728 EVOLUTION OF ANIMALS Part V 



to egg-laying mammals (e.g., duckbill), marsupial mammals (e.g., kangaroos), 

 and placental mammals (e.g., man). The central reptilian stock (Archosauria) 

 were the seemingly insignificant progenitors of the midgets as well as the giants 

 of the Reptilian Age. The amphibious dinosaurs were plant feeders that moved 

 heavily on four legs in their marshy homes. Certain of them were only 30 to 

 40 feet long, but the fossil skeleton of one measures about 80 feet. The car- 

 nivorous dinosaurs ran upright on their hind legs, as do some modern lizards, 

 pricked into speed by hunger and fighting (Figs. 35.15, 35.16). They became 

 increasingly large and the fossil of Tyrannosaurus shows a monster that reared 

 upward 19 feet, no doubt using its great teeth and front claws on the unarmed 



Fig. 35.16. Drawings of three frilled lizards. (Chlamydosaurus) and another 

 species (Grammatophora) at right showing the bipedal habit in living reptiles. 

 Drawings made from photographs of exhilarated lizards running at full speed. 

 Millions of years ago reptiles walked on two legs. In succeeding ages nearly all 

 the reptiles abandoned the habit but in the birds that originated from them, walk- 

 ing was continued with success. An ostrich can run. (Courtesy, Young: The Life 

 of the Vertebrates. Oxford, England, The Clarendon Press, 1950.) 



plant feeders. Some small reptiles were no larger than chickens and squirrels. 

 A little dinosaur whose fossilized skeleton was about one foot long was dis- 

 covered a few years ago near South Hadley, Massachusetts, in a region where 

 footprints of giant dinosaurs are found in the sandstone. Some flying reptiles 

 were the size of sparrows; some had wingspreads of 20 feet. 



After some 140 million years, the Age of Reptiles came to an end and the 

 hordes of these ruling animals gradually disappeared. A cataclysm or a gradual 

 change of climate or great competition for food and space between the reptiles 

 and other animals may have brought about their disappearance. By that time 

 there was a host of active, warm-blooded mammals with appetites for reptilian 

 eggs and meat. These mammals originated from one or more strains of reptiles. 

 From the reptiles also had come the shelled egg which could be incubated in a 

 dry place, yet the developing embryo would be surrounded by fluid. The 

 shelled egg and the embryonic membranes were the great contributions of the 

 reptiles to the evolution of vertebrates. 



