Geoscience Reference
In-Depth Information
the evolution of vegetation. This has been interpre-
ted as due to the early Silurian global transgression
which could have destroyed, reduced or moved
the habitats of the earliest plants (Steemans 2000).
After this global transgression, a global regression
was initiated up to the early Devonian. Plants colo-
nized new areas which would have become pro-
gressively available during the regression. Most
probably, plants producing trilete spores were
better adapted to the new ecological conditions as
they had increasingly dominated the vegetation
since the Wenlock.
According to the palaeobotanical record
(Edwards & Richardson 2004), cryptospore-
producing plants were probably bryophyte-like
stem group embryophytes and trilete spore-
producing plants were tracheophytes and their
immediate ancestors ('protracheophytes'). After
the Aeronian-Telychian event the vegetation
landscape radically changed; the previous veg-
etation dominated by very small bryophyte-like
plants was replaced by a vegetation dominated by
tracheophytes which colonized a wider range of
biotopes and began to rapidly increase in size.
spore and cryptospore producers has been much
debated with slightly different scenarios developed
by Steemans (1999), Wellman et al. (2000b) and
Edwards & Richardson (2004).
Palaeogeographic considerations based
on the earliest miospores: a model
Generalities of cryptospore dispersion
It has been explained in Steemans et al. (2007) how
improbable effective cryptospore transport would
be over long distances (at least several hundred
kilometres). Effective transport implies that cryp-
tospores survive during the transport, land in a suit-
able area where they will germinate and produce a
viable gametophyte. Cryptospores are believed to
be produced by bryophyte-like plants. Such plants
now predominantly inhabit at least temporarily
wet habitats. Their spores have a low resistance
to desiccation, and therefore do not survive long
distance transport by high altitude winds. The ear-
liest bryophyte-like plants were of small size
(a few millimetres high) limiting the probability
of their spores being redistributed far by wind as
they quickly fell down on a wet ground. Finally,
cryptospores are usually of a large size diameter
(.25 mm), further reducing their ability to be trans-
ported over long distances. According to those con-
siderations, long distance transport of cryptospores
is considered to be very improbable. However, as
stated in Steemans et al. (2007):
Considering long geological time spans, however, the
possibility of long distance dispersal via a chance
event is of course not impossible. Even if effective
long-range dispersal of one species is possible, long-
range relocation of a complete cryptospore assemblage
is much less probable.
The palaeogeographic model developed below is
therefore constructed on the basis of this hypothesis
which implies that the terrestrialization of the tec-
tonic plates is possible only if they are not separated
by large distances.
Early Lochkovian
The biodiversity curve published in Steemans
(1999) is based only on independently dated ass-
emblages. They have been isolated from marine
near-shore sediments in which there are other
palynomorphs that is, chitinozoans and acritarchs,
or macrofossils such as graptolites, and so on.
However, some Lochkovian studies report on conti-
nental sediments for which there are no independent
age controls. The contents of those assemblages
were not included when constructing this biodiver-
sity curve. This is important because cryptospore
biodiversity in these continental deposits is much
higher than in marine deposits.
It has been shown that the near-shore marine
miospore assemblages are dominated by trilete
spores while continental ones are usually dominated
by cryptospores (e.g. Richardson 1996; Wellman &
Richardson 1996; Rubinstein & Steemans 2002;
Steemans et al. 2007). This has been interpreted as
resulting from differences in the biotopes in which
the plants producing trilete spore and cryptospores
grew. The latter probably lived in or around wet
confined environments (lakes and swamps) with a
low potential for their spores to be transported by
rivers to the near-shore marine basins. This behav-
iour is still observed in the extant nature (van
Zanten & P´cs 1981). On the other hand, trilete
spore-producing plants may have inhabited valleys
and alluvial plains; their spores were therefore dis-
seminated more easily into rivers and transported
out to sea. The ecology and habitats of the trilete
The colonization of the land by the first
embryophytes
The oldest cryptospores believed to be produced by
embryophytes are Llanvirn in age from Saudi
Arabia on the Gondwana continent (Fig. 1a).
Some specimens are also known from the Czech
Republic. At that time, Avalonia was still a part of
the Gondwana Plate or at least very close to it
(Vecoli & Samuelsson 2001; Samuelsson et al.
2002). Diversified cryptospores are reported from
the Caradoc of the UK by Wellman (1996). It is
therefore possible that
the earliest bryophytes
