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with size-frequency distribution graphs,
Cisne concluded that the whole population
was preserved but the juveniles were more
strongly represented because of higher
juvenile mortality than deaths in
adulthood. However, more recent studies
made by Briggs and Edgecombe (1992,
1993) have shown that the protaspids do
not belong to
Triarthrus
but to
Stenoblepharum
, adult remains of which
are extremely rare in Beecher's Trilobite
Bed. So, it appears that
Triarthrus
protaspids lived elsewhere, perhaps in the
plankton, but the protaspids of
Stenoblepharum
were benthic while the
adults of this trilobite were not.
Other fossils
The trilobites in Beecher's Bed come
mostly from the lower few millimetres of
the bed, called the mud assemblage; the
upper part of the bed contains the silt
assemblage which has much rarer trilobites
but a few echinoderms. Graptolites (
60
)
and brachiopods (
61
) are present in both
assemblages.
60
60 The graptolite Climacograptus.
Note the double-row of thecae,
one on each side of the stipe, each
of which would have borne an
individual polyp-like animal in life
YPM. Animal is about 2.5 cm
1 in long.
61
61 Graptolite and orthid
brachiopod YPM. Brachiopod is
about 6 mm 0.2 in wide.
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