Uraniacanthus curtus ( Powrie, 1870 ) Newman & Davidson & Blaauwen & Burrow, 2012

Newman, Michael J., Davidson, Robert G., Blaauwen, Jan L. Den & Burrow, Carole J., 2012, The Early Devonian Acanthodian Uraniacanthus curtus (Powrie, 1870) n. comb. from the Midland Valley of Scotland, Geodiversitas 34 (4), pp. 739-759 : 744-751

Uraniacanthus curtus ( Powrie, 1870) n. comb. ( Figs 1 ; 3-13 )

Euthacanthus curtus Powrie, 1870: 293 , pl. 12, figs 7, 7a. — Woodward 1891: 32. — Traquair 1892: 34. — Paton 1976: 6. — Denison 1979: 27.

? Diplacanthus curtus – Woodward & Sherborn 1890: 65, 81.

Euthacanthus Curtus – Powrie 1881: 169.

? Uraniacanthus sp. – Paton 1976: 15.

TYPE SPECIMEN. — Powrie (1870) mentioned two syntypes when he first erected the species but described and figured only one, namely NMS G.1891.92.249 ( Figs 1 ; 3 ) from Farnell, the headless specimen which Traquair (1892) considered the type (and hence the lectotype). Paton (1976) described NMS G.1891.92.249 as the syntype.

REFERRED SPECIMENS. — Traquair (1892) also registered NMS G.1891.92.250 ( Figs 4 , 5 ), a very small complete specimen from Turin Hill. Although it is from Powrie’s collection, we cannot be certain it is the other one mentioned by Powrie (1870), as he provided no description. The third specimen NMS G.1964.31.30A/B ( Fig. 6 ), from Hayston Hill, is a partial articulated fish lacking the head. NMS G.1964.31.30B is the counterpart of the above but is less well-preserved

TYPE LOCALITY. — Pow Burn near Farnell, Brechin, Angus, Scotland (National Grid reference NO 622 549).

STRATIGRAPHIC HORIZON AND AGE. — Uraniacanthus curtus n. comb. occurs in the Arbuthnott Group of the Early Devonian (Lochkovian) of the Midland Valley of Scotland.

OCCURRENCE. — Apart from the type locality, Uraniacanthus curtus n. comb. has been collected from Turin Hill (probablyTillywhandland Quarry), Forfar (National Grid reference NO 528 537) and from a small, in-filled quarry north of Hayston Hill near Glamis, Dundee (National Grid reference NO 413 460).

REVISED DIAGNOSIS. — Short, deep bodied diplacanthiform acanthodians; dorsal fin spines have numerous longitudinal grooves and ridges, with the anterior dorsal fin spine having up to eleven ridges and the posterior up to nine ridges per side; four opercular covers on each side; flank scales are from 0.2 to 0.5 mm long; all body and fin scales are ornamented: scales have a flat crown with deep, wide, U-shaped grooves that sometimes extend the length of the crown, fanning out towards the anterior edge; large multiple pores in the anterior half of the grooves open into wide ascending canals that lead into wide circular canals, interconnected by short radial canals, in the lower crown; branched dentine tubules extend within each crown growth zone into the vascular canals; a row of 1-3 pores are on each side of the scale neck constriction.

DESCRIPTION

General structure

Specimens are from 20mm to an estimated 110 mm long. Whilst all three specimens are deep bodied, they are not complete enough to give an accurate depth to length range. All three specimens are preserved in lateral view suggesting the body was deeper than wide. The lateral sensory line is visible only on NMSG.1891.92.249 ( Fig.3A ) as a shallow depression running between the scales. Nothing is known of the sensory lines of the head. The endoskeleton is not preserved, and is presumed to be unmineralized.

Head

On NMSG.1964.31.30A ( Fig. 6 ) the head region is mostly missing except for some scattered broken dermal plates visible at the edge of the slab, but these are too fragmentary to be recognisable or reconstructed into their original arrangement. The plates are larger than the body scales. The head is completely missing from the holotype NMS G.1891.92.249 ( Fig. 3A ). As the outline of the head is not preserved on any of the specimens we cannot ascertain the slope of the head between the rostrum and the anterior dorsal fin. NMS G.1891.92.250 ( Fig.4 ) is the only specimen with recognisable, intact structures in the head region, but they are extremely small and the head is slightly disarticulated and rotated dorsally. Four spatulate opercular covers are preserved ( Fig. 4C )

cf

ad.fs

ppv.fs2

with the larger ones in the middle of the series. They are broader towards the anterior, tapering to a point at the posterior end. Their ornamentation is difficult to discern, but appears to consist of concentric, thin ridges. The opercular series has rotated slightly dorsally and would have originally been closer to the scapulocoracoid.

