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 ...
138 ADVANCES IN PALAEOICHTHYOLOGY Fig. 7. Poracanthodes sulcatus sp. nov. SEM micrographs of scales (A-B), crown views, anterior upwards, and thin section photos (C-J). A, holotype, LIGG 25-A-2613; B, LIGG 25-A- 2614; C-F, vertical longitudinal section of scale. Thin section 3776. C, general view; D, detail of the anterior crown part and base apex at higher magnification; E, magnified detail of the posterior crown part and base; F, detail of scale primordium at higher magnification; G-J, vertical transverse section of scale. Thin section 3790. G, general view; H, the right part of crown at higher magnification; I, the left part of crown and the adjacent base portion; J, detail of the base apex at higher magnification. Lūžņi-4 borehole, depth 176.5 m. Přidoli, Jūra Regional Stage, Lūžņi Beds of the Targale Formation. Abbreviations: arcpc, arcade pore canal, avc, ascending vascular canal, b, bone, d, dentine, oc, osteocyte cavity, pc, superficial pore canal, rpc, radial pore canal, rvc, radial vascular canal, sf, Sharpey’s fibre traces. Scale bars: 0.1 mm. to open on the surface. The arcade pore canals (Fig. 7 H-I) are just as wide as the radial canals and connect the latter and superficial ones to form the complicated net piercing the crown tissue. The high-pyramidal base is composed of densely lamellar cellular bone incorporating varying numbers of osteocytes and pierced by long traces of Shapey’s fibres. The osteocytes are densest in the apex of the pyramid, where they are largest and
J. Valiukevičius. Silurian acanthodians from Lūžņi-4 139 most clearly oriented along the almost horizontal growth lines, transforming into a Stranggewebe-like tissue(Fig. 7 E, F, J). Remarks. This punctatiform Poracanthodes is closestly related to P. punctatus Brotzen discovered in many regions (Brotzen 1934; Lehman 1937; Gross 1947, 1956, 1971; Märss 1986; Valiukevicius 1998, 2003 a; Burrow et al. 1999; Vergoossen 1999 a, b, c; 2000; Burrow 2003). From typical P. punctatus it differs in having an antero-median sulcus, random arrangement of pores on crowns, and winding “vermiculate” oblique superficial pore canals seen through the thin transparent outer layer in the posterior crown part. The histological microstructure is generally uniform with the scales of P. punctatus. Poracanthodes marssae sp. nov. Fig. 8 A-E 1986 Gomphonchus hoppei (Gross); Märss, pl. 29, fig. 7. 2003 c Gomphonchus boekschoteni Vergoossen; Valiukevicius, p. 51. Etymology. In honour of Dr T. Märss (Tallinn), the first discoverer of scales. Holotype. LIGG 25-A-2559, flank scale (Fig. 8 A). Type horizon. Venzava Beds of the Targale Formation, Pridoli, Upper Silurian. Range. Targale Formation and its Estonian equivalent. Material. 6 scales. Diagnosis. Punctatiform Poracanthodes having scales with smooth, flat, pear-shaped crowns with a long, narrow posterior spine-like corner; symmetrical lateral projections in crowns placed at the beginning of its narrowing; very small pores visible on the posteriormost crown part; scale bases deep, protruding beyond crowns laterally and far anteriorly; necks of medium height and linearly porose on all faces; sharp keel on the posterior neck mirroring the crown shape. Scale crowns are composed of acellular mesodentine or dentine, and durodentine; ascending canals with rare branches; branches of radial canals extend down to the base; one? radial pore canal with few superficial pore openings running back from the scale primordium; high-pyramidal base is composed of dense, thin-lamellar cellular bone. Description. Morphology. Scales are of medium to large size (crown length is 0.34- 0.81 mm). Crown is smooth, flat, without inclination, pear-shaped, consisting of a rhomboidal main plate and very narrow, spine-like posterior part that may be as long as the main plate (Fig. 8 A). Two symmetrical lateral projections of triangular form occur in crowns of some scales at the place where the prominent narrowing of the crown begins (Fig. 8 C, see alo Märss, pl. 29, fig. 7). Crowns of most scales have a shallow medial concavity extending almost the entire length. A few ?linearly placed small pores, visible only at high magnification, open centrally on the posteriormost crown part (Fig. 8 B). Very deep rhomboidal bases protrude beyond crowns laterally and far anteriorly. The base is always deepest in advance of the crown. The base/neck junction is outlined by a clear anterior rim. Necks are well developed, of medium height and porose on all sides (Fig. 8 A). Pores are large, forming linear rows. A sharp vertical keel on the
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138<br />
ADVANCES IN PALAEOICHTHYOLOGY<br />
Fig. 7. Poracanthodes sulcatus sp. nov. SEM micrographs of scales (A-B), crown views, anterior<br />
upwards, <strong>and</strong> thin section photos (C-J). A, holotype, LIGG 25-A-2613; B, LIGG 25-A-<br />
2614; C-F, vertical longitudinal section of scale. Thin section 3776. C, general view; D, detail<br />
of the anterior crown part <strong>and</strong> base apex at higher magnification; E, magnified detail of the<br />
posterior crown part <strong>and</strong> base; F, detail of scale primordium at higher magnification; G-J, vertical<br />
transverse section of scale. Thin section 3790. G, general view; H, the right part of crown at<br />
higher magnification; I, the left part of crown <strong>and</strong> the adjacent base portion; J, detail of the base<br />
apex at higher magnification. Lūžņi-4 borehole, depth 176.5 m. Přidoli, Jūra Regional Stage,<br />
Lūžņi Beds of the Targale Formation. Abbreviations: arcpc, arcade pore canal, avc, ascending<br />
vascular canal, b, bone, d, dentine, oc, osteocyte cavity, pc, superficial pore canal, rpc, radial<br />
pore canal, rvc, radial vascular canal, sf, Sharpey’s fibre traces. Scale bars: 0.1 mm.<br />
to open on the surface. The arcade pore canals (Fig. 7 H-I) are just as wide as the radial<br />
canals <strong>and</strong> connect the latter <strong>and</strong> superficial ones to form the complicated net piercing<br />
the crown tissue. The high-pyramidal base is composed of densely lamellar cellular<br />
bone incorporating varying numbers of osteocytes <strong>and</strong> pierced by long traces of Shapey’s<br />
fibres. The osteocytes are densest in the apex of the pyramid, where they are largest <strong>and</strong>
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