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 ...
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62 ADVANCES IN PALAEOICHTHYOLOGY<br />
The main visible difference between spines with a triangular cross-section as<br />
depicted by Rohon (1893) <strong>and</strong> Otto (1991) is in the arrangement of ridge-like tubercles,<br />
which are arranged either in rows or irregularly, <strong>and</strong> in the difference in depth.<br />
The spine described by Gross (1969: text-fig. 5 E) is of the same type as that from the<br />
Ruhnu bore hole. They are flattened, sculptured on both flattened sides <strong>and</strong> distally<br />
aro<strong>un</strong>d the whole spine. Proximally they have a cavity that extends one third of their<br />
length. This spine has some similarities with the cornual plate of placoderms.<br />
Other spine-like elements described by Gross <strong>and</strong> also those in the Tallinn collection,<br />
were attached to tesserae-like elements in the skin. These have a concave visceral side<br />
<strong>and</strong> are not fin-spines.<br />
The spine assigned to the sarcopterygian Psarolepis (Zhu <strong>and</strong> Schultze 1997) has<br />
a very narrow overlapped area. These spines are not inserted deeply in the skin in<br />
contrast to spines of sharks <strong>and</strong> most acanthodians. Zhu et al. (1999) emphasized the<br />
occurrence of spines in basal osteichthyans (including Lophosteus), acanthodians <strong>and</strong><br />
sharks. Advanced sharks <strong>and</strong> the Early Devonian chondrichthyan Leonodus (Soler-<br />
Gijón <strong>and</strong> Hampe 2003) possess only symmetrical spines, whereas spines in front of<br />
paired fins are known in Early Devonian sharks (Sahney <strong>and</strong> Wilson 2001: putative<br />
chondrichthyan; Miller et al. 2003: Doliodus).<br />
A convex plate (GIT 382-25, Fig. 1 B, C) represents a new element. Its maximum preserved<br />
length is 6.6 mm. The plate is convex externally <strong>and</strong> concave internally; the lateral margins<br />
are broken. It has a short anterior overlapped area. Oval, smooth, abraded tubercles are<br />
distributed in indistinct rows posterior to that area (Fig. 1 C); smaller tubercles are placed<br />
between larger ones. Fine ridgelets occur radially at the margin of each tubercle. The<br />
deeper part of an elongate narrow keel on the concave inner side is partly broken off (Fig.<br />
1 B). Lineations that may represent growth lines are visible in front of the keel. The plate<br />
is symmetrical, belongs to the dorsal midline, <strong>and</strong> may be comparable to a fulcrum.<br />
An interesting head plate (GIT 232-16 = Pi 6186, Fig. 2) of L. superbus was described<br />
<strong>and</strong> figured by Märss (1986: 58, pl. 35, fig. 1 a, b). The arched plate shows an open lateral<br />
line canal which bifurcates anteriad aro<strong>un</strong>d the anterior invagination. The course of the<br />
lateral line nerve is picked out on the inner side by pores that transmitted branching<br />
nerves to the neuromasts in the lateral line canal. The course of the pores <strong>and</strong> thus the<br />
course of the lateral line nerve on the inner side from a posterior to an anterior invagination<br />
does not show the bifurcation of the canal on the outer side of the plate. Such plates<br />
with an open lateral line canal have already been described <strong>and</strong> figured by Gross (1969:<br />
figs 5 H, 6 D, 7 A, B).<br />
Lophosteus cf. superbus<br />
2000 Lophosteus sp. cf. L. superbus — Burrow <strong>and</strong> Turner: 170, 172; fig. 3.3.<br />
Material: part of one scale UQY 8667 (Burrow & Turner 2000: fig. 3.3).<br />
Locality: Bullock Creek, northern Queensl<strong>and</strong>, Australia.<br />
Horizon: Ancoradella ploeckensis Conodont Zone, Jack Formation, lower Ludlow,<br />
Upper Silurian.<br />
Description: Burrow <strong>and</strong> Turner (2000) gave no description. The scale is reminiscent<br />
of those of L. superbus <strong>and</strong> have a pustulate anterior field <strong>and</strong> elevated oblique ridges