Encyclopedia of Evolution.pdf - Online Reading Center

Burgess shale
jointed legs, Anomalocaris is now considered an arthropod.
At more than a meter in length, it was one of the largest
arthropods ever to live, though not as large as some modern
crabs.
• Hallucigenia sparsa. As the name might suggest, when the
fossil was first studied it seemed as strange as a hallucination.
At first, Conway Morris thought that it walked upon
strange stilt-like legs and had tubes coming out of its back.
No such organism exists anywhere today, so Hallucigenia
might have represented an extinct phylum. New discoveries
in China revealed that the tubes were feet, and the stilts
were actually defensive spines. Hallucigenia appears to
have been related to onychophorans.
• Opabinia regalis. This animal also had swimming flaps but
had a long jointed snout with which it apparently ate, and
a cluster of five eyes on its head. Because of its general similarity
to Anomalocaris, Opabinia is also now considered
an arthropod.
• Wiwaxia corrugata. It resembled a flat snail with an armorplated
body that had sharp spines sticking up from it. It
may have had a radula, which is the structure with which
mollusks scrape single-celled algae or other cells from surfaces
and eat them. Because of its similarities to annelids
and to mollusks, it may have been a relative of the common
ancestor of these two groups.
• Odoraia alata was almost completely surrounded by a
large shell, from which two stalked eyes and a tail with
three flukes emerged. It was probably also an arthropod.
The Burgess shale fossil Pikaia gracilens resembled a
modern lancelet, which is an invertebrate chordate. Because
true fishes have been found from the Chengjiang deposits (see
fishes, evolution of), it appears that Pikaia was not part
of the group ancestral to modern vertebrates.
Besides being a window into the tremendous diversity
of marine animal life in the earliest period of multicellular
life on the planet, the Burgess shale fossils also became the
focal point of controversy about the general pattern of evolution.
Most of the scientifically literate public had not heard
of the Burgess shale until paleontologist Stephen Jay Gould
published Wonderful Life, in which he presented numerous
details about it. He interpreted most of the animals that
were difficult to classify as representing phyla of animals that
are now extinct. Life, he said, was experimenting during the
Cambrian period with a great diversity of different structures,
like a bush with profuse branches; what happened in
the Ordovician period and afterward was the extinction of
many of them, and domination by a few of the more familiar
types of animals. Because life during the Cambrian had such
wild diversity, there was no way to tell which of the many
body plans might have survived. If Anomalocaris or Opabi-
nia had survived, and vertebrates had not, what would animal
life look like on the Earth today? There would probably
be no vertebrates, and therefore no humans. Life could have
evolved an almost unlimited number of ways, very few of
which would have produced a world similar to the one that
actually exists.
One of the members of the Whittington group, Simon
Conway Morris, initially thought along similar lines but
ultimately took a very different view not only of the Burgess
shale but of the direction of evolution in particular. As noted
above, he and others at Cambridge have now concluded that
many of the weird and wonderful animals of the Burgess
shale were actually members of animal phyla familiar to scientists
today, such as arthropods. Walcott had not been too
far off, it turns out, for classifying Burgess shale animals into
modern groups. If primitive vertebrates had become extinct,
the terrestrial vertebrates (such as birds and mammals) would
not have evolved, but Conway Morris maintains that something
similar would have evolved from a different ancestor.
Only a limited number of animal structures will work successfully,
and the diversity of animal life on Earth seems to
converge upon them (see convergence).
The Burgess shale animals also give an idea of what
might have caused the Cambrian Explosion. The affinities
of the Ediacaran organisms are unclear, but all researchers
agree that they appeared quite defenseless. The near or complete
extinction of the Ediacaran organisms may have been
caused by the evolution of the first predators. Many of the
Burgess shale animals appear almost outlandishly defended.
In a world without predators, a low diversity of unprotected
organisms may be possible; the advent of predators sparked
an arms race of coevolution between ever more efficient
predators and ever more cleverly defended prey.
If scientists had only the animals with hard parts preserved
from the Cambrian period, they would be able to see
less than one-fifth of the animal diversity that is now known
to have existed. This might have led to some serious errors
in reconstructing evolutionary history. The importance of
the Burgess shale, the Chengjiang deposits, and Sirius Passet
deposits can scarcely be exaggerated.
Further Reading
Conway Morris, Simon. The Crucible of Creation: The Burgess Shale
and the Rise of Animals. New York: Oxford University Press,
1998.
Gon, Sam. “The Anomalocaris homepage.” Available online. URL:
http://www.trilobites.info/anohome.html. Accessed March 23,
2005.
Gould, Stephen Jay. Wonderful Life: The Burgess Shale and the
Nature of History. New York: Norton, 1989.