Travel Reference
In-Depth Information
our own. The start of the Cambrian was like the beginning of a concert after
an unconscionably long period during which the orchestra seems to have
been merely tuning up. The sudden commencement of this full-throated
evolutionary concert was so dramatic that geologists have named it the
Cambrian explosion.
We have a good idea of what the bottom of early Cambrian seas might
have looked like. Shale beds from the Chengjiang area that lies to the south
of Kunming in southern China are fi lled with a wide variety of beautifully
preserved fossils, prevented from decay by sudden underwater landslides.
They have been dated to 525 million years ago, a mere 17 million years after
the start of the Cambrian. Thriving communities of arthropods, mollusks,
worms, chordates, and many other animals covered the bottom. Muck-div-
ers in those shallow seas would have been entertained by this great diversity
of creatures, though because of the lack of smart predators they would prob-
ably not have been as colorful as the creatures of Lembeh today.
The Cambrian explosion and the roots
of animal divergence
The fossil record appears at fi rst blush to show that the diversity of animal
phyla arose rapidly at the start of the Cambrian. But the diversifi cation of
these animals began well before the Cambrian. At Chengjiang it is already
clear that our chordate ancestors and the early mollusks were taking dif er-
ent paths.
A little chordate-like creature called
Cathaymyrus
from Chengjiang is the
earliest animal with ai nity to ourselves that has yet been found anywhere.
But even this early hemichordate was already the proud possessor of gills, a
heart, and a dorsal nerve chord.
The underwater landslides at Chengjiang preserved clusters of
Cathay-
myrus
. These little animals apparently burrowed together in groups in the
mud, like a present-day primitive hemichordate called
Amphioxus
that they
resembled.
