THE ORIGIN OF THE OLDEST FOSSILS. 375 



(5) The bottom fauna soon produced progressive development among 

 pelagic animals. 



(G) After tlie establishment of the fauna of the bottom elaboration 

 and diftereutiation among the rei^resentatives of each primitive type 

 soon set in and led to the extinction of connecting forms. 



Many of the oldest fossils like the pteropods are the modified 

 descendants of ancestors with hard parts, and there is no reason to 

 supjiose that the lirst animals whicli were capable of preservation as 

 fossils have been discovered, but it is interesting to note that the oldest 

 known fauna is an unmistakable a])proximation to the primitive fauna 

 of the bottom. 



The Lower Cambrian fossils are distributed through strata more 

 than 2 miles thick, some, at least, of them showing by their tine grain 

 and by the perfect preservation of tracks and burrows which were 

 made in soft mud, and of soft animals like jelly-fish, that they were 

 deposited in water of considerable depth. The sediment was laid down 

 slowly and gently in water so dee]) as to be free from disturbance and 

 under conditions so fiivorable that it contains the remains of delicate 

 animals not often found as fossils. 



While the fauna of the Lower Cambrian undoubtedly lived in water 

 of very considerable depth, it was not oceanic but continental, for we 

 are told by Walcott that ''one of the most important conclusions is 

 that the fauna of the Lower Cambrian lived on the eastern and west- 

 ern shores of a continent that in its general configuration outlines the 

 American continent of to-day." " Htrictly speaking, the fauna did not 

 live upon the outer shore facing the ocean, but on the shores of interior 

 seas, straits or lagoons that occupied the intervals between the several 

 ridges that ran from the central i)latform east and west of the main 

 continental land surface of the time. 



This fauna was rich and varied, but it was not self supporting, for 

 no fossil i)lants are found, and the primary food supply was j)elagic. 

 Animals adapted for a rapacious life, such as the pteropods, were 

 abundant, and prove the existence of a rich supply of pelagic animals. 

 All the forms known from the fossils are either carnivorous, like the 

 medusa-, corals, Crustacea, and trilobites, or they are adapted, like the 

 sponges, brachiopods, and lamellibranchs, for straining minute organ- 

 isms out of the water or for gathering those whicli rained down from 

 above, and the conditions under which they lived were very similar to 

 those on the bottom at the present day. 



Walcott's studies show that the earliest known fauna had the follow- 

 ing characteristics: It consisted, so far as the record shows, of animals 

 alone, and these were dependent upon the pelagic food supply for sur- 

 port. While small in comparison with many modern animals, they 

 were gigantic compared with primitive pelagic animals. The species 

 were few, but they represent a very wide range of types. All these 

 types have modern representatives, and most of the modern types are 