A small bone 3 mm long, interpreted as an occlusal plate on the right lower jaw ( Fig. 4B ) of NMS G.1891.92.250, has been displaced anterodorsally and is seen in lateral view. It is fairly well preserved and is a single ossified element with an antero-posterior axis that is slightly convex dorsally. Midway along the upper edge is a sub-triangular dorsal ‘coronoid’ process. Anterior of this process the bone is thickened on the dorsal edge. The posterior end of the bone forms a spoon-shaped blade. A shallow trough runs anteriorly from the posterior blade; the ventral edge of the bone is thickened.

Towards the dorsal anterior of the head is an impression of a thick triangular bone ( Fig.4F ). If it had rotated in the same direction as the rest of the bony elements of the head, it would have been positioned originally just behind the orbit. This element is thus interpreted as a postorbital plate, although no ornamentation can be observed because the internal rather than the external surface is exposed.

Scapulocoracoid

The dorsal end of the scapulocoracoid, a simple cylindrical shaft, is just visible on the edge of the slab of NMSG.1964.31.30A ( Fig. 6 ). The scapulocoracoid of NMSG.1891.92.250 ( Fig. 4D ), whilst being very small at only 2 mm high is more complete and the following description is based on this specimen. The scapulocoracoid is a perichondrallyossified structure. The scapular shaft is shorter in NMS G.1891.92.250 than U. probaton n. comb., but this may be an ontogenetic feature as the partial remains in NMS G.1964.31.30A indicate the scapulocoracoid was longer and had a thin dorsal tip in the adult form ( Fig. 6 ). The cylindrical shaft extends down toward the contact with the pectoral fin spine. The anterior edge of the scapulocoracoid is concave, as the element widens anteroventrally. It is not clear if an anterior lamina is present; the scapula flares out posteriorly toward the pectoral spine contact ( Fig. 4D ). The scapulocoracoid is tilted forward. Poor preservation obscures details of the articulation to the pectoral fin spine.

Spines

The dorsal, anal and pectoral fin spines are long and slender.The ornamentation consists of straight, smooth, deep grooves and rounded longitudinal ridges. The grooves and ridges terminate on the inner (trailing) edge of all the fin spines, as clearly evident on the anterior dorsal fin spines of both NMS G.1964.31.30A and NMS G.1891.92.250 ( Fig.7 ). The posterior dorsal fin spine is about fourfifths the length of the anterior dorsal fin spine. In the holotype NMS G.1891.92.249 ( Fig. 3 ) these are the only two spines preserved, with the base of the anterior dorsal fin spine missing at the edge of the slab. In this specimen the spines are preserved as coarse moulds and it is impossible to count the number of grooves and ridges, although it is clear that the anterior dorsal fin spine is quite broad at the base and has a distinctive posterior-pointing curvature at the tip. The spines are better preserved in NMS G.1964.31.30A ( Figs 6 ; 7A ) but only the dorsal and anal fin spines are present. The tip of the slightly curving anterior dorsal fin spine is buried in matrix and the base is broken, but it appears to have a deep base of insertion. This spine is fairly broad with at least 11 thin ridges and grooves visible on one side ( Fig. 7A ). The posterior dorsal fin spine of NMS G.1964.31.30A is straight and also fairly robust. Although not as well preserved as the anterior dorsal fin spine, nine thin ridges and furrows are visible on one side; the inserted end is covered in scales. The central canal of the spine has been mineralized and exposed towards the base of the spine, showing that it was about half the radius of the original spine. The anal fin spine is slightly curved, shorter (about 60%) and thinner than the posterior dorsal fin spine. Whilst not particularly well preserved, four ridges and grooves are visible on one side.

The small specimen NMS G.1891.92.250 ( Figs 4 ; 5 ; 7B ) is only 20 mm long. The fin spines are poorly preserved, but correspond to the relative dimensions and shapes of the spines on the two larger specimens while having fewer grooves and ridges. The anterior dorsal fin spine ( Fig. 7B ) has six grooves present on one side as well as a posterior-pointing curvature at the tip.

Newman M. J. et al.

