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32 ADVANCES IN PALAEOICHTHYOLOGY The branchial plates in Obruchevia were briefly described and schematically figured by Halstead Tarlo (1965, fig. 48). The Moscow collection contains five branchial plates (PIN # 87/9-13, O. Lebedev pers. comm., 2003), four of them fairly complete. The plates are of either a wider (Fig. 5C, 5D, PIN 87/11) or narrower type (Fig. 5A, 5B, PIN 87/ 10). In either case they are broadly triangular with a somewhat concave medial margin and convex lateral margin. Where preserved the posterior margin is transverse and shows a well-developed lobe towards the lateral edge of the plate. The dorsal surface is convex and generally unornamented, as it was mostly covered by soft tissue in life. In PIN 87/11 (Fig. 5C, a right branchial plate) some growth lines are visible on the dorsal side of the vertically oriented lateral portion of the plate, which was presumably exposed. The growth lines on the concave ventral side of the plate are more strongly developed. On this side the plate shows radially arranged furrows, similar to those on the external surface of the ventral plate (Fig. 5D). The branchial plates have strongly downturned lateral free margins, which would have functioned as runners. One of the Riga specimens is an incomplete right branchial plate (Fig. 4A, 4B, Pl 10/8). It includes part of the distal edge and posterior margin, although much of the proximal part of the plate is missing. It is 133 mm long, 70 mm wide, and 11 mm in maximum thickness and is ‘j’ shaped in cross-section. The concave ventral surface (Fig. 3A) is ornamented with irregular rounded pits and is shiny due to the addition of pleromin to the outer part of the spongy aspidin. A strongly developed growth line parallels the outer margin of the plate. Laterally the plate margin is slightly concave and strongly downturned. The margin is broken, revealing a cross section of the bone and showing that it has a central spongy layer (2.5-5 mm thick), covered on both sides by compact pleromin (2 mm thick). The margin is finely vertically striated, more strongly on the dorsolateral side, giving evidence of abrasion of the “runners” against the bottom sediment. The microstructure of the Obruchevia dorsal plate (Obruchev 1941, pl. II) does not differ very much from that known in other psammosteids (except in the absence of dentine tubercles). The upper part of the spongy aspidin layer is compact and solidified by the deposition of pleromin; further down the spongiosa is coarser. The basal layer is comparatively thin and finely laminated (in Obruchev 1941, pl. II fig. 1 the laminated structure is not visible: in the photo the basal layer is completely black). Halstead Tarlo also figured pleromin in Obruchevia heckeri (1964, pl. VII, fig.1, 2, 4; pl. XII, fig. 5). His fig. 4 (pl. VII) shows pleromin in polarized light. Discussion. Pleromin in the free margins of Obruchevia branchial plates shows the same phenomenon known in other psammosteids in which particularly the lateral corners of the branchial plates but also the central part of the ventral plates and the posterior ends of ventral ridge scales show additional deposition of this hard tissue. However, pleromin is not thought to have developed in obrucheviids as a tissue to counteract wear only. The tissue had several different functions, to counteract wear of the carapace and squamation, but also to reinforce the fabric of the carapace (Ørvig 1976; Mark-Kurik 1984). Ørvig established that pleromin started to appear between and below the tubercles before the exoskeletal plates were attacked by wear. This can be seen in several of the specimens described here in which pleromin can be seen on surfaces that were not subjected to wear (e.g. the growth lines visible on the lateral dorsal surface of PIN 87/11, Fig. 5C).
D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia 33 Fig. 5. Obruchevia heckeri . Branchial plates (PIN 87/10, 87/11) in dorsal (A, C) and ventral (B, D) views. Scale bars equal 50 mm.
Page 1 and 2: LATVIJAS UNIVERSITĀTES RAKSTI 679.
Page 3 and 4: LATVIJAS UNIVERSITĀTES RAKSTI 679.
Page 5 and 6: Contents Ervīns Lukševičs and Ģ
Page 7 and 8: ACTA UNIVERSITATIS LATVIENSIS, 2004
Page 9 and 10: Ervīns Lukševičs, Ģirts Stinkul
Page 15 and 16: Olga Afanassieva. Microrelief on th
Page 23 and 24: D.K. Elliott, E. Mark-Kurik, E.B. D
Page 31: D.K. Elliott, E. Mark-Kurik, E.B. D
Page 47 and 48: C.G.Miller, T. Marss, H. Blom. New
Page 59 and 60: H.-P. Schultze, T. Marss. Revisitin
Page 81 and 82: O.A. Lebedev. A new tetrapod from R
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O.A. Lebedev. A new tetrapod from R
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ACTA UNIVERSITATIS LATVIENSIS, 2004
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E. Lukševičs, I. Zupiņš. Sedime
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J. Valiukevičius. Silurian acantho
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Ž. Žigaite. New telodont from Tuv
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