REPTILES, BIRDS, AND MAMMALS 453 



lateral rows of spikes and their stiff tails ending in a huge clublike mass of bone. 

 The ceratopsians or horned dinosaurs were the last group to appear; their entire 

 history was confined to the Cretaceous period. They were heavy-bodied quad- 

 rupeds with an enormous skull which projected as a bony shield over the neck 

 and shoulders. The earliest known member of the group was about 6 feet long 

 and had merely the rudiment of a nasal horn. From such a type came several 

 lines of horned dinosaurs, one of which culminated in Triceratops of the late 

 Cretaceous — a 30 foot monster with a skull 8 feet long armed with three sharp 

 horns. Like all the ornithischians the horned dinosaurs were herbivores, and their 

 armament was purely defensive. 



The end of the Age of Reptiles. The Cretaceous reptiles were abun- 

 dant and diversified, and their rule seemed firmly established. Ichthyo 

 saurs, plesiosaurs, and mosasaurs dominated the seas, the various archo- 

 saur groups held sway over the lands, and pterosaurs soared in the skies. 

 Yet all this varied reptilian life disappeared with apparent suddenness 

 at the end of the era. Only the turtles, the lizards and snakes, the croco- 

 diles, and the relict tuatara are known to have survived the transition to 

 Cenozoic times. What caused this downfall of a whole dynasty? Many 

 hypotheses have been advanced in explanation, but there was probably 

 no single, simple cause. Basically the cause was inability of the reptiles 

 to adapt to change; and there were many changes that came toward the 

 close of the Cretaceous period. The first effects of the revolution that 

 ushered in the Cenozoic era began to be felt ; climates slowly shifted from 

 the uniformly warm, equable conditions of Mesozoic time toward cooler, 

 more zonal ones with fluctuating temperatures, and tropical lowlands 

 gave place to rolling uplands. With these physical changes came altera- 

 tions in the biotic environment. Cycads and conifers retreated before the 

 advance of the newly arisen hardwood forests; and a host of small, 

 active, warm-blooded and bigger-brained mammals appeared to compete 

 with the cold-blooded, smaller-brained, less efficient reptiles. The changes 

 occurred slowly; no individual would have noticed them, nor several 

 generations, but they continued and their effects were cumulative. Not 

 any one of these things, but all of them together and doubtless others still 

 unsuspected, probably caused the disappearance of the reptilian hordes. 



THE HISTORY OF BIRDS 



There is little in the appearance or actions of modern birds to suggest 

 the reptile. These alert, active, highly coordinated creatures share domi- 

 nance of the modern world with the mammals and higher insects and 

 seem a far cry from the dinosaurs and their kin. Yet it was long ago 

 pointed out that aside from their special modifications for flight, with 



