Travel Reference
In-Depth Information
Mollusks too were clearly dif erentiated into their own lineage by the
start of the Cambrian. Millions of tiny mollusk shells, mixed with the hard
parts of other organisms, are found in the very earliest Cambrian sedimen-
tary rocks. The almost overnight suddenness of their appearance is astound-
ing. Andrew Knoll and his colleagues have found beds of shale in southern
China dated just nine million years before the Cambrian that are empty of
such fossils, and contain only traces of a few simple algae. Then, nine million
years later, these “small shelly fossils” suddenly appeared around the world.
We have few clues about the details of the bodies of the tiny creatures that
inhabited these little shells. But the better-preserved fossils from Chengjiang
show that 25 million years later mollusks were as advanced in their own way
as
Haikouella
. Some of them resembled present-day clams. Others were more
like present-day nudibranchs, using toothed mouth parts called radulas to
scrub tiny organisms from rocks. The radulas are clearly preserved in many
of these fossils.
Some of these early snail-like animals, unlike the nudibranchs, were
wildly armored. One of the strangest was the Cambrian creature called
Wiwaxia
. This animal was a total mystery to geologist Charles Walcott, who
found the fi rst complete specimens in 1911 in Canadian shale deposits that
were laid down twenty million years later than Chengjiang.
Wiwaxia
was
oval-shaped, covered with armored plates, and decorated with twin rows of
fl attened spines that jutted up vertically. It looks like a helmet suitable for a
punk rock singer. Walcott thought at fi rst that it must have been a strangely
armored marine worm, and later investigators put it into a totally new phy-
lum. But close examination by Simon Conway Morris and others eventually
revealed that
Wiwaxia
had a radula-like pair of feeding structures. Argument
continues, but it seems likely that the previously mystifying
Wiwaxia
is a kind
of primitive and well-armored mollusk.
12
If these little fi ve-centimeter-long animals crawled along the bottom and
scraped their food from rocks, like present-day nudibranchs, then why did
they need such elaborate armor? For protection, it seems. There are signs
that some
Wiwaxia
shells may have been crushed and damaged by preda-
tors before they were buried and fossilized. There were some formidable
predators in those Cambrian seas, especially the “fi erce crab”
Anomalocaris
.
