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dense accumulation of trilobites, brachio-
pods, and both benthic (bottom-living)
dendroid and planktonic graptoloid
graptolites, but no orthocone nautiloids.
The fauna has been compressed by
compaction of the shale, but the the
trilobites remained three-dimensional
enough while pyritization occurred to
preserve them close to their original shape
in life. The dorsal exoskeleton and
appendages of
Triarthrus
are pyritized, but
not the ventral body cuticle (Whittington
and Almond, 1987). The compaction has
rotated the limbs away from their position in
life, and while all authors since Beecher
have recognized the importance of com-
pression in the taphonomy of the trilobites,
each has imposed a different degree of
retrodeformation in their reconstruction:
Beecher (1893 -1896) unsquashed
Triarthrus
the least, Cisne (1974, 1981)
recognized and took it into account
somewhat, but Whittington and Almond
(1987) used photographs of three-dimen-
sional enrolled specimens of
Triarthrus
beckii
, a close relative of
T. eatoni
(see below),
published by Ross (1979) to get a truer
picture of the convexity of the trilobite in
life.
Briggs
et al
. (1991) studied the
taphonomy of the trilobite bed because
the preservation of soft parts by pyrite is a
rare occurrence in the fossil record. They
took samples from freshly exposed rock
surfaces along a continuous sequence
from about 20 cm (approx. 8 in) below to
about 15 cm (6 in) above the trilobite bed
(
54
) and measured the pyrite sulfur,
reactive iron, and organic carbon content.
In order to determine the relative timing
of formation of the pyrite, the ratio of
sulfur isotopes was measured. During
the reduction of sulfate by microbes,
32
S is
reduced more rapidly than
34
S. Therefore,
sulfate dissolved in water is richer in the
lighter isotope,
32
S, than the heavier
34
S,
and is preferentially incorporated into
pyrite (FeS
2
) until the pore waters in the
sediment become distant from the open
water, e.g. by further accumulation of
sediment. Once the lighter isotope is
unavailable, the sulfate-reducing bacteria
use the heavier isotope. So, the ratio of the
two isotopes (expressed as
34
S) indicates
whether the system was open to diffusion
or closed, and therefore the relative timing
of pyrite formation. The trilobite bed
showed the highest level of the heavier
isotope (+30.7%) in the sequence studied,
indicating that pyrite precipitation
continued there longer than elsewhere.
In order for soft parts to be replaced by
iron pyrite there needs to be a strong
contrast between the chemistry in the
region of the carcass and that of the
surrounding sediment. Briggs
et al
. (1991)
measured low organic carbon
concentrations throughout the sequence
while the amount of reactive iron was
unusually high. Thus, there was insufficient
organic matter to feed sulfate-reducing
microbes and, indeed, the presence of
burrows in the sediments above and below
the trilobite bed and the high reactive iron
content shows that the sea floor was well
oxygenated. So, because of the low organic
matter content, little pyrite was formed until
the influx of the trilobite bed. This brought
in loads of dead trilobites which provided
plenty of organic matter for sulfate-
reducing bacteria to cause the plentiful iron
to diffuse towards the carcasses, react with
sulfur, and produce iron pyrite.
δ
34
S values
for the dorsal exoskeleton and the limbs of
the trilobites differed in that those of the
exoskeleton were always lower in all
specimens of
Triarthrus
measured (Briggs
et
al
., 1991). This was because dissolution of
the calcium carbonate in the dorsal
exoskeleton enhanced the production of
pyrite. The pyritization process continued
while the sediment was being compacted
but eventually stopped and the sediment
compacted further. It is likely to have been
a matter of some months before pyritization
ceased.
δ
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