Part 3
Coals mainly occur in the Westphalian. However, at the end of the
Westphalian the considerable changes in the composition of the
flora can be seen. The hitherto widespread tree-like lycophytes
largely became extinct; only very few forms locally persist into
the Stephanian. In the Rotliegend (Early Permian) of Europe and
North America only a single tree-like lycopsid is known:
Sigillaria brardii. This plant was very tolerant
with regard to its habitat, although it always grew in humid
environments.
Although this floral change has thoroughly been documented in
recent years, also by palynological data (Phillips et al. 1985),
the causes have never been explained in a satisfactory way. One
of the problems is that there are hardly successions in which
the Westphalian-Stephanian transition is developed without hiatuses.
A further problem is that the Westphalian and
Stephanian have been defined in different types of basins; the
Westphalian in a so-called paralic basin and the Stephanian in an intramontane
basin. Floras in such fundamentally different types of basins are not
necessarily well comparable. Virtually all our Stephanian floras are
known from intramontane basins.
Several authors relate the almost complete extinction of
arborescent lycopods to climatic changes (e.g., DiMichele et al.
1992). From the Westphalian onwards northwest and central Europe
drifted in northward direction. In the Westphalian this area was
still in an equatorial position, by the end of the Permian it had
reached a position of approx. 30° N. This means that the
continental plates wandered through the climatic zones. Although
this is certainly an important factor, also tectonic processes
have recently been discussed in relation to this floral change
(Kerp 1996).
By the end of the Westphalian the northward moving southern, so-called
Gondwana continental plate collided with the Eurasian (or Laurussian) plate
(Ziegler 1990). As a result of this collision sediments were folded up and
the Variscan Mountain Chain was formed (Fig. 4). This also resulted a strong
reduction of the Paralic Coal Basin which was situated immediately north of
the folded area; only a few small relict basins were left. The sediments
in these small basins express much higher sedimentation dynamics. The
folding and formation of the Variscan Mountain Chain had resulted in a
stronger erosion and the sediments of the uppermost Westphalian and
Stephanian are therefore generally sandier than the underlying Westphalian
deposits. In such sandy sediments the ground water level will
have been lower than in the predominantly clayey sediments of the
Westphalian, in which stagnation of the ground and rain water locally resulted in the
formation of suitable biotopes for arborescent lycophytes. The development of
the Variscan Mountain Chain will certainly also have influenced atmospheric
circulation patterns. Moreover, it should also be realised that the
reduction of a once widespread wetland area will have had some effect. The
replacement of an extended wetland area with a luxurious vegetation, a more
or less self-regulating ecosystem with a considerable water-storage
capacity, by a mountainous area will have lead to changes in atmospherical
humidity. Arborescent lycophytes and other plants strongly adapted to very
stable ecological conditions could apparently not persist under the higher
sedimentation and climate dynamics during the Stephanian and became extinct.
It is clear that there is not a single reason for the extinction of the
arborescent lycophytes, but that it is the result of a complex many, often
interrelated factors. Abb. 4a Abb. 4b
Abb. 4: Palaeogeographical reconstructions of the Westfalian and Stefanian. Modified after Ziegler (1990).
Although the Stephanian is still partly coal-bearing, in the Rotliegend a
few thin coal seams are only very locally developed. In practice it appears
to be very difficult to define a clear boundary between the Stephanian and
Rotliegend because several plant taxa which have previously been regarded as
marker fossils now appear to be strongly facies-dependant. These therefore
rather reflect different ecological conditions than that they indicate
different ages. The biostratigraphical value of these fossils is thus very
limited. Another major problem is the facies development in the individual
basins. Especially the Rotliegend is characterised by many rapid lateral and
horizontal changes in facies; this can also have influenced the composition
of the floras.
The individual, mostly rather small intramontane basins, especially the
Rotliegend basins, are often very difficult to correlate with each other.
In some cases such correlations are also strongly hampered by the presence
of considerable hiatuses. Stephanian floras are much more common and
therefore much better known than Rotliegend floras. Especially in younger deposits plant fossils become
rare. Although we know a relatively large number of plant
localities from the Lower Rotliegend, there are only very few localities
known from the Upper Rotliegend. Floras which are most probably coeval can
be developed very differently. In these cases palynology, the study of
fossil spores and pollen does not help any further, as palynomorphs are
usually absent or very poorly preserved in redbed sequences.
Fig. 5: A conifer from the
Rotliegend of the Saar-Nahe Basin.
As early as 1930 Gothan & Gimm noticed the existence of
different types of floral associations in the European Rotliegend. These
are the peat-forming floras which indicate humid biotopes and the
hinterland floras with elements which grew under drier
conditions. Detailed studies have resulted in the recognition of
a larger number of other typical associations (e.g., Barthel
1976). The hinterland or extra-basinal floras are usually poorly
and incompletely preserved as these plants grew outside the
sedimentation basins and the material was transported for a
longer distance before it was embedded in the sediment. The
preservation potential of such floral associations is therefore
much lower than that of those growing in basinal areas.
Although many Rotliegend floras predominantly consist of hygro- and
hydrophilous elements, the number of taxa adapted to drier
conditions generally increases in younger deposits. The hygro-
and hydrophilous floras of the Rotliegend have a rather archaic
character and include many long-ranging, essentially Stephanian
taxa. Evolutionary innovations, especially within the
gymnosperms, took place in the extra-basinal areas. Several
groups which first appeared during the Stephanian and Rotliegend,
such as Cycadophytes and Ginkgophytes
consist of typical hinterland elements. It is very difficult,
if not impossible, to reconstruct the development of hinterland floras
because of the paucity of the fossil record.
Although the general picture is more or less clear, many detailed questions
on Stephanian and Rotliegend floras still remain unanswered. Anyhow, it is
obvious that there was a clear differentiation in basinal and hinterland
floras. The first are characterised by the dominance of tree ferns such as
Psaronius in the petrified forest of Chemnitz (Rößler
1996); the latter were dominated by conifers.
During the Namurian and the Westphalian the subsidence of the
depositional basins was more or less equal to the sedimentation
rate. Therefore thick peat deposits could develop over large
areas. These peat deposits later became coal. Only occasionally
transgressions occurred, which, however, could lead to the
flooding of large areas. In the mainly terrestrial succession of
the Namurian and Westphalian marine horizons occur, some of which
even have been traced over a area of 600,000 km². However, these
rather short-termed floodings have had little effect on the
flora. Most plant species could apparently retreat to refugia
from which they could migrate back after the lowering of the sea
level. In the Westphalian the marine influences diminished and
the youngest marine intercalations are known from the Stephanian.
terrestrial and nearshore marine deposits
Variscan fold belt
Land areas
Front of the Variscan fold belt with overthrust
Volcanism
Coal deposits
The flora of the Stephanian can be characterised by a sudden
decrease of arborescent lycophytes and a dominance of tree ferns;
also seed ferns and cordaites are well represented. During the
Stephanian the conifers, a group of gymnosperms whose earliest
representatives looked very similar to modern araucarians, became
gradually more common. The earliest conifers are known from the
Middle Westphalian, but these records consist of small fragments
which have apparently been transported over considerable
distances. The increase in dominance of conifers continues in the
Rotliegend, although completely conifer-dominated floras are rare
in the Rotliegend. Other new elements are the more modern
types of pteridosperms, like the callipterids. Since the Stephanian a
reduction of the number of species can be seen.