Geoscience Reference
In-Depth Information
The Burgess
Shale
B
ACKGROUND
:
THE
C
AMBRIAN
E
XPLOSION
Although multicellular animals had only
appeared at the very end of the
Precambrian (Chapter 2), their evolution
at the beginning of the Cambrian was
so rapid that this event is known as the
'Cambrian Explosion'. In an astonishing
orgy of evolution, a period of little more
than 10 million years at the beginning of
the Cambrian saw the appearance of
almost every animal phylum and body
plan known today, along with some
other bizarre forms which soon became
extinct, suggesting that this was an
experimental phase of evolution.
Approximately 35 different animal phyla
are known today; in the Cambrian seas
there were undoubtedly several more and
some authorities would claim up to 100.
The sudden appearance of this diverse
fauna within the geological record has
long posed perplexing questions. Darwin
supposed that these phyla had been
gradually evolving throughout the
Precambrian, but had simply not
been preserved as they were entirely soft-
bodied. Perhaps they simultaneously
acquired preservable hard shells or
skeletons at the onset of the Cambrian
(in response to a critical change in
atmospheric oxygen or oceanic chemistry)
so that the Cambrian Explosion was no
more than an artefact of preservation?
However, the discovery of the
Precambrian Ediacaran biota (Chapter 2)
in the latter half of the twentieth century
showed that while soft-bodied animals did
exist at the very end of the Precambrian,
these were mostly primitive annelids and
cnidarians and were unlikely ancestors for
the characteristic Cambrian animals such as
archaeocyathids, brachiopods, trilobites,
and mollusks. Moreover, even soft-bodied
animals should leave trace fossils, which are
also absent from all but the latest
Precambrian sediments.
The first wave of the evolutionary
explosion is evident in the basal Cambrian
Tommotian Stage with the sudden
appearance of many small shelly fossils.
It may be, as discussed by Fortey (1997)
and Conway Morris (1998), that these
did have soft-bodied ancestors with a
long Precambrian history, but which
were so small that neither their bodies nor
their traces would be preserved. Even so it
is unlikely that such tiny animals, often only
a few millimetres in length, were equipped
with the huge range of body plans that were
evident later in the Cambrian; perhaps
these 'small shellies', as they are known, are
simply the hard spikes or spines of soft-
skinned animals.
Very soon after their appearance the
main burst of evolution began and a
variety of possible triggers has been put
forward. Gradually increasing oxygen
Search WWH ::
Custom Search