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shows these features, as well as internal
ripples and laminae which are known in
microturbidites (
63
).
Cisne (1973) studied the orientation of
the fossils in the trilobite bed (
64
) and
showed that the carcasses were equally
divided between those which lay dorsal-
side up and those ventral-up. The carcasses
were strongly aligned in a single direction:
the long axes of the trilobites were mostly
parallel to each other, but the head could
be oriented either way (
64
). This means
that the trilobites were laid down in a
current which oriented them parallel to its
direction of flow. Being dead animals,
there was no difference in stability whether
they were dorsal- or ventral-up; had they
been only dorsal exoskeletons, they would
have been more stable dorsal-side up (i.e.
more easily flipped over if ventral-up) and
more easily disarticulated.
So we can envisage the mud assemblage
fauna consisting of benthic trilobites,
with
Triarthrus
as the commonest,
some
Cryptolithus
, and protaspids of
Stenoblepharum
.
Triarthrus
most likely fed
on organic matter on the sea floor: a
scavenger and predator, digging into the
mud and passing food forward with its
coxae to the mouth at the front.
Cryptolithus
, too, was a bottom-feeder. In
the water column there were planktonic
graptolites and nektonic orthocone
nautiloids, as well as possibly some swim-
63
63 Cross-section through the
trilobite layer showing cross-
bedding and a burrow
bottom left YPM.
Thickness 3 cm 1.2 in.
64
64 Plan view of slab of trilobite
layer showing orientation of
Triarthrustrilobites and
Climacograptusgraptolites.
Stippled trilobites indicate dorsal
side uppermost, black indicates
ventral-up. Rose diagram to
right shows the orientation of
fossils. Note that there is a
preferred orientation aligned
with the topbottom of the
page, and that there is no
preferred way-up about half of
the trilobites are ventral-up, half
dorsal-up. Average trilobite
length about 3 cm 1.2 in. After
Cisne 1973a.
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