Nostolepis watsoni, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 159-162

Nostolepis watsoni n. sp. ( Figs 23 A-F; 24)

HOLOTYPE. — LIG 35-2020. Scale ( Fig. 23A ). Pioneer Island , sample P-10-1. Lower Devonian, Member 8 ( Klubov et al. 1980).

PARATYPES. — LIG 35-1937, 1938, 1940, 1968 and 1969: scales ( Fig. 23 B-F). All from the same sample as the holotype.

ETYMOLOGY. — In honour of D. M. S. Watson, acanthodian researcher.

MATERIAL EXAMINED. — 83 scales.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 5, bed 55; Spokojnaya River, outcrop 48, beds 5, 7 and outcrop 49, bed 11; Pod”emnaya River, observation point 26-3. Pioneer Island: samples P-12-7, 12-12, 10-1, 9-1 and observation point 2478. Lower Devonian, Emsian, Rusanov and Al’banov formations of the October Revolution Island and supposed analogues, in age, of the first one on Pioneer Island (Members 6-8).

DIAGNOSIS. — Nostolepis with scales of moderate size, rhombic crowns ornamented by six short, rounded, sub-parallel or sub-radial ridges, low neck and strongly anteriorly advanced base. Extremely thinlamellose cellular to acellular base bone, with a simple network of mesodentine and Stranggewebe in crown which can be replaced by dentine.

DESCRIPTION

Species described from disarticulated scales. Their size varies in length from 0.4 to 0.85 mm. Crown is rhombic, with a slightly elongated posterior area. Six short, rounded, sub-parallel or sub-radial converging ridges ( Fig. 23A, B ) occur along the proximal margin. Another scale variety ( Fig. 23D, F ) shows more regularly parallel, lower and longer ridges reaching the middle crown part. The scale base is massive, strongly vaulted anteriorly and protruding the crown ( Fig. 23A, C ). A distinct sub-rhombic rim, separating it from a very low neck that thickens posteriorly, outlines it. Postero-lateral neck walls may bear vertical grooves.

The two noted scale varieties have a slightly different histological structure. The short-ridged crowns (like the holotype) are composed of mesodentine with a typical network of tubules interconnected with osteocyte spaces ( Fig. 24C, D ). The small posterior area demonstrates a Stranggewebe only superficially developed. It never occurs in the primordial lamella. The extremely thin-layered base is composed of cellular bone. The second scale variety has bases consisting of almost acellular bone and crowns made of dentine ( Fig. 24A, B ). The number of growth lamellae is the same: eight. Ascending vascular canals are smooth, with long widened principal and shorter side branches. No lacunae or osteocytes are met. A complex central knot ( Fig. 24A ) of dentine canals is seen in the primordial growth lamella.

DISCUSSION

By its crown ornamentation and its specific, largely advanced, massive and anteriorly vaulted base, Nostolepis watsoni n. sp. is comparable to several representatives belonging to different genera. The closest one is Nostolepis sp. B defined by Vyushkova from the Telengitian upper Salairka beds and Belovo Horizon of Salair ( Vyushkova 1992: pl. 1, figs 7-9, pl. 3, fig. 1). The histological structure of scales has not been investigated. Morphologic differences concern the more numerous and regular subparallel ridges occurring in the Salair specimens. This is also supposedly said about the weakly ridged scales of Trundlelepis cervicostulata morphotype 2 ( Burrow 1997), identified from the Lochkovian Trundle beds of New South Wales, Australia ( Burrow 1997: pl. 1, fig. 17a, b, pl. 3, fig. 3). As opposed to N. watsoni n. sp., they bear six to 12 anterior ridges, and about 10% of scales contain crown pore openings, which are not observed in N. watsoni n. sp. Trundlelepis cervicostulata scale bases are also composed of cellular ( Burrow 1997: fig. 5C and explanatory

A B C D

fig. 5- 3 in addendum 1997) to acellular bone (fig. 5D and fig. 5-4). Their simple mesodentine (figs 5-2 and 5- 5 in addendum 1997) in crown contains short winding dentine tubules with lacunae expansions comparable only to the first variety of scales in N. watsoni n. sp. Stranggewebe or dentine tissue with long ascending vascular canals is not seen in T. cervicostulata .

Several morphologic similarities unite N. watsoni n. sp. with the scales of Cheiracanthoides sp. indet. from the Emsian La Grange Limestone of Armorican Massif, France ( Vidal et al. 1994: fig. 4.11). However the crown ridges of figured specimen seem sharper when compared to N. watsoni n. sp. The histological structure of these scales is not taken into account.

By most morphologic and histological features Nostolepis watsoni n. sp. resembles scales of Watsonacanthus Valiukevicius, 1979 . It can be marked as an intermediate form between certain Nostolepis and Watsonacanthus lineages. By its converging rounded and flattened ridges it is closer to W. oervigi Valiukevicius, 1979 (pl. 11, figs 1-8, pl. 12, figs 1-4). W. oervigi scale bases are characterised by a highly cellular bone ( Valiukevičius 1979: fig. 1a-d; 1994: fig. 75.5), containing dense multiangular osteocytes, and a simple mesodentine network in crowns without forming main branches. Stranggewebe has also not been observed.

A B

BIOSTRATIGRAPHIC SIGNIFICANCE

Key species of the beds with Watsonacanthus costatus n. sp. based on acanthodian, a dispersed scale assemblage, an age analogue of the nominate acanthodian zone, marked earlier as Watsonacanthus oervigi Zone can be proposed. On Severnaya Zemlya Archipelago it is attached to the Rusanov Formation and lower Al’banov Subformation. The Conodont-grounded age, based on ranges of the dominating Pandorinellina expansa (Uyeno & Mason, 1975) , P. exigua exigua (Philip, 1966) with a smaller numbers of Pelekysgnathus Klapper & Johnson, 1980 and Steptotaxis Uyeno & Klapper, 1980 , gives an Emsian age, within the dehiscens-inversus zones of the standard conodont scale.

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. 24. — Nostolepis watsoni n. sp., scale histology, Pioneer Island, sample P-9-1, Lower Devonian, Member 8; A, B, vertical transverse sections through the scales with elongated flattened ridges (see Fig. 23 D), thin sections 3286 (A) and 3266 (B); C, D, vertical longitudinal sections through the scales with short, rounded, higher ridges (see Fig. 23A), thin sections 3267 (C) and 3268 (D). Abbreviations: abo, acellular bone; avc, ascending vascular canal; cbo, cellular bone; gl, growth lamella; oc, osteocyte cavity; prgl, primordial growth lamella; smd, simple mesodentine; stg, Sranggewebe (oriented mesodentine). Scale bar: 0.1 mm.