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Aglosperma-type). This dominance of the Moresne-
tia-type over other seed types in the Belgian neritic
deposits confirms the British observations, and thus
contradicts the isolated event hypothesis.
It is tempting to conclude that the Moresnetia-
type dominated, with its abundance in the fossil
record corresponding to its Upper Devonian abun-
dance. The fossil record is however strongly
biased. The Belgian Upper Devonian deposits are
all mostly marine. The fossil assemblage is thus
drifted, sometimes quite far from its growing site.
Moresnetia has often been found in connection
with its branching system. In contrast, the other
Belgian seeds (Aglosperma type, Dorinnotheca-
type and Condrusia-type) are most often found as
isolated seeds. This suggests that the relative
abundance of the Moresnetia-type is more likely
due to preferential fossilization than to its original
abundance in the environment.
Rothwell & Sheckler's (1988) inferences on the
Upper Devonian ecological successions weremainly
based on Moresnetia-type plants. Their conclusions
might only apply to this morphological type, with
those plants being pioneer colonizers of newly
emerged, primary successional habitats near shore-
lines. This concept could probably also be extended
to river margins. By contrast, the other seed-types
are never or very rarely collected in attachment to
extensive branching systems, suggesting that they
were probably thriving in other habitats. Their
ecology and precise environment are however impo-
ssible to characterize precisely.
(Ziehe & Demetrius 2005; Demetrius & Ziehe
2007). They propose instead a new set of parameters
named the 'Directionality theory'. In this model,
populations are compared to a thermodynamic
process and their characterization is based on a par-
ameter called demographic entropy. This allows the
quantification of the population parameters that can
then be compared to each other. Plant populations
that spend most of their life history in stationary
growth (vegetative growth) are characterized by
high demographic entropy values. By opposition,
plant populations that spend most of their life
history in the exponential growth phase are charac-
terized by low demographic entropy values. This
model can be qualitatively applied to the Upper
Devonian environment.
As discussed above, the landscape was ecologi-
cally partitioned at a high taxonomic level that is,
in habitats ranging from those devoted to opportu-
nistic species to those occupied by equilibrium
species. Archaeopteris populations were probably
defined by the highest demographic entropy values
as shown by the large trunks and the extensive
photosynthetic apparatus. By contrast, the herbac-
eous plants such as Eviostachya hoegii or Gille-
spieae randolphensis had very low if not the
lowest demographic entropy values. Rhacophyton
and/or seed plants had middling values.
The Upper Devonian environment was therefore
probably much more complex than currently recog-
nized. Indeed, using a quantitative palynology
approach, Streel (1999) was able to demonstrate
the occurrence of at least four distinct ecological
niches in Laurentian terrestrial environments. More-
over, all fossil plant communities considered in the
present paper, including continental localities as
well as the sediments studied in Streel (1999),
only record nearshore environments.
The outstanding records of the Refrath borehole
are an exception to this assessment, however
(Fairon-Demaret & Hartkopf-Fr¨der 2004). The
Refrath borehole was drilled in Germany near
Cologne and passed through marlstone/mudstone
sediments attributed to the upper Knoppenbissen
Formation (Fairon-Demaret & Hartkopf-Fr¨der
2004). The core has been dated palynologically and
assigned to the LL biozone (Hartkopf-Fr¨der 2004).
A slow and light maceration of the immature sedi-
ments yielded a rich and diverse plant mesofossil
association. It revealed the occurrence of an unex-
pected diversity with many leaf types, advanced
sporangial morphologies and several xylem types
occurring. They all document morphologies still
unknown in Upper Devonian assemblages attributed
to virtually unexplored vegetations. This assem-
blage most probably provides a glimpse of a
presumably more continental ('upland') flora and,
by extension, a completely unconsidered set of
Seed plants and their environment: an
ecological hypothesis
Chaloner & Sheerin (1981) suggested that the
increase of seed size in the Carboniferous could
be explained by the progressive switch from r
selection to K selection. This r/K selection model
classifies organisms into equilibrium (K) and
opportunistic (r) species (Pianka 2000). Upper
Devonian and Lower Carboniferous small-sized
seeds would therefore be considered as r-selected
and the Upper Carboniferous large-sized seeds as
K-selected. All subsequent studies which focused
on the earliest seeds confirmed this trend (Rothwell
& Scheckler 1988; Rowe 1997; Cressler 2006).
Most Upper Devonian spermatophytes probably
roughly conform to the Elkinsia polymorpha recon-
struction (Serbet & Rothwell 1992), that is,
moderate-sized plants producing a large number of
small seeds. These characteristics conform to the
morphology associated with pioneer opportunistic
species.
Recent researchers have discussed the predic-
tive and explanatory limitations of the r/K model
