An Explosion Defused?  By Carl Zimmer|Sunday, December 01, 1996
From Darwin’s time to today, one group of fossils--those of the
early Cambrian Period, beginning 540 million years ago--have
baffled researchers. Before the Cambrian, fossils tell us, there
existed only a few types of simple, blobby animals. Then, within
just 10 million years, almost every major group, or phylum, of
animals appeared. Almost all the major body plans seem to have
sprung into existence--including phyla such as arthropods (which
came to include insects and crustaceans), annelids (leeches and
worms), mollusks (squid and clams), echinoderms (starfish and
sea urchins), and chordates (fish and humans). Now dubbed the
Cambrian explosion, it’s looked upon as the great event in
animal evolution--and, in its suddenness, one of evolution’s
greatest mysteries.  Yet some scientists have long found such an
explosion hard to believe. They share Darwin’s own suspicion
that, for some reason, we haven’t yet found fossils of pre-
Cambrian chordates, a