Figures

Figure 1. Styracopterus cranial material. Unlabelled scale bars equal 1 cm. Medium-grey infill indicates the extent of thick ganoine bands covering other ornament. Light-grey infill indicates unidentified skull material. A, GSE 8731; B, NMS 1891.53.49. C, GM 1902.85md maxilla; D, NMS 1891.53.50; E, NMS 1891.53.51 denticles and jaws; F, GSE 2136; G, GSE 5663.

Figure 2. Styracopterus paired fins. Unlabelled scale bars equal 1 cm. A, GSE 8731 pectoral; B, NMS 1891.53.50 pectoral; C, NMS 1891.53.51; D, GSE 5672 pectoral; E, GSE 5664 pectoral; F, GSE 5663 pectoral; G, NHM P1663 pelvic; H, GSE 8731 pelvic; I, GSE 5663 pelvic.

Figure 3. Styracopterus median fins and caudal material. Unlabelled scale bars equal 1 cm. A, GSE 8731 dorsal; B, GSE 5672 dorsal; C, GSE 5664 dorsal; D, GSE 5663 dorsal; E, NHM P1663 anal; F, GSE 5664 anal; G, GSE 5663 anal; H, GSE 5664 tail; I, GSE 5663 axial lobe; J, GSE 5663 ventral caudal fin.

Figure 4. Styracopterus reconstructions and squamation. Reconstruction drawings of various size classes are based on all available specimens around each length, and are near actual size at full resolution. Squamation photographs are shown at three times the size, at the scale of reconstructions. Roman numerals on the reconstructions indicate the position of scales with matching labels. Medium-grey infill indicates the documented extent of thick ganoine plates covering other ornament. A, 8-cm, scales from GSE 8731; B, 12 cm, scales from GSE 5672–3; NHM 1891.53.49 and NHM 1891.53.49–50; C, 16 cm, scales from GSE 2136 and GSE 5663–4.

Figure 10. Fouldenia anal fins. Unlabelled scale bars equal 1 cm. A, NMS 1980.40.30; B, GSE 2187; C, NMS 1965.4.3 (part); D, NMS 1965.4.3 (counterpart); E, NMS 1984.67.61; F, NMS 1980.40.31; G, NMS P61549; H, NHM P61002; I, NMS 1984.67.62; J, NHM P61548.

Figure 11. Fouldenia tails and caudal fins I. Photographs and interpretative drawings for specimens of 4–7 cm in total length. Unlabelled scale bars equal 1 cm. A, NMS 1980.40.30; B, NHM P13187; C, NMS 1965.4.2; D, GSE 2187; E, NMS 1980.40.27; F, NMS 1965.4.3.

Figure 12. Fouldenia tails and caudal fins II. Photographs and interpretative drawings for specimens of 8–12 cm in total length. Unlabelled scale bars equal 1 cm. A, NHM P13183; B, NHM P13178; C, NMS 1980.40.31; D, NHM P61549; E, NHM P13185; F, NHM P61548.

Figure 13. Fouldenia reconstructions and squamation. Reconstruction drawings of various size classes are based on all available specimens within each class. Squamation photographs are shown at three times the size, at the scale of reconstructions. Roman numerals on the reconstructions indicate the position of scales with matching labels. Mediumgrey infill indicates the documented extent of thick ganoine plates covering other ornament. A, 3–4 cm, scales from GSE 2143; NHM 1965.4.2 and NHM 1980.40.30; B, 6–7 cm, scales from NMS 1965.4.3, NMS 1965.4.6, NHM P14562, and GSE 2187; C, 8–9 cm, scales from NHM P13183, NHM P61546, NHM P13178, and GLAHM V8327; D, 11–12 cm, scales from NHM P61002, NHM P61548, and NMS 1984.67.64.

Figure 15. Styracopteridae and other Eurynotiform taxa from North America and Europe. Medium-grey infill represents the extent of the thick ganoine plates along the jaws based on specimens listed in the Appendix S1, lines along leading edges of fins represent depth of fringing fulcra and consolidated fin spines. A, Fouldenia from the late Tournaisian of Scotland. Lines on the median fins represent the extent of consolidated, spine-like lepidotrichia. Lines on the pectoral fins represent fringing fulcra depth. B, Styracopterus from the early Visean of Scotland. C, Benedenius from the early Visean of Belgium (based on Traquair, 1878, Fraipont, 1890, and the specimens listed in Appendix S1). D, Eurynotus from the Visean–Permian of Euramerica and Russia (based on Traquair, 1879 and the specimens listed in Appendix S1). E, Proteurynotus from the early Visean of Scotland (based on Moy-Thomas & Bradley Dyne, 1938). F, Mesolepis from the Serpukhovian–Pennsylvanian of Europe (based on Traquair, 1907 and the specimens listed in Appendix S1). G, Paramesolepis tuberculata from the early Visean of Scotland (based on Moy-Thomas & Bradley Dyne, 1938 and the specimens listed in Appendix S1). H, Paramesolepis rhombus from the early Visean of Scotland (based on Moy-Thomas & Bradley Dyne, 1938). I, Wardichthys from the late Visean and Serpukhovian of Scotland (based on Traquair, 1907 and the specimens listed in Appendix S1). J, Cheirodopsis from the early Visean of Scotland (based on Moy-Thomas & Bradley Dyne, 1938 and the specimens listed in Appendix S1). K, Amphicentrum from the late Visean–Permian of Euramerica (based on Coates, 1988 and the specimens listed in Appendix S1).

Figure 18. Comparison of early actinopterygian skulls III. A, Fouldenia. B, Strepheoschema (after Gardiner, 1985). C, Aetheretmon (after Gardiner, 1985). D, Phanerosteon (after Gardiner, 1985).