Thelodus, Agassiz, 1839

Karatajūtē-Talimaa, Valentina & Märss, Tiiu, 2002, Upper Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 405-443 : 438-442

Thelodus ex gr. schmidti (Pander, 1856) ( Figs 21-23 G-M)

SYNONYMY. — For Thelodus schmidti see Turner (1976), Karatajūtē- Talimaa (1978) and Märss (1986b).

DESCRIPTION AND COMMENTS

In the Ludlow deposits of October Revolution and Pioneer islands, levels containing abundantly scales of Thelodus were discovered. Morphological sets composed from scales of different levels have some similarities but establishing their identity is complicated (compare Figs 21 and 22 ). The authors came to the conclusion that at the present stage of the study the identification of species or, even more, establishing any new taxon is not justified. At first it is necessary to revise Thelodus schmidti and its closely related species, Thelodus carinatus and Thelodus marginatus Karatajūtē- Talimaa, 1978, distributed in the uppermost Wenlock and Ludlow of Europe ( Estonia, Latvia, Lithuania, southern Sweden, Gotland, Norway, North Timan, bore cores of Timan-Pechora Region, the Urals [ Talimaa 2000]).

Some samples from the Ludlow of Severnaya Zemlya (e.g., Spokojnaya River, sample 47-14) yield so many different morphological varieties of these scales that it is not easy to find the limits for a species of mentioned Thelodus . For this reason, in the present paper we treat Thelodus ex gr. schmidti s.l.

Most typical for Thelodus schmidti is the scale set from the Spokojnaya River, sample 47-14 ( Fig. 21 ) and Ushakov River, sample 31-1. The scales are medium-sized, the biggest reach 1.0- 1.3 mm in length, rarely 1.4 mm. Head scales ( laevis - type) are rounded, oval or rhomboidal ( Fig. 21 A-D). Crown surface is usually smooth; rarely the antero-lateral margins of the crown are crenulated. Sometimes a fine striation occurs on its surface ( Fig. 21F ). In a few scales, fine vertical ridgelets on the postero-lateral parts of the neck were observed. The base is of moderate height, with centrally positioned large pulp opening. A part of head scales has a base smaller than crown. The scales with strongly swollen anterior part of the base are rare ( Fig. 21B ).

We ascribe to the transitional scales of the group ( Fig. 21G, H ) the scales (type “A”) with large smooth central area and narrow postero-lateral downstepped lateral ones (lateral rims). They also exist in Baltic T. carinatus ( Märss 1986b: fig. 19: 2-4).

The trunk scales (type “B”) ( Fig. 21 I-N) are the ones with longitudinally furrowed crowns. In part of the scales the central area of the crown is separated from the lateral rims by comparatively deep grooves; on the surface of the central area, one to three fine groovelets occur. Lateral rims can be smooth or with fine groovelets. Often the lateral rim is downstepped ( Fig. 21L ) like in type “A”. The trunk scales of type “C” are covered with longitudinal ridgelets ( Fig. 21 O-X), in narrow scales their number is four to five, in wider scales it increases up to six to seven. Anteriorly ridges may bifurcate. The subdivision of scales into types “A” and “B” is rather conventional because the ridges can also be divided by rather deep grooves. Part of trunk scales has developed fine vertical ridgelets (four to eight on each side) on the postero-lateral neck wall close to the base. There is a lot of scales having smooth surface of the neck. Base is moderately high; pulp opening is large. Some scales have root-like projections of the base ( Fig. 21N ).

Together with typical T. schmidti-T. carinatus , in the sample also a smaller number of scales of trilobatus - and pugniformis - type are associated.

Fig. 22 presents a morphological set of scales from the Ludlow of Pioneer Island (sample 5m). Among them, head scales ( Fig. 22A, B ) and trunk scales of three types: type “A”, characteristic for T. carinatus ( Fig. 22C, D ), type “B” ( Fig. 22 E-G), and type “C” ( Fig. 22H , J-O, S, T), and also modified scales with strongly developed sculpture of the crown and small low base ( Fig. 22I , P-R) are found. This morphological set contains all typical scales for T. schmidti .