The base of insertion is very deep. The posterior dorsal fin spine is too broken and crushed to be described. This is the only known specimen where the pectoral, pelvic and prepelvic fin spines are preserved. A pair of pelvic fin spines are present with the right spine being better preserved, showing three or four grooves and ridges on one side. The pelvic spines are about 75 percent the length of the anal spine. The pectoral fin spines are too crushed and broken to be described in detail and are jumbled up with another pair of crushed, broken fin spines. The latter must represent the anteriormost pair of intermediate (i.e. either admedian or first prepelvic) fin spines observed by Traquair (1892), but they cannot be visually separated from the pectoral spines. Some deterioration possibly has occurred since the specimen’s collection over 130 years ago, the rock matrix being susceptible to damage particularly from humidity. The second pair of prepelvic spines just anterior to the pelvic spines are better preserved, with three grooves and ridges visible on the side of one spine; they are about half the length of the anal fin spine.

The only other visible spines, positioned in front of the scapulocoracoids ( Fig. 4E ), are the right first and second prepectoral spines, the latter being larger. They appear to be isolated, with no attachment to any dermal bone, and surrounded by scales. The preservation is not good enough to allow further description.

Caudal fin

The caudal fin of Uraniacanthus curtus n. comb. is not well preserved in any of the specimens. In NMS G.1891.92.249 and NMS G.1964.31.30A the dorsal surface is only slightly deflected from the body. The tail is heterocercal as in all acanthodians, with the main or chordal lobe longer than the hypochordal lobe. At first glance, the caudal fins on NMS G.1891.92.249 and NMS G.1891.92.250 appear trilobed, but this is an artefact of the folding of the fin web and sediments. The tail has a slightly forked appearance.

Squamation

The scales in NMS G.1891.92.250 are not well enough preserved to be described while in NMSG.1891.92.249 ( Fig.8 ) they are only preserved as coarse impressions. In NMS G.1964.31.30A/B most of the scales are preserved with their bases showing, as often occurs when a specimen is split into two halves. Where exposed ( Fig. 9 ), the well preserved scale crowns have three to five grooves that are either quite close ( Fig. 9B ) or widely separated ( Fig. 10D ). Scales are from 0.2-0.5 mm long and wide with straight lateral crown edges converging to a single posterior point ( Figs9 B-D). On mid-flank scales the grooves extend the length of the scale crown, fanning out from the posterior corner to the anterior edge of the crown. Scales of the caudal fin have a crown ornament similar to body scales, but with grooves only on the anterior end of the crown ( Fig. 9D ). ESEM study of individual scales ( Fig. 10 ) from three regions on NMS G.1964.31.30A – dorsal caudal ( Fig. 10 A- E), dorsal posterior to the pdfs ( Fig. 10 F-K), and mid-flank below the posterior dorsal fin spine (pdfs) ( Fig. 10 L-O) – shows that the crown ornament is formed by 2-5 deep U-cross section grooves in the flat crown. The posterior point extended beyond the base ( Figs 9D ; 10J ). Crowns on some scales show short shallow incipient grooves along the anterior margin ( Fig. 10G, H ). The number of grooves increases with size of the scale ( Fig. 10E, O ). Several wide pores open out in the anterior part of the crown grooves ( Fig. 10A, C, D, F, O ), and also around the “waist” of the deep scale neck ( Fig. 10A, G , J-L). The pores in the crown grooves lead into wide canals within the crown, as exposed in the fracture on the scale in Figure 9B . The base is shallow or flat. Thin sections of the scales reveal a well-developed network of wide circular canals in the lower crown ( Fig. 11 A-F), interconnected by short radial canals which also lead to the pore openings in the neck ( Fig. 11A, E, F ); ascending canals lead to the pore openings in the crown grooves ( Figs 11C, D ). Other wide longitudinal canals extend back from the anterior of the ring canals towards the posterior tip of each growth zone ( Fig. 11B ). All the wide canals are interconnected to form a single network, rather than separate “pore” canal and vascular canal systems. Dentine tubules extend from the wide canals through each crown growth zone; each scale has 2-4 growth zones ( Fig. 11A, B, D ). Sharpey’s fibre canals extend through the acellular base ( Fig. 11 A-D).

Figures

FIG. 1. — Uraniacanthus curtus (Powrie, 1870) n. comb. NMS G.1891.92.249, holotype from Farnell. A reproduction of Powrie’s figure (1870: pl. 12, fig. 7) of the complete specimen and his enlargement of the scales (Powrie 1870: pl. 12, fig. 7a).

FIG. 3. — Uraniacanthus curtus (Powrie, 1870) n. comb., NMS G.1891.92.249, holotype from Farnell: A, photograph of the specimen; B, line drawing of the specimen. Abbreviations: ad.fs, anterior dorsal fin spine; cf, caudal fin; ll, lateral line; pd.fs, posterior dorsal fin spine. Scale bars: 10 mm.

