Mistaken Point - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 26

too removed from Glaessner's 1960s ideas
that many of them were jellyfish or sea-
pens, and that the Ediacaran biota
represented 'the exposed roots of the
metazoan tree of life'. Those few forms
which do not fit into such a phylum
probably can be accommodated in extant
phyla, as has been shown by Jim Gehling
who has identified early sponges,
echinoderms, and mollusks in addition to
the cnidarian-like forms (see page 38).
Any view expressed about the
Ediacaran animals, however, will always
find its opponents, and the microscopic
architecture recently described in
small frondose fossils by Narbonne
(2004), including tertiary branches only
150 microns in diameter (far too small to
accommodate cnidarian polyps), have led
him back to Seilacher's original view that
certainly the fronds and spindles (and
maybe other forms) represent a clade
(the 'rangeomorphs') that is unrelated to
any modern group of organisms. Perhaps
this older clade became extinct in the
terminal Proterozoic in competition
with the early 'cnidarian-like' animals
represented in the younger Ediacaran
assemblages both in Newfoundland,
Australia, and elsewhere; if so, it
represents a failed experiment in the
evolution of multicellularity.
What happened to the Ediacaran
biota after the end of the Precambrian?
Some forms apparently continued into
the Cambrian. For example the frond-
like sea pen, Thaumaptilon , from the
Burgess Shale (Chapter 3), is believed by
Simon Conway Morris (1993) to be
related to Charniodiscus . Moreover,
Ediacaran-like fossils have been
described from strata as young as the
Upper Cambrian from County Wexford
in Ireland (Crimes et al ., 1995). However,
there is little doubt that the majority of
Ediacarans did not survive to the
Phanerozoic. Whether this was the result
of the first mass extinction event, or
whether the Ediacarans were simply
eaten or outcompeted by Cambrian
animals, or whether an increase in

Cambrian bioturbation simply closed this
taphonomic window, will continue to be
debated for some time.

P ALEOECOLOGY OF THE M ISTAKEN
P OINT BIOTA
The Avalonian Terrain, which forms
present-day eastern Newfoundland,
together with England, Wales and parts of
Northern Europe, and which was cleaved
apart in the Mesozoic by the opening of
the Atlantic Ocean, is thought to have
been located adjacent to the Amazonian
Craton during the late Neoproterozoic at
about 40-65°S (Wood et al ., 2003). The
presence of abundant ash layers in the
successions suggests deposition associated
with an arc-related basin.
Although Ediacaran assemblages
from other parts of the world have
been interpreted as marine benthic
communities living in the shallow
euphotic zone between fair-weather wave
base and storm wave base, those from
Mistaken Point seem to have been living
as a deep-water slope biota, well below
storm wave base (i.e., more than 50 m),
on a southeast-facing slope. This
interpretation is based both on the
absence of any wave-generated features
and by the fact that the succession is
dominated by turbidites (graded beds
deposited by downslope currents, with
coarse sands and greywackes fining
upwards into a muddy top). Most workers
have previously suggested a submarine
fan setting, but Wood et al . (2003) argue
that the Drook and Briscal formations
accumulated in a basin-floor axial
turbidite system, while the finer-grained
Mistaken Point, Trepassey, and Fermeuse
formations formed on the marginal
slopes of such a system.
The lack of evidence of uprooting of
the tethered fronds or of damage to other
soft-bodied forms, as might be expected
in a turbidity flow, suggests that this
assemblage lived, died, and was preserved
in situ near the base of the slope at a
depth of several hundreds of metres. Such
an untransported census population of