Beechers Trilobite Bed - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 50

of the nautiloids, but little pyrite elsewhere
outside the trilobite bed (Briggs et al .,
1991). These graptolitic horizons probably
represent long periods of low
sedimentation during which time the
remains of planktonic (floating, e.g.
graptolites) and nektonic (swimming, e.g.
nautiloids) animals accumulated on the
sea floor. It is thought that the pyrite
accumulated in the nautiloids because
there was still some residual, undecayed
organic matter present in their chambers
which triggered the precipitation of pyrite.

The graptolites, on the other hand,
decayed high in the water, so there was
little soft tissue left to decompose by the
time they landed on the sea floor (Briggs
et al ., 1991).
The trilobite body parts which are found
in the shales outside the trilobite bed are
most likely molted exoskeletons, so also
lack soft tissue, but the Triarthrus
specimens in the trilobite bed are com-
plete, not molts. Near the base of the
trilobite bed itself is the so-called trilobite
layer (Cisne, 1973a) ( 54 ) which contains a

53

53 An orthocone nautiloid
from the Walcott
Quarry YPM. Animal is
about 2 cm 0.8 in long.

54
sandstone

54 Stratigraphic
section of Beechers
Quarry after Bottjer
et al., 2001.

siltstone

mudstone

trilobites

brachiopods

burrows
ripple marks

Trilobite Layer