FIG.4. — Uraniacanthus curtus (Powrie,1870) n. comb.,NMS G.1891.92.250,a very small complete individual from Turin Hill (Tillywhandland),Angus,Scotland:A, complete specimen;B, ossification of the right Meckel’s cartilage with the coronoid process visible on the upper surface;C, external view of the spatulate opercular plates with a trace of thin ridge ornamentation;D, scapulocoracoids;E, possible prepectoral spines with the posterior one being larger; F, internal surface of a postorbital plate.Scale bars:A,5 mm;B-D,F, 1 mm; E, 0.1 mm.

FIG. 5. — Uraniacanthus curtus (Powrie, 1870) n. comb., NMS G.1891.92.250, a very small complete individual from Turin Hill (Tillywhandland), Angus, Scotland: Line drawing of the specimen. Abbreviations: ad.fs, anterior dorsal fin spine; a.fs, anal fin spine; cf, caudal fin; sc, scapulocoracoid; oc, opercular covers; op, occlusal plate; pd.fs, posterior dorsal fin spine; p.fs, pectoral fin spine; pp, postorbital plate; pp.s1, first prepectoral spine;pp.s2, second prepectoral spine;ppv.fs2, second prepelvic fin spine;pv.fs, pelvic fin spine. Scale bar: 5 mm.

FIG. 6. — Uraniacanthus curtus (Powrie, 1870) n. comb., NMS G.1964.31.30A, the part of an incomplete, moderate sized specimen from Hayston Hill, Angus, Scotland: A, photograph of the specimen; B, line drawing of the specimen. Abbreviations: ad.fs, anterior dorsal fin spine; a.fs, anal fin spine; cc, central canal; cf, caudal fin; hp, head plates; scs, scapula shaft; pd.fs, posterior dorsal fin spine. Scale bars: 10 mm.

FIG. 7. — Uraniacanthus curtus (Powrie, 1870) n. comb. anterior dorsal fin spines: A, NMS G.1964.31.30A, from Hayston Hill, An- gus, Scotland; B, NMS G.1891.92.250, from Turin Hill (Tillywhandland), Angus, Scotland. Abbreviation: scs, scapula shaft. Scale bars:A, 10 mm; B, 5 mm.

FIG. 8. — Uraniacanthus curtus (Powrie, 1870) n. comb., NMS G.1891.92.249, holotype from Farnell: Magnified impressions of the body scales. Scale bar: 1 mm.

FIG. 9. — Uraniacanthus curtus (Powrie,1870) n. comb., an incomplete moderate sized specimen from Hayston Hill, Angus,Scotland: A, B, NMS G.1964.31.30B, in situ but scattered body scales on the counterpart; C, D, NMS G.1964.31.30A, in situ scattered scales in the tail region of the part. Scale bars: 0.5 mm.

FIG. 10.— Uraniacanthus curtus (Powrie,1870) n. comb., NMS G.1964.31.30A,an incomplete moderate sized specimen from Hayston Hill, Angus, Scotland. Scales removed from the dorsal caudal region (A-E), posterior to the pdfs (F-K), and mid-flank (L-O), photographed under ESEM: A, lateral view; B, anterolateral view; C, posterocrown view; D, anterocrown view; E, posterior view with the back of the scale crown broken off.; F, crown view; G, crown view; H, posterior view; I, lateral view; J, crown view; K, basal view; L, laterocrown view; M, lateral view; N, crown view; O, crown view. Abbreviations: c, canal; p, pore opening. Scale bars: 0.1 mm.

FIG. 11. — Uraniacanthus curtus (Powrie,1870) n. comb., NMS G.1964.31.30A,an incomplete moderate sized specimen from Hayston Hill, Angus, Scotland. Various thin sections of scales: A, vertical longitudinal section; B, vertical longitudinal section with the anterior of the crown missing; C, vertical transverse section; D, vertical transverse section; E, mid crown horizontal section; F, low crown horizontal section. Abbreviations: ac, ascending canal; cc, circular canal; cz, crown growth zone; dt, dentine tubules; lc, longitudinal canal; p, pore opening; rc, radial canal. Scale bars: 0.1 mm.

FIG. 12. - Uraniacanthus Meckel’s cartilage:A, U.probaton (Bernacsek & Dineley,1977) n. comb., UALVP42095.Modified from Hanke & Davis (2008:fig.9D) by being flipped horizontally 180 degrees and relabelled;B, U.curtus (Powrie,1870) n.comb., NMS G.1891.92.250. Abbreviations: cp, coronoid process; lt.mk, left Meckel’s cartilage; rt.mk, right Meckel’s cartilage. Scale bars: 1 mm.

FIG. 13. — Reconstruction of Uraniacanthus curtus (Powrie, 1870) n. comb.