CHAPTER VIII 

 THE FAUNAL RELATIONS OF THE EARLY VERTEBRATES 



S. W. WILLISTON 

 The University of Chicago 



The environmental conditions affecting the evolution of the early 

 air-breathing vertebrates offer at the present time many peculiarly 

 difficult problems, problems which must depend in large measure 

 upon the geologist for solution. They are very different from those 

 which confront the student of the neozoic vertebrates, since we have 

 better data for comparisons and conclusions in the living faunas as 

 well as in our existing climatic and geographic conditions. And 

 especially are the problems more involved and complicated when we 

 attempt to deal with the marine or aquatic air-breathers of those 

 early times. Here we can practically predicate little as to the con- 

 ditions of the oceans and climates in which they lived. But these 

 early vertebrates do offer, it seems to me, much that is suggestive 

 regarding the migrations and evolution of faunas, involving theories 

 as to paleogeographic conditions and changes, and, within certain 

 limits, the climatic conditions which surrounded and controlled the 

 migrations. And it is of this phase of the subject that I would choose 

 to speak now. 



As has been said, the evidence offered by the vertebrates, when 

 available, is often, if not usually, more decisive than that of any 

 other class of organisms in the determination of the relationships 

 and correlations of faunas. A single species of the higher vertebrates 

 found to occupy two remote provinces would furnish more positive 

 evidence of contemporaneity and the possibilities of faunal migrations 

 than would scores of others of lower types. But of species in verte- 

 brate paleontology we can say little; the term with us is usually a 

 far more vague and indefinite one than it is among students of living 

 faunas, partly because much of the evidence which the neozoologist 

 has, the paleozoologist has not, partly because the taxonomy of living 

 creatures is still based too much upon superficial resemblances. And 



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