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J. Valiukevičius. Silurian acanthodians from Lūžņi-4
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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.