Molecular Decay of Plant Biopolymers - Biopolymers: A Molecular Paleontology Approach - page 13

environmental samples 1 year old or more. This result differs from those discussed
above (Opsahl and Benner 1995 ). A number of factors may account for this dis-
crepancy, including both biotoc and abiotic as well as environmental and preserva-
tion conditions. Additionally, earlier work used chemolysis rather than pyrolysis to
analyse the samples. As cuticles constitute a multi-layered structure with a diverse
chemistry, a chemical degradation approach targeted at specifi c bipolymeric moi-
eties may not be adequate to rank the preservational potential of each category of
biomolecules in a single chemolytic protocol. In addition to the different analytical
techniques employed and different natural setting (i.e., marine vs. lacustrine), the
discrepancy may refl ect differences in the duration of decay. Using HR-MAS NMR
(high resolution- magic angle spinning- nuclear magnetic spectroscopy) spectros-
copy, Kelleher et al. ( 2006 ) documented an increase in the proportion of aliphatic
cutin components of pine needles during the fi rst 10 weeks of their natural decay
when this concentration plateaued at a high level throughout the experiment. Thus,
it appears that cutin of plant cuticles survive well at least during the early states of
transformation as our new data indicate. If lignin were more robust than cutin, a
fossil leaf should yield a primarily aromatic signal from biopolymeric and diage-
netically altered lignin. An aliphatic signal should not dominate after the loss of
cutin. However, as discussed above, most fossil leaves reveal a signifi cant aliphatic
content. The results of our study may explain why fossil cuticles are often pre-
served with a ubiquitous aliphatic composition, even where internal tissue has
degraded. Even in examples where no cuticular material is evident under SEM, an
aliphatic composition is still evident in fossil leaves due to lipid incorporation of
internal tissue material from cellular membrane components (Gupta et al. 2007c ).
Moreover, Gupta and Pancost ( 2004 ) observed that internal tissues, despite being
protected by the cuticle, are susceptible to decay as a result of microbial entry
through stomata.
Analyses of modern Metasequoia leaves have revealed the presence of the struc-
tural polyester cutin, guaiacyl lignin units and polysaccharides as the primary bio-
molecules. Metasequoia leaves that have undergone natural decay at a lake-sediment
setting to various degrees over a year long period revealed that guaiacyl lignin units
and cellulose had degraded more relative to cutin. These data suggest that cutin and
cuticular components (and their diagenetically altered products) are likely more
stable than both lignin and cellulose during early diagenesis. Electron microscopy
revealed changes in the cellular structure and cuticle of the modern and decayed
leaves and confi rmed there was no obvious change in the epidermis and covering
cuticular membrane. In addition, arm palisade cells and sponge cells largely col-
lapsed, the vascular bundle had largely decayed and the cell wall in the xylem had
thinned after a year of decay. Fossil leaf of Metasequoia from the Miocene Clarkia
locality revealed that the internal tissue, including the epidermis, has decayed, but
the cuticular membrane was still clearly evident, supporting the preservation
insights provided by the decay experiment.
Analysis of Tertiary Metasequoia fossils from the Eocene of Republic
(Washington State) showed a signifi cant aliphatic component without detection of
biopolymeric lignin and polysaccharides. Fossils from the Eocene of Axel Heiberg