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 ...
30 ADVANCES IN PALAEOICHTHYOLOGY Specimen Pl 10/10 is a fragment of Obruchevia ventral plate (Fig. 4C, 4D) from the posterior portion of the right side; it is 110 mm wide, 105 mm long, and has a maximum thickness of 17 mm. The margins of the fragment are broken, except for the smooth and slightly convex mesial margin that shows the presence of a posterior notch. Five growth lines are visible on the external surface, the middle one being the strongest. They parallel the outer margin of the plate but successively die out against the inner margin. The two earliest growth zones are smooth but the later ones have short radial Fig. 3. Obruchevia heckeri. Fragments of the dorsal plate showing variations in dorsal surface ornament and the presence of growth lines. 1, PI 10/11; 2. PI 10/12. Scale bars equal 50 mm. furrows. The fragment is thickest (about 20 mm) at its anterior margin and above the earliest two growth lines. The plate becomes gradually thinner towards the lateral margin. The ventral plate was probably rather flat. As in Obruchevia dorsal plates, the growth of the ventral plate took place along the outer edges under the margins in such a way that one or several flanges (actually growth zones) were formed. The center of the ventral plate and the flanges that were formed later are thinner. Due to this method of growth the spongy layer is laminated. Weathering of the plate has caused the thin basal layer on the visceral surface of the ventral plate to peel off and the spongy layer has also peeled off in patches (Fig. 4D). The ventral plate, not known before in Obruchevia, had a deep posterior notch similar to that found in Schizosteus, Pycnolepis, Pycnosteus, Ganosteus, and Tartuosteus (see Obruchev and Mark-Kurik 1965; Halstead Tarlo 1964, 1965) (Fig. 10B).
D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia 31 Fig. 4. Obruchevia heckeri. Branchial plate (PI 10/8) in ventral (A) and dorsal (B) views; posterior lateral fragment of a ventral plate (PI 10/10) in dorsal (C) and ventral (D) views. Scale bars equal 50 mm.
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D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia<br />
31<br />
Fig. 4. Obruchevia heckeri. Branchial plate (PI 10/8) in ventral (A) <strong>and</strong> dorsal (B) views; posterior<br />
lateral fragment of a ventral plate (PI 10/10) in dorsal (C) <strong>and</strong> ventral (D) views. Scale<br />
bars equal 50 mm.
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