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ornamented trilete spores previously reported
were from the Homerian (Burgess & Richardson
1991). The Saudi Arabian material
The three main events of miospore evolution will
be discussed in the following sections: the Hirnan-
tian glaciation, the Aeronian-Telychian event and
the early Lochkovian.
is currently
under investigation.
Rhuddanian assemblages are numerous on the
Gondwana plate (North Africa, Saudi Arabia,
South America), and on the Euramerican plate (USA
and UK). Assemblages are similar to those from the
Ashgill. During the late Aeronian there is an impor-
tant decrease in cryptospore biodiversity. It con-
tinues during the Telychian where the first patinate
trilete spores (Archaeozonotriletes spp.) occur
(Richardson 1988; Wellman et al. 2000a; Steemans
& Pereira 2002; Mendlowicz Mauller et al. 2004a).
Data on the Wenlock are scarce, especially con-
cerning the Sheinwoodian. They are mainly from
the UK (e.g. Richardson & Lister 1969; Burgess &
Richardson 1991; Wellman & Richardson 1993)
and Libya (Richardson & Ioannides 1973). Biodi-
versity is still very low and assemblages are similar
to those from the Telychian. Numerous inceptions
of new trilete spores and cryptospores occur
during the Homerian. Records exist from the UK
(e.g. Richardson & Lister 1969; Burgess & Richard-
son 1991; Wellman & Richardson 1993; Burgess &
Richardson 1995), from the Czech Republic (Dufka
1995) and from Libya (Richardson & Ioannides
1973), and so on. For the fist time, the biodiversity
of trilete spores is greater than that of cryptospores.
In the Ludlow, Pridoli and younger stratigraphic
levels, data become more numerous. Trilete spores
continue to diversify, and are generally more abun-
dant and diverse than cryptospores. Data are mainly
from the UK (e.g. Richardson & Lister 1969;
Wellman 1993; Wellman & Richardson 1993;
Burgess & Richardson 1995), from Libya (e.g.
Richardson & Ioannides 1973; Rubinstein & Stee-
mans 2002), from Turkey (Steemans et al. 1996),
from Spain (Richardson et al. 2001), from Brazil
(Steemans et al. 2008a) and so on. Clearly, trilete
spores continued to rapidly diversify and dominate
miospore assemblages. Finally, during the Lochko-
vian, data are very abundant and will not be exhaus-
tively listed here.
Hirnantian glaciation
The Hirnantian glaciation is known to have been
responsible for the strong decrease in biodiversity
of most fossil groups (Webby et al. 2004). Ashgil-
lian to lower Aeronian miospore assemblages
are very similar in all localities, showing that the
glaciation had little effect on miospore assemblages.
This appears to be the case whatever the climatic
conditions were, for example, close to the southern
pole in Paraguay or Brazil (Le H´riss´ et al.
2001a; Steemans & Pereira 2002; Mendlowicz
Mauller et al. 2004a, b) or in temperate regions on
the Avalonia plate (Wellman 1996; Steemans
2001). No decrease in miospore biodiversity has
been observed after the Hirnantian glaciation (Stee-
mans & Wellman 2004): pre and post-glaciation
assemblages are very similar. This has been expla-
ined by the ability of the earliest cryptospore-
producing land plants to grow and to reproduce
under diverse climates (Steemans 2000; Wellman
& Gray 2000). The areas where the vegetation was
destroyed during the advancing glaciation were
recolonized as soon as the ice retreated.
Aeronian - Telychian event
The miospore diversity curve shows a minimum
value during the Aeronian and the Telychian (Stee-
mans 1999) because of a high rate of Aeronian
extinction and the small number of inceptions
of new species around the Llandovery-Wenlock
transition. The low biodiversity value could reflect
the global marine sedimentological conditions
after the melting of the Hirnantian glaciers. It is
probably partially true that miospore numbers are
underestimates as reports from marine sediments
dominate this interval. However, assemblages of
miospores are very different before and after this
'event': (1) trilete spores which were previously
rare become increasingly abundant, even if diversity
remains low up to the Homerian; (2) cryptospores
enclosed in a membrane (e.g. Velatitetras spp,
Segestrespora spp., etc.) which were very abundant
became rare after the event; (3) cryptospore tetrads
and dyads from the mid Ordovician up to the middle
part of the Aeronian had tightly adpressed spores
(in younger strata these polyads consist of loosely
adherent spores); and (4) monads which were
previously rare became much more abundant.
This strong cryptospore biodiversity decrease
which was accompanied by a major change in the
vegetation could be considered as a major event in
Miospore biodiversity evolution
A detailed analysis of the literature enables con-
struction of a biodiversity curve from the Ordovi-
cian up to the Lochkovian (Steemans 1999, 2000;
Steemans & Wellman 2004). Recently, a similar
curve compiled by Strother has been published in
Traverse (2007). This is generally consistent with
the previous curves, although there are some small
discrepancies that are probably a result of Strother's
wider cryptospore concept which includes all con-
tinental palynomorphs, that is, continental algae
(Strother & Beck 2000).
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