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et al.'s (2006) analysis, and led those authors
to propose several hypotheses (ecological, palaeo-
geographic, palaeoclimatic, data-biased) to explain
the similarities between the analysed assemblages.
They suggested 'a true palaeogeographic proximity'
(allowing biological exchange by propagules)
between Bolivian and British assemblages. This is
consistent with the absence of a palaeogeographical
barrier between the ORSC and Gondwana as
proposed by Steemans et al. (2007). Plants of the
South Euramerican-Northwest Gondwanan unit
were very small and mostly characterized by narrow
dichotomous axes with terminal sporangia (e.g.
Cooksonia, Pertonella). On the contrary, plants
from the equatorial North Euramerican and the
Northeast Gondwanan units were characterized by
much larger sizes. They belonged to the Lycophyta
clade (zosterophylls or early Lycopsida). The
simultaneous existence of floras at markedly
different grades of organization adds another note
of caution to the use of early land plants for
biostratigraphical purposes.
The early Lochkovian record of land plants is not
significantly different from that of the Late Silurian
(compare the floral lists of Raymond et al. 2006
for the late Silurian and of Edwards & Wellman
2001 for the Lochkovian). It includes various
genera of diminutive plants with terminal sporangia
('rhyniophytoids') and plants of Lycophyta affi-
nities. However, note that in the Lochkovian these
Lycophyta had spread to higher latitudes and were
no longer restricted to the equatorial zone.
The Lochkovian assemblages from Great
Britain (Wales, England, Scotland; see Edwards &
Wellman 2001 or Edwards et al. 2001 for a
summary) and fromBrazil (Paran´ Basin; Gerrienne
et al. 2001, 2006) bear a striking resemblance. At
least five different taxa are found in both areas:
Caia, Cooksonia, Pertonella, Tarrantia and a new
genus to be created to accommodate the specimens
assigned to Cooksonia caledonica (work in pro-
gress, University of Li`ge). Those similarities
imply major floral exchanges between Southern
Euramerica and Gondwana that can probably be
explained by a close proximity of those palaeocon-
tinents as illustrated in Figure 1d.
The Hirnantian glaciation had little observable
effect on cryptospore biodiversity which suggests
that the plants that produced them were adapted to
a wide range of climates. In contrast, the destruction
of biotopes by the global Early Silurian trans-
gression strongly affected their biodiversity and
favoured the emergence of the trilete spore-
producing plants which have been dominating the
vegetation from the Homerian. The variability
observed in trilete spore assemblages since the
Lochkovian, compared to a lack of variability
among cryptospore assemblages, is better explained
by a latitudinal effect than by the presence of
palaeogeographical barriers.
The zonation of the Late Silurian land plant
floras was probably climatically induced. Striking
similarities between the Lochkovian assemblages
from Great Britain and Brazil suggest major
floral exchanges between Southern Euramerica
and Gondwana, and a close vicinity of
these
palaeocontinents.
This work is supported by the Eclipse II project 'The
terrestrialization process'. The authors are grateful to
J. Marshall and the anonymous reviewer for improvement
of the manuscript.
References
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Conclusions
The colonization of the land by the earliest ter-
restrial vegetation can be explained by the close
proximity of the different palaeoplates, from
the migration of these plates and the collisions
between them. Models that show large oceans,
which would probably have been impassable
barriers for the transport of miospores, cannot
explain the progressive invasion of the continents
through time.
