Watsonacanthus costatus, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 168-171

Watsonacanthus costatus n. sp. ( Figs 23 G-K; 32)

Watsonacanthus oervigi Sobolev, Karatajute-Talimaa & Valiukevicius, 1988 : pl. 1, figs 12, 13. — Valiukevičius 1994: 165, pl. 22, figs 1-5, text-figs 75.5, 76.1-4; 1995a: 394, 397, text-figs 2, 4, 6.

HOLOTYPE. — LIG 35-2074. Scale ( Fig. 23K ). October Revolution Island, Spokojnaya River, outcrop 48, bed 5.

PARATYPES. — LIG 35-1967, 2140, 1422, 1993: scales ( Fig. 23 G-J).

ETYMOLOGY. — From Costatus (Latin): ridged, with reference to the crown ornamentation.

MATERIAL EXAMINED. — 35 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. Pioneer Island: samples P-9-1 and 10-1. Lower Devonian, Emsian, lower Al’banov Subformation of the October Revolution Island and supposed age analogues of the Rusanov Formation on the Pioneer Island (Member 8).

DIAGNOSIS. — Watsonacanthus with scales ornamented by three to six sharp radial ridges pointing posteriorly or converging at crown midlength. Neck low, base massive, strongly anteriorly protruding crown. Scale crown composed of simple mesodentine network without principal canal branches and lacunae (anterior part, primordial lamella included), and dentine containing long ascending and radial vascular canals (posterior part). Base bone lowly cellular.

DESCRIPTION

Species described from isolated scales. Small and medium-sized scales with crown length of 0.4- 0.8 mm, dominate. Rare examples exceed 1 mm. Crown is of a rhombic, rarely elongated-rhombic ( Fig. 23H ) form, flat. Maximum elongated crowns overhang bases posteriorly. Three to six sharp radial ridges, highered anteriorly and gradually lowered, may point at the posterior edge or converge at the crown midlength. Their anterior transverse sections make characteristic triangle forms. Base massive, rhombic in outline, anteriorly vaulted and strongly protruding crown. Neck low.

Maximum six lamellae of superpositional growth in crown composed of simple networked mesodentine (anterior part, including primordial lamella) and dentine (posterior part). Mesodentine without principal branches of canals ( Fig. 32B, C ) and without lacunae. Rare osteocyte spaces observed only in primordial lamella ( Fig. 32A ). Posterior crown part penetrated by long widened ascending canals with numerous smaller side branches. For this of dentine composed part characteristic also long, multibranched radial vascular canals displaced at the junction strip between base and crown ( Fig. 32B, C ). Extremely thin-lamellar base bone includes small numbers of osteocytes.

DISCUSSION

Detailed study of Watsonacanthus scales from Tajmyr and Severnaya Zemlya has led to a final opinion assigning them to separate species differing from W. oervigi of Spitsbergen ( Valiukevičius 1979). W. costatus n. sp. has essentially higher, sharper and mostly longer crown ridges. W. oervigi differs both by its crown and base tissue structure. Simple networked mesodentine lacking main vascular canals composes entirely three to four growth lamellae in crown of W. oervigi . It includes osteocytes ( Valiukevičius 1994: text-fig. 75.5). Dentine-type tissue has not been observed. Scales from Tajmyr are identical in their morphological features to those of Severnaya Zemlya, but they differ in their crown mesodentine structure, which exhibits lacunae and osteocytes in the earliest growth lamellae ( Valiukevičius 1994: text-fig. 76.1-4). Stranggewebe is developed in very restricted area. W. sibiricus scales ( Valiukevičius 1994: pl. 22, figs 7, 8) differ from the species under consideration by their tiny size and the flatness of scales, ornamented by a pair of short medial ridges and two pairs of longer lateral ones almost reaching the posterior edge, but with no posterior point.

BIOSTRATIGRAPHIC SIGNIFICANCE

Index species among the acanthodian assemblage. It allows to distinguish on Severnaya Zemlya Archipelago the early Emsian beds with Watsonacanthus costatus n. sp. (lower Al’banov Subformation on October Revolution and Pioneer islands and Member 8, supposed Rusanov Formation on the Pioneer Island). In Tajmyr (Dolgan beds, formerly Favosites regularissimus Zone ) and Timan-Pechora (Varandei Formation) regions – index species for the nominate acanthodian Zone (earlier W. oervigi ; Valiukevičius 1995a: text-figs 2-6). It corresponds on Pioneer Island to dehiscens-inversus and in Tajmyr to gronbergi-inversus zones of the standard conodont scale.

Order DIPLACANTHIFORMES Berg, 1940

Acanthodian fishes having scales of the “ Diplacanthus ” - type histological structure ( Valiukevičius 1985, 1995b).

Family DIPLACANTHIDAE Woodward, 1891 Genus Diplacanthus Agassiz, 1844

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. 32. — Watsonacanthus costatus n. sp., scale histology, Pioneer Island, sample P-10-1, Lower Devonian, Member 8; A-C, vertical longitudinal sections of scales, anterior to left, thin sections 3289 (A, scale like figured on Fig. 23K), 3302 (B, scale with shorter ridges) and 3303 (C, scale like figured on Fig. 23H); D, vertical transverse section through the central part of scale like figured on Fig. 23K. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; gl, growth lamella; oc, osteocyte cavity; rvc, radial vascular canal; smd, simple mesodentine. Scale bar: 0.1 mm.