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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86 ADVANCES IN PALAEOICHTHYOLOGY<br />
Tulerpeton (Lebedev <strong>and</strong> Clack 1993) <strong>and</strong> Densignathus (Daeschler 2000). Its anterior<br />
<strong>and</strong> posterior edges are broken off; the dorsal margin shows small fused parts of the<br />
surangular. The ornament consists of shallow pits <strong>and</strong> anastomosing ridges, the pits<br />
become more elongated radially from the ossification centre in the area dorsal to the<br />
m<strong>and</strong>ibular seismo-sensory canal groove (Fig. 4A). Ventral to it the ridges become<br />
more swollen <strong>and</strong> pits are deeper than above the canal. The canal is deep <strong>and</strong> wide, a<br />
short part of it close to the ossification centre is exposed to the surface, <strong>and</strong> the remaining<br />
course comm<strong>un</strong>icates to the surface with a single row of large ro<strong>un</strong>ded or elongated<br />
foramina. This pattern conforms to that in Densignathus (Daeschler 2000), but contrasts<br />
with that fo<strong>un</strong>d in Tulerpeton, in which the canal is completely housed in an open<br />
groove (Lebedev <strong>and</strong> Clack 1993). In Acanthostega, on the contrary, most of the canal<br />
is enclosed in the bone <strong>and</strong> only its anterior <strong>and</strong> posterior sections at the angular are<br />
exposed in the grooves (Ahlberg <strong>and</strong> Clack 1998). In Ventastega the canal through this<br />
bone is not exposed at all (Ahlberg et al. 1994). The ventral edge of the preserved part<br />
of the bone is almost straight. The mesial lamina is narrow <strong>and</strong> smooth; its lateral edge<br />
is separated from the lateral lamina with a straight ridge of <strong>un</strong>iform width. The notches<br />
for the Meckelian foramina are shallow, <strong>and</strong> there are at least three of them seen in the<br />
preserved fragment (Fig. 4B).<br />
Pectoral girdle. The cleithrum (PIN 2657/343) (Fig. 5) is generally well preserved,<br />
only its lateral surface <strong>and</strong> the anterior edge are slightly worn. On its mesial surface the<br />
posterodorsal corner of the dorsal lamina (Fig. 5B) bears a deep elliptical pit, which<br />
may be a trace of lifetime damage, as the edges of the bony bars at the pit bottom<br />
constituting the tissue are smooth, as happens in healed tissue. However, there are no<br />
traces of regeneration to support this suggestion.<br />
In its general shape the cleithrum is very similar to the bones (LDM 81/522 <strong>and</strong><br />
LDM 57A/1984) attributed to Ventastega (Ahlberg et al. 1994). The general outline of<br />
the cleithrum is feather-shaped; it is sigmoidally curved in the lateral aspect. In the<br />
anterior aspect (Fig. 5C) the bone is only very slightly curved laterally, its dorsal lamina<br />
is comparatively thin; the maximum thickness of the bone is attained at the contact with<br />
the supraglenoid buttress. The bone outline <strong>and</strong> thickness are most consistent with that<br />
of Ventastega, however it is significantly less curved laterally. In Hynerpeton bassetti<br />
Daeschler, Shubin, Thomson et Amaral, 1994 <strong>and</strong> Acanthostega g<strong>un</strong>nari Jarvik, 1952<br />
(Coates 1996) the dorsal lamina is much more robust. The maximum length of the bone<br />
is 57 mm, maximum width across the ventral point of the anterodorsal edge is 17 mm.<br />
The lateral surface of the bone (Fig. 5A) is covered with tiny pores of blood supply<br />
vessels in combination with longitudinally directed small anastomosing ridges. At the<br />
mesial surface similar sculpturing is observed only in the anterodorsal part of the bone;<br />
it marks here the contact area for the anocleithrum. The rest of the mesial surface dorsally<br />
from the scapulocoracoid bears numerous minute subparallel blood vessel grooves<br />
organised in a fan-shaped manner, which radiate from the dorsal edge of the<br />
scapulocoracoid attachment area.<br />
The exp<strong>and</strong>ed dorsal blade occupies more than half of the bone length, as in<br />
Ventastega <strong>and</strong> in contrast to other known Devonian tetrapods (Ahlberg et al. 1994;<br />
Lebedev <strong>and</strong> Coates 1995; Jarvik 1996; Coates 1996; Daeschler et al. 1994). The<br />
anterodorsal edge of the cleithrum, forming the contact with the anocleithrum, is