Paralogania wilsoni, Karatajūtē-Talimaa & Märss, 2002

Karatajūtē-Talimaa, Valentina & Märss, Tiiu, 2002, Upper Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 405-443 : 418-421

Paralogania wilsoni n. sp. ( Figs 9 ; 10 )

Paralogania sp. 2 Märss & Karajūtē- Talimaa 2002: fig. 1. HOLOTYPE. — Scale Pi 7598 ( Fig. 9M ).

ETYMOLOGY. — Named in honour of Dr. M. V. H. Wilson, Edmonton, Alberta University, a Furcacaudiform researcher.

TYPE LOCALITY AND AGE. — Sample MF 157-2, Spokojnaya River, October Revolution Island, Severnaya Zemlya Archipelago, Krasnaya Bukhta Formation, Pridoli, Upper Silurian.

MATERIAL EXAMINED. — Over 300 scales, relatively well preserved.

DIAGNOSIS. — Head scales with crenulated margins and a convex crown surface. Transitional scales have oakleaf-like configuration and crown surface convex. Trunk scales anteriorly smooth in outline. Lateral portions of the crown are wide, medial crown plate smooth or with a very shallow longitudinal groove. There occur three to four proximally wide spines postero-laterally of the crown and one beneath the posterior crown apex. Dentine tubules very fine, hardly branching, and situated closely to each other. They do not form lacunae-like structures. Pulp canals present. Dentine canals start from the pulp canal. Aspidine layer in the base is thin.

DESCRIPTION

Morphology

There is only a few head scales in our collection ( Fig. 9A ). They are small (length 0.6 mm and width 0.4 mm). Their crown margins are crenulated anteriorly and serrated posteriorly; their crown surface is slightly convex. Transitional scales ( Fig. 9 B-D) are 0.6-0.7 mm long, 0.4- 0.6 mm wide and around 0.4 mm high. Anterolateral notches give the scales an oak-leaf-like shape. Postero-lateral downstepped margins are slightly serrated. Base is convex in mature scales. Trunk scales ( Fig. 9 I-P) with length of 0.6- 1.0 mm and width of 0.4-1.0 mm (holotype is 0.8 mm long and 0.5 mm wide) have slightly convex or flat smooth medial crown plate, or it can also carry a shallow longitudinal furrow. Lateral crown portions are rather wide and end posteriorly with three or four spines. The latter are rarely preserved. Trunk scales are flattened, their base has spur-like projection directed anteriorly downwards. Pulp depression is situated posteriorly in the base.

Scales with smooth flat crown plate that can have notches antero-laterally and a base high anteriorly and fading out posteriorly ( Fig. 9 E-H) originate from the leading fin edges. Small scales (length 0.3-0.5 mm and width 0.1-0.3 mm) probably originate from the posterior fin edges. There are two types of very small scales: with a medial crown plate and lateral portions ( Fig. 9Q ), or with a smooth plate only. The last ones might be scales covering the trunk: they just become small but retain the same characteristic features as trunk scales, and so do the fin scales.

Histology

Pulp canals, one main and three pairs of lateral ones, distinct ( Fig. 10B, D, G, H ). Main or medial pulp canal are comparatively wide, the lateral ones slightly narrower. They enter into the posterior spines ( Fig. 10E, F ). Dentine canals arise from the pulp canals and from the pulp depression. In the upper part of the crown they are fine, comparatively straight, in the lower part, branching and widened ( Fig. 10B, D, G ). Dentine tubules are very fine ( Fig. 10A, C ). Light thin surface layer of the crown contains numerous straight dentine tubules ( Fig. 10H ).

COMPARISON

Paralogania wilsoni n. sp. differs from the other species of this genus in having wide lateral portions of the crown of trunk scales and, anteriorly, smoothly rounded transitional scales of oak-leaf-like outline of the crown. The scales lack a rib above the spines which is present in P. consimilis and P. ludlowiensis . P. wilsoni n. sp., P. consimilis , P. martinssoni and P. kummerowi all have only one spine beneath the posterior crown apex while P. ludlowiensis has two parallel spines. The head and anterior transitional scales of P. ludlowiensis and P. wilsoni n. sp. strongly differ, the former species having here a “traquairi”- type of scales on the head and trilobatiform scales on the transitional body area.

Figures

FIG. 9. — Paralogania wilsoni n. sp.; A, head scale; B-D, transitional scales; E-H, most probably the scales of leading edges of the fins; I-P, trunk scales; Q, a fin scale; A-D, F, G, I-Q, the scales in crown view; E, H, the scales in lateral view; A, Pi 7603; B, Pi 7594; C, Pi 7596; D, Pi 7601; E, Pi 7593; F, Pi 7591; G, Pi 7597; H, Pi 7602; I, Pi 7592; J, Pi 7599; K, Pi 7600; L, Pi 7595; M, holotype, Pi 7598; N, Pi 7590; O, Pi 7589; P, Pi 7587; Q, Pi 7588; sample MF 157-2, Spokojnaya River, October Revolution Island, Krasnaya Bukhta Formation, Pridoli, Upper Silurian. Scale bars: A-F, I, L, N-Q, 0.1 mm; G, H, J, K, M, 0.2 mm.

FIG. 10. — Paralogania wilsoni n. sp., microstructure of the scales; A, Pi 7637; B, Pi 7638; C, Pi 7639; D, Pi 7640; E, F, Pi 7641; G, Pi 7642; H, Pi 7643; sample 157-2, Spokojnaya River, October Revolution Island, Krasnaya Bukhta Formation, Pridoli, Upper Silurian. Abbreviations: b, base; cr, crown; dc, dentine canal; dt, dentine tubules; n, neck; mpc, main pulp canal; pc, lateral pulp canal. Scale bars: 0.1 mm.