Fouldenia ischiptera, (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 : 172-187

FOULDENIA ISCHIPTERA ( TRAQUAIR, 1881)

Holurus ischipterus Traquair, 1881

Styracopterus ischipterus Traquair, 1890

Fouldenia ottadinica White, 1927

Styracopterus ottadinica Moy-Thomas, 1937

Styracopterus ischipterus Moy-Thomas, 1937

Styracopterus fulcratus Gardiner, 1985

Holotype: GSE 2187 (M1122b, holotype of ‘ Styracopterus ischipterus ’), British Geological Survey , Edinburgh, UK, nearly complete articulated individual in part (EBL: 6 cm).

Paratypes: NHM P13178 (holotype of ‘ Fouldenia ottadinica ’), Natural History Museum , London, UK, nearly complete, flattened articulated individual showing dorsal aspect in part (EBL: 9 cm); NHM P13179, impression of complete articulated skull in part (EBL: 8 cm); NHM P13187 (part) and NHM P13188 (counterpart), nearly complete articulated individual (EBL: 5 cm); NHM P13180 (part) and NHM P14560 (counterpart), incomplete articulated anterior half of individual (EBL: 8 cm); NHM P13182 (part) and NHM P13183 (counterpart), incomplete articulated anterior half of individual (EBL: 8.5 cm); NHM P13186, articulated posterior half of individual in part (EBL: 10 cm); NHM P13186, articulated caudal portion of individual in part (EBL: 10 cm) .

Additional material: GSE 2143, articulated anterior portion of individual, including skull (EBL: 3 cm); NHM P14562, incomplete articulated individual in part (EBL: 6 cm); NHM P14564, nearly complete articulated individual in part (EBL: 9 cm); NHM P61546, incomplete articulated individual in part with trunk squamation and skull (EBL: 8 cm); NHM P61549, complete articulated individual in part (EBL: 10 cm); NHM P61002, incomplete articulated individual in part with trunk squamation, paired fins, and posterior skull (EBL: 10.5 cm); NHM P13181 (part) and NHM P14561 (counterpart), incomplete articulated anterior third of individual (EBL: 12 cm); NHM P61548, nearly complete articulated individual in part and counterpart (EBL: 12 cm); NMS 1980.40.30, National Museums of Scotland, Edinburgh, Scotland, UK, articulated postcranium of individual in part and counterpart (EBL: 4 cm); NMS 1965.4.2, nearly complete articulated individual in part (EBL: 5 cm); NMS 1965.4.2, articulated postcranium of individual in part (EBL: 5.5 cm); NMS 1980.40.27, articulated postcranium of individual in part and counterpart (EBL: 6.5 cm); NMS 1965.4.3, nearly complete articulated individual in part and counterpart (EBL: 7 cm); NMS 1956.5.1, incomplete articulated individual in part and counterpart (EBL: 7.5 cm); NMS 1984.67.61, nearly complete articulated individual in part (EBL: 7.5 cm); NMS 1984.67.63, nearly complete articulated individual in part and counterpart (EBL: 9 cm); NMS 1980.40.31, nearly complete articulated individual in part (EBL: 9.5 cm); NMS 1984.67.65, nearly complete articulated individual in part and counterpart (EBL: 9.5 cm); NMS 1984.67.64, incomplete anterior two-thirds of individual in part (EBL: 11 cm); NMS 1984.67.62, nearly complete articulated individual in part, lacking axial lobe (EBL: 11.5 cm); GLAHM V8327 , Hunterian Museum , Glasgow, Scotland, UK, nearly complete articulated postcranium of individual in part (EBL: 8 cm) .

Type localities and horizon: River Tweed below Coldstream, Berwickshire, Scotland, UK. Calciferous Sandstone Series, Cementstone Group, Schopfites claviger-Auroraspor macra (CM) miospore biozone (348–347 Mya; Smithson et al., 2012), Tournasian 3 (TN3) zone, Ivorian regional substage, Dinantian regional stage, late Tournaisian stage, Mississippian subsystem, Carboniferous system ( Traquair, 1881; Gardiner, 1985; Smithson et al., 2012).

Other localities and horizons: Foulden Fish Bed, Foulden Burn, Berwickshire, Scotland, UK. Calciferous Sandstone Series, Cementstone Group, CM biozone, TN3 zone, Courceyan regional substage, Dinantian regional stage, late Tournaisian stage, Mississippian subsystem, Carboniferous system ( White, 1927; Gardiner, 1985; Wood & Rolfe, 1985; Dineley & Metcalfe, 1999; Smithson et al., 2012); Coomsden Burn, Redewater, Northumberland, England, Cementstone Group, Dinantian regional stage, late Tournaisian stage, Mississippian subsystem, Carboniferous system ( White, 1927; Moy-Thomas, 1938; Gardiner, 1985).

Diagnosis (emended from Traquair, 1881): As for genus.

