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and Harvey 1998 ) is decay-prone, except where it has undergone substantial
cross-linking, as in the jaws of polychaetes (Briggs and Kear 1993 ). Protein and
polysaccharide remnants have been shown to survive in archaeological plant
remains 1,400 years old (Bland et al. 1998 ), in weevil samples as old as 24.7 m.y.
(Stankiewicz et al. 1997b ; Gupta et al. 2007a ) and even in kerogen 140 million years
old (Mongenot et al. 2001 ) where preservation of labile moieties was facilitated by
encapsulation within a resistant aliphatic matrix. The presence of diketodipyrrole
and pyrimidine in the periderm of Rhabdopleura is diagnostic of collagen
(Stankiewicz et al. 1997a ), but these moieties are absent in the periderm of grapto-
lites. Thus, the ultrastructure may refl ect the original composition even though the
molecular components have been transformed to an aliphatic polymer.
The phenols in the graptolite periderm are likely the product of diagenesis of
aromatic structures in the fossil, and not derived from original amino acids. Although
encapsulation may promote protein preservation by steric protection of labile
compounds (Knicker et al. 2001 ; Mongenot et al. 2001 ; Riboulleau et al. 2002 ), no
nitrogen-bearing compounds were detected in the pyrolysates. Thus proteins,
including collagen, appear not to have been preserved within the resistant aliphatic
matrix that makes up graptolites, and it is likely that protein moieties do not survive
in Early Palaeozoic fossils.
Analysis of the periderm of Rhabdopleura confi rmed that it is proteinaceous in
composition and contains no resistant aliphatic components (Briggs et al. 1995 ).
The graptolites reveal a composition with a dominant aliphatic component similar
to Type II kerogen [Bustin et al. ( 1989 ) noted that the hydrogen and oxygen indices
of graptolite periderm (as determined by Rock-Eval pyrolysis) were similar to
Type II kerogen]. Although sulphur-bearing compounds were not detected during
pyrolysis, such components were detected in the pyrolysate of Monograptus and
Amphigraptus in a previous study (Briggs et al. 1995 ) due to diagenetic incorpora-
tion of inorganic sulphur species under anoxic conditions in their samples; such
conditions were likely absent in our samples. This facilitates further cross-linking
of the macromolecule, as is often observed in natural vulcanization of kerogen
(e.g. Kok et al. 2000 ).
In the absence of a diagenetically-stable aliphatic biopolymer in the living
relative, the preservation and aliphatic character of the graptolites cannot be
explained by selective preservation. The organic content of the sediment differs
from that of the fossils, so migration from an external source can be excluded. Thus,
the aliphatic component probably has been derived from compounds present in the
organism itself. This is supported by the molecular structure. Thermochemolysis
of modern Rhabdopleura and the graptolites investigated revealed a very similar
saturated fatty acyl distribution ranging from C 7 to C 18 with a maximum abundance
of C 16 and C 18 moieties (Table 9.1 ). The unsaturated fatty acyl components
(e.g., C 16:1 and C 18:1 ), on the other hand, are more abundant in Rhabdopleura than in
the fossils, suggesting there was loss of unsaturated compounds during diagenesis
(Wakeham et al. 1984 , 1997 ).
Preservation of graptolites involves transformation of labile aliphatic compo-
nents (such as fatty acids) into a recalcitrant crosslinked polymer with a dominant
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