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

94 ADVANCES IN PALAEOICHTHYOLOGY
situation especially refers to the near-shore environments of the Russian platform
localities: Ketleri-Pavari, Rybnitsa and Gornostayevka. Coarse or fine-grained
embedding rock may also provide information on the duration of transportation.
Before 1984 it was generally accepted that the earliest tetrapods dwelled exclusively
in fresh-water basins. The discovery of Tulerpeton (Lebedev 1984; Lebedev and Clack
1993) demonstrated that the habitats of these animals were more diverse than thought
earlier, and tetrapods could also live in the brackish-water conditions close to marine.
As deduced from sedimentological and invertebrate information, analysed localities
are separated by salinity level into presumably fresh-water (Scat Craig, Ala-Archa and
East Greenland), that follows from their intramontane position and absence of geological
information on connection to marine basins, and presumably brackish (most of the
remaining ones). Gornostayevka is considered here as mostly marine with occasional
fresh water influxes from nearby uplands. The presence of macrofloral remains in East
Greenland, Red Hill, Ningxia and Ala-Archa might support their presumed fresh-water
or almost fresh water environmental conditions.
At this time within Laurussia, epicontinental marine, deltaic near-shore and fluvial
vertebrate assemblages are known. The poorly known East Gondwanan Jemalong locality
is believed to be fluvial (Campbell and Bell 1977); the Kazakhstan Ala-Archa is supposed
intramontane. Thus, the original idea of the rigid relation of tetrapods to fresh-water
basins may be rejected, as tetrapod habitats show a great variety of environmental
conditions. Tolerance of these animals to non fresh water corroborates Thompson’s
(1980) and Milner’s (1993) idea of the possibility they might also live in the littoral
zone that gave them wide possibilities of migration from one continent to another. This
implies that the faunal exchange between the Laurussian, Gondwanan and North Chinese
block was rather active, and the Tethys intercontinental basin was comparatively shallow,
making possible migrations.
The second part of Table I deals with assemblages’ composition. The most striking
feature, as noted earlier by Lebedev (1985) is the presence of Remigolepis antiarch
fishes in all Devonian tetrapod localities known at that time and his suggestion of the
potential for a tetrapod find in Ningxia. Such a close association, as we now know, is
not invariable. Examples of this are the absence of this genus from Scat Craig, Ketleri
and Red Hill. The first of these localities dated as Frasnian that explains the situation,
as Remigolepis is not yet recorded from such early deposits. At Red Hill, in contrast to
the localities of the Russian platform and Greenland, the assemblage includes abundant
groenlandaspidids, but no Holoptychius or Bothriolepis (Daeschler et al. 2003;
Daeschler, pers. comm.), highly characteristic of these central and east Laurussian
communities. Possibly, the biocoenosis structure at Red Hill was quite different from
that of the Russian platform and Greenland or groenlandaspidids might in some respects
substitute for Remigolepis in the community. The sarcopterygian Sauripterus might
occupy the ecological niche of Holoptychius and Hyneria that of Eusthenodon.
At Ketleri the dominant members of the assemblage are Holoptychius and
Bothriolepis. These genera are also highly characteristic of East Greenland tetrapod
localities, but Bothriolepis is not the dominant antiarch there. Direct substitution of
Remigolepis with Bothriolepis in the ecological niche does not seem convincing, as in
many other cases these groups, which are rather different morphologically, coexist in
the same community, obviously playing their own separate roles. On the other hand,