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86 ADVANCES IN PALAEOICHTHYOLOGY Tulerpeton (Lebedev and Clack 1993) and Densignathus (Daeschler 2000). Its anterior and posterior edges are broken off; the dorsal margin shows small fused parts of the surangular. The ornament consists of shallow pits and anastomosing ridges, the pits become more elongated radially from the ossification centre in the area dorsal to the mandibular seismo-sensory canal groove (Fig. 4A). Ventral to it the ridges become more swollen and pits are deeper than above the canal. The canal is deep and wide, a short part of it close to the ossification centre is exposed to the surface, and the remaining course communicates to the surface with a single row of large rounded or elongated foramina. This pattern conforms to that in Densignathus (Daeschler 2000), but contrasts with that found in Tulerpeton, in which the canal is completely housed in an open groove (Lebedev and Clack 1993). In Acanthostega, on the contrary, most of the canal is enclosed in the bone and only its anterior and posterior sections at the angular are exposed in the grooves (Ahlberg and Clack 1998). In Ventastega the canal through this bone is not exposed at all (Ahlberg et al. 1994). The ventral edge of the preserved part of the bone is almost straight. The mesial lamina is narrow and smooth; its lateral edge is separated from the lateral lamina with a straight ridge of uniform width. The notches for the Meckelian foramina are shallow, and there are at least three of them seen in the preserved fragment (Fig. 4B). Pectoral girdle. The cleithrum (PIN 2657/343) (Fig. 5) is generally well preserved, only its lateral surface and the anterior edge are slightly worn. On its mesial surface the posterodorsal corner of the dorsal lamina (Fig. 5B) bears a deep elliptical pit, which may be a trace of lifetime damage, as the edges of the bony bars at the pit bottom constituting the tissue are smooth, as happens in healed tissue. However, there are no traces of regeneration to support this suggestion. In its general shape the cleithrum is very similar to the bones (LDM 81/522 and LDM 57A/1984) attributed to Ventastega (Ahlberg et al. 1994). The general outline of the cleithrum is feather-shaped; it is sigmoidally curved in the lateral aspect. In the anterior aspect (Fig. 5C) the bone is only very slightly curved laterally, its dorsal lamina is comparatively thin; the maximum thickness of the bone is attained at the contact with the supraglenoid buttress. The bone outline and thickness are most consistent with that of Ventastega, however it is significantly less curved laterally. In Hynerpeton bassetti Daeschler, Shubin, Thomson et Amaral, 1994 and Acanthostega gunnari Jarvik, 1952 (Coates 1996) the dorsal lamina is much more robust. The maximum length of the bone is 57 mm, maximum width across the ventral point of the anterodorsal edge is 17 mm. The lateral surface of the bone (Fig. 5A) is covered with tiny pores of blood supply vessels in combination with longitudinally directed small anastomosing ridges. At the mesial surface similar sculpturing is observed only in the anterodorsal part of the bone; it marks here the contact area for the anocleithrum. The rest of the mesial surface dorsally from the scapulocoracoid bears numerous minute subparallel blood vessel grooves organised in a fan-shaped manner, which radiate from the dorsal edge of the scapulocoracoid attachment area. The expanded dorsal blade occupies more than half of the bone length, as in Ventastega and in contrast to other known Devonian tetrapods (Ahlberg et al. 1994; Lebedev and Coates 1995; Jarvik 1996; Coates 1996; Daeschler et al. 1994). The anterodorsal edge of the cleithrum, forming the contact with the anocleithrum, is
O.A. Lebedev. A new tetrapod from Russia 87 Fig. 5. Jakubsonia livnensis gen. and sp. nov., right cleithrum with partly preserved scapulocoracoid, holotype PIN 2657/343. A, lateral view; B, mesial view; C, anterior view. Abbreviations: aclmc, contact area for the anocleithrum; adc, anterodorsal crest; amd, anteromedial depression; an, anterior notch; avp, anteroventral process; dl, dorsal lamina; ma, muscle attachment tubercles; map, muscle attachment pits; sgb, supraglenoid buttress; sgf, supraglenoid foramen; ssf, subscapular fossa; vap, ventral anteromedial process. subvertical and arched; more ventrally it forms a slight notch as in Hynerpeton (Daeschler et al. 1994) and Tulerpeton (Lebedev and Coates 1995). In the Carboniferous forms this notch is found in the cleithrum of, for example, Proterogyrinus (Holmes 1984). In Ventastega, Ichthyostega and Acanthostega this notch is unknown. As in Ventastega, Elginerpeton (Ahlberg 1998) and Hynerpeton, and in contrast to Acanthostega (Coates and Clack 1991; Coates 1996) and Ichthyostega (Jarvik 1980) the anterior edge ventrally forms no branchial lamina. Instead, in Jakubsonia this edge is rounded, thickened and bears massive elongated tubercles for the attachment of muscles directed anterodorsally, seemingly for the m. cuccularis or m. depressor mandibulae. The most prominent tubercle bearing rugosity is situated opposite the anterodorsal part of the supraglenoid buttress (Fig. 5B). Ventrally from it the anteroventral process tapers quickly. Laterally it limits the deep anteromedial depression, which housed the posterior lamina of the
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LATVIJAS UNIVERSITĀTES RAKSTI 679.
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LATVIJAS UNIVERSITĀTES RAKSTI 679.
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Contents Ervīns Lukševičs and Ģ
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ACTA UNIVERSITATIS LATVIENSIS, 2004
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Ervīns Lukševičs, Ģirts Stinkul
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Olga Afanassieva. Microrelief on th
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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
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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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Page 101 and 102: E. Lukševičs, I. Zupiņš. Sedime
Page 121 and 122: J. Valiukevičius. Silurian acantho
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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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