Geoscience Reference
In-Depth Information
levels after 3 billion years of
photosynthesis by cyanobacteria and later
by plants may have allowed the evolution
of more mobile and larger, complex
animals which were able to exploit empty
niches significantly devoid of
competitors. Alternatively, the
emergence of the first predators may
have initiated the first 'arms race', in
which animals had either to escape by
moving faster and/or getting bigger or to
defend themselves by developing hard
shells. Changes in continental
configuration (and therefore of ocean
currents) have also been suggested. Or
maybe genetic mechanisms were simply
more flexible at that time, leading to
accelerated diversification.
Much of our knowledge of the fauna
and flora during the Cambrian Explosion
(called 'the crucible of creation' by
Conway Morris, 1998) is gleaned from
perhaps the best known of all Fossil-
Lagerstätten - the Burgess Shale of British
Columbia in Canada. By an accident of
geological history, this thin layer of shale
has provided a window into the richness
of a Middle Cambrian marine ecosystem
at a most vital time in the evolution of life
on Earth. Here, by a process still not
completely understood, decay was
arrested so that the complete diversity of
the Cambrian seas, including many soft-
bodied animals (with their internal
organs and muscles) has been exquisitely
preserved.
These are the 'weird wonders'
popularized by Stephen Jay Gould
(1989) in his topic
Wonderful Life
. And if
one considers that approximately 85%
of Burgess Shale genera are entirely soft-
bodied and thus absent from other
Cambrian assemblages, it becomes
apparent just how misleading the fossil
record would be had this particular
Lagerstätte not been preserved or
discovered.
H
ISTORY OF DISCOVERY OF THE
B
URGESS
S
HALE
It was an American, Charles Doolittle
Walcott, then Secretary of the Smithsonian
Institution in Washington DC, who first
discovered the Burgess Shale, high in the
Canadian Rockies (
28
). The romantic
story suggests that at the end of the 1909
field season his wife's horse, descending
the steep Packhorse Trail that leads off the
ridge between Mount Wapta and Mount
Field, in what is now the Yoho National
Park, stumbled on a boulder. Walcott
dismounted to clear the track and on
splitting the offending boulder revealed a
fine specimen of the soft-bodied 'lace
crab',
Marrella
, glistening as a silvery film
on the black shale.
Unfortunately his diary does not
support this story, but certainly Walcott
discovered the first fossils in September
1909 and began excavating in earnest
during the summer of 1910. Annual field
seasons with his family continued until
1913 and he returned in 1917, 1919, and
1924. By the time of his death in 1927 he
had amassed a collection of 65,000
specimens, which were transported back
to Washington DC, where they remain
today in the Smithsonian Institution.
Search WWH ::
Custom Search