Acanthacanthus ornatus, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 167-168

Acanthacanthus ornatus n. sp. ( Figs 30A, B ; 31 )

HOLOTYPE. — LIG 35-A-347. Specimen preserved in lateral view, exposing only the squamation ( Fig. 30A, B ). Spokojnaya River, outcrop 41, bed 12.

ETYMOLOGY. — From ornatus (Latin) : ornamented, decorate.

LOCALITY AND AGE. — The single holotype specimen. Lower Devonian, lower Lochkovian, top of the Severnaya Zemlya Formation.

DIAGNOSIS. — Climatiid with large-sized scales of elongated rhombic or drop-shaped crown, ornamented by two posteriorly pointed radial ridges, separating lateral lowered crown areas, and one to two pairs of radial, also acute ridges on the flattened medial area. Crown composed of dominating Stranggewebe, containing wide ascending vascular canals, forming large knots-chambers at the junction with circular canals. Base bone thin-lamellar and moderately cellular.

DESCRIPTION

Scales with rhombic, elongated-rhombic or drop-shaped crowns 0.7-1.2 mm long. The anterior margin is widened, posteriorly stretched, distantly overhanging base, at a low angle relative to it. Rare examples are as wide as long. Crown plates are flat or slightly longitudinally convex. Two symmetrical marked radial ridges cross over the whole crown; they point posteriorly and separate narrow lowered lateral areas. One or two pairs of medial ridges are running parallel to laterals point centralwards but do not reach the posterior edge of crown. All ridges are rounded, not sharp. The shorter central ridges form sometimes a small forecrown-like area, essentially protruding others. The scale neck is low anteriorly and somewhat higher posteriorly; the rhombic base, anteriorly convex, strongly advances. Scales are set densely in lines, with small overlap areas.

Scale crowns demonstrate superpositional (mainly on the two earliest lamellae of the medial area) and areal growth (lateral lamellae, up to four) ( Fig. 31 A-D). Almost all crowns are composed of Stranggewebe with large stretched lacunae, and only the two latest growth lamellae contain strips of simple networked mesodentine ( Fig. 31B, D ). It also composes the very restricted anterior crown area ( Fig. 31E ). Osteocytes are observed even in the latest – lateral – lamellae ( Fig. 31D ). Characteristic widened ascending vascular canals; at crossing points with circular ones they make large knotschambers ( Fig. 31B, D ) and are connected with oriented lacunae or outward-streamed dentine tubules. The Stranggewebe of crown gradually turns into base bone with average cell cavities pierced by long Sharpey’s fibres and densely lined by thin growth lamellae.

DISCUSSION

Through their characteristic crown ornamentation Acanthacanthus ornatus n. gen., n. sp. scales differ from all the known Climatiidae . Distant affinities are noted with scales of Climatius reticulatus Agassiz, 1845 , which show radially ridged crowns, but the symmetrical pairs of ridges are not sharp ( Denison 1979: figs 16C, 17B). Another comparable taxon, Nostolepis costata Goujet, 1976 , shows high, sharp, sometimes posteriorly pointed ridges and has lowered lateral areas ( Goujet 1976: pl. 62, figs 1-10). But these ridges are of an inconstant length – from short to extend over all the crown – or symmetrically meeting in pairs. By their characteristic vascular canal widening, named “pulp chambers” by Denison (1979), Acanthacanthus ornatus n. gen., n. sp. is comparable to N. costata , however these last scales are composed of simple mesodentine, without Stranggewebe ( Goujet 1976: text-fig. 55A-D) and Euthacanthus macnicoli Powrie, 1864 ( Denison 1979: fig. 10A), of which it distinctly differ by the mesodentine structure.

BIOSTRATIGRAPHIC SIGNIFICANCE

Key species of the Poracanthodes menneri Subzone , a unit based on association of articulate acanthodians from the topmost part of the Severnaya Zemlya Formation. It corresponds to the lower part of the Nostolepis minima Zone , widely traced through the regions. Age is early Lochkovian. Tentatively correlated to the woschmidti Zone of standard conodont scale.

Genus Watsonacanthus Valiukevicius, 1979

Figures

FIG. 30. — A, B, Acanthacanthus ornatus n. gen., n. sp., Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; A, holotype, specimen LIG 35-A-347 in lateral preservation, squamation, anterior to left; B, enlarged area (right corner) of the same specimen, scales, crowns; C-F, Acanthospina irregulare n. gen., n. sp., scales,specimen LIG 35-A-345, Pod”emnaya River,outcrop 67, bed 12, Severnaya Zemlya Formation; C, D, LIG 35-1245, crown (C) and base (D), anterior upwards; E, F, LIG 35-1246, crown (E), anterior upwards and side (F), anterior to left. Scale bars: A, 10 mm; B, 2 mm; C-F, 1 mm.

FIG. 31. — Acanthacanthus ornatus n. gen., n. sp., scale histology, Pod”emnaya River, outcrop 67, bed 12, Severnaya Zemlya Formation; A, vertical transverse section through the central part of medium wide scale, thin section 689; B, enlarged crown of the same section; C, vertical transverse section through the mostly widened scale, thin section 688; D, magnified right crown area of the same section; E, vertical longitudinal section through the maximum elongated scale, thin section 687; F, magnified crown area of the same scale. All scales from the holotype specimen LIG 35-A-347. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; gl, growth lamella; kchvc, knot-chamber of vascular canals; oc, osteocyte cavity; smd, simple mesodentine; stg, Stranggewebe (oriented mesodentine). Scale bars: 0.1 mm.