Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

64 ADVANCES IN PALAEOICHTHYOLOGY
Diagnosis: Scale ridges short, not reaching the posterior border, where they override
older ridges. Alternating ridgelets cover the entire ridge and even the crest; anterior
overlapped field pustulate. Tubercles of the plate have nodules at the base of the
ridgelets.
Description: The holotype (Fig. 3 A) represents a deep scale from the left anterior flank
of the fish. It is 3.0 mm deep and 0.7 mm wide. The ridges arising posteriad have convex
surfaces and do not form a crest. The anterior overlapped field is wide and pustulate,
the free field is covered with two generations of ridges. There are 16 ridges. The longer
anterior (younger) ridges overlie posteriorly shorter (older) ridges; both cover only part
of the free field. The ridges are covered by ridgelets that alternately reach the crest (Fig.
3 B). The dorsal margin of the scale forms a broad overlapped area (“peg”). The overlay
of ridges is more intimate in other scales (Fig. 3 C), where the two or three generations
are not so clearly separated as in the holotype. The inner surface of the scale carries an
elongate elevation. Only a few ridges are present on the scale (0.9 mm deep and 0.6 mm
wide); the ridges are more elongate than in the holotype, and the ridgelets are more
distinct. The scale has a pillow-like elevation on the inner surface. The spine assigned
to the species, clearly shows the overlay of flat ridges (Fig. 3 C) that are covered
anteriorly with pronounced ridgelets. Fine pores of vascular canals open around the
ridges in the basal plate.
A bony plate with star-like tubercles (Fig. 3 D, E) is assigned to L. canadensis on the
basis of comparison with L. superbus and of the co-occurrence with the scales of L.
canadensis at the same locality. Only one, the longest margin of the plate, is preserved
intact, it is arched downwards; the other margins are broken, so that the position of the
plate cannot be established. Its maximum measurements are: width 8.0 mm and length
4.0 mm. The plate is covered with closely packed stellate tubercles of different sizes;
they are tiny on the arched margin. The middle area of the plate is covered by tubercles
of 2 to 3 generations, older ones are partly covered by younger ones. Each stellate
tubercle carries fine ridgelets converging towards its smooth apex. On the bony basal
plate, the ridgelets terminate in small nodules; small nodules are also placed irregularly
between the tubercles. The same is the case in L. superbus (Gross 1969: figs 8 A, D, 9 G).
The concentric lines around the tubercles (on the ridgelets just above the basal plate)
are a peculiar feature and reminiscent of growth lines (Fig. 3 E). Three deep open
grooves of the sensory line system occur on the plate, two of them form an angle of
about 45° on one side, and one short section is preserved on the opposite side (Fig. 3
D). The bony base appears rather compact with small canal openings as seen on the
margins of the broken plate.
Comparison: Lophosteus canadensis n. sp. and Lophosteus superbus have scales of
nearly the same size. The ridges on the free field of the scales are slightly closer to each
other in Lophosteus canadensis n. sp. than in L. superbus. The ridges are smaller in L.
connexus, and joined at the base on some elements. Alternating ridgelets reaching the
top of the ridges are characteristic of L. canadensis. This feature distinguishes the
species from all other species of the genus. A crest is not developed like in L. incrementus.
Moreover, the bony pustules on the anterior overlapped area and between the ridges
are lower and smaller in L. superbus and L. canadensis than in L. incrementus.
On the bony plates, nodules on the ridges of the stellate tubercles are present in
four species of Lophosteus (Märss 1986: pl. 35, figs 1-14, for the nodules see fig. 13 a);