Distribution of Cutan in Modern Leaves - Biopolymers: A Molecular Paleontology Approach - page 18

paleoatmospheric carbon dioxide levels (Royer et al. 2001 ). Underpinning all of this
research is the assumption that either (i) the fossil record of leaves is representative
of ancient plants and vegetation or (ii) the biases in the leaf fossil record are under-
stood. Some aspects of taphonomic bias that result from transport processes or
growth environment are partly understood and can be taken into account through a
detailed understanding of facies associations. For example, high proportions of sten-
ophylls and non-entire margined leaves at streamsides may yield an anomalously
cool climate signal compared to the surrounding vegetation (Liang et al. 2003 ).
The outer cuticle of leaves is made up of the structural biopolymer cutin (primar-
ily C 16 and C 18 hydroxy fatty acids; Kolattukudy 1980 ), which is hydrolysable under
basic conditions, surface waxes (soluble in organic solvents) and, in some cases, the
resistant non hydrolysable aliphatic biopolymer cutan (Nip et al. 1986a , b ). The
outer cuticle protects the internal tissues composed of more labile biopolymers—
polysaccharides, lignin, and proteins. Chemical analyses of fossil leaves and cuti-
cles have shown that although relatively 'younger' fossil material may preserve
carbohydrates and lignin (Briggs et al. 2000 , and references therein) fossil leaves
and cuticles older than the Tertiary very often yield a dominant aliphatic signal (Nip
et al. 1986a , b ; Tegelaar et al. 1991 ; Logan et al. 1993 ; van Bergen et al. 1994 ;
Mösle et al. 1997 , 1998 ; Collinson et al. 1998 ; Stankiewicz et al. 1998b ; Gupta et al.
2007a , b ).
Tegelaar et al. ( 1991 ) argued that the leaf fossil record is biased in favor of leaves
containing the highly resistant, highly aliphatic, non-hydrolysable macromolecule
cutan in their cuticles. Any bias due to the presence or absence of cutan must be
understood if data from fossil leaves are to be applied reliably in multidisciplinary
research.
Cutan is a long chain aliphatic biopolymer resistant to hydrolysis (i.e., an ali-
phatic residue is recovered after base and acid hydrolysis: Nip et al. 1986a , b ;
Tegelaar et al. 1989c ; McKinney et al. 1996 ; Mosle et al. 1997 , 1998 ; Schouten
et al. 1998 ) and was therefore interpreted as a diagenetically stable polymer that can
survive in the fossil record with little chemical change (Nip et al. 1986a , b ; Tegelaar
et al. 1989a ). Its diagenetic stability is due to largely non-hydrolysable ether link-
ages and sterically protected ester functional groups (for structural details refer to
McKinney et al. 1996 ; Schouten et al. 1998 ). Cutan was fi rst described and docu-
mented by Nip et al. ( 1986a , b ) from the leaf cuticle of modern Agave americana , a
monocotyledon fl owering plant. Nip et al. ( 1986a ) reported that the leaf cuticle of
another monocotyledon, Clivia miniata , gave similar results to Agave and also con-
tained cutan. Tegelaar et al. ( 1991 ) reported a characteristic cutan signature (an
aliphatic signal of an extended homologous series of n -alkanes, n -alk-1-enes and
n -alkadienes) in chromatograms generated by Curie-point pyrolysis of isolated
complete cuticles of 11 of 13 modern plants analysed. However, this result was
based on analysis using solely pyrolysis GC-MS, and did not involve isolating the
cutan biopolymer from the modern plants using hydrolytic techniques. Tegelaar
et al. ( 1991 ) equated the aliphatic signal with the material defi ned as non-ester cutin
in maturing Clivia miniata cuticles by Schmidt and Schönherr ( 1982 ) and inter-
preted it as cutan. The apparently widespread occurrence of an aliphatic signal in