Gomphonchus nordicus, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 174-179

Gomphonchus nordicus n. sp. ( Figs 23 N-P; 35)

HOLOTYPE. — LIG 35-2007. Scale ( Fig. 23P ). Pioneer Island, sample 12-76.

ETYMOLOGY. — From Nordicus (Latin): nordic, referring to the area of origin.

MATERIAL EXAMINED. —About 115 scales.

LOCALITY AND AGE. — October Revolution Island: Spokojnaya River, outcrop 41, bed 1, outcrop 45, bed 20, sample from talus (supposedly top of outcrop 45) and outcrop 51a, bed a. Pioneer Island: sample 12-76. Upper Silurian, Pridoli: Krasnaya Bukhta Formation of the Spokojnaya River series and supposed age analogues on Pioneer Island (Downton, Member 3 as taken by Klubov et al. 1980).

DIAGNOSIS. — Gomphonchus scales of medium size with rhombic, sub-rhombic to oval crown, high neck and convex base; crown rarely protruding anteriorly. Crown and base frequently wider than long. Flat crown with shallow central, random small lateral dd depressions and postero-lateral concentric linear groovelets. Scale base composed of cellular bone; crown made of dentine containing smooth major ascending vascular canals, with upstreamed interspersed branches and complicated radial canals. Durodentine only developed in the central superficial crown part.

DESCRIPTION

Species described from isolated scales. Mid-size examples (0.4-0.8 mm) of scales dominate; only rare ones reach 1 mm. The crown shape varies from regularly rhombic, rhombic elongated posteriorly, rhombic widened, to leaf-like or oval. Crown plate is flat, but not even, with central slightly lowered area and small incon- stantly shaped random shallow deepenings. Some parts, especially the extreme posterolateral ones, look like the edges of the growing plates in forming concentrically linear groovelets ( Fig. 23P ). Several specimens have shallow convex crowns. The posterior crown tip rarely overhangs the scale base. A large amount of the material is composed of scales with widened crowns, width exceeding length. These have also strongly widened and anteriorly placed bases. Bases have a rhombic outline. The advancing crowns rarely protrude them to the front. In general, bases are centrally convex. The neck is high, frequently with vertical grooves.

Scales microstructure shows a classical cellular bone in bases and dentine in crowns. Mature scales are composed of 10 to 12 growth lamellae in crown ( Fig. 35A, B ), pierced by a system of well developed vascular canals: principal ascending ones are smooth, long, mainly with upstreamed intersperse branches. Centripetal multibranched radials occur just above the base and smaller branchings are directed toward the base. Part of canals may be unexpectedly widened (older scales?) ( Fig. 35D ). A durodentine strip is absent or developed only superficially in the scale centre ( Fig. 35A, C ). Several scales demonstrate a highly cellular base bone ( Fig. 35A, B ) and several contain only single osteocytes ( Fig. 35C ).

DISCUSSION

By its combination of dentine and durodentine in crown and cellular bone in base, G. nordicus n. sp. steps out as a specific representative of Ischnacanthidae . This species is distantly similar to Poracanthodes punctatus Brotzen, 1934 , which also shows a transition from cellular to acellular base bone, but the crown is composed of a different mesodentine ( Gross 1956: text-figs 116-118). When compared to the known Gomphonchus species, it is closest to G. sandelensis ( Gross 1971: pl. 5, fig. 11, pl. 9, figs 15-17), but differs in shape, base and crown proportions (new species – alike a transitional form between Gomphonchus and Acanthoides ?) and variable morphologic details of crown. The graphic illustrations of typical G. sandelensis scales histological structure in Gross (1971: text-figs 19E-G, 20A-E), really demonstrate acellular base bone though this author has pointed out Nostolepis , Gomphonchus and Poracanthodes as having cellular base bone, without definite differences ( Gross 1971: 53). G. sandelensis possesses regularly shaped ascending canals with short upward directed interspersed branches ( Gross 1971: text-fig. 20B, C), different from the long and also upstreamed ones in G. nordicus n. sp. G. tauragensis , described from the Early Devonian of Baltic, differs in its radially ridged crowns, general scale flatness, strongly advanced bases and the large area of durodentine completely composing some late grown lamellae of crowns and thin-lamellar, compact acellular bone in bases ( Valiukevičius 1998: pl. 4, figs 10-16, pl. 5, fig. 1, pl. 14, figs 7, 8).