Remarks: The genus Fouldenia includes all specimens from Foulden previously assigned to Styracopterus ( Moy-Thomas, 1937; Gardiner, 1985), as well as specimens from the Calciferous Sandstones at Coldstream originally attributed to Holurus ischipterus , subsequently to S. ischipterus (or ‘ischypterus’), and finally to S. fulcratus ( Traquair, 1881, 1890; Moy-Thomas, 1937; Gardiner, 1985). The Coldstream specimens are here found to be indistinguishable from juveniles of Fouldenia , as originally predicted by Moy-Thomas (1937), and are thus reassigned to the latter taxon. The older name for the single known species within Fouldenia is therefore ischipterus ( Traquair, 1881) , not ottadinica . This has been corrected to ischiptera to agree with the gender of the genus. Likewise, the appropriate holotype for Fouldenia is GSE 2187, as designated for Holurus ischipterus by Traquair (1881), rather than NHM P13178 as proposed for Fouldenia by White (1927).

DESCRIPTION

Skull

The construction of the skull of Fouldenia is largely as illustrated by White (1927) and Gardiner (1985), but there are several discrepencies. Starting at the anterior end, Fouldenia bears an elongate, nearly erect median rostral, featuring sigmoidal lateral margins ( Figs 5–7 , 13 ). The rostral exhibits a slight curve in smaller specimens, but is significantly inclined halfway along its length in larger individuals, where it is ornamented with wide, horizontal ganoine bands ( Figs 7B , 13D ). The elongate, curved nasals of Fouldenia are bounded by the premaxilla and lachrymal ventrally, and frontal and dermosphenotic dorsally ( Figs 5C, D, F, G , 6C , 7C, D, E ). The nasal widens towards the gape and does not exhibit marginal indentitions for the nostrils, contrary to Gardiner’s (1985) reconstruction.

In the smallest specimens of Fouldenia (e.g. NHM P14562; Fig. 5B ), the frontals and parietals are relatively wide and nearly equal in size, but in larger fish (e.g. NHM P13181; Fig. 7D ) the frontals appear longer and narrower. These are joined at a diagonal suture line formed by the posterolateral processes on the frontals. The supraorbital canal runs through the lateral midline of these bones, and the mesial surface of the canal is apparent in the holotype NHM P13178 ( Fig. 6B ) and NHM P61549 ( Fig. 6H ). Frontal ornament consists primarily of horizontal striations, although a curved ridge at the midpoint marks a granulated or wavy posteromedial ornament field in both NHM P14562 ( Fig. 5B ) and NHM P13181 ( Fig. 7D ). The parietal ornament consists of intercalating wavy ridges laterally and more longitudinal bands dorsally, as is apparent in NHM P13179, NHM P13180, and NHM P13181 ( Figs 5F, G , 7D ). The dermopterotic is more like that described by White (1927) than Gardiner’s (1985) reconstruction; the posterior portion is wide and the margins are smooth ( Figs 5–7 ). The lateral margin curves to form an anterior point. This margin sits at the level of the curved ridge on the frontal in NHM P14562 ( Fig. 5B ) and NHM P13181 ( Fig. 7D ), whereas the straight posterior margin contacts the extrascapular. This spatial relationship appears fixed in individuals of all sizes. Fouldenia possesses one pair of rectangular extrascapulars ( Figs 5B, F, D , 6A, C, F, B , 7D ) that sit at the level of the anterior margin of the opercular series, ornamented with short horizontal striations. The post-temporal is wide with a diagonal posterior margin. Although the general form is blunt and rounded in smaller fish (e.g. NHM P14562, NHM P13179, NHM P13180; Fig. 5B, F, G ), it tapers to a rounded lateral point in larger specimens (e.g. NMS 1984.67.64, NHM P61002, NHM P61548; Fig. 7A, B, D ). The post-temporal is also long, extending past the opercular series.

The orbit in Fouldenia is bounded by the nasal, the dermosphenotic, and two infraorbitals (the lachrymal and jugal), but not by the premaxilla as described by Gardiner (1985). The dermosphenotic is ornamented with horizontal ridges ( Fig. 6C ) and stretches from the nasal to the preoperculum, leaving little room for any unobserved supraorbitals or suborbitals. In specimens under 7 cm in total length (e.g. NHM P14562 and NMS 1965.4.3; Fig. 5B, C ), the dermosphenotic is a homogenous, thick crescent, whereas in larger specimens (e.g. NHM P14564 and NMS 1984.67.64; Figs 6C , 7C ) it features a distinct rectangular ventral portion and an elongated anterior process. Contrary to the previous description by Gardiner (1985) and coding for a t-shaped dermosphenotic by Gardiner & Schaeffer (1989), there is no evidence for a distinct posterior process (NHM P14562, NMS 1965.4.3, NMS 1956.5.1, and NHM P14564; Figs 5B, C, D , 6C ).

The jugal is unfortunately incomplete in specimens of Fouldenia , but largely conforms to that described by Gardiner (1985). It forms a thick arch extending from the dermosphenotic to the midpoint of the orbit in NMS 1984.67.65 and NHM P13179 ( Figs 5F , 6F ), and widens with size (NHM P61548 and NHM P13181; Fig. 7D, E ). It is not clear if the horizontally ridged ornament in NMS 1965.4.3, NHM P13181, and NHM P61548 ( Figs 5C, D , 7E ) has any relationship with a branching suborbital canal, as in Amphicentrum ( Coates, 1988) . The lachrymal is rectangular and meets the premaxilla and nasal at an anterior margin that is blunt in small individuals (NHM P14562; Fig. 5B ), but is tapered in the largest specimens (NHM P13181; Fig. 7D ).

