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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88 ADVANCES IN PALAEOICHTHYOLOGY<br />
dorsal process of the clavicle (Figs. 5B, C). The ventral part of the anterior edge of the<br />
cleithrum is convex. Mesially from the anteroventral process <strong>and</strong> posteriorly from the<br />
anteromedial depression there is a process parallel to the former, named here the ventral<br />
anteromedial process. Its mesial surface is smooth; it is straight <strong>and</strong> the ventral tip<br />
projects below the apex of the anteroventral process.<br />
The corresponding part of the posterior edge of the cleithrum is, on the contrary,<br />
concave <strong>and</strong> its outline almost follows in parallel that of the anterior edge. The contact<br />
with the scapulocoracoid can be easily traced; it follows the sharp ridge of the<br />
posteroventral edge. The posterodorsal edge of the bone is less acute <strong>and</strong> smoothly<br />
arched in plan view.<br />
The scapulocoracoid (Fig. 5B) is incompletely ossified <strong>and</strong> only its scapular portion<br />
is partly preserved, in accordance with the condition noted by Holmes (1980) in<br />
Proterogyrinus. This bone is solidly fused to the mesial side of the cleithrum, however,<br />
in contrast to Ichthyostega, Acanthostega <strong>and</strong> Hynerpeton <strong>and</strong> in agreement with the<br />
condition in Elginerpeton (Ahlberg 1998) its sutures are fairly well discernible. As<br />
noted above, the posterolateral suture of the scapulocoracoid r<strong>un</strong>s parallel to the<br />
posteroventral edge of the cleithrum. Posterodorsally it is obscured by three shallow<br />
muscle attachment pits, possibly for the analogue of the m. levator scapulae. These pits<br />
are separated anteriorly from the anterodorsal part of the supraglenoid buttress by a<br />
prominent transverse ridge. The anterodorsal crest at this point is robust, ventrally it<br />
reduces in height forming a concavity at the anterior edge of the subscapular fossa,<br />
which would have been large <strong>and</strong> comparable in configuration to that in Hynerpeton<br />
(Daeschler et al. 1994) <strong>and</strong> Elginerpeton (Ahlberg 1998). The anteroventral part of the<br />
anterior edge of the scapulocoracoid abuts the dorsal part of the ventral anteromedial<br />
process of the cleithrum. A small supraglenoid foramen is situated close to the<br />
scapulocoracoid-cleithrum suture in the same position as in Hynerpeton <strong>and</strong> foramen<br />
“C” in Acanthostega (Coates 1996).<br />
Only the distal part of the left femur (PIN 2657/344) is preserved. The major feature<br />
distinguishing it from all other known Devonian tetrapods is the absence of an<br />
intercondylar fossa (Figs. 6, 7) <strong>and</strong> correspondingly distinctly expressed condyles.<br />
Instead, the dorsal (extensor) surface of the femur in its distal part is almost flat (Fig.<br />
6B). Two shallow ridges, one r<strong>un</strong>ning along the posterior edge of the dorsal surface <strong>and</strong><br />
the other parallel to it roughly in the middle of the surface, form a shallow groove with<br />
<strong>un</strong>even surface. The rest of the surface adjoining the tibial condyle is also flat <strong>and</strong><br />
pierced with several large blood vessel foramina. Its anterior edge forms an abrupt<br />
bend to the condyle surface. More proximal part of the dorsal surface in the shaft area<br />
is ro<strong>un</strong>ded <strong>and</strong> passes smoothly into the shaft portion of the anterior surface of the<br />
femur. The massive adductor crest r<strong>un</strong>ning obliquely towards the anterior bone surface<br />
delimits the ventral margin of the anterior surface (Fig. 6A). The distal part of the<br />
anterior surface is dominated by the bulky perichondral mass of the tibial condyle<br />
strongly overhanging <strong>and</strong> even partly enclosing ventrally the large <strong>and</strong> deep triangularshaped<br />
popliteal fossa (Figs. 6A, C). The ventral surface of the bone enclosed between<br />
the adductor crest <strong>and</strong> a sharp ridge separating this surface from the posterior one is<br />
narrow distally but widens in the proximal direction. The latter ridge is situated<br />
topographically in the same position as a well marked <strong>un</strong>named ridge situated at the<br />
ventral side of the femur receiving the distal extremity of the adductor crest <strong>and</strong> separating