BIOSTRATIGRAPHIC SIGNIFICANCE

Key species of the Poracanthodes punctatus Zone , identifying upper Pridoli in different regions ( Märss 1997). On Severnaya Zemlya Archipelago this zone is attached to the lower part of Krasnaya Bukhta Formation.

Genus Poracanthodes Brotzen, 1934

Poracanthodes punctatus Brotzen, 1934 ( Figs 28 A-D; 36; 37)

For synonymy see Valiukevičius (1998), and for diagnosis Gross (1947) and Valiukevičius (1998).

MATERIAL EXAMINED. — LIG 35-A-391: exposing jawbone fragment with main cusps in lateral preservation ( Fig. 28A), also isolated teeth (from the jawbone and palatine, Fig. 28D, C) and scales ( Fig. 28B). More than 50 isolated scales from different samples.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 2, bed 1 and outcrop 4, bed 3; Spokojnaya River, outcrop 51a, beds a-c, outcrop 45, bed 20 and sample from talus (top of outcrop), outcrop 41, beds 1 and 12. Komsomolets Island: outcrop 18077, bed 9; Pioneer Island, sample 12-76. Upper Silurian: Ludlow, Ust’-Spokojnaya Formation and Pridoli, Krasnaya Bukhta Formation. Lower Devonian, Lochkovian to Pragian, Severnaya Zemlya, Pod”emnaya and Spokojnaya formations.

REMARKS

The lower jawbone fragment is 23 mm long and up to 5 mm high stretching backwards and widening frontward medial with an oval section. It is composed of highly vascularised bone. Lateral main cusps are higher (to 3 mm) than wide, and have an oval parabasal form. They are slightly medially curved and striated by wavy groovelets. They have distinct sharp flanges supposedly on their anterior side only. Palatine teeth are tiny, elongated (to 0.3 mm), and oval in cross section, slightly recurved and present sharp sides. The main cusps are composed of trabecular dentine ( Fig. 36A, B ) with multibranched widened pulpar vascular canals, restricted in mature examples by concentric denteons. They stream outerward dense and tiny branchy dentine canals ( Fig. 36B ). Palatine teeth demonstrate up to six growth lamellae of indeterminable dentine with enlarged vascular canals in the basal part, and a base composed of cellular bone ( Fig. 36C, D ).

Scales small (0.2-0.5 mm), exhibit rhombic flat crowns, containing pore openings densely distributed in concentric lines or randomly. The pore canal system consists of widened radial canals, arcade, and narrower superficial ones, opening on the surface. Vascular canals are of dentine type ( Fig. 37B, C ). Their base is composed of cellular bone. The osteocyte may inconstantly reduce in number. Scale growth is mainly superpositional, but sometimes appears areal? ( Fig. 37C ), especially in posterior scale part.

BIOSTRATIGRAPHIC SIGNIFICANCE

Long-ranging species, with spanning time from the Late Silurian to the end of Early Devonian; last occurrences on Severnaya Zemlya Archipelago are in the Spokojnaya Formation (Pragian).

Poracanthodes sp. cf. P. porosus Brotzen, 1934 ( Figs 28N, O; 38 )

MATERIAL EXAMINED. — 88 isolated scales.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 2, bed 1; Spokojnaya River, outcrop 51a, beds a-c, outcrop 45, sample from talus (top of outcrop) and outcrop 41, bed 1. Pioneer Island: sample 12-76. Upper Silurian, Ludlow and Pridoli, Ust’-Spokojnaya and Krasnaya Bukhta formations.

REMARKS

Severnaya Zemlya specimens differ from the typical P. porosus Brotzen, 1934 by less ornamented scale crowns. The Late Silurian P. porosus is characterised by especially ridged scales with short, parallel anterior ridges and concentrical grooves, lining the postero-lateral crown sides ( Märss 1986: pl. 29, figs 11-17, pl. 30, figs 1-5). They contain pore openings both

B-D

arranged in radial and concentrical lines. Severnaya Zemlya examples rarely demonstrate all these peculiarities commonly represented. Their microstructure ( Fig. 38 ) shows a crown mesodentine of superpositional growth containing a small number of canals, but with main ascending branches. Radial, arcade and superficial pore canals may sometimes develop only on the restricted distal crown area. Densely thinlamellar base bone includes only rare osteocyte spaces.

