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