Reconstruction of Anomalocaris, an enigmatic
arthropod from the Burgess Shale Formation in BC
Taken from Briggs et al. 1994. This and other
forms are part of the vast explosion of new forms
that developed from the Cambrian Explosion.
WAS IT PREDATION?
Several authors including Simon Conway Morris
(1998) have suggested that predation caused
an ‘arms race’ that increased the pace of
evolution: As creatures developed protective
shells predators had to adapt to overcome these
new defensive structures. Late Precambrian
fossils of Cloudina have tiny holes bored into
them suggesting active predation in metazoans
(creatures consisting of more than 1 cell) was
initiated just prior to the Cambrian (Bengtson
and Zhao, 1992).
The Cambrian Explosion is the name given
to the rapid diversification of life that
occurred at the base of the Cambrian
around 540 million years ago. Although
many soft bodied forms existed just prior
to this event, creatures with hard parts and
skeletons suddenly appear at this time
(Ausich and Lane 1992). The question that
remains to be answered is why this
diversification occur then and not earlier.
OXYGEN EXPLANATIONS
In order for creatures to grow large and complex
oxygen must be able to diffuse to all tissues. This
requires a certain concentration of oxygen in the
oceans. Logan et al. (1995) suggest that the
evolution of the metazoan guts and facel pellets
permitted rapid burial of waste material that
would have otherwise used up oxygen as the
waste products decayed in the water column.
The Cambrian diversification of animal life.
Purves et al. 2003
IS OCEAN CHEMISTRY THE KEY?
The chemistry of the Earth’s oceans has
changed many times. A significant event is
recorded just before the base of the Cambrian
(Narbonne et al. 2004). It is possible that this
change may have permitted creatures to
develop hard part skeletons at this time.
However, direct evidence of any relationship has
yet to be proved (Morris 2000).
REBOUND FROM EXTINCTION
Other Authors such as Kerr (2002) believe that
the apparent Cambrian explosion is merely the
radiation of new forms into niches left unoccupied
by creatures following a mass extinction in the
Precambrian.
CONCLUSIONS
It is likely that there is more than one specific
reason for the Cambrian explosion. Biotic
components and abiotic components probably
had a role to play. In addition, other large scale
factors such as paleo-plate configuration and
perhaps unknown astronomical factors may
have had influence on the sudden radiation of
new life forms. Additional data from new Early
Cambrian sections in China may help throw new
light this important event.
REFERENCES
Ausich W. I. and Lane, G. N. 1999. Life of the Past, fourth edition.
Prentis Hall, 321 pp. ISBN 0-13-896069-0.
Briggs, E. G., Erwin. D. H. and Collier, F. J. 1994. The fossils of the
Burgess Shale.Smithsonian, 238 pp. ISBN 1-56098-364-7.
Kerr, R. A. 2002. Evolution: A Trigger for the Cambrian Explosion?
Science, 298: 1547-1547.
Logan G.A., Hayes J.M., Hieshima G.B. and Summons R.E. 1995.
Terminal Proterozoic reorganisation of biogeochemical cycles. Nature,
376: 53-56.
Morris, C. M. 1998 The Crucible of Creation. Oxford Univ. Press, 276
pp. ISBN 0192862022.
Morris, C. M. 2000. The Cambrian "explosion": Slow-fuse or
megatonnage? PNAS, 97(9): 4426-4429
Cloudina fossils
demonstrating borings
Bengston and Zhao, 1992
Trichophycus Pedum trace fossils at
Fortune Head Newfoundland. These
earliest Cambrian traces demonstrate
that metazoans were actively interacting
with the environment at the time of the
Cambrian explosion.
Narbonne, G. M., Kaufman, A. J. and Knoll, A. H. 1994.
Integrated chemostratigraphy and biostratigraphy of the Windermere
Supergroup, northwestern Canada: Implications for Neoproterozoic
correlations and the early evolution of animals. Geological Society of
America Bulletin, 106: 1281-1292
Precambrian Ediacaran Creatures.
Did their extinction permit the Cambrian
Explosion? Image from Morris 2000.
Purves, W. K., Sadava, D., Orians, G. H. and Heller, C. H. 2003,
Life: The Science of Biology, fourth edition. Freeman, 1121 pp. ISBN
0-7167-9856-5.
Runnegar, B. 1992. Evolution of the earliest animals in Schopf J. W.
[editor], Major Events in the History of Life. Jones & Bartlett, 208 pp.
ISBN 0867202688