Loganellia matura, 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 : 387-389

Loganellia matura n. sp. ( Figs 5 A-O; 6A, B)

HOLOTYPE. — Trunk scale Pi 7492 ( Fig. 5G ).

ETYMOLOGY. — From matura, Latin word maturus, meaning mature, referring to the fact that the genus Loganellia had already attained all characteristic features for the scales in the Llandovery.

TYPE LOCALITY AND HORIZON. — Sample MF 3-8, Srednii Island, Golomyannyj Formation, middle part of Llandovery, Lower Silurian.

MATERIAL EXAMINED. — 47 well preserved scales.

OCCURRENCE. — A sample MF 3-8, Golomyannyj Formation, middle part of Llandovery, Srednii Island. DIAGNOSIS. — Head scales with strong ridges directed to the centre of the crown. Transitional scales with two to three elongated grooves anteriorly of the convex crown, shallow neck and strong, often high vertical base. Trunk scales tripartite: medial part with central longitudinal furrow, and wide lateral downstepped portions of the crown extending far posteriorly. Scales having two lateral ribs, each successively situated on a lower level, are also present. Crown margins are gently serrated. Pulp canal and dentine canals open into the pulp depression. Pulp canal continues with several fine dentine tubules backwardly. Dentine canals wide in the lower part of the scale, forming complicated lacunae-like widenings in the middle part of it. Dentine canals and dentine tubules relatively straight in the upper part of the scale.

DESCRIPTION

Morphology

Scales in the collection are of small to medium size (length 0.4-0.6, mainly 0.5 mm). Anterior head scales ( Fig. 5A, B ) are with strong ridges directed to the centre of the crown, the base is square with a central pulp cavity. Transitional scales and/or scales of the leading edges of the fins ( Fig. 5 C-E) are elongate with two deep and one shallow notches anteriorly of the crown. The base can have a vertical spur ( Fig. 5D ). Trunk scales ( Fig. 5 F-M, O) are rhomboidal, their crown is with up to three lateral ribs which converge in the posterior crown apex, the medial one not reaching the posterior end of the crown. In Fig. 5O the posterior half of a scale in lateral view exposes a series of ridges arising from the neck upwards and backwards. The base is swollen anteriorly, often with a spur-like process, which can constitute one third of the whole length of the scale. Scales with a smooth and flat central part are missing in our collection, hence the reason for establishing a new species. The scales most similar to L. scotica are bigger, mainly 0.6 mm and up to 0.8 mm in length. In Fig. 5N there is a very narrow and rather long scale with a slightly higher medial part and one or two downstepped lateral ones, that could be a fin scale.

Histology ( Fig. 6A, B )

In the anteriorly placed scales of the squamation only one pulp canal occurs. Posterior scales have up to three canals. Pulp canals, a medial and a pair of lateral ones, spring from the pulp depression up and backwards. The size of medial and lateral pulp canals is the same and all canals are independent. Dentine canals are relatively wide, and they possess lacunae-like expansions in the middle part of the scale. Pulp canals continue as a bunch of dentine tubules posteriorly. Dentine tubules are relatively straight. Distinct tubules of Sharpey’s fibres occur in the base, which are parallel to each other and perpendicular to the base surface. Aspidine “pegs” in the anterior part of the base are arranged in a criss-cross network.

COMPARISON

Loganellia matura n. sp. differs from other Loganellia species in having rather wide lateral portions of the crown extending far posteriorly. It differs from L. scotica (Traquair, 1898) in having ridged sculpture instead of the plate of the crowns. L. matura n. sp. has up to three pulp canals while L. scotica has one, comparatively short pulp canal.

Loganellia grossi Fredholm, 1990 ( Fig. 2 N-S)

SYNONYMY. — See Fredholm (1990) and Märss (1996).

MATERIAL EXAMINED. — About 200 scales of varying preservation.

OCCURRENCE. — Samples 2-62, 2-67, 2-68, Matusevich River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian.

REMARKS

Scales are small. Head scales have round or oval configuration ( Fig. 2N ), crenulated margin and slightly convex crown surface. The neck is very low, pulp depression has a central position in young scales. Transitional scales ( Fig. 2O ) anteriorly with notches, postero-lateral margins of the crown smooth. Outline of the scales is more rhomboidal. Trunk scales ( Fig. 2 Q-S) are relatively small (length 0.2-0.5 mm), medial rhomboidal plate is smooth and flat, anterior to it a short and shallow notch can occur. Short lateral down-stepped rims unite beneath and behind the apex of medial plate. Neck is not very distinct. Base is, depending on the stage of development of the scale, concave or convex.

Genus Shielia Märss, 1998

TYPE SPECIES. — Shielia taiti (Stetson, 1931) .

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. 6. — Microstructure of the scales; A, B, Loganellia matura n. sp.; C-F, Shielia multispinata n. sp.; G, Paralogania consimilis n. sp.; A, Pi 7624; B, Pi 7625; C, Pi 7626; D, Pi 7627; E, Pi 7628; F, Pi 7629; G, Pi 7630; A, B, sample MF 3-8, Srednii Island, Golomyannyj Formation, middle part of Llandovery, Lower Silurian; C-F, sample MF 157-7, Spokojnaya River, upper part of Samojlovich Formation, Wenlock, Lower Silurian; G, sample MF 46-23, Ushakov River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian. Abbreviations: asp, aspidine “pegs” in the base; b, base; bc, canal in the base; cr, crown; dc, dentine canal; dt, dentine tubules; fpc, fan-shaped pulp canals; lac, lacunae of dentine canals; mpc, main pulp canal; n, neck; pc, pulp canal; pco, pulp canal opening; sp, spine. Scale bars: 0.1 mm.

FIG. 2. — A-M, Stroinolepis maenniki n. gen., n. sp.; A, head scale; B-E, transitional scales; F-M, undivided trunk and fin scales; A-G, I, L, M, scales in crown view; H, lateral oblique view; J, K, lateral view; A, Pi 7476; B, Pi 7479; C, Pi 7477; D, Pi 7484; E, Pi 7480; F, Pi 7472; G, Pi 7478; H, I, Pi 7481; J, Pi 7474, scale is lost; K, Pi 7470; L, Pi 7469; M, Pi 7473; A, C, F, G, J-M, sample MF 49-7, Strojnaya River, Ozernaya Formation, Middle-Upper Ordovician; B, E, H, I, sample MF 49-9; D, sample MF 49-11, Strojnaya River, October Revolution Island, Strojnaya Formation, Upper Ordovician; N-S, Loganellia grossi Fredholm, 1990; N, head scale; O, transitional scale; P-S, trunk scales; N, O, Q-S, scales in crown view; P, a scale in base view; N, Pi 7530; O, Pi 6839; P, Pi 7533; Q, Pi 7532; R, Pi 6837; S, Pi 7535; sample 2-68, Matusevich River, October Revolution Island, upper part of Samojlovich Formation, Wenlock, Lower Silurian. Scale bars: 0.1 mm.