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128 ADVANCES IN PALAEOICHTHYOLOGY Fig. 3. Nostolepis latvica sp. nov. SEM micrographs of scales (A-G), crown views, anterior upwards, and vertical longitudinal thin section photos (H-M). A, holotype, LIGG 25-A-2574; B, LIGG 25-A-2575; C, LIGG 25-A-2576; D, LIGG 25-A-2578; E, LIGG 25-A-2609; F, LIGG 25-A-2610; G, LIGG 25-A-2655; H-K, thin section of a holotype-like scale. Thin section 3788. H, general view; I-K, details of the posterior (I), central (J) and anterior (K) crown parts and the bordering base portions at higher magnification; L-M, thin section of a scale like that in Fig. 3 F. Thin section 3763. L, general view; M, detail of the posterior crown part at higher magnification. Lūžņi-4 borehole, depth 272.7 m. (G-K), 280.3 m (A-D and L-M) and 330.4 m (E-F). Přidoli, Minija Regional Stage, Varniai (A-D and G-M) and Šilalė (E-F) Beds of the Minija Formation. Abbreviations: avc, ascending vascular canal, cvc, circular vascular canal, dt, dentine tubule, gl, growth lamella, oc, osteocyte cavity, rvc, radial vascular canal, sg, Stranggewebe, smd, simple mesodentine. Scale bars: 0.1 mm.
J. Valiukevičius. Silurian acanthodians from Lūžņi-4 129 three short anterior riblets (Fig. 3 C, E-G). The ribbed crown slopes sharply down to the base anteriorly. The base is rhombic in outline and has clear anterior rims; it is shallow, centrally convex, often wider than long and protrudes beyond the antero-lateral margins of the crown. No pore or vascular canal openings are visible on the scales. Histology. The microstructure of flank scales is mainly uniform despite the marked differences in crown sculpture. Four to six lamellae in crowns of the superpositional growth type are composed of Stranggewebe and networked mesodentine. The Stranggewebe area of the posterior crown part seems slightly reduced as compared to the classical “Nostolepis”-type histology. It is replaced by the bone-like cellular mesodentine in the basal crown parts (area of the posterior neck, Fig. 3 I-J), with plenty of large osteocyte cavities connected by short winding processes almost excluding space for the dentinal canaliculi here. The Stranggewebe strips are not covered by the mesodentinal network (Fig. 3 M). The anterior part of all crown lamellae except the scale primordium is entirely composed of very dense simple mesodentine incorporating frequent osteocytes among the interwoven dentine tubules (Fig. 3 J-K). The main system of wide vascular canals is poorly developed except for the ascending and radial canals basally in the neck (Fig. 3 M). The low-pyramidal base has large numbers of spindleshaped osteocyte cavities of the cellular bone oriented along the thin-lamellar growth lines (Fig. 3 K). This pattern, combined with the lack of Stranggewebe in the lower areas of the crown, gives a sharp border between the crown and base tissues. Remarks. In the crown sculpture characters (slightly concave medial area carrying short anterior riblets, presence of lateral steps formed by the oblique neck ridges), N. latvica sp. nov. resembles N. striata sensu Gross 1947 (pl. 26, figs 8-11), but distinctly differs from Pander’s original N. striata (Pander, pl. 6, figs 7 a-c). This difference led to my prior definition of this species as “prestriata” (Valiukevicius 2003 c) according to its lower stratigraphic occurrence in the sequence. (N. striata is present only in the Garzde and Lužni Beds of the topmost Pridoli in the Lužni-4 borehole). N. latvica, judging by both morpho- and histological characters, belongs to the N. striata group of nostolepids. In this sense, it can be covered by the discussion of nostolepid species having medial crown area on scale crowns given by Valiukevicius (2003 d). The holotypelike scales of N. latvica differ from those of other nostolepids by the uniform subparallel, sharp and constant number of ridges fading out at the mid-length of crown, and also by details of the Stranggewebe and simple mesodentine structure (reduced area of the first tissue in the posterior crown part, density of the dentinal network of the second one in the anterior part, reduction of the system of main vascular canals higher in crowns). Vergoossen (2002 a, pl. 6, fig. 63; 2002 b, pl. 3, fig. 36) recorded morphologically similar, but not histologically examined scales from the Late Silurian of Sweden which he assigned to N. striata. The Lužni scales, especially holotype-like ones, differ from those of Sweden by the uniformity and stable number of crown ridges. Occurrence. See Table.
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LATVIJAS UNIVERSITĀTES RAKSTI 679.
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LATVIJAS UNIVERSITĀTES RAKSTI 679.
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Contents Ervīns Lukševičs and Ģ
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ACTA UNIVERSITATIS LATVIENSIS, 2004
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Ervīns Lukševičs, Ģirts Stinkul
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Olga Afanassieva. Microrelief on th
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D.K. Elliott, E. Mark-Kurik, E.B. D
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C.G.Miller, T. Marss, H. Blom. New
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H.-P. Schultze, T. Marss. Revisitin
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Page 81 and 82: O.A. Lebedev. A new tetrapod from R
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Page 101 and 102: E. Lukševičs, I. Zupiņš. Sedime
Page 121 and 122: J. Valiukevičius. Silurian acantho
Page 127: J. Valiukevičius. Silurian acantho
Page 149 and 150: V. Pernegre, V. Dupret. Biostratigr
Page 159 and 160: Ž. Žigaite. New telodont from Tuv
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