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ecological niches (Fairon-Demaret & Hartkopf-
Fr¨der 2004).
Famennian spermatophytes are characterized by
extensive branching systems bearing a very large
number of small seeds and very rarely presenting
secondary growth (Fairon-Demaret & Scheckler
1987; Serbet & Rothwell 1992; Prestianni C. pers.
comm., 2009). In the Famennian environment,
seed plants were confined to low entropy values
which are characterized by an early age of sexual
maturity and a large net progeny set (Demetrius &
Ziehe 2007). The forest builder Archaeopteris
dominated the highest demographic entropy values,
and consequently excluded seed plants from them.
The biotic environment thus acted as a selective
force that limited the outcome of higher entropy
values in spermatophyte populations.
As demographic entropy is linked to the age of
sexual maturity and to the number of propagule pro-
duced, variations that do not modify these values are
therefore allowed. Consequently, seed plant evol-
ution within the limitations of their demographic
entropy values was probably only triggered by
abiotic constraints with low or no interspecific com-
petition corresponding to n-selection (DiMichele
et al. 1989). This type of selection favoured the
establishment of the wide cupular and ovular mor-
phological range of Famennian seeds as these vari-
ations only had a limited impact on demographic
entropy values. In contrast, all seed plant vegetative
systems correspond to the stereotypic morphologies
mentioned above. The modification of these is
directly linked to the age of sexual maturity and
the number of propagule produced. As a conse-
quence, instead of being an illustration of a non-
selective evolution (which can be concluded from
the observation of seeds only), their radiation
more probably illustrates a 'canalized' evolution
that is, one confined by other plants to restricted
entropy values. If we only consider the fossil
record, we can hypothesize that upper Devonian
seed plants diversified in ecospaces left free by the
dominating vegetation.
If the hypothesis that all seed plants are oppor-
tunistic species is correct, they should be confined
to disturbed environments. These environments
are diverse: seashore, river margins, disturbed
parts of forests and fern glades, forest margins,
and so on. The morphological array of these basal
spermatophytes potentially illustrates adaptations
to the different types of disturbed environments as
suggested by the differential morphological parti-
tioning in the fossil record. Such a hypothesis is
however difficult to test. Finally, it is notable that
the largest spermatophyte diversity is recorded in
Belgium (Fairon-Demaret 1996a; Prestianni 2005;
Cressler 2006). The Belgian Famennian environ-
ment (Blieck et al., 2010) was characterized by a
generally arid climate with periodic rain falls. This
may provide an additional, indirect support for the
opportunistic (disturbance) theory.
The 'dark and disturbed' hypothesis (Feild et al.
2004) explains early angiosperm establishment as
an adaptation to disturbed tropical understory
environments. Ancestral angiosperms were, as
hypothesized by Feild et al. (2004), shrubs or trees
with particular adaptations to shaded conditions
(f. e. leaf anatomy). It allowed angiosperms to
'gain a root-hold in the well-established Mesozoic
plant communities' (Feild et al. 2004).
The early seed plant evolution and its subsequent
establishment might have occurred in a comparable
way. However, we possess drastically less infor-
mation about early spermatophytes than about
early angiosperms. Any physiological assumptions
are therefore dangerous to infer. Indeed, if we
have a general idea of early seed plant morphology,
very little is known about their anatomy. Moreover,
all ecological information tentatively summarized
here is very indirect and all biased by geological
processes. Although different, the ecology of the
Upper Devonian forest can nevertheless be com-
pared with that of the Mesozoic gymnosperm-
dominated environment. This suggests that the
very early angiosperms adapted to shade disturbed
understory environments might have simply
repeated what their ancestors did in the Upper
Devonian disturbed environments.
Conclusion
The upper Devonian seed plant radiation was rapid
and led to the establishment of a whole set of
cupular, integumentary and ovular morphologies.
These reflect the particular conditions in which
early spermatophytes evolved. We here tentatively
apply the concept of demographic entropy to the
Famennian Archaeopteris-dominated forest. This
progymnosperm occupied the highest demographic
entropy values where the lowest values were occu-
pied by populations of herbaceous plants. In that
environment, seed plants and ferns were confined
to middling values. This leads to the development
of a canalized type of evolution with a strong
biotic pressure that limited the available habitats
but with only weak abiotic selective pressure inside
specific niches. Seed plant thus evolved a stereo-
typical morphology corresponding to the biotic
pressure. This monotonous plant architecture bears
surprisingly variable seed types corresponding to
the weak abiotic selective pressure. We suggest
that this evolution occurred in disturbed environ-
ments and that it was comparable to the Cretaceous
'dark and disturbed' pattern of
angiosperm
evolution.
