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

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ADVANCES IN PALAEOICHTHYOLOGY
Histology. Three or four superposed growth lamellae of scale crowns are composed
of simple bone-like mesodentine and only poorly formed Stranggewebe (scales with
medial groove, Fig. 1 H-J), and of Stranggewebe enveloped in each lamellae by a strip
of simple networked mesodentine (holotype-like scales, Fig. 2 A-D). Both scale varieties
contain large main radial, circular and ascending vascular canals positioned basally
in the growth zones (Fig. 1 I-J, 2 C). Particularly long and wide radial vascular canals
are displaced over the base, separating the different tissues of base and crown. The
crown tissues are highly cellular up to the outer layer. Osteocyte cavities are multiangular,
large, and incorporated into a short-tubular mesodentine network or
Stranggewebe distinguishing by dense and long Stranglacunae (Fig. 2 B, D). The
cellular bone of bases is densely lamellar and contains plenty of multi-angular
osteocyte cavities (Fig. 1 L).
Remarks. The sculpture characters of the crown, crown/neck/base proportions, as
well as the histologic microstructure of tissues, including highly vascularized bonelike
mesodentine and Stranggewebe in crowns and cellular bone in bases, suggest an
attribution of scales to the genus Nostolepis. Among the known Silurian nostolepids,
N. alifera sp. nov. seems most similar to N. alta (Märss 1986). One variety of the latter
also has a raised medial area sculptured by sub-parallel ridges and the lowered lateral
areas on scale crowns (Märss 1986: pl. 28, figs 12-14), but their neck and base features
separate them distinctly. Unlike N. alta, no scales of N. alifera have tall necks or small
and flat bases that sometimes are pierced by large vascular canals. The histologic
structure of both species is similar except for the vascular canals in scale bases and
reduced Stranggewebe area in crowns of N. alta (Märss 1986: pl. 31, figs 1-4), and the
poorly developed mesodentine network strips in the outer layers of growth lamellae
in N. alifera.
N. alifera resembles N. amplifica and N. musca (Valiukevicius 2003 b: fig. 2 A-H and
fig. 5 J-R respectively), both from the Baltic Silurian in the development of the medial
area on scale crowns. However, the ridge ornamentation is different: N. amplifica and
N. musca have shorter, more rough and rounded ridges which are less uniformly arranged
than in N. alifera. Converging ridges on the postero-medial crown part are not observed
in both compared species. The histologic structure of all species is similar except for
wider and more numerous vascular canals of the principal system in N. alifera, that are
almost absent in N. musca (Valiukevicius 2003 b: fig. 8 A-G), and thicker layers of simple
networked mesodentine enveloping the Stranggewebe in N. amplifica (Valiukevicius
2003 b: fig. 3 A-G). The Stranggewebe of N. musca shows longer and more uniform
Stranglacunae distributed more evenly in the growth lamellae.
Several Devonian or Siluro/Devonian nostolepids recently described from the Old
Red Sandstone Continent are similar to N. alifera in having medial areas on scale
crowns. In N. decora (Valiukevicius 2003 a: fig. 17 C-G) this area is concave, carrying
only one or two short central anterior riblets, whereas the lateral edges are often spiny
with the spinelets directed obliquely upward. The principal histologic difference is that
the Stranggewebe is not overlain by the mesodentinal network in the outer parts of
growth lamellae in N. decora (Valiukevicius 2003 a: fig. 18 A, C, F). N. adzvensis
(Valiukevicius 2003 d) is distinguished by characteristic posterior crown/neck structures
comprising oblique ridges and oblique or vertical neck riblets. The crown tissues of the