148 



INTRODUCTION TO EVOLUTION 



from adequate fossil material the highest invertebrate group of animals is 

 represented. This fact indicates that a great proportion of the evolution of 

 invertebrates had already occurred, although our records of its history 

 are almost entirely lacking. Since trilobites appeared on the scene with 

 such apparent abruptness their origin is uncertain. It seems reasonable to 

 believe that they evolved from annelid worms, the other great group of in- 

 vertebrates having segmented bodies. Evidence that annelid worms were 

 present in the Proterozoic will be recalled. 



FIG. 8.2. Typical Cambrian trilobites. (Reprinted by permission from Textbook of 

 Geology, Part II, Historical Geology, by Louis V. Pirsson and Charles Schuchert, pub- 

 lished by John W\\ey & Sons, Inc., 1915, p. 595.) 



Our knowledge of life in Cambrian seas would be confined almost ex- 

 clusively to shells and exoskeletons were it not for a most fortunate and 

 unusual fossil discovery in the Burgess shale of British Columbia. The fos- 

 sils consist of thin carbon films (see p. 129) showing in amazing detail the 

 structures of the animals from which they were formed. Both hard and soft 

 parts are shown by these carbonaceous films. Among the remains are trilo- 

 bites preserved with their limbs and antennae, dehcate arthropods Hke the 

 modern brine shrimp, annelid worms complete with setae (bristles) and 

 details of the internal organs, sponges, and such soft-bodied creatures as 

 jellyfish. One of the most interesting members of the assemblage is an 

 onychophoran. The onychophorans are peculiar, wormlike arthropods ex- 

 emphfied by the modern Peripatus (Fig. 8.4). Their particular interest 



