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40 ADVANCES IN PALAEOICHTHYOLOGY Fig. 10. Perscheia pulla gen. et sp. nov. Reconstruction of dorsal (A) and ventral (B) plates based on the new material. Dorsal plate outline based on the holotype of Obruchevia heckeri (Obruchev 1941; pl. I.1). Abbrev: ldc, lateral dorsal sensory canal; mdc, medial dorsal sensory canal. Scale bars equal 100 mm.
D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia 41 two most important in reconstructing the plate are the holotype CMN-NUFV101 (Figs. 6A, 6B, 7A), which is probably from the posterior margin, and CMN-NUFV104 (Figs. 6E- G, 7C), which is from the posterolateral margin. CMN-NUFV101 can be placed in the midline based on the presence of symmetrically placed longitudinal sensory canals that are probably the medial dorsal canals. They run parallel to the radial ornament and are joined by a transverse commissure that runs parallel to the growth lines. CMN-NUFV104 is a fragment of the lateral part of the plate and contains a shallow embayment that is probably equivalent to the lateral notch present in Obruchevia. It is suggested here that this fragment comes from the left side of the plate based on the fact that: (1), the sensory canal thought to be the lateral dorsal canal continues in one direction (anterior) while petering out in the other (posterior); (2), the growth lines suggest a very convex margin to the plate beyond the lateral notch and if this was the anterior part of the fragment then it would suggest a very anterior position for the notch which is not the case in Obruchevia; and (3), the diagonal ridges across the embayment, if related in some way to the branchial duct, are more likely to be oriented posteriorly. Specimens CMN-NUFV102 (Figs. 6C, 7B) and CMN-NUFV103 (Figs. 6D, 7D) have no natural edges and can only be positioned on the dorsal plate based on their ornament and the sensory canals present. CMN-NUFV103 probably extends from the midline towards the lateral margin and contains a section of the lateral dorsal canal. CMN-NUFV102 may represent a more posterior part of the plate close to the posterolateral notch and contains parts of the medial and lateral dorsal canals. Based on these interpretations the dorsal plate might be as much as 600 mm long and 550 mm wide, which is close to the size of the dorsal plate of Obruchevia. The ventral plate is represented only by CMN-NUFV105 (Fig. 8), which clearly represents the left posterior part of the plate including the margin of the posterior median notch. It is difficult to estimate the overall size of the plate from this one fragment, but based on other species that have notched ventral median plates, it could have been as much as 450 mm long and 350 mm wide (Fig. 10B). Remarks on the phylogenetic relationships of obrucheviids The presence of a well-developed and apparently connected canal system on the dorsal plate of Perscheia is an unusual feature, as the psammosteid canal system is generally poorly known (Obruchev and Mark-Kurik 1968). Where it is known it normally consists of a pair of medial dorsal canals and one to three pairs of transverse commissures, all of which are present below the surface layer of dentine tubercles. The presence of this system as open grooves on the surface in Perscheia is also unusual, although not unexpected in a species in which the surface covering of dentine tubercles is missing. An intermediate stage can be seen in Traquairosteus pustulatus, a Frasnian species from Scotland in which the outer surface of aspidin is thrown up into conical mounds, each surmounted by a small crenulated dentine tubercle. The dentine tubercles are thus very sparse. In the holotype (BM P.8297; Halstead Tarlo 1965, pl. XVII, fig. 1) two canals can be seen as open grooves on the surface, indicating that as the dentine tubercles were reduced, the canals became exposed and were only covered by dermis. Given
Page 1 and 2: LATVIJAS UNIVERSITĀTES RAKSTI 679.
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Page 5 and 6: Contents Ervīns Lukševičs and Ģ
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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
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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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V. Pernegre, V. Dupret. Biostratigr
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Ž. Žigaite. New telodont from Tuv
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