THE BIOLOGIST POSES SOME PROBLEMS 91 



tons. Although most skeletons in the Cambrian, as today, were 

 certainly built of that useful substance, there were also calcium 

 phosphate skeletons, as in the ecardinate brachiopods, and quite 

 likely in some Trilobites and Aglaspida. Moreover, the first really 

 certain siliceous organisms, the sponges and Radiolaria, appear 

 about the same time. Claims have been made of radiolarians and 

 sponge spicules in the pre-Cambrian, but they are not regarded as 

 acceptable by most modern authorities on the groups (Raymond, 

 1935). Moreover, the general organization of the hard parts of the 

 lowest fossilized arthropods clearly indicates that there must 

 have been a considerable amount of organic matrix in the exo- 

 skeleton, and we know from the Burgess Shale deposits that 

 during the middle Cambrian there was a vast fauna of arthropods 

 with organic exoskeletons, strong enough to permit muscle attach- 

 ments, but not heavy enough, or sufficiently mineralized, ordinarily 

 to fossilize. It has often been supposed that the appearance of 

 fossilizable skeletons in the lower Cambrian must reflect changes 

 in chemical composition of the ocean also occurring at that time. 



Phosphate is a very minor constituent of sea water and is an 

 essential element in all organisms, whereas calcium is probably 

 essential for all animals if not for all organisms, so that efficient 

 mechanisms for collecting both calcium and phosphate would be 

 early evolved. From these facts it seems extremely likely that 

 calcium phosphate skeletons, if they had been desirable, could 

 have been evolved under conditions of considerably lower calcium 

 concentration than occur in the modern ocean, while the apparent 

 contemporary introduction of siliceous skeletons at the same time 

 as that of calcareous skeletons can have no simple geochemical 

 explanation. For this reason it seems better to look for a biological 

 explanation rather than one based on geochemical considerations. 



When we examine the lower Cambrian fauna as a whole, the 

 impression is very strong that practically all the organisms known 

 to be present were microphagous, feeding either on detritus or 

 minute organisms in sediments or on plankton. The mollusca 

 presumably had radulae, since Neopilina has such an organ, but 

 it is unlikely that such radulae were very efi^ective organs of 

 predation except possibly on any unskeletonized sponges and 



