Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ... Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
132 ADVANCES IN PALAEOICHTHYOLOGY Material. Approximately 170 scales. Diagnosis. Nostolepis having mid-sized scales with highly raised dominant medial area on crowns ornamented by one to four short anterior ridges; narrow lateral areas are outlined by long oblique neck ridges pointed posteriorly. Scale crowns (maximum four growth lamellae) are composed of dense, finely-networked, cellular simple mesodentine (dominant tissue) and outer durodentine; no Stranggewebe; vascular canals are well developed; mesodentine grades into the cellular bone of scale bases without sharp border. Description. Morphology. Scales have elongated triangular crown plates slightly sloping down anteriorly with the tapered posterior crown part far overhanging the base. Crown length varies from 0.38 to 1 mm; most frequently 0.6-0.8 mm. Crown width is 0.2-0.62 mm. A wide medial area forms most of the crown, which is highly raised compared to the narrow lateral slopes. Only the anteriormost strip of the medial area is sculptured by a few (one to four) short rounded ridges. The lateral areas are outlined by the oblique long neck ridges which converge with the medial ones extremely posteriorly (Fig. 5A- C). The posterior crown overhang can reach a half of its length. Scale necks are low and bases rhomboidal, of medium convexity, with the deepest point forward of centre. Histology. Three to four growth lamellae of the crown are composed of simple mesodentine and durodentine (canalless dentine) which is developed in thin strips superficially centrally (Fig. 5L). Stranggewebe is lacking in the posterior crown parts. Simple mesodentine forms a particularly dense and fine network of dentinal canaliculi (Fig. 5 E-F) incorporating numerous random, large, multi-angular osteocyte cavities. Mesodentine character in this respect is similar to the base bone pattern except for more regular orientation of osteocytes along the growth lines in bases (Fig. 5 E-G). There is no sharp border between the crown and base tissues; mesodentine and bone make a gradual transition. The system of principal vascular canals in crowns (ascending, radial and circular) is well developed forming long and wide branches in each lamella (Fig. 5 I, L). Remarks. Gross (1947) regarded Diplacanthoides elegans as a junior synonym of Nostolepis striata. However, as proposed by Vergoossen (1999-2002), the ‘elegans’ group of scales is distinguishable from N. striata by diagnostic morpho- and histological characters. N. striata sensu Gross (1947, 1971) can not compare with the other nostolepid taxa or any acanthodian in having so numerous and excessively diverse scale forms, and undoubtedly incorporates multiple biological species. For more extended morphological comments on N. elegans scales and comparisons with other nostolepids, I direct readers to Vergoossen’s (1999-2002) remarks. N. arctica Vieth (1980: pl. 5, figs 1-9; fig. 20 A-E) differs from N. elegans in having more ornamented scales which crowns are highly inclined, longer and often longitudinally bent. Now, after a study of histologic structure of scales (N. elegans was not previously examinated), the differences from N. arctica are furtherly supported by the absence of Stranggewebe in crowns of N. elegans. Occurrence. See Table.
J. Valiukevičius. Silurian acanthodians from Lūžņi-4 133 Fig. 5. Nostolepis elegans (Brotzen) 1934. SEM micrographs of scales (A-C), crown views, anterior upwards, and thin section photos (D-M). A, LIGG 25-A-2624; B, LIGG 25-A-2629; C, LIGG 25-A-2623; D-G, vertical transverse section of a scale like that in Fig. 5 A. Thin section 3743. D, general view; E, detail of the left crown part and the base portion; F, detail of the right crown part and base portion; G, detail of the right part of the base; H-M, vertical longitudinal sections of scales. H-J, thin section 3742. H, general view; I, detail of the posterior crown part at higher magnification; J, detail of the central base part at higher magnification; K-M, thin section 3740. K, general view; L, detail of the central crown part (scale primordium and the later growth lamellae) and the bordering base strip at higher magnification; M, detail of the crown/base junction from the left side at higher magnification. Lūžņi-4 borehole, depth 197.2 m. Přidoli, Jūra Regional Stage, Garzde Beds of the Targale Formation. Abbreviations: avc, ascending vascular canal, b, bone, dt, dentine tubule, gl, growth lamella, oc, osteocyte cavity, p, scale primordium, rvc, radial vascular canal, smd, simple mesodentine. Scale bars: 0.1 mm.
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J. Valiukevičius. Silurian acanthodians from Lūžņi-4<br />
133<br />
Fig. 5. Nostolepis elegans (Brotzen) 1934. SEM micrographs of scales (A-C), crown views,<br />
anterior upwards, <strong>and</strong> thin section photos (D-M). A, LIGG 25-A-2624; B, LIGG 25-A-2629; C,<br />
LIGG 25-A-2623; D-G, vertical transverse section of a scale like that in Fig. 5 A. Thin section<br />
3743. D, general view; E, detail of the left crown part <strong>and</strong> the base portion; F, detail of the right<br />
crown part <strong>and</strong> base portion; G, detail of the right part of the base; H-M, vertical longitudinal<br />
sections of scales. H-J, thin section 3742. H, general view; I, detail of the posterior crown part<br />
at higher magnification; J, detail of the central base part at higher magnification; K-M, thin<br />
section 3740. K, general view; L, detail of the central crown part (scale primordium <strong>and</strong> the<br />
later growth lamellae) <strong>and</strong> the bordering base strip at higher magnification; M, detail of the<br />
crown/base j<strong>un</strong>ction from the left side at higher magnification. Lūžņi-4 borehole, depth 197.2<br />
m. Přidoli, Jūra Regional Stage, Garzde Beds of the Targale Formation. Abbreviations: avc,<br />
ascending vascular canal, b, bone, dt, dentine tubule, gl, growth lamella, oc, osteocyte cavity, p,<br />
scale primordium, rvc, radial vascular canal, smd, simple mesodentine. Scale bars: 0.1 mm.
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