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O.A. Lebedev. A new tetrapod from Russia
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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.