Jaws and dentition

A badly preserved palatoquadrate is found in NHM P13183 ( Fig. 6A ), but little can be determined about the morphology. The dorsal surface of the parasphenoid is fairly wide in NMS 1984.67.65 ( Fig. 5E ), with a shallow central depression and lateral margins marked by prominent ridges that might represent parabasal canals. The bone is laterally thick in NHM P13180 ( Fig. 5G ) and NMS 1984.67.65 ( Fig. 6E ), widening towards the posterior wall of the orbit. Phylogenetically important information concerning the extent and condition of the ascending process remains unknown ( Patterson, 1982; Gardiner & Schaeffer, 1989; Coates, 1999).

The maxilla is well preserved in nearly all specimens of Fouldenia and overlaps the mandible, particularly a rounded posteroventral process. The maxilla features a rounded, triangular posteriorly expanded portion and a rectangular anterior ramus ending in a blunt margin. In the smallest individuals (e.g. NMS 1980.40.30 and NHM P14562; Fig. 5A, B ), the two portions are nearly equal in height to the premaxilla, and the bone features rounded, edentulous ventral margins, as best shown in NHM P13180 and NMS 1984.67.62 ( Figs 5G , 7C ). The premaxilla bears rounded lateral processes that sit above the maxillae ( Figs 7C , 13 ), but these are less prominent than reconstructed by Gardiner (1985), and do not contact the orbit.

The surangular is not visible in any specimen of Fouldenia , and there appears to be little room for such a bone between the dentary and angular. The dentary is robust in specimens over 10 cm in total length ( Fig. 7 ), expanding dorsally such that the symphysial margin is diagonal and the jaw margin is prominent. In smaller specimens ( Fig. 5 ), a more gracile, blunt dentary is almost entirely overlapped by the maxilla. The crescent-shaped angular in Fouldenia ( Figs 5D, G , 6A, F, G ) is obscured by the maxilla in smaller specimens, as described by White (1927) and Gardiner (1985). However, it extends posteriorly in larger fish (e.g. NHM P13185, NHM P14562, and NMS 1984.67.65; Figs 6B, C , 7B ), with its juncture with the quadratojugal exposed.

Bands of smooth ganoine sit parallel with the jaw margins in all specimens, and the prominence of these is apparent in NHM P14564 ( Fig. 6C ), NHM 1984.67.65 ( Fig. 6E ), NHM P61549 ( Fig. 6G ), NHM 1984.67.64 ( Fig. 7B ), and NHM P61548 ( Fig. 7E ). The smallest specimens (e.g. NMS 1980. 40.30 and NHM P14562; Fig. 5A, B ) have a single rectangular tract on each side of the gape. This increases to two maxillary and three mandibular bands in mid-sized individuals, such as NHM P13179 ( Fig. 5F ), with an additional tract added along the anterodorsal margin of the maxilla in larger individuals like NMS 1984.67.65 ( Fig. 6E ). In the largest specimens of Fouldenia (e.g. NMS 1984.67.64; Fig. 7B ), the bands are fused and intercalated, joined by shorter segments of ornament, forming a solid enameloid ‘beak’. Similar ganoine cover is found in Styracopterus (see above) and the Visean styracopterid Benedenius ( Van Beneden, 1871; Traquair, 1877 –1914, 1878; De Koninck, 1878; Moy-Thomas, 1937; L.C.S., pers. observ.; Appendix S1), as well as the Cheirodopsis , Amphicentrum, Paramesolepi s, and Eurynotus , albeit without distinct bands (L.C.S., pers. observ.; Appendix S1; Fig. 15 ). A line of tiny holes in the dentary ornament marks the mandibular canal in NMS 1984.67.64 ( Fig. 7B ), as do similar openings in Eurynotus (NMS 1957.1.5686; L.C.S., pers. observ.), Amphicentrum ( Coates, 1988) , and Styracopterus (L.C.S., pers. observ.). The rest of the maxilla is ornamented with concentric ridges at the margins (as in NMS 1984.67.65; Fig. 6E ) and a distinct field of lateralized wavy ridges at the centre of the posterior portion. The angular in Fouldenia is almost completely covered by enameloid patches in fish over 9 cm in total length, such as NMS 1984.67.65, NHM P61002, and NMS 1985.67.64 ( Figs 6F , 7A, B , 13C, D ), yet is naked in smaller individuals.

The gape in all specimens of Fouldenia appears edentulous in lateral aspect. White (1927) described and illustrated small, blunt teeth from the posterior portion of the dermal mandible in NHM P13183 ( Fig. 6A ). These possess flat, bulbous crowns and thin waists ( Fig. 14 ), resembling the pediculate dentition of the Palaeozoic actinopterygians Mesolepis ( Traquair, 1879; Boulenger, 1902; Coates, 1988; L.C.S., pers. observ.; Appendix S1), Eurysomus , as illustrated by Traquair (1879: pl. IV; fig. 6.5), and Benedenius , as described by Boulenger (1902). Given the position of these ‘teeth’ in the Fouldenia specimen NHM P13182–3 ( Fig. 6A ), away from the dentary and just ventral to the palatoquadrate, they are likely associated with coronoid-derived mandibular tooth plates like those found in Styracopterus and other eurynotiform fishes ( Moy-Thomas & Miles, 1971; Coates, 1988; L.C.S., pers. observ.; Appendix S1; Fig. 14 ).

