REPTILIA 221 



underwent a secondary increase in proportions, for the culminating 

 type of the Sauropoda, Brachiosaurus (Fig. 124, G), had the fore legs 

 even heavier and longer than the hind legs. This immense creature 

 rivaled the modern whales for sheer bulk, and was possibly the most 

 ponderous creature of all time; unquestionably it was the largest 

 by all odds of the known terrestrial giants, dwarfing the largest ele- 

 phants by contrast. 



"In the final extinction of the herbivorous sauropod type," says 

 Osborn, "we find an example of the law of elimination, attributed to 

 the fact that these types had reached a cul-de-sac of mechanical evo- 

 lution from which they could not adaptively emerge when they en- 

 countered in all parts of the world the new environmental conditions 

 of advancing Cretaceous time." 



The Ornithischia. While both of the groups of dinosaurs just 

 described are alike in having the typical reptilian pelvis and are 

 therefore grouped together as Saurischia, another great group of 

 contemporary dinosaurs had the avian type of pelvis and are called 

 Ornithischia. These dinosaurs appear to have been an offshoot of the 

 early herbivorous types and had retained that habit, using probably 

 the harder vegetable foods, as is attested by the development of a 

 heavy, chitinous beak much like that of the modern bird. These 

 Beaked Dinosaurs radiated adaptively into three distinct structural 

 types: Ornithopoda (bird-footed), Stegosauria (armored dinosaurs), 

 and Ceratopsia (horned dinosaurs). 



The Ornithopoda were unarmed, bipedal forms doubtless capable 

 of great speed. As examples of these forms we may cite the fam- 

 iliar Iguanodon (Fig. 126) Trachodon, the duck-billed dinosaur, and 

 Corythosaurus (Fig. 124, H) the " hooded duck-bill. " All of them show 

 well the bird-like feet and pelvis. It is significant in this connection 

 to note that it is from this group that some authors would derive 

 the birds, a theory that is presented in discussing the ancestry of 

 the bird* Of all of the Ornithischia perhaps the most bird-like 

 type is the "Ostrich Dinosaur" Struthiomimus (Fig. 124, E). 



The Stegosauria seem to have reacquired the quadrupedal habit 

 in correlation with the massive weight of their armature. Stegosaurus 

 (Fig. 127) represents the culmination of the evolution of the armored 

 types, and has been fittingly chosen as the exemplar of one type 

 of preparedness, distinguished by possessing "all armor and no brain," 

 an equipment as little apt to meet with success in those antique days 



