Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 153

locality of the Burgess Shale in British Columbia, one can find the
weirdly shaped (and named) wiwaxiids, unknown from Chengjiang.
Looking like an animal with a penchant for punk rock, its spiky 'hairdo'
perfectly matches an organism whose affinities remain enigmatic—it
might be an armoured (polychaete) worm, or perhaps a mollusc, or
perhaps neither.
Among the Chengjiang fossils there is also a little fish-like animal,
called Haikouichthys . The first part of its name is from the locality, near
the town of Haikou in Yunnan where the fossils were first found. The
second part of its name, 'ichthys', means 'fish'. There is a debate among
scientists about whether fossils of Haikouichthys preserve enough
information to say that this is a true fish, but that need not trouble us.
For Haikouichthys does preserve gills, a notochord, a dorsal fin, and a
distinctive head. Small and insignificant in this world of Chengjiang
organisms, it nonetheless shares a common ancestry with all of the
fish living in the seas today, with the blue whale, and also, of course,
with us.
Becoming Complex
If Cambrian marine ecosystems were inherently more complex than
those of the Precambrian, they were also unstable. Much of the ocean's
life remained in a narrow belt on the continental shelves fringing the
ancient continents: the deep oceans remained oxygen-poor and prob-
ably sustained few large multicellular organisms. Cambrian ecosys-
tems were also species-poor by comparison with those of modern
seas. This may have contributed greatly to their vulnerability. Conser-
vation biologists often talk of 'keystone species', a term coined by
Washington University zoologist Robert Paine in a now classic paper
published in 1969. The effect of keystone species on the ecosystems in
which they reside is profound. In marine settings, keystone species
range from top predators, such as sharks, to more humble creatures