There is also evidence for an upper jaw dentition obscured from lateral view. In NHM P61549 ( Fig. 6G ), a homogeneous set of closely situated, small round elements is exposed by a gap in the anterior ramus of the maxilla. In NMS 1984.67.61 ( Fig. 5E ), tall cylindrical teeth with expanded crowns are found in the same area, with roots attached to what is either the mesial surface of the maxilla or reinforced palatal bones. NMS 1980.40.31 ( Fig. 6D ) has imprints of tooth or denticle bases overlying an impression of the maxilla and remnants of ganoine ornamentation. These teeth are positioned similarly to the maxillary dentition found in Amphicentrum and Styracopterus , but may also be associated with ectopterygoid-derived tooth plates, as in the same taxa ( Bradley Dyne, 1939; Coates, 1988; L.C.S., pers. observ.; Appendix S1). In light of the mandibular tooth plate denticles in NHM P13183, it is probable that Fouldenia possesses both maxillary teeth and palatal tooth plates ( Fig. 14 ), but the fossil evidence and/or morphological similarity prevents definite differentiation.

The jaws of Fouldenia are supported by a nearly vertical suspensorium, as reconstructed by Gardiner (1985). The hyomandibula is exposed in a number of individuals of different sizes, including NMS 1980.40.30, NHM P14562, NHM P14564, and NMS 1985.67.64 ( Figs 5A, B , 6C , 7B ). The thick hyomandibula features an expanded dorsal head located near the dermopterotic, as in Amphicentrum ( Bradley Dyne, 1939) , and covered laterally by a teardropshaped dermohyal with an ornament of vertical ridges ( Figs 6G , 7B ). Whereas the hyomandibula is nearly straight in smaller specimens (e.g. NMS 1980.40.30; Fig. 1A ), it exhibits a high degree of curvature in the large individuals listed above.

The preoperculum in Fouldenia , best preserved in NMS 1980.40.30, NHM P13180, NHM P14564, and NHM P61002 ( Figs 5A, G , 6C , 7A ), is tall with an indistinct dorsal arm ending near the skull roof and a thick posteroventral process to the quadratojugal. The anterior margin is shallowly curved, whereas the posterior margin mirrors the hyomandibula and is lined by a wide ridge. A horizontal pit line is visible just above the maxilla in many specimens (e.g. NHM P61002, NHM P13180, NHM P13179, NMS 1956.5.1, and NMS 1985.67.64; Figs 5D, F, G , 7A, B ), and bisects the ornament into a ventral field of vertical ridges and a dorsal field of horizontal and inclined lines (see NHM P14564 and NHM P61002; Figs 6C , 7A ) .

Opercular series

The operculum of Fouldenia is similar to that reconstructed by White (1927), in that it is slightly inclined with a rounded apex just under the extrascapular and a sigmoidal ventral margin at the level of the horizontal pitline (see NMS 1984.67.64, NHM P13179, NMS 1980.40.30; Figs 5A, F , 7B ). The bone is wider and much less tapered in individuals over 8 cm ( Figs 6 , 7 ), so that it appears more plate-like than ovoid. The ornamentation consists of intercalated diagonal ridges running posteroventrally, as seen in NHM P61002 ( Fig. 7A ).

The suboperculum of Fouldenia is largely rectangular in NHM P61002 ( Fig. 7A ). It extends ventrally to the quadratojugal and posteriorly to the level of the post-temporal. The posterior margin is slightly rounded, whereas the ventral margin is sigmoidal, bearing an anteroventral process reaching towards the jaw joint, as originally illustrated by White (1927; Figs 6E , 7A , 13 ) and also found in other styracopterids, eurynotiforms, and the Palaeozoic taxa listed in the description of Styracopterus (L.C.S., pers. observ.; Appendix S1). Like the other styracopterids (L.C.S., pers. observ.; Appendix S1), the ornamentation of the suboperculum in NHM P13180 ( Fig. 5G ) and NHM P61002 ( Fig. 7A ) is divided: the ganoine ridges on the process and near the ventral margin are posteroventrally diagonal, whereas the ornamentation elsewhere radiates horizontally or dorsally from the anterior margin of the bone.

Gulars and branchiostegals

Fouldenia has small leaf-shaped median gular with ventral ornament of concentric bands mirroring the same shape, as revealed by the displaced bones of NMS 1984.67.64 and NHM P61548 ( Fig. 7B, E ). The morphology of rectangular lateral gulars in NHM P13180, NMS 1984.67.65, and NMS 1984.67.64 ( Figs 5G , 6E , 7B ) is difficult to distinguish from the branchiostegals. The largest individuals of Fouldenia may have up to 12 rectangular branchiostegal rays with rounded distal margins, although no specimen has a complete set ( Fig. 7 ). The rays increase in length posteriorly and are held at an almost 45° angle to their origin in 10–12-cm fish, as shown by NMS 1984.67.64 ( Fig. 7B ). This ‘flaring’ is unusual as the branchiostegals are directed inwards towards the ventral midline in most early actinopterygians ( Gardiner, 1984). It might be caused by diagenetic crushing or other taphonomic processes, as all Foulden actinopterygians share this trait ( White, 1927; Moy-Thomas, 1938; Gardiner, 1985; Fig. 17 ). However, the open positioning of the branchiostegals in these fishes may be induced by a robust clavicle and shoulder girdle, forcing the branchiostegal bases above the ventral body margin ( Gardiner, 1985; L.C.S., pers. observ.; Appendix S1; Figs 5E, B , 7C, E , 13C, D ). It is notable that individuals of Fouldenia under 8 cm in length do not exhibit such flaring, and likewise have more ventrally positioned gapes ( Figs 5C, D , 13A, B ).

