Diplacanthus solidus, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 173-174

Diplacanthus solidus n. sp. ( Figs 20 H-J; 34)

HOLOTYPE. — LIG 35-1283. Scale ( Fig. 20H, I ). Matusevich River, outcrop 5, bed 151 (top).

ETYMOLOGY. — From Solidus (Latin): solid, compact, referring to scale shape.

MATERIAL EXAMINED. — 12 scales.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 5, beds 136-153; Ushakov River, outcrop 27, bed 16. Middle Devonian, Eifelian?, Vstrechnaya Formation.

DIAGNOSIS. — Diplacanthus having small scales with triangle, trapezoid- to leaf-like crown and high neck. Two to four pairs of symmetric narrow linear grooves expanding slightly radially over all crown, except for a smooth rim along the proximal margin. Crown mesodentine resembles dentine by its style of ascending and horizontal vascular canals. Radial canals over the base almost not developed.

DESCRIPTION

Species described from isolated scales. They are small in size (0.3-0.4 mm), with rounded triangular, trapezium-like (with rounded and widened anterior edge), to leaf-like crown, a high neck and a slightly convex, centrally vaulted, base. Crown is ornamented with two to four symmetric pairs of linear grooves, crossing through the all length, in a slightly radially or fan-like pattern. Grooves increase in depth and width anteriorly. Along the proximal margin a narrow smooth strip ( Fig. 20H ) may be present. Another variety of scales has comparatively wide medial area with a shallow longitudinal concavity ( Fig. 20J ), marked by a central pair of grooves. All grooves seem wider as in other Diplacanthus . A smooth proximal rim is absent. In several specimens the postero-lateral neck walls contain two rounded bulbs.

By their shape the ascending vascular canals in crowns are closely resembling these of dentine: long, smooth, with principal branch, basally placed in each growth lamella ( Fig. 34A, B ), numbering four. Smaller horizontal branches contain numerous short upstreamed dentine tubules. Long and wide vascular canals, centripetally oriented towards the flattened pyramidal tip, also pierce the base.

DISCUSSION

The new species differs from the abovedescribed D. poltnigi n. sp. by the reduced number and more radially directed narrower grooves, the presence of an unornamented anterior strip on scale crowns and a higher neck. Histological structure of the compared species is close by the style of vascular canals ascending and penetrating the base, and by the lack of lacunae in crowns. Their principal difference concerns the neck radial canals: they are multibranched, complicated in D. poltnigi n. sp. and almost not developed in D. solidus n. sp. See also the discussion of preceding species to compare them to other Diplacanthus taxa.

BIOSTRATIGRAPHIC SIGNIFICANCE

Index species of the beds with the Diplacanthus solidus n. sp. acanthodian association, occurring from the upper part of Vstrechnaya Formation (Middle Devonian, Eifelian?) on Severnaya Zemlya Archipelago. The species is also met in Baltic area within the Nostolepis kernavensis (Eifelian, kockelianus Zone of conodont standard) and Diplacanthus gravis (Givetian) acanthodian zones.

Order ISCHNACANTHIFORMES Berg, 1940 Family ISCHNACANTHIDAE Woodward, 1891 Genus Gomphonchus Gross, 1971

Figures

FIG. 20. — Drawings of scales; A-G, Nostolepis fragilis n. sp., holotype; A-C, LIG 35-1275, crown (A), side (B) and base (C), anterior upwards (A, C) and to left (B); D, E, LIG 35-1276,crown (D, anterior upwards) and side (E, anterior to left); F, G, LIG 35-1277 and 35-1279, crowns, anterior upwards. For locality and stratigraphic arrangement see Fig. 19; H-J, Diplacanthus solidus n. sp.; H, I, holotype, LIG 35-1283 crown (H, anterior upwards) and side (I, anterior to left); J, LIG 35-1285, crown, anterior upwards, Matusevich River, outcrop 5, bed 151 (top), Vstrechnaya Formation; K, L, Diplacanthus carinatus Gross, 1973, crowns, anterior upwards; K, LIG 35-1254; L, LIG 35-1282, Matusevich River, outcrop 5, bed 149, Vstrechnaya Formation; M-O, Cheiracanthus sp. cf. Ch. longicostatus Gross, 1973; M, LIG 35-1286, crown, anterior upwards; N, O, LIG 35-1284 crown, anterior upwards (N) and side, anterior to left (O). Matusevich River, outcrop 5, bed 153 (M) and top of bed 151 (N, O), Vstrechnaya Formation. Abbreviation: fos, fossa. Scale bars: 0.5 mm.

FIG. 34. — Diplacanthus solidus n. sp., scale histology, vertical transverse sections through the central parts of scales, Matusevich River, outcrop 5, bed 151, Vstrechnaya Formation; A, thin section 667; B, thin section 668. Abbreviations: abo, acellular bone; avc, ascending vascular canal; bcan, vascular canal piercing base of scale; dt, dentine tubule; gl, growth lamella. Scale bar: 0.1 mm.