222 



THE ORIGIN AND EVOLUTION OF LIFE 



Fig. loo. A Pair of Upper Cretaceous Iguano- 



DONTS FROM MONTANA. 



After a lapse of 500,000 years of Cretaceous time the 

 Camptosaurus (Fig. gg) evolved into the giant " duck- 

 billed " dinosaur Trachodon, described by Leidy and 

 Cope from the Upper Cretaceous of New Jersey and 

 Dakota. 



Two skeletons of Trachodon annedens (upper) discovered 

 in Montana, as mounted in the American Museum of 

 Natural History, and restoration of the same (lower) 

 by Osborn and Knight. (Compare Fig. 74.) 



spread all over the 

 northern hemisphere 

 and attained an extra- 

 ordinary adaptive radi- 

 ation in the river- and 

 shore-living "duck- 

 bill" dinosaurs, the 

 iguanodonts of the Cre- 

 taceous Epoch (Fig. 

 loi). The adaptive 

 radiation of these ani- 

 mals has only recently 

 been fully determined; 

 it led into three great 

 types of body form, all 

 unarmored. First, the 

 less specialized types 

 which retain more or 

 less the body structure 

 of the earlier Jurassic 

 forms and the famous 

 iguanodont of Bernis- 

 sart, Belgium. Related 

 to these are the krito- 

 saurs of the Cretaceous 

 of Alberta, with a com- 

 paratively narrow head, 

 the protection of which 

 was facilitated by a 

 long, backwardly pro- 

 jecting spine. Second, 

 there are the broadl\' 