BIOSTRATIGRAPHIC SIGNIFICANCE

Long-ranging species, on Severnaya Zemlya Archipelago incoming into acanthodian assemblages of the Ludlow and Pridoli (Upper

A

B-D

Silurian), the last of which being marked as Poracanthodes punctatus Zone. In other regions, e.g., Baltic ( Märss 1986; Valiukevičius 1998), the species extends to the Lower Devonian, Emsian.

Figures

FIG. 23. — A-F, Nostolepis watsoni n. sp., scales, Pioneer Island, sample P-10-1, Lower Devonian, Member 8. (Here and further members of the Pioneer Island geological sequence taken after Klubov et al. 1980); A, holotype, LIG 35-2020, crown; B, LIG 35-1968, crown; C, LIG 35-1938, base; D, LIG 35-1937, crown; E, LIG 35-1969, side; F, LIG 35-1940, crown; A, B, D, F, anterior upwards; C, to left; E, to right; G-K, Watsonacanthus costatus n. sp., scales; G, LIG 35-1967, crown; H, LIG 35-2140, crown; I, LIG 35-1422, side; J, LIG 35-1993, crown; K, holotype, LIG 35-2074, crown; anterior upwards (G, K, J), to the left (H) and to right (I); G-I, Pioneer Island, sample P-10-1, Lower Devonian, Member 8; J, K, October Revolution Island, Spokojnaya River, outcrop 48, bed 5, lower Al’banov Subformation; L, M, Acanthodii indet., scales, crowns, anterior upwards; L, LIG 35-1997; M, LIG 35-1998, Komsomolets Island, outcrop 18077, bed 9, Spokojnaya Formation; N-P, Gomphonchus nordicus n. sp., scales; N, LIG 35-2006, crown, anterior upwards; O, LIG 35-2008, side, anterior to left; P, holotype, LIG 35-2007, crown, anterior upwards, Pioneer Island, sample 12-76, Upper Silurian, Pridoli, Member 3. Scale bar: 1 mm.

FIG. 35. — Gomphonchus nordicus n. sp., scale histology, Pioneer Island, sample 12-76, Upper Silurian, Pridoli, Member 3; A, vertical transverse section through the anterior scale part, thin section 3368; B, C, vertical longitudinal sections through the central parts of scales, thin sections 3369 (B) and 3367 (C); D, vertical transverse section through the flattened older? scale, thin section 3366. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dd, durodentine; lsh, longitudinal Sharpey’s fibres; oc, osteocyte cavity; rvc, radial vascular canal. Scale bar: 0.1 mm.

FIG. 36. — Poracanthodes punctatus Brotzen, 1934, histology of teeth, Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; A, longitudinal section through the main cusp, thin section 3459; B, magnified apex area of the same section; C, D, longitudinal sections of palatine teeth, thin sections 3461 (C) and 3460 (D), paratype LIG 35-A-391. Abbreviations: cbo, cellular bone; dto, denteon; gl, growth lamella; pvc, pulpar vascular canal; trd, trabecular dentine; vc, vascular canal. Scale bars: 0.1 mm.

FIG. 37. — Poracanthodes punctatus Brotzen, 1934, scale histology; A, vertical longitudinal section through the old? scale with randomly displaced pores, thin section 3365; B, magnified right area of the same section; C, vertical longitudinal section through the scale with concentrically displaced pores, thin section 3462; D, vertical transverse section through the posterior part of scale with concentrically displaced pores, thin section 3464; E, vertical longitudinal section of scale, thin section 973; F, magnified central area of the same section. Paratype LIG 35-A-391; Pioneer Island, sample 12-76. Upper Silurian, Pridoli, Member 3 (A, B) and Spokojnaya River, outcrop 41, bed 12; Severnaya Zemlya Formation (C-F). Abbreviations: arcpc, arcade pore canal; avc, ascending vascular canal; oc, osteocyte cavity; po, pore opening; prgl, primordial growth lamella; rpc, radial pore canal. Scale bars: 0.1 mm.

FIG. 38. — Poracanthodes sp. cf. P. porosus Brotzen, 1934, scale histology, vertical longitudinal sections of scales, Spokojnaya River, outcrop 51a, beds a-c, Krasnaya Bukhta Formation; A, scale with radially linear pore openings, thin section 794; B, scale with pore openings lined both radially and concentrically, thin section 3010. Abbreviations: avc, ascending vascular canal; gl, growth lamella; oc, osteocyte cavity; po, pore opening; rpc, radial pore canal. Scale bar: 0.1 mm.