154 



INTRODUCTION TO EVOLUTION 



suture 



septum 



FIG. 8.8. Pearly nautilus, with shell cut longitudinally to show internal chambers. 

 (After Hancock.) 



portant feature of the fauna until the Silurian. These remarkable arthro- 

 pods were small for the most part, though one had a body length of 7 feet 

 and was probably the largest arthropod that ever lived. 



Closely similar to eurypterids in many ways, and probably descended 

 from them, were scorpions not unlike those of the present day (Fig. 8.12). 

 These and millipedes existing at the time may have been terrestrial, 

 thereby earning the distinction of being the first known animals to live on 

 land. The evidence is inconclusive, however. Similarly, evidence is inde- 

 cisive as to whether or not terrestrial plants existed at this time, though 

 their occurrence seems not unlikely in view of the great numbers of them 

 found in the next period. From our earlier discussion of the dependence of 

 animal life upon plant life (p. 84) it will be evident that no considerable 

 development of terrestrial animals could be expected prior to development 

 of terrestrial plants. 



Vertebrates are represented in Silurian deposits by remains of osfraco- 

 derms. These were little creatures with jawless mouths, their nearest mod- 

 ern relatives being lampreys and hagfishes. They became much more 

 abundant in the next period. Placoderms, the first fishes possessing jaws, 

 were probably also present at this time (see p. 156). 



