Molecular Preservation in Graptolites - Biopolymers: A Molecular Paleontology Approach - page 149

Dictyonema peltatum

o

C 18:1 FA

C 16:1 FA

o

C 9 FA

o

o

o

o o

o

o

o

o

o

C 15

m/z 74+87+85+83

C 16 FA

C 12 FA

o

o

C 14 FA

o

o C 18 FA

o

C 7 FA

o o

o

o

o

o

o

o

Retention time

Fig. 9.3 Partial ion chromatogram showing the TMAH/pyrolysis-GC/MS analysis of Dictyonema
peltatum revealing the distribution of fatty acyl moieties in relation to the alkane/alkene homo-
logues highlighted by the mass chromatogram m/z 74 + 87 + 85 + 83. Note the similarity in distribu-
tion of the fatty acyl moieties in the fossils and Rhabdopleura . For key to symbols see Fig. 9.1

The predominance of fatty acyl moieties emphasizes the importance of ester
functional groups in crosslinking the aliphatic polymer in these fossils (Versteegh
et al. 2004 ; Gupta et al. 2007b ). To determine the nature of these bonds the fossils
were hydrolysed in basic conditions and a residue was recovered. The weight of
residue after hydrolysis was similar to that after extraction and thermochemolysis
of this residue yielded fatty acids in distribution similar to that after extraction.
Thus, the aliphatic polymer in the fossils is non-extractable and largely non-hydro-
lysable attesting to its recalcitrant nature. Pyrolysis failed to detect any ketones
(Fig. 9.2 ) suggesting low amounts of ether linkages in the graptolite periderm.
The primary oxygen-containing functional group in the graptolites may be as esters.
The n -alkyl chains may protect the ester functional group in 3-dimensions by steric
hindrance, accounting for their immunity to base hydrolysis (McKinney et al. 1996 ;
Gupta et al. 2007a , b ).

Discussion

The ultrastructure of graptolite periderm suggests that its original composition was
collagen (Towe and Urbanek 1972 ; Crowther and Rickards 1977 ). Collagen, like
other proteins, (Tegelaar et al. 1989 ; de Leeuw and Largeau 1993 ; but see Nguyen