Distribution of Cutan in Modern Leaves - Biopolymers: A Molecular Paleontology Approach - page 34

et al. 2000 ) and Populus (Collinson 1992 ; Mai and Walther 1991 ) leaves are also
infrequent but are known from the Paleogene onwards. Studies of fossil Pinus tend
to focus on cones (Mai 1995 ) but leaves are also recorded in the Paleogene and
Neogene (e.g. Kva

ek and Rember 2000 ; Walther 1999 ). Leaves of Acer , Quercus
and Metasequoia are abundant, diverse and widespread in the fossil record. Acer
and Quercus are particularly well represented in North America, Asia and Europe
from the Oligocene onwards (Prochazka and Buzek 1975 ; Daghlian and Crepet
1983 ; Wolfe and Tanai 1987 ; Kva

č

ek and Walther 1989 , 1998 ; Mai and Walther
1991 ; Mai 1995 ; Tanai 1995 ; Liu et al. 1996 ; Fotyanova 1997 ; Walther 1999 ;
Kva

č

ek and Rember 2000 ; Hably et al. 2000 ). Metasequoia is absent in the
Cainozoic of Europe (Kva

č

ek and Rember 2000 ; LePage et al. 2005 ) but has an
extensive record from the Cretaceous onwards elsewhere in the Northern Hemisphere
(LePage et al. 2005 ; Yang et al. 2005 ). It is important to note that these leaf fossils
include both compressions and impressions. Impression fossils lack original organic
material. If the presence of cutan in the cuticle were an important factor in leaf
preservation leaves lacking cutan might be represented by a predominance of
impression fossils over compression fossils but this is not the case.
The above data indicate that the presence of cutan in the leaf is not a strong pre-
dictor for a particularly abundant, widespread or diverse leaf fossil record of that
taxon, nor is the presence of cutan a prerequisite for such a leaf fossil record. This
does not mean that the presence of cutan in some leaves plays no role in the forma-
tion of fossils; cutan is resistant to a variety of diagenetic reactions and may infl u-
ence leaf preservation. However, the lack of a correlation between the presence of
cutan and a fossil record indicates that a variety of other factors, such as proximity
to a depositional setting, redox conditions in the depositional setting and rates of
burial, are far more important.

č

Explanations for the Aliphatic Component in Fossils

Most fossil leaves and cuticles are characterised by a strong aliphatic signal irre-
spective of plant type, enclosing lithology, depositional environment, locality and
age (Table 2.2 ). The similarity between this aliphatic composition and that of cutan
was one of the main reasons to invoke cutan as a primary control on fossil preserva-
tion (Tegelaar et al. 1991 ). It is now unclear if it was appropriate to place such an
emphasis on the aliphatic composition of fossil organic matter, because the aliphatic
content is generally overestimated when analysed by conventional pyrolysis and
solid state 13 C NMR (e.g. forest soils investigated by Poirier et al. 2000 ). Nonetheless,
it remains critical to identify the source of fossil aliphatic macromolecules in order
to understand the chemical reactions that underpin fossil leaf taphonomy.
Selective preservation —The aliphatic composition of leaf fossils (Table 2.2 ) was
interpreted previously as a direct consequence of decay resistance and selective pres-
ervation of the diagenetically stable aliphatic biopolymer cutan (Nip et al. 1986a , b ;
Tegelaar et al. 1991 ; de Leeuw and Largeau 1993 ). Cutan occurs in modern Podocarpus