Molecular Decay of Plant Biopolymers - Biopolymers: A Molecular Paleontology Approach - page 9

Fig. 1.5 SEM photograph of transverse section of a common compression fossil leaf of
Metasequoia collected from the Clarkia Miocene deposit, showing the presence of external cuticu-
lar membrane ( arrowheads ) although all other internal tissue has decayed. Scale bar 6 μ m

material beneath the adaxial epidermis. Sponge cells had also collapsed. The vascular
bundle has largely decayed (see also Fig. 1.4c ). The parenchymatous bundle sheath
showed the least change: almost all the cells were preserved, but the cell wall had
thinned (compare with Figs. 1.3a , 1.4a-b ). Cells in the xylem had not collapsed
much but the cell wall had thinned. Cells in the phloem had collapsed, causing the
phloem to appear more compressed (see also Fig. 1.4b ). Sclerenchymatous cells
were not evident. The resin canals were more or less unaltered but the surrounding
cells were crushed. A common compression fossil leaf of Metasequoia from the
Miocene Clarkia locality was examined under SEM (Fig. 1.5 ) for comparison.
It shows that the internal tissue, including the epidermis, has decayed, but the
cuticular membrane is still clearly evident.

Chemistry of Fossil Tissue

The fossil Metasequoia from Clarkia (Fig. 1.6a ) did not yield any polysaccharides
including levoglucosan, but revealed the presence of lignin, alkyl phenols, vinyl
phenol, benzene, pristene and an aliphatic component up to C 27. Analysis of fossil
Metasequoia from the Eocene of Axel Heiberg (Fig. 1.6b ) detected the presence of
biopolymeric lignin, levoglucosan from cellulose and vinyl phenol, benzene,
pristene, alkyl phenol derivatives and an aliphatic component up to C 29 . Pyrolysis of
fossil Metasequoia from the Middle Eocene of Republic (Fig. 1.6c ) revealed no
trace of biopolymeric lignin or polysaccharides, but yielded benzene derivatives,
alkyl phenols and an n- alkyl component consisting of alkane and alkene homo-
logues up to C 23 . Thus, n -alkyl components indicate the occurrence of a signifi cant
long chain aliphatic geopolymer in the fossil leaves that is not evident in the modern
sample, indicating that it is a product of diagenesis.