It is a primitive fish-like creature, one of several that live here. Some look like eels, some like hagfish. They are the first chordates, or vertebrates. They are our ancestors.

CAMBRIAN LIFE

The distribution of fossils from the fantastic deposits in Chengjiang, China, has given us a new window into the origin of the animal phyla on Earth. To continue the analogy, this is a window to a floor lower than that of the Burgess Shale. The approximately 12 million years separating the age of these two deposits thus gives us a new view of how animals diversified. Because both Chengjiang and the Burgess preserve soft parts and skeletonized animals, we have a good picture of what was there and in what relative abundance. Without this added view yielded by the preservation of soft parts, we would never be sure about the relative abundance of various kinds of animals, for perhaps there was a huge abundance of creatures like soft worms and jellyfish, forms that had no skeletons. Thus we are surprised at what appears to be a clear view of the nature of the fauna at both sites. Over 50,000 fossils have been collected from the Burgess Shale (and a lesser number from Chengjiang). In their summary of the Burgess fauna, Derek Briggs, Doug Erwin, and Fred Collier, in their 1994 book The Fossils of the Burgess Shale, list a total of 150 species of animals. Almost half are arthropods or arthropod-like. But an even more interesting number relates to the number of individual fossils. Well over 90 percent of all Burgess fossils are arthropods, followed in number by many fewer sponges and brachiopods. Like the earlier Chengjiang, the Burgess sea bottom was dominated in the kinds and numbers of animals by arthropods. Arthropods are among the most complex of all invertebrates and yet in these almost earliest of fossil deposits in the time of animals they are diversified and common.

THE PUZZLE OF THE TRILOBITES AND THE ORIGIN OF SEGMENTATION

Our visit back to the Cambrian leads to an inescapable conclusion: in sheer numbers of individuals and species (described as diversity) and



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement