Distribution of Cutan in Modern Leaves - Biopolymers: A Molecular Paleontology Approach - page 35

and a similar aliphatic signal has been reported in a Neogene cuticle (Wijninga 1996 )
suggesting that cutan survived into the fossil record (Boom et al. 2005 ). However, the
fossil sample was not identifi ed as Podocarpus , but was one of fi ve unidentifi ed iso-
lated dispersed cuticles (Wijninga 1996 , Fig . 2.4 ) , some of which were found in asso-
ciation with Podocarpus wood. There is no direct evidence for cutan preservation in
fossils. Combined with the lack of cutan in many leaves with diverse fossil records,
the above indicates that selective preservation of cutan is no longer tenable as an
explanation for the highly aliphatic signal found in most leaf fossils.
Migration from sediment —Given the widespread occurrence of aliphatic compo-
nents in sediments, insect, and plant fossils, the occurrence of aliphatic components
in fossil leaves might be attributed to migration (Baas et al. 1995 ; van Bergen et al.
1995 ). This possibility, however, has been countered by several lines of evidence:
(1) Aliphatic polymers are characteristically insoluble, and therefore relatively
immobile (see Briggs 1999 for discussion); (2) An aliphatic signal was detected in
Tertiary Hymenaea leaves trapped in amber (Table 2.2 ), where they are protected
from external contamination (Stankiewicz et al. 1998a ); (3) The aliphatic signatures
in co-occurring plant and insect fossils from the Upper Carboniferous of North
America are different, indicating that they could not have been introduced solely
from the matrix (Stankiewicz et al. 1998b ) and the internal morphology of the cuti-
cle is altered indicating diagenesis; (4) The composition of artifi cially matured
insect tissue is aliphatic (Stankiewicz et al. 2000 ) showing that endogenous organic
matter can generate an aliphatic composition, as observed in fossils; (5)
Thermochemolysis (TMAH assisted pyrolysis: Challinor 1989 , 1991a , b ; de Leeuw
and Baas 1993 ; Martin et al. 1994 ; Almendros et al. 1998 , 1999a ; McKinney et al.
1996 ) of co-occurring insect and plant fossils and the associated organic rich matrix
revealed differences in the distribution of the constituent fatty acyl components
indicating that the aliphatic component of the fossil is endogenously derived (Gupta
et al. 2007b ); (6) Logan et al. ( 1995 ) showed that leaf lipids in the Miocene Clarkia
sediments were concentrated on the leaf surfaces without migrating into the sur-
rounding sediment. Introduction from other sources such as sediment is not tenable
as an explanation for the highly aliphatic composition of leaf fossils.
In-situ polymerisation of labile aliphatics —In the absence of a diagenetically-
stable aliphatic biopolymer in the living relatives, the preservation and aliphatic
character of the fossil leaves cannot be explained by selective preservation.
Migration from an external source can also be excluded. Thus, the aliphatic compo-
sition of the fossil leaves and cuticles (Table 2.2 ) must have been derived endoge-
nously from other compounds present in the leaf tissues.
Pyrolysates of fossil leaves from the Tertiary of the Ardèche showed the domi-
nance of C 16 and C 18 fatty acyl moieties (Gupta et al. 2007a ). Thermochemolysis
released the fatty acyl moieties, ranging in carbon number from C 8 to C 32 with a
predominance of C 16 and C 18 homologues, that form part of the geopolymer. C 16
and C 18 fatty acyl homologues in equivalent modern leaves occur in cutin, phospho-
lipid fatty acids (PLFA) and as triacylglycerides, steryl esters, other complex lipids
and free fatty acids (FA). Thus, polymerisation of labile aliphatic components pres-
ent in the cuticle and internal leaf tissue (cutin, PLFA, FA) during diagenesis is a