Shoulder girdle

The inclined, rounded supracleithrum in Fouldenia extends to a pointed margin near the midline of the suboperculum ( Fig. 7B ). There is no evidence of the course of the posterior lateral line, as it is likely to be obscured by the dense, vertical ganoine ridges spread out around the midline in NHM P61548 ( Fig. 7B ). Neither White (1927) nor Gardiner (1985) described postcleithra in Fouldenia , but a single postcleithrum is preserved in NMS 1984.67.62, NMS 1984.67.65, NHM P61549, and NHM P13180 ( Figs 5C, G , 6E, G ). This is similar to the postcleithra in Paramesolepis ( Moy-Thomas & Bradley Dyne, 1938) , Amphicentrum ( Coates, 1988) , and Styracopterus (L.C.S., pers. observ.; Appendix S1), in that it is long with rounded margins and ends around the first branchiostegal.

The cleithrum in Fouldenia is inclined posteriorly, and bears a broadly crescentic dorsal process with a pointed dorsal limit and a rounded posterior process underlying the pectoral fin insertion. ( Figs 5A, B, E , 6B , 7A ), as in other Foulden actinopterygians ( White, 1927; Moy-Thomas, 1938; Gardiner, 1985; L.C.S., pers. observ.; Appendix S1). The cleithrum is ornamented with vertical stripes of ganoine that mirror the curved posterior margin. A process of the endoskeletal shoul- der girdle is exposed where the cleithrum curves around the point of pectoral fin insertion in NHM P14562 ( Fig. 5B ) and NHM P61546 ( Fig. 5H ), and is covered with longitudinal ridges distinct from any nearby dermal ornament. The cleithrum joins with the laterally convex posterior margin of the clavicle at the level of the opercular series in NMS 1984.67.62 ( Fig. 7C ) and NMS 1984.67.65 ( Fig. 6F ). The morphology of the clavicle is largely obscured, but it appears very thick (as discussed above) and somewhat triangular in the lateral dimension in NMS 1984.67.61, NHM 1984.67.65, NHM P61002, and NHM P14564 ( Figs 5E , 6C, F , 7A ).

Paired fins

The pectoral fin is situated near the ventral body margin and has a nearly vertical insertion into the shoulder. The fin is supported by at least six radials: posterior/ventral radials preserved in NHM P61002 ( Fig. 8J ) and NMS 1984.65.61 ( Fig. 8C ) are elongate, with narrow, somewhat overlapped proximal ends and rounded distal portions. The middle of each radial in NHM P61002 ( Fig. 8J ) consists of a narrow cylindrical body with a thin ventral flange. GLAHM V8327 ( Fig. 8E ) shows the squat anterior/ dorsalmost radials, which sit in a shorter space between the primary fin rays and the dermal shoul- der. The presumptive propterygium is surrounded by fin rays, as is diagnostic for actinopterans ( Coates, 1999; Gardiner et al., 2005). The difference in length between the anterior and posterior radials matches the diagonal orientation of lepidotrichial bases in NHM P13183, GLAHM V8327, and NHM P14564 ( Fig. 8F, G ), an arrangement found in other early actinopterygians such as Mimipiscis ( Choo, 2011) and Styracopterus (L.C.S., pers. observ.; Appendix S1).

As noted by White (1927), no specimen of Fouldenia exhibits a complete pectoral fin, obscuring its exact shape. The proximal fins in GSE 2143, GLAHM V8327, NHM P13183, and NHM P14564 ( Fig. 8A, E, F, G ) suggest that it was wide-based with a slightly curved anterior margin lined by fringing fulcra that are barely distinguishable from lepidotrichial terminals. The lepidotrichia are thin, cylindrical, and densely packed, with long segments throughout their length. Thus, the pectoral fin more closely resembles that of Phanerosteon ovensi ( White, 1927; L.C.S., pers. observ.; Appendix S1) and the Visean Tarrasius (Sallan, 2012) , than Styracopterus .

Only a handful of specimens have incomplete pelvic fins. The curved anterior edge is preserved in NMS 1984.67.61 ( Fig. 8D ), with an unsegmented primary fin ray and further lepidotrichia similar to those of the pectoral fin. Large, leaf-shaped fringing fulcra are differentiated from lepidotrichial terminals by linear central furrows. Two large, ovoid basal fulcra overlap the leading edge of the pelvic fin in NMS 1984.67.65 ( Fig. 8I ). These are ornamented with concentric, longitudinal striations that fuse at the posterior margin. The size of the fin base is restricted by the short distance between the pelvic and anal basal fulcra in all fish, even though the position of the pelvic fin changes with size ( Fig. 13 ). In the 4-cm individual NMS 1980.40.30, the pelvic fin is situated near the distal pectoral fin, whereas in larger specimens, such as NMS 1984.67.65, it originates posterior to the body midline ( Fig. 8 ).

