Thelodus calvus, Märss & Karatajūtē-Talimaa, 2002

Märss, Tiiu & Karatajūtē-Talimaa, Valentina, 2002, Ordovician and Lower Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 381-404 : 400-403

Thelodus calvus n. sp. ( Figs 5 P-S; 14; 15)

HOLOTYPE. — Pi 7503 ( Fig. 5Q ).

ETYMOLOGY. — From the Latin word calvus , “bald”, referring to the smooth unsculptured surface of the scales.

TYPE LOCALITY AND HORIZON. — Sample MF 46-23, 32a-208, Ushakov River, October Revolution Island, Severnaya Zemlya Archipelago; upper part of Samojlovich Formation, Wenlock, Lower Silurian.

MATERIAL EXAMINED. — More than 300 scales of good preservation.

OCCURRENCE. — Samples MF 46-23, 32a-204, 32a- 208, Ushakov River, and MF 157-7, Spokojnaya River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian; (?)MF 34-1, Cape October, and MF 46-26, Ushakov River, October Revolution Island, Severnaya Zemlya Archipelago, Ust’-Spokojnaya Formation, Ludlow, Upper Silurian.

DIAGNOSIS. — Scales small; average length 0.4 mm, width 0.4 mm and height 0.4 mm. Scale crowns squarish to oval, smooth and flat or slightly convex. Neck very distinct, rather high, base placed at anterior of scale. Pulp opening in centre or slightly posteriorly in base. Pulp cavity complicated, having openings of dentine canals. Dentine canals occur in base, neck and lower part of crown. Bunches of dentine tubules arise from dentine canals. Both aspidine “pegs” and tubules of Sharpey’s fibres present.

DESCRIPTION

Morphology

The morphology of the scales is very simple. The length of the scale ranges from 0.3 to 0.9 mm, width 0.2-0.7 mm and height 0.2-0.6 mm; in the holotype the length/height measurements are 0.4/ 0.4 mm. The crown is mainly rhombic with slightly rounded anterior corner but they also can be oval. The scales reveal no sculpture. The neck is very distinct, smooth and high. The lateral margins of the crown are slightly elevated ( Fig. 5Q, S ). The base is anteriorly longer than the crown and protrudes forward. The rounded pulp opening is situated in the centre or in the posterior part of the base. Fig. 14 exhibits the set of scales of T. calvus n. sp.

Histology ( Fig. 15 )

The pulp cavity continues posteriorly as a pulp canal at a right angle from pulp cavity ( Fig. 15F ). The walls of the pulp cavity are uneven because of openings of dentine canals ( Fig. 15A, B ). Dentine canals that start in the cavity enter into the base, anterior portion of the crown ( Fig. 15A ) and the neck ( Fig. 15B ). The posterior region of the neck has very few canals if any. Numerous dentine tubules are distributed as bunches going out from the dentine canals. Dentine tubules in the posterior crown portion are directed upwards and backwards, then they turn anteriorly and again posteriorly, before running straight up towards the crown surface ( Fig. 15B ). Horizontal thin sections ( Fig. 15E, G ) of the uppermost part of the crown, show rather straight dentine tubules. In a little deeper section ( Fig. 15C, D ) dentine canals are also visible. In the base, aspidine “pegs” and tubules of Sharpey’s fibres occur.

COMPARISON

Scales of T. calvus n. sp. are to some extent similar to smooth flat small scales of Thelodus parvi-

B

dens Agassiz, 1839. They differ from other Thelodus species in lacking any sculpture. Among Thelodus species , dentine canals occur only in T. calvus n. sp. Similar scales with a smooth crown and high smooth neck have been found in the Wenlock of East Baltic (see e.g., Märss 1986; Karatajūtē- Talimaa & Brazauskas 1995).

Thelodus ? sp. ( Fig. 7 T-V)

Ridged scales without spines ( Figs 7 T-V) have been found in the Samojlovich Formation, Wenlock, Lower Silurian, Ushakov River section on October Revolution Island, and have been identified as Thelodus ? sp. in this work. The possibility cannot be excluded that such scales might also belong to Paralogania . Both Thelodus and Paralogania scales are present in the same sample.

Figures

FIG. 5. — A-O, Loganellia matura n. sp.; A, B, head scales; C-E, transitional scales; F-M, O, trunk scales; N, fin? scale; A, C, F-H, J-L, N, scales in crown view; B, D, E, M, scales in lateral view; I, a scale in base view; N, posterior half of a trunk? scale in lateral view; A, Pi 7486; B, Pi 7487; C, Pi 7499; D, Pi 7496; E, Pi 7489; F, Pi 7494; G, holotype Pi 7492; H, Pi 7490; I, Pi 7497; J, Pi 7493; K, Pi 7501; L, M, Pi 7495; N, Pi 7485; O, Pi 7500; sample MF 3-8, Srednii Island, Sedov Archipelago, Golomyannyj Formation, middle Llandovery, Lower Silurian; P-S, Thelodus calvus n. sp.; P, LIG 35-907; Q, holotype Pi 7503; R, Pi 7505; S, Pi 7504; P, a scale in crown view; Q, S, in lateral view; R, in base view; P, sample 32a-208; Q-S, sample MF 46-23; Ushakov River, October Revolution Island, Samojlovich Formation, Wenlock, Lower Silurian. Scale bars: 0.1 mm.

FIG. 14. — Set of scales of Thelodus calvus n. sp.; A, B, LIG 35-180; C, D, LIG 35-181; E, LIG 35-182; F, LIG 35-183; G, LIG 35-184; H, LIG 35-185; I, LIG 35-186; J, LIG 35-187; K, L, LIG 35-188; M, LIG 35-191; N, LIG 35-192; O, P, LIG 35-189; Q, LIG 35-190; R, LIG 35-193; sample 32a-208, Ushakov River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian. Scale bar: 0.3 mm.

FIG. 15. — Thelodus calvus n. sp., microstructure of the scales; A, Pi 7631; B, Pi 7632; C, D, Pi 7633; E, Pi 7634; F, Pi 7635; G, Pi 7636; sample MF 46-23, Ushakov River, October Revolution Island, Samojlovich Formation, Wenlock, Lower Silurian. Abbreviations: asp, aspidine “pegs” in the base; b, base; cr, crown; dc, dentine canal; dt, dentine tubules; n, neck, pc, pulp canal; pcv, pulp cavity. Scale bars: 0.1 mm.

FIG. 7. — A-S, Shielia multispinata n. sp.; A, head scale; B-D, F, transitional scales; E, G-P, trunk scales; Q-S, fin scales; A, B, scales in lateral view; C-N, P, Q, S, scales in crown view; O, a scale in oblique view; R, a scale in base view; A, Pi 7517; B, Pi 7519; C, Pi 7518; D, Pi 7520; E, Pi 7521; F, Pi 6804; G, Pi 7523; H, Pi 7522; I, Pi 7524; J, Pi 7525; K, holotype Pi 6803; L, Pi 6805; M, Pi 6801; N, O, Pi 7526; P, Pi 6802; Q, Pi 7527; R, Pi 7529; S, Pi 7528; sample MF 157-7, Spokojnaya River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian; T-V, Thelodus? sp., scales in crown view; T, Pi 7516; U, LIG 35 908; V, Pi 6814; T, V, sample MF 46-23; U, sample 32a-208, Ushakov River, October Revolution Island, Samojlovich Formation, Wenlock, Lower Silurian. Scale bars: A-I, K-M, P-V, 0.1 mm; J, N, O, 0.2 mm.