Fig. 23 G-M shows trilobatiform scales from sample 8g-1, Pioneer Island. Some of them have developed spur-like projection anteriorly of the base; the crown is displaced at the back of the scale. The scales are smaller if compared with common trunk scales. The crown of trilobatiform scales is composed of a central and several pairs of lateral areas. Posteriorly, the crown is multicuspidate. Trilobatiform scales covered, most probably, a region of the body of Thelodus schmidti but also of Thelodus parvidens (see also Turner 1986 on Thelodus parvidens from Canada). Gross (1967: 18, 19, taf. 2: 13-21) considered Thelodus trilobatus Hoppe, 1931 (thin, three- to five-cuspidate crown posteriorly of the scale and long spur-like base anteriorly of it) as a good independent species for deposits of the Thelodus parvidens Zone, Pridoli.

Thelodus sp. indet. ( Figs 6F, G ; 19E, F ; 23 A-F)

DESCRIPTION AND COMMENTS

On Pioneer Island in beds of Ludlow age, together with Andreolepis hedei were found, rather large scales with a smooth flat surface, distinct neck and base that could be identified as Thelodus parvidens . In the same samples also occur scales with elongated crown covered with longitudinal ridges and having comparatively wide base and pulp depression. Their microstructure reveals similar straight, proximally rather wide dentine tubules ( Figs 6F, G ; 19E, F ). In the present work we identify them as Thelodus sp. indet.

Several samples of the Ust-Spokojnaya Formation on October Revolution Island (Matusevich River, samples 2-38, 2-43; Spokojnaya River, sample 47-14; Ushakov River, sample 31-1) and Ludlow part on Pioneer (samples 5m, 5d) and Komsomolets islands (sample 18041-6) yield tiny scales which crowns reach only 0.08- 0.4 mm in length. The crown of such scales is rounded, oval or rhomboidal. Neck is distinct, sometimes rather high and smooth without spinelets on the postero-lateral walls. Base is more convex anteriorly, pulp cavity central or slightly posterior. The crown surface of most of such tiny scales is smooth ( Fig. 23 A-D). Elongated scales that have unclear longitudinal ridgelets anteriorly ( Fig. 23E 1 ) or grooves ( Fig. 23F ) have also been found.

Usually the scales described above are associated with larger scales of the Thelodus schmidti type but sometimes they predominate (sample 2-38), or the whole sample is composed of tiny scales only (samples 2-43; 18041-6).

Species identification for such tiny scales is difficult. They might have been situated in a certain part of the squamation of representatives of the genus Thelodus , and in this case they must be treated as their corresponding species. Here they are left in open nomenclature.

Family LANARKIIDAE Obruchev, 1949

Genus Lanarkia Traquair, 1898

TYPE SPECIES. — Lanarkia spinosa Traquair, 1898 .

Lanarkia ? sp. ( Figs 3N ; 6H, I )

DESCRIPTION AND COMMENTS

In a few samples (samples 5 M-I, Pioneer Island; MF 157-2, Spokojnaya River, October Revolution Island) in the Ludlow deposits there have been found a few scales that resemble those of Lanarkia ( Märss & Ritchie 1998) . They are conical, with posterior end bending down, a crown finely ridged, wide pulp cavity, narrowing into pulp canal. A scale ( Fig. 6H, I ) that narrows upwards posteriorly (cuneiform scale) has wide pulp cavity that narrows into a long pulp canal. Wide branching dentine canals in the lower part of the crown give bunches with numerous dentine tubules in the upper part of it. Dentine canals open into the pulp cavity, pulp canal and the base. To some extent they resemble the Katoporodus - type of histology.