Median fins

The dorsal fins in the largest specimens of Fouldenia are as described by White (1927): tall and superficially triangular, with the peak in the posterior half of the fin. In these individuals (e.g. NHM P61548 and NHM P61549; Fig. 9E, F ), the anterior margin of the dorsal fin and leading lepidotrichia are straight, and the fin originates in the posterior portion of the body ( Fig. 13C, D ). In the smaller specimens NMS 1980.40.30, NMS 1980.40.27, and NMS 1965.4.3 ( Fig. 9A, B, C ), the dorsal fin is rounded with curved lepidotrichia held nearly parallel with the body axis, and originates at the midpoint of the trunk, opposite the pelvics and well before the anal fin ( Fig. 13A, B ). Robust, erect basal fulcra sit against the dorsal lepidotrichia. Disarticulated fulcra in NMS 1980.40.31 ( Fig. 9D ) exhibit needle-like distal processes with paired cylindrical ridges separated by a deep furrow, bounded ventrally by rounded plates with concentric ornament. The primary dorsal lepidotrichia are thick and unsegmented, presenting a spine-like form when combined with the elliptical fringing fulcra ( Fig. 9 ). The remaining lepidotrichia have rounded bases and short plate-like segments with cylindrical anterior ridges.

The anal fin in Fouldenia is very similar to the dorsal fin, with the first few lepidotrichia likewise consolidated into ‘spines’, a feature readily apparent in NMS 1965.4.3 ( Fig. 10C, D ). Anal basal fulcra in NMS 1984.67.62 and NHM P61548 ( Fig. 10I, J ) are evocative of their dorsal equivalents and those of Styracopterus (see above). However, more anterior basal fulcra in this series of four or five are thick and rounded like the dorsal ridge scales. The anal fin base extends to the base of the caudal peduncle. Like the dorsal fin, the anal fin is rounded in smaller specimens such as GSE 2187 ( Fig. 10B ) and NHM P13183. The anterior margin remains curved in the largest fishes, whereas the posterior margin is deeply concave (e.g. NHM P13183), resulting in a superficially half-crescent form in NHM P61548 ( Fig. 10J ).

Tail and caudal fin

The caudal region of Fouldenia comprises over a third of the total body length in the smallest specimens (e.g. NMS 1980.40.30; Figs 11A , 13A ) and a quarter in the largest (e.g. NHM P61549; Figs 12D , 13D ). As described by White (1927), the tail consists of a thick caudal peduncle, a deep yet shallowly cleft caudal fin, and a very long, tapering axial lobe extending beyond the fin. The dorsal surfaces of the peduncle and axial lobe are covered by enlarged basal fulcra that, as preserved in NHM P13183 and holotype NHM P13178 ( Fig. 12A, B ), are identical to the dorsal fin basal fulcra in NMS 1980.40.31 ( Fig. 9D ). The posteriormost basal fulcra on the ventral surface of the peduncle in NMS P13183 ( Fig. 12A ), originally illustrated by White (1927), are nearly mirror images, except that the paired rami separate to form distinct points distally ( Fig. 12A ), and are thus narrower versions of the same scales in Styracopterus . More posterior basal fulcra on both the peduncle and axial lobe are progressively shorter and have pointed apices, providing a highly overlapped, elliptical appearance in lateral view ( Figs 11 , 12 ).

Prominent fringing fulcra line the entire ventral margin of the caudal fin in NMS 1965.4.2, NMS 1965.4.3, NMS 1980.40.31, and NHM P13185 ( Figs 11A, F , 12C, E ). These are also elliptical, slightly overlapped, and sit perpendicular to the lepidotrichia. Fulcra appear as narrow spines with longitudinal furrows on the base of the axial lobes in NHM P13183 and NMS 1965.4.2 ( Figs 11C , 12A ), becoming smaller and more inclined distally in GSE 2187, NMS 1965.4.3, and NHM P13185 ( Figs 11C, F , 12E ). Fulcra in the second half of the series are paired in larger specimens ( Fig. 12 ), a trait previously reported only in Cheirolepis ( Gardiner, 1984) , but also found along the distalmost tails of Styracopterus , Aesopichthys , and other Foulden fishes (L.C.S., pers. observ.; Appendix S1). In fact, all the axial lobe fulcra are paired in smaller, probably juvenile individuals with relatively shorter axial lobes, such as NHM P13187, NMS 1965.4.2, and NMS 1980.40.27 ( Fig. 11B, C, E ), suggesting fusion during ontogeny in Fouldenia and perhaps other actinopterygians. The large specimens NMS 1980.40.31 and NHM P13185 ( Fig. 12C, E ) are the only individuals with complete axial lobes, which lack fulcra distal to the caudal fin. This arrangement is similar to that of the elongated axial lobe in the deep-bodied Serpukhovian genus Aesopichthys ( Poplin & Lund, 2000; L.C.S., pers. observ.; Appendix S1), as well as other styracopterids and Eurynotus (e.g. NMS 1878.18.12; Traquair, 1879; L.C.S., pers. observ.; Appendix S1).

Both NMS 1980.40.31 and NHM P13185 ( Fig. 12C, E ) bear remnants of a distinct epichordal fin around the distalmost tail. Lepidotrichia found along the ventral surface of the posteriormost axial lobe differ from the caudal lepidotrichia in their tapering morphology, and originate after a considerable gap.

