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J. Valiukevičius. Silurian acanthodians from Lūžņi-4
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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