20 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



should not be permitted to cause an error in the interpretation of the beds. 

 In swamp deposits and deposits of small bodies of water, either fresh or 

 marine, the remains are apt to be those of the local fauna, terrestrial or 

 aquatic. 



The bodies of terrestrial animals which find their way into a stream 

 may be carried long distances with the current and in times of flood may 

 be carried far outside the normal bed of the stream and laid down on flood- 

 plains or in places where the streams spread widely over the subaerial 

 portion of deltas. Even after the cadaver, freed from the distending gases, 

 has sunk or been torn to pieces, the parts would be swept along until finally 

 drawn into some eddy or stranded upon a flat. Where streams pass rapidly 

 from one physiographic region to another this might result in the mingling 

 of fossil forms very distinct in their natural habitat, as the remains of purely 

 mountain or upland forms might to-day be swept out upon the surface of 

 the Great Plains and mingled with remains of animals of radically dijfferent 

 habitat; or animals entirely inland might be swept out by the floods of the 

 Mississippi, Nile, Amazon, or other great rivers, and buried in subaqueous 

 parts of the delta far from shore and intermingled with remains of marine 

 animals. One would not regard as impossible the occurrence of the bones 

 of the American antelope or the bison in the muds of the Mississippi delta, 

 to take an extreme case, or the bones of horses, cows, etc., in muds of the 

 Louisiana bayous where such remains would not naturally occur. An ex- 

 ample of the determination of the physiographic habitat of animals is given 

 by Osborn.^ 



More difficult is the interpretation of the contents of large bone-beds or 

 shell-beds in fluviatile deposits. It would be obviously very dangerous 

 to interpret the surroundings of such a collection from the contents of the 

 bed until a study of the fossils permits the elimination of foreign forms. 

 Much-worn bones or shells would naturally indicate long transportation, 

 but when the cadavers were transported a great distance before being 

 destroyed or the hard parts subjected to much wear this evidence of trans- 

 portation would be less noticeable. 



The fauna of a bed, aside from the accidental inclusions noted above, 

 indicates the character of the deposit — marine, brackish, or fresh water, 

 swamp, or purely terrestrial. But here a new series of factors enters the 

 problem ; the nature of the evidence shifts from the inorganic to the organic. 

 The time element becomes as important a factor as the space element. 



(&) ORIGIN OF THE FAUNA. 



Aquatic invertebrate fauna. — ^Any fossil fauna either originated where it is 

 found by evolution from older types, or it migrated into the region, or it 



^ Osborn, H. F., Cenozoic Mammal Horizons of Western North America. Bull. U. S. 

 Geological Survey, No. 361, Age of Mammals, pp. 84-85, 1909. 



