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explanation for the similar distribution of acids in leaf fossils before and after
hydrolysis. Crosslinking via other such functional groups as vulcanisation via sulfur
incorporation (Kok et al.
2000
) or crosslinking through ether functional groups and
oxidation could be important in kerogen (Gatellier et al.
1993
; Riboulleau et al.
2001
) and algae (Versteegh et al.
2004
; de Leeuw et al.
2006
). A lack of sulfur-
bearing compounds, however, suggests that sulfur may not have been important in
crosslinking the polymer in eurypterid cuticles, and data suggest that crosslinking is
related instead to a combination of ether and sterically hindered ester bonds.
Such incorporation of lipids via
in situ
polymerization appears to be a major fac-
tor in the preservation of eurypterids. Such a process has been proposed to explain
the preservation of fossil leaves (Gupta et al.
2006b
,
2007b
), graptolites (Gupta
et al.
2006c
) and the experimental maturation of modern arthropod cuticles (Gupta
et al.
2006a
). We have not detected characteristic such bacterial markers as hopanes
in the extractable and hydrolysable fraction of organic fossils (Gupta et al.
2007b
);
thus evidence for incorporation of bacterial lipids is weak.
The aliphatic component in the Carboniferous cuticles from Joggins, Nova
Scotia, Canada, and Lone Star Lake, Kansas, ranged to longer chain compounds
(from C
6
to C
30
, Stankiewicz et al.
1998a
) than the samples analysed here, which did
not exceed C
22
. The Joggins and LoneStarLake samples also yielded phenols and
polyaromatic compounds, indicating a greater degree of aromatisation (Table
7.1
).
This is consistent with the Raman spectra (Fig.
7.5
). Increasing thermal maturity
results in progressively more ordered carbonaceous matter (Beyssac et al.
2002
;
Rahl et al.
2005
). The Gpeak is more prominent relative to the D peak in the analysis
of the adelophthalmid from Joggins than in the eurypterids from the Midcontinent,
indicating greater thermal maturity, which is in turn refl ected in greater aromatiza-
tion (Miknis et al.
1993
). The disordered band is more prominent in
E. lacustris
from Ontario indicating that the thermal maturity of these eurypterid cuticles is
lower than that of the adelophthalmid (Stankiewicz et al.
1998a
), corresponding to
their lower aromatic content. The Raman spectrum for the LoneStarLake scorpion
indicates a thermal maturity intermediate between these two. These contrasts are
echoed in other data on the thermal maturity of the rocks that yielded the cuticles.
While thermal maturity varies within the Cumberland Group, which includes the
Joggins Formation, a vitrinite refl ectance value of 0.66 % R
o
was obtained for the
coal nearest the samples analysed here, indicating a burial temperature of >100°,
and the majority of the deposits lie within the oil window (Mukhopadhyay et al.
2003
). The vitrinite refl ectance value of the Williamsburg Coal, source of the Lone
Star Lake scorpion, is 0.51 % R
o
, and the rank of the coal is high-volatile B bitumi-
nous (Brady and Hatch
1997
), indicating a burial temperature of around 100 °C.
The burial temperature of Silurian strata in southern Ontario, however, from which
most of the eurypterids were collected, is only 60-80 °C, based on the Conodont
Alteration Index (Legall and Barnes
1980
). Raman imagery clearly holds promise
for nondestructive, rapid, and inexpensive microscale characterisation of organic
matter. Relative thermal maturity can also be assessed when vitrinite refl ectance and
measurement of such traditional biomarkers as hopane or sterane stereochemical
ratios and other approaches are diffi cult due to small sample sizes.
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