Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

More documents

Recommendations

Info

90 ADVANCES IN PALAEOICHTHYOLOGY the popliteal from the fibular fossa in Tulerpeton (Lebedev and Coates 1995). It supports the ventral corner of the fibular condyle and is termed here the fibular ridge (fr). The fibular ridge limits the posterior face of the distal part of the femur ventrally (Fig. 6D). The ventral part of this surface distally is occupied by a very large triangular elongated fibular fossa, the size of which, as that of the popliteal fossa, exceeds the sizes of corresponding structures in all known Devonian tetrapods. In cross-section the distal part of the femur is shaped roughly as an equilateral triangle, with its base slightly arched outwards (anteroventrally) at the expense of the overhanging tibial condyle. The distal articulating surface is very well ossified, but incompletely preserved. The only intact area is the central part of the tibial facet adjoining the crest, which separates it from the fibular facet. The latter is completely destroyed (Fig. 6). Cranial bone questionably assigned to Jakubsonia livnensis gen. and sp. nov. (Fig. 8). The posterior part of the skull roof PIN 2657/348 includes an unpaired postparietal, the anterior part of the right tabular, right supratemporal, fragments of the right parietal and left supratemporal. The only well preserved margin of this specimen is the posterior. The feature which might suggest attribution of the specimen to tetrapods is the characteristic pit-and-ridges dermal sculpturing of the same type as found in the fragment of the interorbital part of the skull roof described above (PIN 2657/346). However, the Fig. 8. ? Jakubsonia livnensis gen. and sp. nov., posterior part of the skull roof, PIN 2657/348. A, dorsally, B, ventrally, C, line drawing of B. Abbreviations: Pa, parietal; Pp, postparietal; St, supratemporal; Ta, tabular.
O.A. Lebedev. A new tetrapod from Russia 91 roofing bone pattern is unusual, and among Devonian tetrapods may be compared only to that in Ichthyostega, in which the postparietal is unpaired. In contrast to the condition in that animal, the posterior margin in PIN 2657/348 is convex rather than concave. More precisely, its shape is angular and the most conspicuous medial point is formed by the posteromesial process of the postparietal as in Ventastega (Lukševics, pers. comm.). The postparietal is hexagonal and occupies about half of the skull table width in this area. The lateral side contacts the tabular and the supratemporal, the anterolateral one the parietal. Given the skull roof pattern of this specimen, which is characteristic of dipnoans, one could be skeptical of its attribution to a tetrapod. However, no Devonian dipnoan is known to possess pit-and-ridge dermal sculpturing. Discussion Jakubsonia, an aquatic tetrapod In order to support the hypothesis of the primary aquatic life mode of Acanthostega and Ichthyostega, Clack and Coates (1995) put forward a number of morphological arguments relating to limb structure, such as the absence of olecranon on the ulna, correspondence of radius and ulna length and polydactyly. The femoral structure was never discussed from this point of view, as most of the previously known early tetrapod femora are more or less similarly built. In this respect the femur in Jakubsonia demonstrates probably the most unusual morphology in that there is no intercondylar fossa on its dorsal (extensor) surface distally and, respectively the condyles themselves are not expressed as well (Fig. 8). That means that the extensor muscles were not capable of pulling the epipodium forwards and the knee joint movements were more than limited. It is very probable that natural mobility in the joint was passive (that is, the muscles did not really act to deflect the knee effectively enough to reach significant limb bending) and instead of muscles, impeding of the surrounding matter (most likely, water!) was compensated only by exceptionally strong ligaments originating at the popliteal and fibular fossae. Thus, this animal was not capable of walking at all and paddling was the only function of its hind limb. This implies that the earlier the tetrapod is found in the Devonian, the more features support its primarily aquatic mode of life. Palaeoecological remarks Earlier attempts to study palaeoecology of Devonian tetrapods were undertaken by Spjeldnaes (1982) who focused on Ichthyostega, Bendix-Almgreen et al. (1990), Coates and Clack (1995) on Acanthostega, and Lebedev (1985, 1992) who considered Tulerpeton in the Andreyevka biota. Since those times several new localities have been discovered, which yielded previously unknown tetrapod remains (see Introduction). The palaeoecology and taphonomy of the Gornostayevka locality from where Jakubsonia livnensis gen. and sp. nov. originates were recently described by Moloshnikov (2001). This contributed much to our knowledge of the subject. In order to evaluate palaeoecological data the primary taphonomical, environmental and assemblage structure information was assembled in the table (Table I). An attempt to synthesize this accumulated information is made here.
Page 1 and 2:
LATVIJAS UNIVERSITĀTES RAKSTI 679.
Page 3 and 4:
LATVIJAS UNIVERSITĀTES RAKSTI 679.
Page 5 and 6:
Contents Ervīns Lukševičs and Ģ
Page 7 and 8:
ACTA UNIVERSITATIS LATVIENSIS, 2004
Page 9 and 10:
Ervīns Lukševičs, Ģirts Stinkul
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Olga Afanassieva. Microrelief on th
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
D.K. Elliott, E. Mark-Kurik, E.B. D
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40: D.K. Elliott, E. Mark-Kurik, E.B. D
Page 47 and 48: C.G.Miller, T. Marss, H. Blom. New
Page 57 and 58: ACTA UNIVERSITATIS LATVIENSIS, 2004
Page 59 and 60: H.-P. Schultze, T. Marss. Revisitin
Page 81 and 82: O.A. Lebedev. A new tetrapod from R
Page 89: O.A. Lebedev. A new tetrapod from R
Page 101 and 102: E. Lukševičs, I. Zupiņš. Sedime
Page 121 and 122: J. Valiukevičius. Silurian acantho
Page 141 and 142:
J. Valiukevičius. Silurian acantho
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
V. Pernegre, V. Dupret. Biostratigr
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Ž. Žigaite. New telodont from Tuv
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
show all

Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

Create successful ePaper yourself

Delete template?

Save as template?