The first few lepidotrichia of the caudal fin are consolidated and bear elongated fringing fulcra like their equivalents in the median fins ( Figs 11 , 12 ). Likewise, the remaining rays in the ventral lobe have plate-like segments with anterior ridges and taper to a point ( Figs 11 , 12 ). The bases of ventral lobe lepidotrichia are distinctly thick and wide in NHM P61548 ( Fig. 12 ), giving the appearance of an extended peduncle. Lepidotrichia in the dorsal lobe originate from the axial lobe in NHM P13185, NHM P61549, and NMS 1980.40.31, and are similar to the paired fin rays in their thin, cylindrical morphology. These apparently lack bifurcation, although most are truncated at their distal extremities.

The overall appearance of the tail and caudal fin varies between size classes. Small individuals have long peduncles that exhibit a gradual, not well marked, transition into a thin axial lobe ( Figs 11 , 12 , 13A, B ). Their long caudal fins exhibit shallow clefts and small, narrow fringing fulcra. In contrast, the largest individuals, such as NHM P61548 and NHM P13183 ( Fig. 12E, F ), have tall peduncles with nearly vertical posterior margins and a marked transition into a wide-based, distally tapering axial lobe. Caudal fins have deep clefts and are equilobate, and feature robust fringing fulcra. The terminus of the lateral line changes along with the morphology. In the 5-cm individual NMS 1965.4.2 ( Fig. 11C ), the lateral line curves parallel with the posterior margin of the caudal peduncle as it approaches the end of the body, turning onto the midline of the axial lobe. In the 12-cm fishes NHM P13185 and NHM P61548 ( Fig. 12E, F ), the lateral line is largely horizontal, and appears to end on the caudal fin just ventral to the base of the axial lobe, but may continue at the caudal fin base.

Squamation

The largest individual of Fouldenia, NHM P 61548 (~ 12 cm EBL), has 51 sigmoidal scale rows from the skull to the peduncle, with up to 34 scales from the dorsal ridge to the anal midline. However, individuals under 6 cm in length have significantly fewer scales: NMS 1965.4.2 (5 cm EBL) has approximately 34 rows of up to 25 scales. The slightly longer NMS 1965.4.6 (5.5 cm EBL) has around 40 rows, whereas NMS 1980.40.27 (6.5 cm EBL) had almost the maximum number of rows, although the number of scales per row remains at 25. The larger NMS 1984.67.61 (7.5 cm EBL) has 30 scales per row at maximum body depth. Fish of 8–9 cm in total length, such as NHM P61002 and NMS 1984.67.63, have approximately the same counts as the largest specimen. Thus, scale counts increase with body size primarily through the generation of rows up to the midpoint of the observed growth series, at which point addition at the margins and increases in scale height cover changes in body area.

In smaller specimens of Fouldenia , such as GSE 2143 (3 cm EBL), NMS 1980.40.30 (4 cm EBL), NMS 1965.4.6 (5.5 cm EBL), NHM P14562 (6 cm EBL), and GSE 2187 (6 cm EBL), all scales are small and acuminate, with smooth margins ( Fig. 14A, B ), and bear two longitudinal bands of smooth ganoine. These are identical to the scales found on the smallest specimens of the co-occurring Aetheretmon (NHM P61006; L.C.S., pers. observ.). Like Aetheretmon , the smallest Fouldenia individuals, such as NMS 1980.40.30 ( Fig. 9A ), lack scales around the bases of their median fins. This suggests that squamation first developed around the lateral line in these fishes, just as it does in living teleosts ( Koumoundoros, Divanach & Kentouri, 2001).

Juvenile scales develop near the dorsal and ventral midlines and are retained on the posterior half of specimens NMS 1965.4.3, NMS 1984.65.61, and NMS 1956.5.1, which range from 7 to 7.5 cm in estimated body length, and have greater body depths anteriorly ( Fig. 13C ). In individuals over 8 cm in length [e.g. NHM P13178 (9 cm EBL), NMS 1984.67.65 (9.5 cm EBL), NHM P61502 (10 cm EBL), NMS 1984.67.65 (11 cm EBL), and NMS 1984.67.65 (11 cm EBL); White 1927; Fig. 13C ], ‘juvenile’ scales are limited to an anterior ventral field around the pectoral fins and clavicle. The largely scaleless actinopterygian Phanerosteon (e.g. NMS 1970.26.27; L.C.S., pers. observ.; Appendix S1), found at both Foulden and Glencartholm, exhibits rows of small scales in the same area ( White, 1927; Gardiner, 1985). Scale rows in this region in Amphicentrum and Benedenius have a different orientation from those on the rest of the flank ( Traquair, 1878, 1879; L.C.S., pers. observ.; Appendix S1). This anterior ventral field (or pectoral peduncle; Sire & Arnulf, 1990) might represent a distinct region of scale development in some early actinopterygians, just as it does in some living teleosts ( Koumoundoros et al., 2001).

In individuals larger than 8 cm in length, such as NMS 1984.65.61 and NHM P13182 ( Fig. 13C, D ), rhomboid scales on the anterolateral flank have smooth margins and exhibit four or more posteroventrally diagonal bands of ganoine. The scales in larger fish (e.g. NHM P61546) are taller and feature a more complex ornament of diagonal ganoine bands that fuse in nested fashion towards the posteroventral corner and a stripe lining the posterior margin. This ornamentation is repeated vertically on the taller, more rectangular anterior flank scales of the larger specimens. A small notch on the posterior margin marks the transition between ornamentation fields, giving the bilobate appearance described by White (1927) and the impression of scale fusion.

