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Early seed plant radiation: an ecological hypothesis
C. PRESTIANNI* & P. GERRIENNE
Universit´ de Li`ge, unit´ P.P.M., All´e du 6 aoˆt, B18/P40, 4000 Li`ge
*Corresponding author (e-mail: cyrille.prestianni@gmail.com)
Abstract: The earliest steps of seed plant evolution have been extensively studied during
the past 25 years. There is a growing body of evidence indicating that the first major spermatophyte
radiation occurred during the Late Devonian. At least fourteen Late Devonian species are now
recognized, and our knowledge of the diversity of those early seed plants has dramatically
increased. Five morphotypes of seeds have been defined, mostly based on cupule morphology
and on the number and degree of fusion of the integumentary lobes. In this paper, we critically
discuss the abundant environmental information in order to characterize the environment in
which this radiation occurred. Sedimentological information indicates that seed plants evolved
in disturbed environments. It is suggested that early seed plants thrived in the shade of the domi-
nating Archaeopteris, and that their evolution was canalized by this strong biotic pressure. We also
confirm the previous suggestion that the variability of seed morphotypes can be explained by the
weak abiotic selective pressure that existed in the Archaeopteris understory.
The seed habit is the most complex and efficient
plant reproductive strategy. It has been set up as
early as 380 Mya during the Middle Devonian
(Gerrienne et al. 2004; Gerrienne & Meyer-
Berthaud 2007). Since then, seed plant evolution is
an uncontested success story still in the making.
The earliest steps are however obscure and the
subject of much speculation (Chaloner & Pettitt
1987; DiMichele et al. 1989; Stewart & Rothwell
1993; Herr 1995). The very early changes that
allowed the seed habit evolution are complex. The
fossil record is still scarce and incomplete and is
as difficult to understand as Darwin's 'abominable
mystery' of angiosperm development. The well-
documented Lower Carboniferous wide range of
ovular type morphologies has long been considered
as first radiation of seed plants (Long 1975;
DiMichele et al. 1989, 2006). However, prior to
this, Upper Devonian floras included an already
diversified spermatophyte community (Hilton
1998a; Prestianni 2005). This rapid radiation of
seed plants is the subject of palaeobotanical investi-
gations (DiMichele et al. 1989) as well as seed plant
broader evolutionary studies (Rydin et al. 2002; Rai
et al. 2003). The present paper discusses the Upper
Devonian spermatophyte evolution within the
palaeoenvironmental context.
The earliest occurrences of seed plants or of
supposedly close relatives are found in the Givetian
of Poland, Greenland and Belgium (Arkhangels-
kaya & Turnau 2003; Marshall & Hemsley 2003;
Gerrienne et al. 2004; Turnau & Prejbisz 2006).
Granditetrasporites zharkovae, Spermasporites
allenii and Runcaria heinzelinii represent a
'complex' of seed-megaspores and proto-seed that
are representative of the first stages of the evolution
of seed plant and of their putative sister groups.
They presumably witnessed a set of Middle Devo-
nian palaeoenvironmental deep changes such as
the first forests (Meyer-Berthaud & Decombeix
2008), biotic crisis in the marine realm (McGhee
1996) and continental aridity crisis (Marshall et al.
2007). However, these changes will not be discussed
here as their complexity requires an independent
treatment beyond the scope of this paper.
The Frasnian and Lower Famennian fossil
records are strikingly scarce and no seed plants have
been recorded so far (Prestianni 2005). A single
occurrence of the seed species Moresnetia zalesskyi
(see below) is reported from the Petino (or Petin)
horizon near Semiluki, Russia (Thchirkova-
Zalesska¨a 1957; Jurina 1988). This record was
dated Upper Frasnian (Smirnova 1974) on the
basis of a palynological study. A recent review
(Avkhimovitch et al. 1993) of the western Russia
palynological record has correlated several loca-
lities, including Petino, at a large scale. This work
attributed this horizon to the Middle Frasnian
OG zone, more precisely to the CVe subzone.
Even although the age seems to be accurate and
recording a very early occurrence of Moresnetia,
the identification of the plant material still needs
to be confirmed.
A rapid and already diversified assemblage
of seed plant species occurs during the Upper
Famennian VCo and VH biozones (Fig. 1) (Hilton
1998a; Prestianni 2005). Seeds of that age have
mainly been collected from Belgium and northeast
USA (Fig. 2). In Belgium, most of the fossiliferous
outcrops belong to the Evieux Formation, Condroz
