CHAPTER III 



EVOLUTION OF EARLY PALEOZOIC FAUNAS IN RELATION 

 TO THEIR ENVIRONMENT 



CHARLES D. WALCOTT 



CONTENTS 

 Introduction. 



North American Continent at the Beginning and at the Close of Cambrian Time. 

 Life at the Beginning of Known Cambrian Time. 

 Distribution of the Lower Cambrian (OleneUus) Fauna over the North American 



Continental Platform of Cambrian Time. 

 Conditions in Middle and Upper Cambrian Time. 

 Evolution of Faunas. 



INTRODUCTION 



The evolution of early Paleozoic faunas could be treated with far 

 greater effectiveness if the studies now in progress on the Cambrian 

 faunas were nearer completion. That of the brachiopods is well 

 advanced' but the great collections of the U. S. National Museum, 

 representing the Crustacea and other invertebrates, have not been 

 studied as to their mode of occurrence, geographic distribution, and 

 biologic and environmental relations. Only a brief summary of the 

 known evidence afforded by the Cambrian rocks and faunas of North 

 America is considered in this paper. 



Animals and plants, as now known, are profoundly influenced by 

 their environment, hence we will first broadlv outline the conditions 

 under which the known marine organisms of Cambrian time lived. ^ 



NORTH AMERICAN CONTINENT AT THE BEGINNING AND AT THE CLOSE 



OF CAMBRIAN TIME 



The information obtained since the publication of my first map on 

 this subject in iSgi^ has been assembled on the two accompanying 

 maps by Mr. Bailey Willis. The first map outlines a central mass 



1 Smithsonian Miscellaneous Collections, Vol. LUX, No. 4, 1908, pp. 139-65. 



2 Bull. Geol. Soc. Amer., Vol. X, 1899, pp. 199-244. 



3 Bull. U. S. Geol. Survey, No. 81, 1S91, PI. III. 



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