Scales on the posterior flank of fish over 8 cm (e.g. GLAHM V8327 and NHM P13182) are shorter and horizontally elongated ( Fig. 13C, D ), yet exhibit a rhomboid shape and ornamentation similar to the more anterior squamation. In specimens over 10 cm in total length, these scales have the same relative height and morphology as anterior flank scales in 8-cm fish ( Fig. 13C, D ). In large specimens of Fouldenia , the lateral line scales are taller than other members of the same row, but are otherwise indistinguishable apart from a raised cylindrical bump that bisects the ornament horizontally ( Fig. 13C, D ). The lateral line runs from the level of the supracleithrum to the midline of the caudal peduncle in the largest specimens ( Figs 12F , 13D ). The diverse flank scales in Fouldenia are similar in shape and ornament to various iterations in the Permian taxon Acrolepis sedgwicki Agassiz, 1833 –1844, as first illustrated by Agassiz (1833 –1844; NHM P553; L.C.S., pers. observ.; Appendix S1), which is a large marine fusiform actinopterygian that diverges from Fouldenia in most other non-plesiomorphic traits.

All specimens over 8 cm in length possess a distinct ‘hinge line’ in the peduncle squamation, marking the transition between scales with vertically directed pegs and those with posteriorly directed pegs, a trait found in many other early actinopterygians ( Patterson, 1982; Gardiner & Schaeffer, 1989; Coates, 1998, 1999; Figs 12F , 13C, D ). Peduncle rows on both sides of the hinge line contain small rhomboid scales fully covered in ganoine and featuring an anterior ridge ( Figs 12D , 13C, D ). These resemble the microsquamation present on the entire tail surface of Tarrasius (Sallan, 2012; L.C.S., pers. observ.; Appendix S1). Scales on the proximal axial lobe and along the base of the caudal fin are diamond shaped, whereas the more distal squamation consists of very small elliptical scales resembling the ‘juvenile’ flank scales ( Figs 12D , 13 ).

Fouldenia specimens over 8 cm in length possess prominent dorsal ridge scales that run from the nape to the dorsal fin basal fulcra ( White, 1927; Fig. 13C, D ). They are highly overlapped: each has a triangular furrow on the anterodorsal surface to hold the distal end of their neighbour. The anteriormost ridge scales are squat with rounded margins and slightly pointed apices, and feature longitudinal striations. Ridge scales in the posterior half of the series are longer, wider, and vertically oriented, and exhibit more robust ornamentation. The last few ridge scales are thinner and have distal processes like the dorsal basal fulcra, and are similarly erect and overlapped. Thick ganoine ridges on these scales run parallel with the body axis. Thus, the transition between the last two dorsal ridge scales, the basal fulcra, and the primary lepidotrichia appears gradual ( Fig. 13C, D ).

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 5. Fouldenia cranial material I. Photographs and interpretative drawings for specimens of 4–8 cm in total length. Unlabelled scale bars equal 1 cm. Medium-grey infill indicates the extent of thick ganoine ornament. Light-grey infill indicates unidentified skull material. A, NMS 1980.40.30; B, NHM P14562; C, NMS 1965.4.3; D, NMS 1956.5.1; E, NMS 1984.67.61; F, NHM P13179; G, NHM P13180; H, NHM P61546.

Figure 6. Fouldenia cranial material II. Photographs and interpretative drawings for specimens of 8.5–10 cm in total length. Unlabelled scale bars equal 1 cm. Medium-grey infill indicates the extent of thick ganoine plates covering other ornament. Light-grey infill indicates unidentified skull material. A, NHM P13183; B, NHM P13178; C, NHM P14564; D, NMS 1980.40.31; E, NMS 1984.67.65 (part); F, NMS 1984.67.65 (counterpart); G, NHM P61549 (part); H, NHM P61549 (counterpart).

Figure 7. Fouldenia cranial material III. Photographs and interpretative drawings for specimens of 10.5–12 cm in total length. Medium-grey infill indicates the documented extent of thick ganoine plates covering other ornament. Light-grey infill indicates unidentified skull material. Unlabelled scale bars equal 1 cm. A, NHM P61002; B, NMS 1984.67.64; C, NMS 1984.67.62; D, NHM P13181; E, NHM P61548.

Figure 8. Fouldenia paired fins. Unlabelled scale bars equal 1 cm. A, GSE 2143 pectoral; B, 1980.40.30 pectoral; C, 1984.67.61 pectoral; D, NMS 1984.67.61 pelvic; E, GLAHM V8327 pectoral; F, NHM P13183 pectoral; G, NHM P14564 pectoral; H, NMS 1984.67.65 pectoral; I, NHM 1984.67.65 pelvic; J, NHM P61002 pectoral; K, NMS 1984.67.64 pectoral.

Figure 9. Fouldenia dorsal fins. Unlabelled scale bars equal 1 cm. A, NMS 1980.40.30; B, NMS 1980.40.27; C, NMS 1965.4.3; D, NMS 1980.40.31; E, NHM P61548; F, NHM P61549.

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 14. Reconstructions of styracopterid dentitions: A, Fouldenia; B, Styracopterus.

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 17. Comparison of early actinopterygian skulls II. A, Styracopterus. B, Platysomus superbus (after Moy-Thomas & Bradley Dyne, 1938). C, Adroichthys (after Gardiner, 1969).