Geoscience Reference
In-Depth Information
Palaeogeographic and palaeoclimatic considerations based on
Ordovician to Lochkovian vegetation
PHILIPPE STEEMANS
1
*, CHARLES H. WELLMAN
2
& PHILIPPE GERRIENNE
1
1
NFSR Research Associate, Pal´obotanique - Pal´opalynologie - Micropal´ontologie,
University of Li`ge, Bˆ t. B-18, Parking 40, 4000 Li`ge 1, Belgium
2
Dept of Animal & Plant Sciences, University of Sheffield, Alfred Denny Building,
Western Bank, Sheffield S10 2TN, UK
*Corresponding author (e-mail: p.steemans@ulg.ac.be)
Abstract: This paper summarizes research on Ordovician to Lochkovian vegetation development
in a palaeogeographic framework. A terrestrialization model is described. Palaeogeographic maps
are modified to explain the vegetation migrations. Land plants first evolved on the Gondwana plate
and colonized Avalonia and later Laurentia and Baltica when those plates were in close proximity
(Ashgill-Llandovery). South America was colonized during the Llandovery following the
collapse of the ice sheet centred on the southern pole which had previously been an impassable
barrier for miospores. An Aeronian-Telychian event related to the global early Silurian transgres-
sion modified the vegetation by destroying biotopes where the earliest vegetation was thriving.
During the regression, trilete spore-producing plants appear to have been more able to respond
to environmental changes and dominated the vegetation from the Homerian. Cryptospore-
producing plants could survive under a wide range of climates, which helped them to survive
during the Hirnantian glaciation. On the contrary, the earliest trilete spore-producing plants
were probably climatically restricted, as suggested by latitudinal variation in assemblage compo-
sition. By the end of the Silurian, there were several phytogeographic units. Information on the
earliest vegetation favours palaeogeographic reconstructions where plates are in close proximity.
The oldest miospores believed to have been
produced by embryophytes (Steemans 1999, 2000;
Steemans & Wellman 2004) are Ordovician in
age, roughly 465 Ma (Le H
´
riss
´
et al. 2007;
Strother et al. 1996). They have been found in two
localities from Saudi Arabia, in the Hanadir
Member (Qasim Formation) of Llanvirn age. The
assemblage contains monad, dyad and tetrad
cryptospores, naked or enclosed within a laevigate
membrane. Most of the cryptospore morphologies
known in younger Ordovician strata are already
present. Despite the absence of a detailed systematic
description, around eleven taxa can be identified on
the basis of the illustrations. Several tetrads and
monads have also been reported from the
ˇ
´rka
Formation of Llanvirn age in the Czech Republic
(Vavrdov
´
1984).
A rich assemblage of cryptospores has been
described from the Caradoc type area in the Welsh
Borderland of south Britain (fig. 1 in Richardson
1988; Wellman 1996). The assemblage contains
13 genera and 17 species. A new assemblage, cur-
rently under investigation, has been observed in
Saudi Arabia. Its age is still imprecise, however,
and is thought to be either Caradoc or earliest
Ashgill. Its cryptospore composition is similar to
the UK assemblage.
Reports of cryptospore assemblages from
Ashgillian layers are numerous. On the Gondwana
plate they have been described from Libya
(Richardson 1988), Chad (Le H
´
riss
´
et al. 2004),
Turkey (Steemans et al. 1996), Czech Republic
(Vavrdov´ 1982, 1984, 1988, 1989) and Saudi
Arabia (work in progress). An assemblage has also
been published from a Chinese locality (Wang
et al. 1997). On the Avalonia plate, cryptospores
have been found in Belgium (Steemans 2001) and
in south Wales (Burgess 1991). To our knowledge,
no Ordovician diversified cryptospore assemblages
have been published on the Laurentia and Baltica
plates. All Ashgillian assemblages are very similar
in composition: tetrads and dyads dominate; monads
are not frequent; species enclosed within a mem-
brane are abundant; spores of dyads and tetrads
are usually tightly associated together.
An important step in the evolution of vegetation
is the inception of the first trilete spores. Rare Ambi-
tisporites specimens have been found in the Ashgill
(Hirnantian) from Turkey (Steemans et al. 1996).
Trilete spores are also observed from Saudi Arabia
in the latest Caradoc or earliest Ashgill up to the
Ordovician-Silurian boundary (Steemans et al.
2009). Surprisingly, specimens of ornamented
Synorisporites trilete spores are present. The earliest
