130 EVOLUTION OF THE EARTH 



of the far-famed Connecticut Valley, 6 for here there is a pro- 

 fusion of trails of creatures great and small, quadrupeds and 

 bipeds, may of which were long of limb, compact of foot, and 

 impress one as being concerned with weighty matters ques- 

 tions of food or safety which would brook no delay. In 

 several instances the quadrupedal resting posture is clearly 

 indicated by a creature whose normal gait is that of a biped, 

 but in no instance is the impressed hand that of a carnivorous 

 or theropod dinosaur, whose sharply curved grasping talons 

 borne on the fore limbs would have left a highly characteristic 

 impression such as is never seen. That carnivores were pres- 

 ent is an irresistible conclusion, and the inference is that certain 

 numerous footprints made by the hind feet only, in which the 

 claws, including that of a grasping hallux, were comparatively 

 sharp, are attributable to such forms. That aridity does give 

 rise to bipedality is evidenced by the present existence of 

 several bipedal lizards, notably in our own Southwest, and in 

 Australia where the frilled lizard, Chlamydosaurus, reaches 

 a length of five feet and is exceedingly swift of foot. 



Rise of sauropod dinosaurs. Climatic oscillation during the 

 Jurassic gave rise to humid conditions and this, coupled with 

 extensive low-lying delta lands along the shores of shallow 

 seas, tempted certain of the increasingly large Theropoda to 

 forsake the strenuous life of a carnivore for the slothful ease 

 of an amphibious herbivore. These huge creatures, because 

 of the increased burden of the flesh, had reacquired a quad- 

 rupedal gait as had the armored dinosaurs (stegosaurs). 

 They suffered no very great alteration in their dental battery, 

 the teeth of which became spoon-like (Brontosaurus] or, col- 

 lectively, rake-like (Diplodocus] , fit only for securing some 

 sort of abundant vegetation, not for the rending of flesh. The 

 fate of these forms and that of the armored Stegosaurus were 



6 See Lull, R. S., "Triassic Life of the Connecticut Valley." Connecticut State 

 Geol. Nat. Hist. Survey, Bull. 24, 1915. 



