Thelodus visvaldi, Karatajūtē-Talimaa & Märss, 2002

Karatajūtē-Talimaa, Valentina & Märss, Tiiu, 2002, Upper Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 405-443 : 430-435

Thelodus visvaldi n. sp. ( Figs 17 ; 18 ; 19 A-D)

Thelodus sp. 1 Märss & Karajūtē- Talimaa 2002: fig. 1. HOLOTYPE. — Pi 7542 ( Fig. 17F ).

ETYMOLOGY. — Named after Dr. Visvaldis Kuršs, Riga, Latvian State University, who participated in the field work on Severnaya Zemlya in 1978.

TYPE LOCALITY AND AGE. — Sample MF 34-1, Cape October, October Revolution Island, Severnaya Zemlya Archipelago, Ludlow, Upper Silurian.

MATERIAL EXAMINED. — About 700 scales.

OCCURRENCE. — Samples MF 34-1, Cape October; 2-3, 2-4, 2-7, 2-23, Matusevich River, October Revolution Island; 5m-1, 5d, 5i, 5zh- 1,5e, Pioneer Island, Severnaya Zemlya Archipelago, Ust- Spokojnaya Formation, Ludlow, Upper Silurian. On Pioneer Island Thelodus visvaldi n. sp. occurs also in the Wenlock part of the section (see Fig. 1 ).

DIAGNOSIS. — Scale crown quadrangular to oblong, with longitudinal grooves in flat surface or with smooth flat crown surface. Between grooves, longitudinal ridges often split into two parallel ones. The neck is without vertical ridges. Typical Thelodus histology.

DESCRIPTION

Morphology

Crowns of head scales quadrangular or round ( Fig. 17A ) with short ridges and grooves anterolaterally; postero-lateral margins serrated. The scales are small, the length of head scales 0.4- 0.5 mm, their width 0.3-0.5 mm. In the same sample the quadrangular scales with flat and smooth crown surface ( Fig. 18 A-G) are rather numerous. We tend to believe that these scales covered the body behind the rostrum and in front of the dorsal and ventral fins, and correspond to the transitional scales. The measurements of the smooth scales are: in average 0.5 × 0.5 mm, the maximum length 1.4 mm and width 1.0 mm. The rest of the trunk was covered with relatively high scales with a quadrangular or oblong crown that was covered with longitudinal grooves and ridges ( Figs 17B , D-I; 18H-T) nearly parallel in the medial part of the crown. Ridges are often divided into two. Narrow unsculptured margins characterize the trunk scales. The measurements of the scales are: length 0.4-0.9 mm and width 0.4-0.8 mm; in the holotype ( Fig. 17F ) the corresponding numbers are 0.6 and 0.5 mm. The neck is distinct and relatively high. The base ( Fig. 17K, L ) is smaller than the crown, rhomboidal, sometimes rather wide. Narrow and small scales ( Fig. 17 M-O) were covering from the posterior part of the trunk. The scale in Fig. 17O is only 0.4 mm long and 0.2 mm wide.

In Fig. 17J is a very small scale, posteriorly broken (length 0.4 mm), but, still, trilobate shape is recognizable. The crown surface is typical for the new species. Its position on the body is unknown.

Histology ( Fig. 19 A-D)

Dentine tubules start from the pulp cavity and are directed towards the base, neck and crown ( Fig. 19B, D ). The tubules are comparatively wide in the lower part of the crown; they branch on some levels, and become narrower in the upper crown portion. The longest and most straight dentine tubules occur in the posterior crown portion ( Fig. 19 A-C).

COMPARISON

Trunk scales of Thelodus visvaldi n. sp. are quadrangular, their crowns carrying nearly parallel grooves and ridges in the medial part, the ridges being often bifurcated. In outline they resemble T. sculptilis Gross, 1967 , in having relatively short scales but the latter has only few ridges on its crowns. The sculpture of both T. visvaldi n. sp. and T. schmidti (= T. laevis ) (Pander, 1856) is formed by longitudinal steep deep grooves and ridges but in the new species the ridges and grooves are more numerous and more parallel the medial part of the crown. The whole set of scales of T. parvidens is not known and in it richly ridged scales are not described. Thelodus carinatus (Pander, 1856) has gentle ridges on flat crown surface; some scales described here (e.g., Fig. 4J ) do resemble it. Sculpture of T. visvaldi n. sp. and T. admirabilis Märss, 1982 differ in ridge/groove arrangement; the posterior prong is absent in T. visvaldi n. sp.

