CHAPTER XXVII 



ANCIENT ANIMALS 



The history of animals really begins with the Cambrian period, when 

 fossils were first preserved in abundance. This was a time far along in 

 earth history, when life had already been in existence for countless ages— 

 perhaps even for 1,000 million years. It is no Avonder, then, that when the 

 curtain rises, it reveals a scene crowded with living things of many kinds. 

 Every important animal phylum except the chordates was already repre- 

 sented. Since the Cambrian fauna is the oldest known assemblage of 

 animals, it deserves more than passing notice. 



The Cambrian fauna. All Cambrian life was marine. What went on in 

 the oceans we do not know; the record is that of the shallow seas that 

 spread over the face of the lands. Their sands and muds are filled with 

 the fossils of shell-bearing animals, and by a rare and fortunate circum- 

 stance a great many of the soft-bodied creatures were preserved as im- 

 prints in the Burgess shales of British Columbia. All told some 1,200 

 species of Cambrian animals have been described from North America 

 alone, and while these are but a sample of the whole, they suffice to give 

 us a clear picture of the life of the time. 



Most of the animals were either small, delicate forms that drifted in 

 the surface waters (plankton) or bottom dwellers (benthos) that crawled or 

 swam over the sea floor, or attached themselves to some object for sup- 

 port, or burrowed in the mud. There was no group of strong swimmers 

 (nekton) such as lived in later seas. In the plankton there must have been 

 an abundance of protozoa and minute multicellular animals that fed 

 upon the floating algae, but they have left few traces. The smaller plank- 

 ton animals and plants furnished sustenance for larger animals — swim- 

 ming annelid worms and shrimplike Crustacea, of which many kinds have 

 been found; these in turn were captured by jellyfishes very like those of 

 today. On the sea floor lived simple sponges and possibly corals, together 

 with short-stalked echinoderms, snails, bivalve mollusks, and primitive 

 cephalopods. The dominant animals were the brachiopods and the 

 trilobites. 



A brachiopod has a hinged bivalve shell like a clam and, like a clam, gets its 

 plankton food from a water current created by its beating cilia; but there the 



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