Diplacanthus poltnigi, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 171-173

Diplacanthus poltnigi n. sp. ( Figs 29O; 33 )

Nostolepis gracilis — Vieth 1980: 32, 33, pl. 5, fig. 14a, b (partim).

Diplacanthus longispinus — Poltnig 1984: 114, 115, pl. 2, figs 1-7.

HOLOTYPE. — LIG 35-1789. Scale ( Fig. 29O). Matusevich River, outcrop 4, bed 3.

ETYMOLOGY. — In honour of Dr. W. Poltnig, Karl- Franz University of Graz, Austria.

MATERIAL EXAMINED. — 10 scales.

LOCALITY AND AGE. — Out of the holotype locality, Spokojnaya River, outcrop 40, beds 21 and 27. Lower Devonian, Lochkovian, Pod”emnaya Formation.

DIAGNOSIS. — Diplacanthus with small rhombic scales, with crown ornamented by 10 to 23 deep, linear, sub-parallel grooves, extending over all its length or converging at the extreme posterior area. Crown composed of mesodentine with long ascen-

bcan ding vascular canals and small side tubules. Complex centripetal radial canals over the base. Acellular base bone penetrated by long and branchy upstreamed vascular canals.

DESCRIPTION

Species described from isolated scales. Crowns are sub-rhombic, rounded-rhombic to oval, flat, 0.4-0.8 mm long, with width generally exceeding the length. Crowns are ornamented with 10 to 12 deep, linear, sub-parallel, U-formed grooves, slightly deepened anteriorly; in scales from other regions up to 23 grooves can occur. No visible ridges. Grooves extend overall crowns, rarely more shallow posteriorly. Scale bases are usually as large as the crowns, slightly convex with a moderately high necks. Rare examples with very high necks have almost flat bases.

Scale crowns are made of mesodentine close to dentine by several features. One encounters up to eight growth lamellae penetrated by long principal ascending vascular canals, connected by small winding side branches. Primordial lamella contains a complicated knot of vascular canals ( Fig. 33B ). Neither lacunae of canals nor superficial durodentine are observed. Multibranched centripetal radial canals occur at the junction strip between base and crown ( Fig. 33A ). Sharpey’s fibres and numerous long and wide ascending vascular canals ( Fig. 33B ) similar to the ones in crown pierce the thinlamellar acellular bony base.

DISCUSSION

We suppose this taxon has been described under different names from the Canadian Arctic ( Vieth 1980) and Graz, Austria ( Poltnig 1984). In the first case it is identified as Nostolepis gracilis Gross, 1947 , but rather differs from the latter by its crown ornamentation. The Canadian specimens are ornamented with very numerous grooves (to 23). The histological structure of this sort of scale has probably not been studied because the published figures ( Vieth 1980: textfig. 19A-C) are supposedly linked to typical N. gracilis – with crown pores and certain “ Nostolepis ”- type tissue collected together with them in fossil samples. The scales from Graz, defined as Diplacanthus longispinus Agassiz, 1844 ( Poltnig 1984: pl. 2, figs 1-7), cannot be ascribed to this species for the same reason – they have no ridges, but only deep linear grooves (11-16 as marked) on crowns. In comparison, D. longispinus ( Gross 1947: pl. 6, figs 1, 2) is ornamented with numerous gradually posteriorly lowering sharp ridges, disposed in a fan-like pattern and separated by wide shallow grooves.

Scales of the known Diplacanthus species, D. crassisimus Duff, 1842 ( Denison 1979: fig. 21A-C) and D. kleesmentae Valiukevicius, 1986 (Valiukevičius & Karatajūtē- Talimaa 1986: pl. 1, figs 8, 9, pl. 3, fig. 8, pl. 4, fig. 6), have scales with ridges or grooves concentric to the postero-lateral margins of crowns, or possessing thick crowns with high posterior medial keel and radiating lower lateral ridges and grooves like in D. carinatus Gross, 1973 ( Gross 1973: pl. 36, figs 8-10; Valiukevičius 1985: pl. 1, figs 1-5, pl. 3, figs 1-4, pl. 11, figs 7-9, pl. 13, figs 5-8) or in less carinated D. gravis Valiukevicius, 1988 ( Valiukevičius 1988b: pl. 8, figs 1-4). At least, D. horridus Woodward, 1892 has crowns with posteriorly converging ridges like D. longispinus . The histological structure of investigated Diplacanthus scales, belonging to the “ Diplacanthus ”- type, differs in D. kleesmentae (Valiukevičius & Karatajūtē- Talimaa 1986: textfig. 4.4-6) by its high network in crown mesodentine with winding principal ascending canals and smaller branchings containing lacunae, and also by their multibranched radial canals over the base. D. carinatus is distinguished by bushy knots of branchy, radially oriented horizontal vascular canals and by arched forms of circular canals ( Valiukevičius 1985: text-fig. 15.8). In D. longispinus , one observes long, parallel, regularly symmetrically branched horizontal vascular canals ( Gross 1947: text-fig. 14A, B). In D. gravis it is composed of three to four growth lamellae of mesodentine with irregularly placed ascending tubes, sometimes forming a bush, long subparallel horizontal vascular canals with characteristically ascending dentine tubules and complicated radials ( Valiukevičius 1988b: textfig. 1a-e). The species under description differs from the known taxa of the genus both by its parallel grooves on crown (without ridges) and a more abundant dentine-like tissue composing them.

BIOSTRATIGRAPHIC SIGNIFICANCE

Long-ranging species. In Severnaya Zemlya Archipelago it is met only within the upper part of Nostolepis minima Zone ( Diplacanthus poltnigi n. sp. Subzone), a Lochkovian (Pod”- emnaya Formation) association. In Canadian Arctic (locality 18) it is distributed in the Red

Canyon River (A) Formation, dated as upper? Lochkovian ( Vieth 1980) or upper Lochkovian/ Pragian ( Langenstrassen & Schultze 1996). The youngest finds occur in Waldsdorf from Graz ( Austria), the latest Emsian after Poltnig (1984).

Figures

FIG. 33. — Diplacanthus poltnigi n. sp., scale histology, Matusevich River, outcrop 4, bed 3, Pod”emnaya Formation; A, vertical transverse section through the central part of scale like holotype, thin section 921; B, vertical longitudinal section through the scale with maximum high neck and flattened base, thin section 922. Abbreviations: avc, ascending vascular canal; bcan, vascular canal piercing base of scale; gl, growth lamella; rvc, radial vascular canal; smd, simple mesodentine. Scale bar: 0.1 mm.