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 ...
84 ADVANCES IN PALAEOICHTHYOLOGY trace of medial suture. The frontals are short; they are only 2.4 times longer than wide. The posterior half of their lateral contact area is occupied by the postfrontal contact, the anterior part served for the prefrontal attachment (Fig. 3). This condition is not observed in the Devonian tetrapods Ichthyostega (Jarvik 1952), Acanthostega (Clack and Coates 1993) or Ventastega curonica (Lukševics, pers. comm.); however, it may be found in the long-snouted anthracosauromorphs, for example Proterogyrinus scheelei Romer, 1970 (Holmes 1984), Eoherpeton watsoni Panchen, 1975 (Smithson 1985) and Anthracosaurus russelli Huxley, 1863 (Panchen 1977). However, in these species the frontal is very long, because of the general elongation of the preorbital part of the skull. In Jakubsonia, on the contrary, a short frontal in combination with a long contact to the prefrontal might suggest large orbits, rather than a long snout. The anterior edge of the frontals shows no sutural contact; on the contrary, the bone becomes thinner here suggesting participation in the posterior margin of an internasal fontanelle as in Tulerpeton (Lebedev and Clack 1993) or in Ventastega (Lukševics et al. 2003). The lateral side of the left frontal, however, shows a rather deep notch, which houses the infraorbital canal opening. This notch might be interpreted as a contact for the left nasal. The lateral parts of the fronto-parietal suture are visible at the ventral side of the skull roof. Mesially a sheath of spongy matter of the connecting ethmosphenoid obscures the suture (Fig. 2B). Fig. 3. Jakubsonia livnensis gen. and sp. nov., reconstructed interorbital fragment of the skull roof. Abbreviations: Fr, frontal; Pa, parietal; Poc, contact margin for the postorbital; Pof, postfrontal; Prf, position of prefrontal; or, orbit; soc, supraorbital canal; son, notch housing the anterior opening of the supraorbital canal in the frontal. From the frontals the supraorbital canal turns laterally and enters the narrow, semilunate postfrontals. The shape of this bone is characteristic of the Devonian tetrapods Ichthyostega (Jarvik 1952), Acanthostega (Clack and Coates 1993) and Tulerpeton (Lebedev and Clack 1993). In the later Carboniferous tetrapods, for example
O.A. Lebedev. A new tetrapod from Russia 85 Proterogyrinus (Holmes 1984), Crassigyrinus (Panchen 1985; Clack 1998) and Anthracosaurus (Panchen 1977) the posteromesial part of the postfrontal widens, deeply wedging into the parietal. However, in contrast to the condition in Acanthostega and in accordance with that in Ventastega (Lukševics, pers. comm.) the lateral sides of the prefrontals must have adjoined the lateral sides of the frontal and postfrontal together, and the lateral process of the postorbital must have abutted the posterolateral part of the postfrontal, rather than wedged between it and the parietal. The contact surface for the postorbital is preserved at the posteriormost tip of the postfrontal on the left side of the specimen. The infraorbital canal leaves the bone shortly anteriorly to this contact area. A pronounced swollen ridge forms the orbital margin. Only the anterior parts of the parietals are preserved. Posteromedially the parietals become thinner at its centre and form a depression on the dorsal side, probably running forwards from the pineal foramen. On the ventral side the most impressive features are deep grooves running alongside the parietal-postfrontal contact. These grooves bear blood vessel impressions, which lead to moderately large foramina opening into the canals inside the bone. The mesial part of the parietals is occupied with a narrow impression of the sphenethmoid. The only lower jaw element preserved is an incomplete left angular (PIN 2657/ 347) (Fig. 4). In most respects it agrees with the corresponding bones in Ichthyostega (Jarvik 1952), Acanthostega (Clack and Coates 1993), Ventastega (Ahlberg et al. 1994), Fig. 4. Jakubsonia livnensis gen. and sp. nov., incomplete angular, PIN 2657/347. A, lateral view; B, mesial view.
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O.A. Lebedev. A new tetrapod from Russia<br />
85<br />
Proterogyrinus (Holmes 1984), Crassigyrinus (Panchen 1985; Clack 1998) <strong>and</strong><br />
Anthracosaurus (Panchen 1977) the posteromesial part of the postfrontal widens, deeply<br />
wedging into the parietal. However, in contrast to the condition in Acanthostega <strong>and</strong> in<br />
accordance with that in Ventastega (Lukševics, pers. comm.) the lateral sides of the<br />
prefrontals must have adjoined the lateral sides of the frontal <strong>and</strong> postfrontal together,<br />
<strong>and</strong> the lateral process of the postorbital must have abutted the posterolateral part of<br />
the postfrontal, rather than wedged between it <strong>and</strong> the parietal. The contact surface for<br />
the postorbital is preserved at the posteriormost tip of the postfrontal on the left side of<br />
the specimen. The infraorbital canal leaves the bone shortly anteriorly to this contact<br />
area. A prono<strong>un</strong>ced swollen ridge forms the orbital margin.<br />
Only the anterior parts of the parietals are preserved. Posteromedially the parietals<br />
become thinner at its centre <strong>and</strong> form a depression on the dorsal side, probably r<strong>un</strong>ning<br />
forwards from the pineal foramen. On the ventral side the most impressive features are<br />
deep grooves r<strong>un</strong>ning alongside the parietal-postfrontal contact. These grooves bear<br />
blood vessel impressions, which lead to moderately large foramina opening into the<br />
canals inside the bone. The mesial part of the parietals is occupied with a narrow<br />
impression of the sphenethmoid.<br />
The only lower jaw element preserved is an incomplete left angular (PIN 2657/<br />
347) (Fig. 4). In most respects it agrees with the corresponding bones in Ichthyostega<br />
(Jarvik 1952), Acanthostega (Clack <strong>and</strong> Coates 1993), Ventastega (Ahlberg et al. 1994),<br />
Fig. 4. Jakubsonia livnensis gen. <strong>and</strong> sp. nov., incomplete angular, PIN 2657/347. A, lateral<br />
view; B, mesial view.
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