146 INTRODUCTION TO EVOLUTION 



Animal fossils from this era are conspicuous by their rarity. Fossils of 

 radiolarians and foraminiferans (protozoans which secrete shells of silica 

 and calcium carbonate, respectively) have been reported, as have also 

 fossils of brachiopods (see p. 147), the spicules of sponges, and even the 

 impression of a jellyfish. While there is every reason to expect that such 

 animals would have been living in Proterozoic oceans, confirmation of the 

 reported fossils themselves seems to be lacking, in some cases at least. 



The fossils most generally accepted as derived from a Proterozoic ani- 

 mal consist, not of the remains of the animals themselves, but of casts of 

 the homes of the animals. In the bottoms of shallow portions of modern 

 oceans annelid worms, marine relatives of our common earthworm, live in 

 burrows. These burrows are not temporary affairs like the tunnels of earth- 

 worms but have definite walls secreted by their inhabitants. Casts which 

 seem to have been formed in burrows of this type are found in Proterozoic 

 deposits. 



If we are correct in interpreting the Proterozoic worm burrows as evi- 

 dence that annelid worms existed in this era, we must conclude that much 

 evolution had occurred prior to this time. Annelid worms are placed well 

 up in the scale of invertebrate animals. They have considerable complexity 

 of structure. Evidently, therefore, an extensive evolutionary history lead- 

 ing up to them must have taken place prior to and during the Proterozoic 

 era. Unfortunately a fossil record of that history was for the most part 

 never formed or has been irrecoverably lost. This statement applies equally 

 to the other phyla of invertebrates. Yet although fossil remains of them 

 are so scanty it is likely that most of the invertebrate phyla were repre- 

 sented in late Proterozoic seas. One reason for thinking so lies in the wealth 

 of invertebrate life found in the seas of the next era. 



PALEOZOIC ERA 



The beginning of the Paleozoic era is known to us from 

 the earliest deposits bearing abundant fossils. It will be noted (Table 7.1, 

 p. 137), that this era is much the longest of the ones following the Protero- 

 zoic, and that it is divided into seven periods, of which the Cambrian is 

 the first or oldest. 



Cambrian Period 



Between the rocks remaining to us from the Proterozoic era and the first 

 ones representing the Cambrian period occurs a gap in the geologic record 