Figures

FIG. 21. — Thelodus ex gr. schmidti (Pander, 1856), morphological set of scales; A-F, K, N, head scales; G, H, trunk scales of type “A”; I, J, L, M, trunk scales of type “B”; O-X, trunk scales of type “C”; A, C-X, scales in crown view; B, scale in lateral view; LIG 35- 254-LIG 35-277. Scale bar: 0.3 mm.

FIG. 22. — Thelodus ex gr. schmidti (Pander, 1856), morphological set of scales; A, B, head scales; C, D, trunk scales of type “A”; E-G, trunk scales of type “B”; H, J-O, S, T, trunk scales of type “C”; I, P-R, modified trunk scales; A1, B, C1, D-G, H1, I1, J, K1, L, M, N1, O-Q, scales in crown view; H2, I2, K2, N2, scales in base view; A2, S2, scales in lateral view; C2, scale in posterior view; A-T, LIG 35-055-LIG 35-077; sample 5m, Pioneer Island, Ludlow, Upper Silurian. Scale bar: 0.3 mm.

FIG. 23. — A-F, Thelodus sp. indet., morphological set of scales; A-D, head scales; E, F, trunk scales; A, B1, C1, D1, E1, F, scales in crown view; B2, C2, E2, scales in base view; E3, scale in lateral view; D2, scale in posterior view; A-F, LIG 35-079-LIG 35-084; sample 5m, Pioneer Island, Ludlow, Upper Silurian; G-M, Thelodus ex gr. schmidti (Pander, 1856), trilobatiform trunk scales in crown view, LIG 35-113-LIG 35-119; sample 8d-1, Pioneer Island, Ludlow, Upper Silurian. Scale bar: 0.3 mm.

FIG. 6. — Microstructure of the scales; A-E, Paralogania ex gr. martinssoni (Gross, 1967); F, G, Thelodus sp. indet.; H, I, Lanarkia? sp.; A, Pi 6378; B, Pi 6362; C, Pi 6370; D, Pi 6368; E, Pi 6446; F, Pi 6377; G, Pi 6373; H, I, Pi 6371; sample 2055 zh, Pioneer Island, upper part of Ludlow, Upper Silurian. Abbreviations: b, base; cr, crown; dc, dentine canal; dt, dentine tubules; mpc, main pulp canal; n, neck; pcv, pulp cavity; wdt, wide dentine tubules. Scale bars: 0.1 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. 3. — A-M, Paralogania ex gr. martinssoni (Gross, 1967); A, transitional scale; B-H, L, M, trunk scales; I, fin scale; J, K, scales of unknown position; A-E, G, I-L, scales in crown view; F, a scale in base view; H, M, scales in lateral view; N, Lanarkia? sp., a scale in lateral view; O-T, Phlebolepis elegans Pander, 1856; O-R, scales in crown view; S, magnified scale of Fig. 3Q showing the finely striated crown margin; T, scale in base view; O-Q, S, cephalo-pectoral scales; R, precaudal scale; T, precaudal? scale; A, Pi 7612; B, Pi 7609; C, Pi 7610; D, Pi 6845; E, Pi 7616; F, Pi 7617; G, H, Pi 7615; I, Pi 6847; J, Pi 6846; K, Pi 7608; L, LIG 35-900; M, LIG 35-901; N, LIG 35-902; O, Pi 7564; P, Pi 7560; Q, Pi 7562; R, Pi 7563; S, Pi 7562; T, Pi 7566; A-K, sample MF 11-1, Matusevich River,October Revolution Island, Ust-Spokojnaya Formation, Ludlow; L-N, sample 5m-1, Pioneer Island; O-T, sample MF 157-4, Spokojnaya River, October Revolution Island, Ludlow, Upper Silurian. Scale bars: A-D, F, G, K, S, 0.1 mm; E, H-J, M-O, Q, R, T, 0.2 mm; L, P, 0.3 mm.