Thelodus matukhini, Karatajūtē-Talimaa & Märss, 2002

Karatajūtē-Talimaa, Valentina & Märss, Tiiu, 2002, Upper Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 405-443 : 435-437

Thelodus matukhini n. sp. ( Figs 2 A-I; 20)

HOLOTYPE. — Trunk scale LIG 35-860 ( Fig. 2B ).

ETYMOLOGY. — Named in honour of Dr. R. G. Matukhin (Novosibirsk, Russia), researcher of Severnaya Zemlya and participant of the field work to the region in 1978.

TYPE LOCALITY AND AGE. — Sample 18041-14, Komsomolets Island, Severnaya Zemlya Archipelago, Ludlow, Upper Silurian.

MATERIAL EXAMINED. — About 200 scales of good preservation.

OCCURRENCE. — Samples 18041-14, Komsomolets Island; Andreolepis sp. ( A. hedei Gross, 1968 most probably) in this sample allows the age determination as Ludlow without any doubt; MF 34-1, Cape October area, Ludlow, Upper Silurian.

DIAGNOSIS. — Medium-sized scales (length 0.9- 1.2 mm, width 0.5-0.7 mm; the length of smallest scales 0.3-0.7 mm). Head scales oval, with smooth and flat surface; their margins are sometimes crenulated. Transitional and trunk scales oval or rhomboidal. Crown surface is ridged, sometimes with longitudinal grooves with elevated spiny posterior apex. The base is not very high; it protrudes as a narrow strip behind the crown. Pulp opening large. A comparatively high neck is developed only in a few scales. Fine vertical ridgelets of the postero-lateral neck walls are present in very rare scales. Morphological set includes trilobatiform and pugniform scales.

DESCRIPTION

Head scales are represented by those with oval and rhomboidal configuration and smooth surface of the crown ( Fig. 20B, C ), some of them having slightly crenulated margins ( Fig. 20A ). The base is rather low and forms a wall around large pulp opening. Rhomboidal head scales ( Fig. 20C ) have a base considerably smaller than the crown, a relatively high neck and a smooth surface. As transitional scale we treat rounded or elongated oval ones ( Figs 2D ; 20 D-G). Especially characteristic are the scales with a median ridge ( Fig. 20D 1 ) that can end with an elevated spine ( Fig. 2D ). Laterally from the ridge, oblique ridgelets are developed. They start at the base of the crown and are directed towards the median ridge. In some of transitional scales the anterior margin is deeply crenulated ( Fig. 20E ) or covered with ridgelets ( Fig. 20G ). Sometimes the whole crown surface is covered with fine longitudinal ridgelets ( Fig. 20F ). The base of transitional scales is not very high but it can be rather wide and protrude as a narrow strip over the whole perimeter of the crown ( Fig. 20D 2 ). Pulp opening is large and elongated. The main part of the collected scales is composed of trunk scales. Among them occur a lot of very small (0.3-0.7 mm long), narrow and elongate, and modified scales, including trilobatiform ( Fig. 20S ) and pugniform ( Fig. 20T 1 , 2 ) types; the latter has laterally compressed and smooth crown surface. Characteristic T. matukhini n. sp. trunk scales are shown in Figs 2 A-C, F-H and 20H-M. The largest scales reach the length of 1.2 mm. The crown is distinctly ridged, with a keel- or spinelike and arosen posterior apex. The central crown area is a narrow groove. Lateral to it, two to three pairs of lateral ridges are developed; they can reach the posterior tapered crown apex. Like a low and narrow strip, the base protrudes the crown perimeters. Sometimes it has short peg-like projection anteriorly, directed obliquely downwards and anteriorwards ( Figs 2E ; 20K, L ). Pulp opening is large, often elongated and situated medially. Among trunk scales with ridged crown are often met scales with narrow or spine-like crown ( Fig. 20N, O, R 1 , 2 ). Some of them have a wide base with anterior spur-like projection ( Fig. 20R 1 , 2 ). Comparatively small part of trunk scales have a crown covered with longitudinal grooves ( Fig. 20P 1 , 2 ). In them, the central crown area is separated from the lateral ones by deeper grooves extending up to the posterior end of the crown ( Fig. 20P 2 ). This variety has fine vertical ridgelets on the postero-lateral neck walls ( Fig. 20P 1 ).

COMPARISON

The most characteristic features for the scales of Thelodus matukhini n. sp. are the specific proportions of the crown, neck and base. Only the trunk scales of this species are characterised by the ridged keel- or spine-like crown and low wall-like base that protrudes as a narrow strip over the whole perimeter of the crown. A relatively large, often elongated pulp opening is also characteristic for the new species. Vertical ridgelets on the postero-lateral crown (or neck) wall are often present in Thelodus schmidti ( Thelodus ex gr. schmidti ), Thelodus parvidens and Thelodus sculptilis but in Thelodus matukhini n. sp. they are observed only in very rare occurences.

Figures

FIG. 2. — A-I, Thelodus matukhini n. sp., scales in crown view; A, LIG 35-859; B, holotype, LIG 35-860; C, LIG 35-861; D, LIG 35- 862; E, Pi 7552; F, Pi 7550; G, Pi 7547; H, Pi 7551; I, Pi 7557; A-D, sample 18041-14, Komsomolets Island, Ludlow, Upper Silurian; E-I, sample MF 34-1, cape October (Octyabrskij), October Revolution Island, Ludlow, Upper Silurian; J-Q, Loganellia cuneata (Gross, 1947); J, L, O-Q, scales in crown view; K, M, scales in oblique lateral crown view; N, a scale in lateral view; J-Q, LIG 35-863- LIG 35-870; sample 2-23, Matusevich River, October Revolution Island, Ust-Spokojnaya Formation, Ludlow, Upper Silurian. Scale bars: A, C, E-H, J-P, 0.1 mm; B, D, I, Q, 0.2 mm.

FIG. 20. — Thelodus matukhini n. sp., morphological set of scales; A-C, head scales; D-G, transitional scales; H-S, trunk scales; T, pugniformis-type of the scale; A-C, D1, E-H, I1, J-O, P2, Q, R2, S, T1, scales in crown view; D2, scale in base view; I2, P1, R1, T2, scales in lateral view; LIG 35-281-LIG 35-300; sample 18041-14, Komsomolets Island, Ludlow, Upper Silurian. Scale bar: 0.3 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.