Preservation of Organic Fossils at the Las Hoyas Formation, Spain - Biopolymers: A Molecular Paleontology Approach - page 169

Discussion and Conclusion
The soft tissues of organisms at Las Hoyas may be preserved either as organic
remains or as a result of authigenic mineralization (Briggs et al. 1997 ) and both
may occur in association. The preservation of the eyes of fi sh as a carbonaceous
bacterial fi lm is reported here for the fi rst time although a similar phenomenon
(where bacteria likewise are not mineralized) occurs in the Jurassic ichthyosaurs
from Holzmaden.
The aliphatic component of the organic remains must have been derived from
compounds present in the organism itself. Migration from an external source can be
excluded (Briggs 1999 ). This is supported by the results of thermochemolysis. Both
modern and fossil fi sh scale revealed a very similar saturated fatty acyl distribution
with a maximum abundance of C 16 and C 18 moieties. The predominance of fatty acyl
moieties emphasizes the importance of ester functional groups in crosslinking the
aliphatic polymer in these fossils. Alkanones, which are generally indicative of
ether linkages, are absent. Thus the primary oxygen-containing intermolecular
bonds in the fossil scale macromolecular organic matter may be esters, although
carbon-carbon bonds are also likely to serve as important cross-linkages. The
n -alkyl chains may protect the ester functional group in three dimensions by steric
hindrance, assisting crosslinking.
Evidence for lipid incorporation has also been reported in fossil leaves dinoclasts,
graptolites, eurypterids, shrimp beetles (Briggs 1999 ) and in kerogens. Thus the evidence
indicates that polymerization and incorporation of labile aliphatic components
present in the organism are responsible for generating the aliphatic macromolecular
component in the fossils during diagenesis.
The molecular components in the various fossils show differences in relative
abundance. Aliphatics in the Las Hoyas fi sh range to C 21/22 . The decapod shows
alkyl benzene and phenols as major components, the aliphatics ranging to C 21 .
Py-GC-MS of beetles from the site, on the other hand, revealed only aliphatics,
ranging from C 8 up to C 31 (Boulton 2003 ). This may refl ect the presence of long
chain waxes (greater than C 30 ) in insect cuticle which may contribute to the long
chain n -alkyl component in the fossil. The Montsechia plant shows a predomi-
nantly aliphatic composition (C 9 to C 25 ) with very little aromatic content.
Differences between the fossil taxa may indicate inherent contrasts in the original
labile aliphatic components and their thermal alteration products derived from the
living organism.
The diagenetic history of organic components varies with starting composition,
but also with environmental setting and time (Briggs 1999 ; Briggs et al. 2000 ).
Degree of aromaticity may refl ect thermal maturity. Thus chemosystematic differ-
ences are not consistent from locality to locality. Differences between plants and
arthropods have been reported in cuticles from the Carboniferous of North America,
for example, but here the contrasts are mainly in the distribution of alkenes/alkanes
rather than in the total chain length or the proportion of aromatics. To investigate
fossil chemosystematics further, a comprehensive analysis of comparable material
from different ages and environments is required.