Shielia multispinata, 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 : 389-393

Shielia multispinata n. sp. ( Figs 6 C-F; 7A-S)

HOLOTYPE. — Pi 6803 ( Fig. 7K ), sample MF 157-7, Spokojnaya River , upper part of Samojlovich Formation , Wenlock, Lower Silurian.

ETYMOLOGY. — From the Latin word multispinata , to indicate that trunk scales of this species have several spines posteriorly of the crown.

MATERIAL EXAMINED. — About 100 scales of good and fairly good preservation.

OCCURRENCE. — Sample MF 157-7, Spokojnaya River, upper part of Samojlovich Formation, Wenlock, Lower Silurian.

DIAGNOSIS. — Scales of medium size. Head scales rounded to oval with crenulated margins. Neck is rather distinct; base deep with vertical rootlets. Transitional scales are oval to rhomboidal with crenulation in the antero-lateral margins and one posterior crown apex. Diagnostic trunk scales bear a central part with a shallow groove, a pair of lateral ribs directed towards the posterior central crown apex, and two-three spines beneath the lateral ribs, one single spine situated below the central crown apex. Each more laterally positioned crown portion (ribs and spines) are downstepped. Anterior horizontal process of the base, sometimes very long, is present in all trunk scales. Neck is low but distinct. Pulp depression large. Opening of the main, medial, pulp canal is situated in the posterior portion of the base. Two to four pairs of lateral pulp canals may be present. Dentine canals start from the pulp depression or pulp canals; they are similar to the orthodentine canals. Fine dentine tubules occur in the crown but can also be found in the base. The crown surface is from a layer of enameloid.

DESCRIPTION

Morphology

Only one sample (MF 157-7) yields the scales of this new species. In the set, comparatively large head and transitional scales are present in contrast to the small trunk and fin scales. The length of scale crowns range from 0.2 up to 0.5 mm. Elongated spiny trunk scales have an anterior horizontal basal spur up to 0.5 mm long and another 0.5 mm makes the crown. The smallest scale is only 0.1 mm wide. Large head scales have a vertical base that is 0.5 mm high at maximum. The holotype ( Fig. 7K ) has a 0.25 mm long horizontal spur and 0.5 mm long crown with width of 0.25 mm. Head scales ( Fig. 7A ) have a round or oval crown and base configuration. The crown surface is flat and smooth with crenulated margins, with the neck in the shape of a shallow groove. The pulp cavity is placed in the middle of the scale and surrounded by vertical rootlets of the base. Transitional scales ( Fig. 7 B-D, F) are oval or rhomboidal with crenulation or two notches antero-laterally of the crown. The base is becoming more horizontal. Trunk scales ( Fig. 7E , G-P) are rather flattened. A longitudinal shallow groove is placed on the medial elongated portion of the crown surface. On both sides of it a slightly downstepped rib starts from the base and is directed towards the posterior medial crown apex; lateral ribs do not join with the latter and a split occurs between them. The second pair of downstepped ribs can be relatively long or consist of up to three spines. The lower side of the posterior crown portion exhibits five ridges. The base has an anterior spur-like process. The pulp depression is large, and openings of pulp canals have a posterior position. Fin scales ( Fig. 7 Q-S) have the same shape as trunk scales but are much smaller.

Histology ( Fig. 6 C-F)

The pulp cavity of the young head and transitional scales can be rather deep and wide, but is narrower in adult scales. The pulp depression of trunk scales is large; from the posterior part of it three to nine forked or fan-shaped pulp canals (a medial and four pairs of lateral) enter into three to nine posterior spines and go far posteriorly ( Fig. 6C, D ). The openings of pulp canals can form a longitudinal row in the base ( Fig. 6F ). Fine, nearly parallel dentine canals go out from the pulp canals or from the base ( Fig. 6E ). Dentine canals are relatively straight and numerous. They resemble more the canals in the orthodentine than in the mesodentine. Dentine tubules straight. In the base, fine tubules of Sharpey’s fibres, perpendicular to the base surface, and aspidine “pegs” are distributed.

COMPARISON

Head and transitional scales of Shielia multispinata n. sp. differ from those in S. taiti (Stetson, 1931) in being larger and more crenulated while in S. taiti they are smaller ( S. taiti has scales up to 0.5 mm long), simpler anteriorly and with two wing-like structures laterally. The new species has trunk scales with lateral ribs directed towards the posterior central crown apex; in S. taiti they start from the neck, tend to run more parallel to the central portion and end with free spines posteriorly. S. taiti has deeper grooves between the ribs. In S. multispinata n. sp. the spines start beneath the lateral ribs.

Genus Paralogania Karatajūtē- Talimaa, 1997

TYPE SPECIES. — Paralogania kummerowi (Gross, 1967) .

Figures

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. 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.