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siltstones), but the Mistaken Point
Formation at the top of this group
represents a transition to the thinner-
bedded and finer-grained shales and
sandstones of the overlying St John's
Group. Although this succession is
interpreted as upwardly shallowing, it is all
considered to be a deep-water regime. The
overlying Signal Hill Group consists mainly
of deltaic and fluvial deposits and thus
continues this upward shallowing trend.
Ediacaran-type fossils range from the
upper part of the Drook Formation in
the Conception Group, through to the
top of the Fermeuse Formation in the
St John's Group, and span some 3 km
(1.9 miles) of strata ( 18 ). Fossils are
known from over 100 different bedding
surfaces and many of these horizons are
overlain by thin volcanic tuffs, which have
allowed accurate uranium/lead (U/Pb)
radiometric dating of included zircon
crystals. The most richly populated fossil
bed, which occurs in the Mistaken Point
Formation, has been dated at 565 million
years, while an ash bed in the upper
Drook Formation has yielded an age of
575 million years, making these the
oldest accurately dated Ediacaran fossils
in the world, and therefore the oldest
known animal communities.
The presence of these tuffs in the
Mistaken Point Formation also has an
important bearing on the taphonomy of
the fossils, which shows marked
differences to the preservation of
Ediacaran assemblages in other parts of
the world where they are preserved on
the soles of sandstone event beds (see
Selden and Nudds, 2004, Figure 5). Here,
on the other hand, specimens are
preserved on the muddy tops of turbidite
beds underneath volcanic tuffs,
suggesting a smothering of benthic life
assemblages by episodic ash falls. Not
only did this ensure that populations
were preserved intact, but the fine ash
also preserved the detailed morphology
of these soft-bodied organisms. Such
'death masks' in the overlying ash tend to
mould the more resistant fronds and to
cast the easily collapsed spindle-shaped
organisms. Morphological detail is
unfortunately limited by the grain size of
the tuff and some forms can often only be
described in terms of gross morphology.
In the Fermeuse Formation, however, the
fossils are associated with thin sandstone
beds in the preservational style seen in
Ediacaran assemblages in other parts of
the world. Some unusual specimens have
recently been discovered by Guy
Narbonne (2004) in the Trepassey
Formation, which are preserved as
uncompressed external moulds and
which exhibit micron-scale features
including previously unseen internal
structures (see p. 37).
D ESCRIPTION OF THE M ISTAKEN
P OINT BIOTA
The diverse fauna of up to 30 species
(most of which have yet to be given formal
names) differs from Ediacaran faunas
elsewhere in the world in that only a few
of the taxa are cosmopolitan (e.g.
Aspidella , Charnia , Charniodiscus ).
Others are known from equivalent strata
at the classic locality of Charnwood Forest
in England (e.g. Bradgatia and Ivesia ),
but most are endemic and restricted to
the Mistaken Point assemblages.
Moreover, the well-known and ubiquitous
Ediacaran genera such as Tribrachidium ,
Dickinsonia , Spriggina , and Kimberella
(see Selden and Nudds, 2004) are
unknown from this locality. Two distinct
assemblages occur: the diverse Mistaken
Point Assemblage consists mainly of
fronds and spindle-shaped forms, while
the younger and less diverse Fermeuse
Assemblage contains mostly small
discoidal forms ( 18 ).
Aspidella
This prolific fossil ranges from the Briscal
Formation to the top of the Fermeuse
Formation ( 18 ) where it reaches its zenith
literally in tens of thousands of individuals.
It was the first fossil to be described from
this assemblage (Billings, 1872) and is
typically a small disc (5 mm [0.2 in]
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