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to chitin, lipids may become incorporated in cutin in
due course (Gupta et al. 2006a). It finally appears
that most of the previously observed fossil cutans
in fact correspond to cuticle lipids which were oxi-
datively linked during diagenesis (van Bergen et al.
2004).
Even relatively simple lipids are not always easy
to relate to their source. Although structural modifi-
cation such as loss of functional groups or changes
in stereochemistry do not usually prevent assign-
ment to their source organisms (e.g. Sinninghe
Damst
´
et al. 1997; Moldowan & Talyzina 1998)
they may disappear from the analytical window.
The corollary of aliphatization is that free lipids
may become part of larger macromolecular struc-
tures so that extra analytical steps are required for
their detection and identification (e.g. Adam et al.
2006).
For the particular organic matter, we may
have visual information on the biological affinities
of the fossils at hand, but to what extent is this
matched by the chemical composition of the
fossils? It seems that much of the aliphatization is
brought about by lipids from the immediate sur-
roundings of the original biomacromolecule, that
is, derived from the source organism. Moreover,
our present understanding of the natural sulphuriza-
tion and oxidative polymerization pathways imply
that these added substances survived relatively
undamaged structurally and isotopically. This
means that there should still be a fair chance of
obtaining information on the nature of the source
organisms, provided the individual products
released upon chemical degradation or (offline)
pyrolysis can be related to a single source and meta-
bolic pathway (van Dongen et al. 2002; van Bergen
& Poole 2002; Poole et al. 2004).
Aliphatization of macromolecular material from
plants and animals therefore seems to be ineluct-
able, which complicates the identification of the
molecular characteristics (and therefore the biosyn-
thetic pathways) of very old organic matter.
difficulties, however, in particular in assigning
Palaeozoic fossil organic matter to its source.
Apart from the fact that the samples have often suf-
fered from thermal alteration, the difficulties mostly
arise from a lack of taxonomic precision of the mol-
ecular biomarkers. Other difficulties arise from the
frequent chemical modification of the material,
despite excellent morphological preservation (e.g.
aliphatization).
All these difficulties easily explain the relatively
large temporal gap which currently exists between
the earliest microscopic plant remains documented
in Middle Ordovician (Strother et al. 1996) and
the earliest unambiguously documented terrestrial
biomarkers in Middle Devonian (Sheng et al.
1992). Despite this, the set of currently identified
molecules of terrestrial origin is already sufficiently
good to discriminate changes in plant associations
during the Carboniferous, revealing further infor-
mation on the terrestrialization process.
It is additionally hoped that condensation
processes, which remove lipids from the pool of
bioavailable products, may conversely facilitate
the survival of specific lipids over long periods of
time and, as such, record the biochemical evolution
related to the terrestrialization in the sediments.
Advancement of the assessment of the stable
carbon and hydrogen isotopic compositions on
lipids or (offline) pyrolysis products increasingly
contributes to unravelling the evolution of biosyn-
thetic pathways and diagenetic overprints. Great
advances will also be made with the development
of micro-scale techniques (microsampling, micro
extractions and nano-SIMS). These techniques
will allow the study of fossils present in very low
amounts such as very early spores and cuticles, or
the study of monospecific fossil associations.
Another rapidly developing approach to resolving
terrestrialization involves genomics: tracing the
evolution of enzymes critical to the biosynthetic
pathways involved in the terrestrialization process.
Terrestrialization and earliest plants had previ-
ously failed to attract many organic geochemists.
However, this is changing as demonstrated by
several recent studies and review papers (van
Bergen et al. 2004; Armstroff et al. 2006; Auras
et al. 2006). It is therefore likely that the right
compounds have not been looked at in the right
place and with the right techniques - yet.
Conclusion
Organic geochemistry plays an important role in the
elucidation of the history of early life (Brocks et al.
1999; Brocks & Summons 2003; Summons et al.
2006). Similarly, it should play a role in understand-
ing the terrestrialization process, in particular for
plants. Numerous molecular biomarkers of terres-
trial plants, deriving either from structural tissues
such as lignin phenols, from epicuticular waxes or
from the large class of terpenoids exist, and they
are widely used in Tertiary and recent sediments.
The study of the terrestrialization process with
organic geochemistry is associated with numerous
We thank M. Vecoli (UST Lille) and G. Clement (MNHN,
Paris) for inviting us to the ECLIPSE II Workshop: Terres-
trialization Influences on the Palaeozoic Geosphere-
Biosphere. We also thank J. de Leeuw (Utrecht University)
and M. Vecoli for constructive comments on the
manuscript. H. Kerp (M¨nster University) is thanked for
his help on the occurrence of resin in early gymnosperms.
Financial support for GJMV by the USTL (Lille) is
gratefully acknowledged.
