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 ...
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D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia<br />
39<br />
forms a zone that is much more compact, <strong>and</strong> the openings reduce rapidly in size <strong>and</strong><br />
number in the lower part of this layer (Fig. 9C, pl). At the external surface spongy<br />
aspidin has been strengthened by the addition of pleromin (Fig. 9A) the hypermineralized<br />
tissue characteristic of psammosteid heterostracans (Ørvig 1976, Mark-Kurik 1984).<br />
Polarized light reveals the characteristic dark “crosses” in the pleromin (Fig. 9B). Similar<br />
“crosses” have been illustrated in the dense pleromin of Psammosteus megalopteryx<br />
(Ørvig 1976, fig. 9). The pleromin in Perscheia appears to be less dense <strong>and</strong> shows<br />
narrower vascular canals than in Obruchevia (Obruchev 1941, pl. 2.1).<br />
Discussion. Although the material consists of fragments, it is possible to use them to<br />
make a partial restoration of the dorsal <strong>and</strong> ventral plates (Fig. 10). Apart from CMN-<br />
NUFV105 (Fig. 8) all the fragments appear to come from the dorsal plate. Of these the<br />
Fig. 9. Perscheia pulla gen et sp. nov. CMN-NUFV106. Vertical transverse thin section of the<br />
surface layer showing dense structure (A) <strong>and</strong> the characteristic cross developed <strong>un</strong>der crossed<br />
polars (B); C, entire vertical transverse thin section (inner surface to the left) showing a thin,<br />
laminar, basal layer followed by a thick spongy layer <strong>and</strong> then a denser surface layer in which the<br />
aspidin has been reinforced by pleromin. Scale bars equal 0.5 mm (A, B) <strong>and</strong> 5 mm (C). Abbrev:<br />
lam, laminar basal layer; pl, pleromin strengthened outer layer.