Geoscience Reference
In-Depth Information
probably has a low preservation potential. Due to its
mixture of aliphatic and aromatic monomers it may
be difficult, if not even impossible, to deduce a
specific suberin fingerprint from the fossil record.
Suberan is a rather enigmatic highly aliphatic
non-hydrolysable biopolymer. It has been described
from bark (Tegelaar et al. 1995). Possibly, this
polymer originates from oxidative polymerization
of unsaturated lipids.
also occur in the filamentous green algae Spirogyra
(Nishizawa et al. 1985) and the phlorotannins occur-
ring in brown algae. Since neither group has direct
relevance for the terrestrialization they will not be
considered further.
Another common and diverse group of products
produced via the phenylpropanoid pathway are
lignans (phenylpropanoid dimers), nor-lignans (with
diphenylpentane carbon skeletons) and lignan
oligomers which, in contrast to lignin (a polymer
of monolignols), are non-structural components
(Lewis & Davin 1999; Suzuki & Umezawa 2007).
Lignans are known from bryophytes such as liver-
worths and hornworths (Lewis & Davin 1999) and
are not known from algae (see also the lignin
section below). Lignan remains may therefore
have been preserved in the earliest land plants and
could be used as tracers.
Another protective strategy is the production of
resins. Plant resins are used typically for protection
(Langenheim 1995). Resin can be exuded passively
(when a plant is wounded) or actively. Often resin
emerges from canals or resin cells. Resin provides
a mechanical and chemical protection against
pathogens. When present on leaves, resin also acts
as a barrier against desiccation and UV damage.
Resins contain a complex mixture of non-
volatile compounds mainly consisting of di- or tri-
terpenoids. In addition, resins contain volatile com-
pounds, including mono- and sesqui-terpenoids,
which can be dominant in fresh resins. These
volatile compounds tend to disappear with time
but a fraction can remain entrapped when the
matrix polymerizes and becomes hard (Anderson
& Crelling 1995). Resin- (and amber-) producing
trees are present both among gymnosperms (e.g.
conifers, cycads) and angiosperms. The most pro-
lific genera all live in the tropical to subtropical
region (Langenheim 1995).
Plant resins have a rich fossil record in the form
of amber or resinite. Amber and resinite are more or
less synonymous terms describing geological
material evolved from plant resin. The main differ-
ence lies in the fact that amber generally describes
macroscopic remains, while resinite describes
microscopic remains observed petrographically
(Anderson & Crelling 1995). This may explain why
'true' amber is rarely described before the Creta-
ceous while resinite has been described in coal sam-
ples as old as Carboniferous (and maybe older).
Among the oldest recognized ambers are Late Trias-
sic ambers from Italy (Roghi et al. 2006), although
some reports of Carboniferous amber exist (Smith
1896). This observation fits well with anatomical
evidence that the earliest plants showing resin
channels or resin-filled cells in their anatomy are
members of the earliest but nowextinct gymnosperm
group, the Pteridospermopsida (Rothwell & Taylor
Signalling and warfare
Living on land also required a new way of transmit-
ting signals between organisms. Previously, signal-
ling was restricted to water soluble compounds; on
land, volatile compounds had to be developed.
Here, nature has expanded in a myriad of molecules
such as repellents, odours and pheromones. To be
effective, these molecules need to provoke a reac-
tion by the receiver, implying that the compounds
must be biologically active. Often such compounds
are already active at low concentrations. For warfare
this is different; the toxins may remain on the organ-
ism outside or in the cells and tend to be lipid or
water soluble. They need not be transportable by
air. In this case, however, the compounds are also
constructed to be biologically active and interfere
with the physiology of the attacking organism.
Many of these compounds are produced via
the phenylpropanoid pathway which experienced
a huge radiation with the adaptation of plants
to land (Cooper-Driver & Bhattacharya 1998;
Lewis & Davin 1999). Compounds produced via
this pathway function for example, as antioxidants,
have antifungal or antimicrobial properties or are
insecticides, nematocides, antifeedants and poisons.
These include the flavonoids mentioned above
(I, III, IV) and their dimer (II) to polymers [the
latter based on flavan-3-ols and/or flavan-3,4-diols
as building blocks (I)], the proanthocyanidins or
condensed tannins (V) (He et al. 2008). In addition,
these all have a strong influence on soil structure
and composition by retarding organic matter break-
down and capturing nutrients and, through this,
substantially modifying the global carbon cycle
(Kraus et al. 2009).
Condensed tannins appear much later in evol-
ution than the flavonoids (Fig. 2). They occur only
in leptosporangiate ferns, gymnosperms and angios-
perms (Popper & Fry 2004; Popper 2008), leaving a
much smaller impact on the carbon budget for the
early evolution of land plants. Such fossil tannins
are only unequivocally known from brown coal
(Wilson & Hatcher 1988).
Two other groups of tannins exist; neither are
derived from flavonoids (de Leeuw & Largeau
1993). These are the hydrolysable tannins typical
for angiosperms (Okuda et al. 2000) but which
