Travel Reference
In-Depth Information
Figure 20
The mysterious
Wiwaxia
, which turns out to be a bottom-crawling armored mollusk.
© Royal Ontario Museum. Photo: J. B. Caron
Possibly
Wiwaxia
had to resort to armor because it did not have the chemical
defenses available to present-day nudibranchs. If, as seems likely, the preda-
tors of the time were too stupid to remember which of their prey tasted bad,
then chemical defenses would have been useless in any case.
Mollusks may be the exception to the rule that nothing resembling pres-
ent-day animals left fossil traces before the Cambrian. A small fl at animal
called
Kimberella
, named after western Australia's ancient Kimberley Range,
lived during the Ediacaran period twenty or thirty million years before the
start of the Cambrian. There is growing evidence that this puzzling organism
had a radula. And it appears to have left scratch marks behind it as it moved
across the sea bottom to feed.
13
The circumstantial evidence is now overwhelming that our ancestors and
those of the mollusks had already parted company long before the beginning
of the Cambrian. But we do not have the smoking gun—the fossil record of
that early divergence has not yet been identifi ed.
