PRE-CAMBRIAN AND PALEOZOIC ERAS 



149 



oippendoige 



FIG. 8.3. Trilobite anatomy. (After Beecher.) 



lies in the fact that they combine characteristics of arthropods with those 

 of annehd worms. Thus they strengthen the evidence that arthropods 

 evolved from annelids. Hence the finding of onychophorans living as con- 

 temporaries of the first arthropods (trilobites) is of much interest. 



Thanks to the rare fortune of the Burgess shale fossils we know that the 

 Cambrian seas supported a wealth of invertebrate life of kinds not ordi- 

 narily preserved as fossils. Perhaps we should have inferred that this world 

 of soft-bodied marine animals existed, but certainty is more satisfying than 

 inference. 



In conclusion we note two general facts about Cambrian animals. They 

 all lived in the ocean; none were land dwellers. They were all inverte- 

 brates; no representatives of Phylum Chordata, comprised of the verte- 

 brates and their kin, were present at this stage in the world's history. 



The Cambrian was a very long period, even for a geologic period. It is 



