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

116 ADVANCES IN PALAEOICHTHYOLOGY
upper part of the formation, suggest that continental conditions similar to semidesert or
desert prevailed at the very end of the Šķervelis time (Stinkulis 2004).
The results of the analysis of rocks of the Pavāri Member indicate their forming in
alternating hydrodynamic regime. A powerful stream flowing from NNE to SSW formed
the erosional channels. Some of them were filled by moderately or well-sorted very
fine-grained to fine-grained sand containing no admixture of clay particles. Judging
from the size and sorting of clastic particles, the material was probably transported in a
low concentration flow. According to data of the grain-size analysis the clastic material
has been sorted by stream and, possibly, also influenced by wave action. Deformations
of sandstone bedding and intrabasinal conglomerate consisting of clay pebbles indicate
the role of slump processes on sides of the erosional channels. Cross-bedding is
characteristic for the infilling of the channels and clearly demonstrates high angle of
dip and bipolar orientation, which is usually explained by the influence of inter-tidal
streams (Reineck and Singh 1980). The structure and poor sorting of the overlaying
beds of clayey silt and sandy-clayey silt suggest accumulation in calm water.
Fining upwards sequence of sedimentary structures is characteristic for the coarsegrained
point bar deposits in fluvial systems (apart of bidirectional orientation of cross
beds) or shallow channels in tidal inlets and tidal deltas in a coastal marine environment
(Reineck and Singh 1980). The remarkable orientation of fossils indicate transportation
in a medium-strong stream with velocity about 1-20 cm/sec, and deposition
of remains on the bottom of an erosional channel at the moments of drastic decrease in
stream power.
Considering the results of taphonomic and sedimentologic analysis of the Pavāri
locality it is concluded that it formed in relatively shallow sea conditions dominated by
stream, not wave influence. The deposits were not accumulated in continental or coastal
freshwater environment, and there is not enough evidence to support the delta hypothesis
regarded previously (Lebedev and Lukševičs 1996). Most likely, vertebrates from the
Ketleri Formation existed in a coastal marine environment, although the possibility
that the remains have been transported for a more considerable distance can not be
excluded. Possibly, rocks of the Pavāri locality formed in a low-tidal terrigenous shelf
environment between low islands where shallow channel might be formed in tidal
processes. It is worth noting that Acanthostega gunnari from East Greenland,
morphologically the most closely resembling Ventastega curonica, have been interpreted
as a mainly aquatic animal dwelling in an active fluvial channels in a fluvial-dominated
environment (Bendix-Almgreen et al. 1990). Comparison of the faunal composition of
the Britta Dal (locality ‘A’) and Ketleri (Pavāri locality) formations shows clear
taphonomic differences between these two localities. The fossil assemblage from the
Britta Dal Formation is dominated by abundant remains of Acanthostega represented
by isolated bones and articulated, sometimes exceptionally well-preserved material
including skulls and even articulated postcranial parts in association with skulls. On the
contrary, the fish material is poorly preserved, consisting only of disarticulated scales,
teeth and placoderm trunk armour plates. The fish fauna of this particular fauna is
represented only by some taxa including Holoptychius, lung-fishes Soederberghia and
perhaps Oervigia, as well as rarely found placoderm Remigolepis. Probably, taphonomic
peculiarities of the formation of Britta Dal oryctocoenosis resulted in the elimination
of a significant part of information on the ancient biocoenosis. In contrast, composition