Molecular Preservation in Graptolites - Biopolymers: A Molecular Paleontology Approach - page 146

external source such as the matrix. This study also provides a further test of the
model of in situ polymerization, as the original composition of graptolite periderm
differs from that of arthropod cuticles and leaves but fossils of all three are com-
posed of dominantly aliphatic macromolecular material.
Methods
The graptolites investigated were selected from the Yale Peabody Museum collec-
tions to provide stratigraphic coverage from the Early Ordovician to the Silurian
and a variety of genera from different localities (Table 9.1 ). They were either on the
rock (e.g., Paleodictyota anastomotica ), in which case samples were scraped from
the surface, or they had been released from the matrix by acid digestion and stored
in glass vials in glycerin (e.g., Dictyonema peltatum ). Samples were extracted ultra-
sonically three times, 15 min each with 2:1- CH 2 Cl 2 (dichloromethane):CH 3 OH
(methanol), to yield an insoluble residue. This residue was analysed by Pyrolysis-
Gas Chromatography/Mass Spectrometry (Py-GC/MS) to reveal the molecular
distribution of compounds. Samples of selected graptolites were subjected to
thermochemolysis (de Leeuw and Baas 1993 ) to permit further structural analysis
of the macromolecule. Graptolites were hydrolysed in 1 M 95 % methanoic NaOH
(saponifi cation) for 1 h at 70 °C to yield a non-hydrolysable residue in order to test
the resistance of the macromolecule to base hydrolysis. Py-GC/MS was carried out
on the periderm of modern Rhabdopleura before and after solvent extraction to
evaluate the molecular distribution in the presence and absence of lipids. Further
structural analysis of Rhabdopleura was carried out by subjecting samples to
thermochemolysis without solvent extraction to reveal the complete molecular
distribution including lipids.
Solvent-extracted graptolite and Rhabdopleura periderm were analysed using
py-GC/MS for analysis of macromolecular compsition, (see Gupta and Pancost 2004
for parameters), and compounds were identifi ed using spectra reported in the litera-
ture (Reeves and Francis 1998 ; Stankiewicz et al. 1997a ). For thermochemolysis, the
extracted residues were transferred to a fresh vial and 1 ml TMAH (tetramethylam-
monium hydroxide) solution was added. They were soaked in TMAH solution for
3-4 h prior to analysis to ensure that suffi cient TMAH was available during on-line
pyrolysis. Thermochemolysis cleaves ester bonds to release constituent fatty acyl
moieties (Challinor 1991a , b ; de Leeuw and Baas 1993 ). For a discussion of the
behaviour of protein compounds under TMAH conditions see Zhang et al. ( 2001 ).
Results
The major pyrolysis products of Rhabdopleura periderm (without lipid/solvent
extraction) included phenols, indoles, pyrimidine, diketodipyrrole and diketopiper-
azine derivatives (Fig. 9.1a , Table 9.1 ). The abundance of diketodipyrrole, a marker