Mistaken Point - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 15

Mistaken Point

B ACKGROUND : THE FIRST
ANIMALS
The development of multicellularity was a
major step in the evolution of life: it
enabled organisms to grow in size, to
develop organ systems through tissue
differentiation, and led to the plants and
animals we are familiar with today.
Until the middle of the last century it
was thought that rocks older than
Cambrian in age, collectively called
Precambrian, were devoid of fossils of
multicellular creatures. The base of the
Cambrian is clearly marked by the sudden
appearance of shelly fossils - brachiopods,
mollusks, trilobites and sponges for
example - but this posed a difficult
question for Darwin whose theory of
gradual evolution required a long period
of pre-Cambrian diversification of these
varied groups.
The discovery of soft-bodied
organisms similar in appearance to
jellyfish and worms in rocks of late
Precambrian age came as a major
surprise. As Andrew Knoll (2003)
eloquently puts it, in some ways it marked
the realization of Darwin's dream, i.e.
animal life before the Cambrian, but it
also deepened Darwin's dilemma,
because the Precambrian forms seem to
bear little resemblance to anything that
came either before or after. These
peculiar organisms were typically flat

creatures, with a high surface area/body
mass ratio, and have posed more
questions than they have answered. How
they relate to the typical Cambrian
animals, and what happened to them at
the end of the Precambrian are questions
that will be explored in this chapter.
This assemblage of primitive organisms
is often known as the 'Ediacaran fauna'
after their initial discovery in the Ediacara
Hills of South Australia (see Selden and
Nudds, 2004), but similar fossils have
since been found at approximately 30
localities on five continents, notably in the
United Kingdom, Namibia, Ukraine, and
in the White Sea area of Russia. One of
the most informative sections, however, is
the spectacular locality of Mistaken Point,
a wave-swept crag on the southernmost tip
of the Avalon Peninsula in southeast
Newfoundland, Canada, which not only
exposes the thickest Ediacaran
successions anywhere in the world, but
also exhibits complete life assemblages,
sometimes of thousands of individuals,
buried beneath beds of volcanic ash, such
that one can walk across a sea floor
apparently frozen in Ediacaran time.
Here then is preserved a record of that
critical period in the Earth's history,
which falls between the microbial
ecosystems of the Precambrian and the
animal ecosystems of the Phanerozoic.