Figures

FIG. 17. — Thelodus visvaldi n. sp.; A, head scale; B, D-I, K-O, trunk scales; C, fin scale?; J, scale of unknown position; A-D, F-J, M-O, scales in crown view; E, N, scales in oblique lateral view; K, scale in lateral view; L, scale in base view; A, Pi 7541; B, Pi 7556; C, Pi 7549; D, E, Pi 7544; F, Pi 7542; G, LIG 35-903, the scale is lost; H, LIG 35-904; I, Pi 7545; J, Pi 7559; K, LIG 35-905; L, LIG 35- 906; M, Pi 7553; N, Pi 7554; O, Pi 7555; A-F, I, J, M-O, MF 34-1, Cape October, October Revolution Island; G, H, K, L, 5m-1, Pioneer Island; Ludlow, Upper Silurian. Scale bars: A-C, F-K, M-O, 0.1 mm; D, E, L, 0.2 mm.

FIG. 18. — Set of scales of Thelodus visvaldi n. sp.; A-G, head scales; H-T, trunk scales; A, B1, C-G, H1, I1, J-M1, N-T, scales in crown view; B2, scale in base view; H2, I2, M2, scales in lateral view; LIG 35-120-LIG 35-139; sample 5m, Pioneer Island, Ludlow, Upper Silurian. Scale bar: 0.3 mm.

FIG. 19. — Microstructure of the scales; A-D, Thelodus visvaldi n. sp.; E, F, Thelodus sp. indet.; A, Pi 7644; B, Pi 7645; C, Pi 7646; D, Pi 7647; E, Pi 6445; F, Pi 6365; A, C, sample 20568, Komsomolets Island, upper part of Ludlow; B, sample 2-3k; D, MF 11-1, Matusevich River, October Revolution Island, Ust-Spokojnaya Formation; E, F, sample 2055zh, Pioneer Island, upper part of Ludlow, Upper Silurian. Abbreviations: b, base; cr, crown; dt, dentine tubules; n, neck; pcv, pulp cavity; rcr, ridge on the crown. Scale bars: 0.1 mm.

FIG. 1. — Distribution of thelodonts in the Silurian section of Pioneer Island; 1, sandstone; 2, sandstone and silty sandstone; 3, argillite; 4, biomorphic limestone; 5, dolomite; 6, dolomitic limestone; 7, oolitic limestone; 8, stromatoporoidal limestone (after Klubov et al. 1980: fig. 2; modified). Cyrillic letters in fig. 2 of Klubov et al. 1980 are in present paper transliterated as follows: A = a; Б = b; B = v; г = g; д = d; E = e; 3 = zh; и = i; K = k; M = m; H = kh.

FIG. 4. — Set of scales of Paralogania ex gr. martinssoni (Gross, 1967); A, head scale; B-J, O, transitional scales; K-N, P, R-Z, trunk scales; Q, fin scale; A-D, E1, F-J, L, N-T1, U, V1, W1, Y, Z, scales in crown view; E2, K, M, V2, scales in lateral view; T2, W2, X, scales in base view; A, LIG 35-007; B, LIG 35-001; C, LIG 35-002; D, LIG 35-005; E, LIG 35-006; F, LIG 35-008; G, LIG 35-009; H, LIG 35- 010; I, LIG 35-0013; J, LIG 35-031; K, LIG 35-016; L, LIG 35-017; M, LIG 35-014; N, LIG 35-021; O, LIG 35-012; P, LIG 35-023; Q, LIG 35-024; R, LIG 35-022; S, LIG 35-025; T, LIG 35-019; U, LIG 35-018; V, LIG 35-028; W, LIG 35-029; X, LIG 35-030; Y, LIG 35- 033; Z, LIG 35-032; A-I, K-W, sample 8d-1, Pioneer Island, Ludlow, Upper Silurian; J, X-Z, sample 5m-1, Pioneer Island, Ludlow, Upper Silurian. Scale bar: 0.5 mm.