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J. Valiukevičius. Silurian acanthodians from Lūžņi-4
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Family PORACANTHODIDAE Vergoossen, 1997
Genus Poracanthodes Brotzen, 1934
Type species. Poracanthodes punctatus Brotzen, 1934. Upper Silurian, Pridoli,
Ohesaare Regional Stage; Ohesaare Cliff, Saaremaa, Estonia.
Poracanthodes sulcatus sp. nov.
Fig.7 A-J
? 1986 Poracanthodes punctatus; Märss, pl. 32, fig. 2.
2003 c Poracanthodes punctatus var. “sulcatus”; Valiukevicius, p. 51.
Etymology. Named from the characteristic antero-median sulcus in scale crowns.
Holotype. LIGG 25-A-2613, flank scale (Fig. 7 A).
Type horizon. Garzde Beds of the Targale Formation, Pridoli, Upper Silurian.
Range. Upper Silurian, Pridoli, Targale Formation and its correlatives of Estonia.
Material. 42 scales.
Diagnosis. Punctatiform Poracanthodes having relatively large scales up to 1 mm long;
pores on crowns are densely and randomly positioned; up to one-third of crown plate to
the anterior is free of pores; well developed antero-median sulcus on crown; superficial
winding “vermiculate” pore canals are visible through the thin transparent outer crown
layer; histologic microstructure of “Poracanthodes”-type is identical to that in
Poracanthodes punctatus.
Description. Morphology. Scale crowns are flat, without inclination, sub-rhomboidal,
with an elongated and stretched posterior part. Their length is 0.36-1 mm; the larger
varieties prevail. The part of the posterior crown overhanging base may reach one-third
of crown length. The antero-median sulcus on crowns is shallow, kite-shaped (Fig. 7 A-
B), with the deepest part projecting slightly in front of the rest of the anterior margins.
Most of the numerous pores opening on the surface are placed randomly, or rarely in
indistinct lines on the posterior crown part, following the growth zones. The anterior
crown part (up to one-third of length) is usually free of pores, but the porose area can
begin just behind the sulcus (Fig. 7 B). Superficial pore canals of a winding “vermiculate”
form, directed obliquely upward at a low angle, are visible through the thin transparent
outer layer of the posterior half of the crown, characterize the scales. Scale necks are
high and porose on all faces. The anterior neck walls carry large pores basally, near to
the base rim. They frequently open in vertical linear grooves that are visible all around
the neck. Large radial pore canals open on the undersurface of the crown in the posterior
part.
Histology. Six to eight lamellae of superpositional growth in scale crowns are composed
of dentine with narrow, long, unbranched ascending and radial canals (Fig. 7 D-E). The
porose posterior crown part contains only a few canals which are short, narrow, winding
and mostly directed upwards. Durodentine is absent. Large radial pore canals, mostly
eight, extend obliquely from the aperture on the posterior crown undersurface to the
scale primordium (Fig. 7 E). They send ascending oblique, short, winding pore canals