Geoscience Reference
In-Depth Information
evidence of land plants is indirectly documented by
cryptospores (Strother 2000). Taylor & Strother
(2008) describe Middle Cambrian palynomorphs
which morphologically and ultrastructurally resem-
ble cryptospores in many ways. However, their
affinity with terrestrial or algal organisms is still
under discussion; the earliest accepted occurrence
of cryptospores is of Llanvirn age (middle Ordovi-
cian) (Strother et al. 1996).
The earliest 'megafossils' are documented in the
late Llandovery (Lower Silurian) (Wellman & Gray
2000; Edwards 2001) (Fig. 1). These plants were
probably small (Wellman et al. 2003) and had no
water conducting organs such as trachea or roots.
In this respect, they were like the modern Bryophyta
which are the most primitive land plants living
today. According to Proctor (2000), many of the
Bryophyta have three water components: symplast
water (in the cells), apoplast water (in the 'free
space' of the cell walls) and external capillary
water which, for much of the time, exceeds the
symplast water by a large but variable quantity.
For these species, water and nutrient transport is
therefore largely via the outside of the plant. This
implies that for these very primitive plants, resist-
ance to desiccation is mostly achieved by drought
tolerance (although this seems not to be the case
for their spores).
However, not all bryophytes rely on external
capillary water. Most thalloid liverworts and some
erect growing mosses with waxy water repellent
surfaces (e.g. many Polytrichaceae and Mniaceae)
rely predominantly on internal water conduction
(Proctor 2000), so that these organisms develop
external coverings which function as water barriers.
With few exceptions, lipids associated with
external coverings are the principal water barrier
component of land plants and animals (Hadley
1989). For plants, these lipids consist primarily of
unbranched fully saturated linear hydrocarbon back-
bones with chain lengths of 20-40 carbon atoms.
Typically these lipids are n-alkanes, n-ketones and
secondary alcohols with a predominance of odd-
numbered chain lengths and primary n-aldehydes,
n-alcohols and n-fatty acids with a predominance
of even-numbered chain lengths. Furthermore, a
wide variety of C
36
-C
60
wax-esters has been
detected. These aliphatic coatings are responsible
for .99% of the water barrier efficiency in plants
(Sch¨nherr 1976; M
´
rida et al. 1981; Jetter et al.
2006). It is therefore no surprise that pollen are
also covered with surface waxes (Piffanelli et al.
1998; Schulz et al. 2000).
For arthropods (earliest evidence: late Cambrian;
McNaughton et al. 2002) the strategy towards desic-
cation is similar to that of plants. Cuticular lipids are
to a large extent identical but mono- and di-methyl
alkanes also occur and may even dominate; for
these organisms, they are also the primary passive
barrier to evaporative water loss (e.g. Ramsay
1935; Hadley 1989; Gibbs 1998, 2002). Long-chain
lipids are also synthesized by various algae such as
long-chain mid-chain diols by Eustigmatophyta
and diatoms (de Leeuw et al. 1981; Versteegh et al.
2000; Sinninghe Damst
´
et al. 2003) long
chain alkenones and alkenoates by Haptophyta (de
Leeuw et al. 1980; Volkman et al. 1980; Rontani
et al. 2007) or long-chain polyunsaturated alkenes
by dinoflagellates (Mansour et al. 1999). However,
the odd predominance of n-alkanes is a typical
feature of land plants, though some microalgae
such as Tetrahedron (Chlorophyta) or Nannochlor-
opsis (Eustigmatophyta) also produce long-chain
n-alkanes with a strong odd predominance (Gelpi
et al. 1970; Gelin et al. 1997). Other algae also
produce long-chain n-alkenes with odd predomi-
nance (Gelpi et al. 1970; see also Volkman et al.
1998) and it has been suggested that such alkadienes
from the green alga Botryococcus braunii have given
rise to an n-alkane odd-predominance in sediments
(Lichtfouse et al. 1994; Riboulleau et al. 2007).
Since the odd predominance of long chain
n-alkanes already occurs in the cuticular waxes of
primitive plants such as liverworts (Matsuo et al.
1974) and mosses (Nissinen & Sew´n 1994), it is
interesting to investigate how this feature developed
through the Palaeozoic. Amarked oddpredominance
of n-alkanes with a maximum in C
23
or C
25
is
observed in the extracts of early mature to mature
coals of Permian age from Brazil and Australia
(Casareo et al. 1996; Silva & Kalkreuth 2005). Con-
trastingly, the n-alkane distribution in the extracts
of many coals of Carboniferous age is not character-
ized by a strong predominance of odd-numbered
compounds; this feature can be ascribed to thematur-
ity of the studied coal samples. However, even early
mature coal samples only show a moderate odd
predominance in the C
25
-C
31
range (e.g. Powell
et al. 1976; Christiansen et al. 1989; Dzou et al.
1995; Fleck et al. 2001; Armstroff 2004).
An exception is the marked predominance in
the C
25
-C
33
range observed in the extracts of very
immature early Carboniferous coals from the
Moscow Basin (Armstroff 2004). The long-chain
predominance is also visible in some Devonian
samples, in particular when (mio)spores are
present in significant amounts: C
25
-C
33
(max C
25
)
in the immature Fammenian marls from Poland
(Marynowski & Filipiak 2007); C
23
-C
31
(max
C
27
) in the middle-late Frasnian early mature
cannel coals from Melville Island, Arctic Canada
(Fowler et al. 1991); C
23
-C
29
(max C
25
) in early
mature Middle Devonian cutinite-rich humic coals
from China containing Zosterophyllum remains
(Sheng et al. 1992). These observations indicate
that
the characteristic signature of epicuticular
