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

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