Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

J. Valiukevičius. Silurian acanthodians from Lūžņi-4
125
Fig. 2. Nostolepis alifera sp. nov. Histological structure of a scale like that in Fig. 1B. Vertical
longitudinal section. Thin section 3760. A, general view; B, detail of the scale primordium; C,
detail of the posterior part of neck with the neck/base junction; D, detail of the central part of
crown. Lūžņi-4 borehole, depth 207.0 m. Přidoli, Jūra Regional Stage, Garzde Beds of the
Targale Formation. Abbreviations: avc, ascending vascular canal, b, bone, gl, growth lamella,
oc, osteocyte cavity, rvc, radial vascular canal, sg, Stranggewebe, sl, Stranglacuna. Scale bars:
0.1 mm.
Description. Morphology. Most (flank) scales have posteriorly elongated rhomboidal
to elipsoidal crowns from 0.28 to 0.85 mm long. Rare specialized varieties have triangular
crowns. The crown plate slopes down slightly anteriorly. A clear elevated triangular
medial area and lower lateral areas are distinguished on the crowns. The medial area
carries five to eight sub-parallel anterior ridges with rounded crests that fade out at
about half of crown length (Fig. 1 C, E-F). Some scales have unevenly long medial
ridges of which some point to or converge with the longest central ridges at the
posteriormost crown part (Fig. 1 B, G). Short anterior bifurcation of ridges occurs
rarely. The medial crown area sometimes has a wide deep central groove (Fig. 1 G).
The lateral crown areas contain one to two ridges extending the whole crown length
(Fig. 1 D, G) or short, more numerous oblique ridgelets which converge with the longest
medial ridges posteriorly (Fig. 1 A-B). The scale bases are rhomboidal (except for
specialized varieties, see Fig. 1 D), slightly (Fig. 1 K) to moderately convex, and
separated from the necks by a clear rim. The necks are short anteriorly because of the
downsloping crown, and frequently contain large pores (up to five basally on each side,
see Fig. 1 C).