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