Geoscience Reference
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of evolution made it a reality. Yet, despite intensive
research primarily focusing on the vertebrate and
embryophyte records, this reality is not fully under-
stood. While the study of fossils remains essential,
new developments in evo-devo (evolutionary devel-
opmental biology) should help to elucidate how
some
Archaeopteris). They make the point that, even at
the dawn of arborescence, several contrasting strat-
egies evolved for making tall plants but only one
(the Archaeopteris strategy) may have impacted
the Devonian environments significantly.
Prestianni & Gerrienne report five different
types of seeds in the Late Devonian which share
the same type of mechanism, an apical extension
of the nucellus, for trapping the pollen. The age
and taphonomy of these records are reviewed. The
authors agree with previous hypotheses suggesting
that early seed plants preferred disturbed habitats,
but they add a 'shady' context, analogous to that
inferred by some authors for the earliest angios-
perms some 300 million years later.
Preceeding Archaeopteris and the seed plants
within the lignophytes, the Aneurophytalean pro-
gymnosperms were widely distributed on the Eura-
merican and Siberian plates during the Middle and
early Late Devonian. Gerrienne et al. report the
first occurrence of the genus Rellimia in Gondwana
which, amazingly, may also represent the oldest
known occurrence of the clade in the late Early
Devonian. The Early Devonian record of plants
from Gondwana is scarce but this discovery indi-
cates that Gondwana may be a good location to
recover early lignophytes.
Three more contributions are devoted to the
palaeoecology and palaeogeography of Late Devo-
nian tetrapod palaeoenvironments. Because of their
diversity and exceptional preservation, Late Devo-
nian vertebrate faunas from East Greenland play a
key role in the current understanding of the mor-
phology and evolution of early tetrapods. A critical
question, which is crucial to understand how this
group became adapted to terrestrial conditions, is
the reconstruction of the environment and ecologi-
cal needs of its major representatives. Astin et al.
review the range of sedimentary environments
present through the Celsius Bjerg Group. They
suggest that the Acanthostega found in situ in the
Britta Dal formation lived in a waterhole, but they
account for the fact that this may not have been
the 'normal' habitat of these organisms.
After years of study, Cressler et al. provide an
updated reconstruction of the Late Devonian land-
scape at Red Hill, an exceptional fossil deposit of
Pennsylvania. This ancient alluvial floodplain, that
was part of the Caskill Delta Complex on the
southern margin of Laurussia, hosted a diverse
fauna of vertebrates and invertebrates. This recon-
struction allows the evaluation of the conditions in
which this fauna thrived in different aquatic habi-
tats, the proposal of hypotheses about the trophic
relationships of the Red Hill ecosystem and the
suggestion of a scenario of the selective pressures
that may have lead the tetrapodomorphs to explore
shallow water habitats.
groups
succeeded
in
their
adaptation
to
terrestrial environments.
A totally new approach to the investigation of
the origin and early evolution of land plants is pre-
sented by Versteegh & Riboulleau, who explore
how the physiological adaptations required by the
transition from a fully aquatic habitat to a subaerial
one might be tracked by the successive appearance
of molecular biomarkers in the sedimentary record.
In their contribution, Strother et al. link the drop
in pCO
2
calculated from the GeocarbIII model for
the Palaeozoic to a stepwise increase of plant
biomass related to three major evolutionary events:
the origination of a bryophyte grade of land plants,
the origination of vascular plants and the advent of
trees. They hypothesize that, through the changes
it induced in the atmospheric pCO
2
, this increase
in terrestrial biomass had a profound effect on the
evolution of the photosynthetic organisms in the
ocean (with a lag response of about 10 million
years for the latter).
An updated synthesis of the changes in diversity
and paleogeographical distribution of land plants
during the 35 million year period of time separating
the occurrence of the earliest cryptospore assem-
blages in Gondwana and the diversification of the
tracheophytes in the early Devonian is presented
by Steemans et al. According to their research,
the migration of land plants from northern Gond-
wana to Baltica in the Late Ordovician was facili-
tated by the northward migration of Avalonia. The
disappearance of the icy barrier that separated
South America from the rest of Gondwana in the
early Silurian might also be related to the spread of
land plants from the northern Gondwanan regions to
western Gondwana. The Aeronian-Telychian event
seems to have triggered the evolution of the trilete
spore-producing plants that are presumed to have
been vascular and more efficiently adapted to their
local habitats.
The three successive articles by Meyer-
Berthaud et al., Prestianni & Gerrienne and
Gerrienne et al. present new insights on land
plant evolution in the Devonian. The increase in
size of land plants and the advent of the seed habit
are frequently cited as two cornerstones in the
terrestrialization of the land plants which played a
major role in the paleoenvironmental changes
occurring at the end of the Devonian. Meyer-
Berthaud et al. discuss the constructional patterns
and putative environmental needs of the two earliest
types of trees (the pseudosporochnalean trees and
