126 FROM FISH TO PHILOSOPHER 



possibly gone as far toward the aglomenilar condition 

 as they can. 



The uricotehc (uric acid-excreting) habitus is char- 

 acteristic of all birds, so far as is known, and of all rep- 

 tiles studied that have persisted in an arid habitat, and 

 we take it to be a fundam^ital biochemical characteris- 

 tic of the reptiHan-avian stock, and one that was evolved 

 before the separation of the avian from the reptiHan 

 stem. The uricoteHc habitus is not present in any known 

 amphibian or mammal, all of which excrete urea (ex- 

 cept for small quantities of uric acid formed in the spe- 

 cific metabolism of nucleoprotein), and it must have 

 been invented by the reptihan branch of the nascent 

 amniotes at the time when they were undergoing adap- 

 tation to terrestrial life, during late Pennsylvanian or 

 early Permian times. 



No other vertebrate class has undergone such spectacu- 

 lar evolution with such rapidity as have the reptiles. Be- 

 fore the end of the Permian they were represented by 

 several highly divergent orders, and in the Triassic they 

 grew in size if not in variety, spawning the first dinosaurs 

 (deinos = terrible; sauros = hzard). These dinosaurs 

 were small in comparison with those of the later Jurassic 

 and Cretaceous— only few reached a length of more than 

 10 to 15 feet— but they all were bipedal and were 

 adapted for running on their hind legs and used the fore- 

 legs for grasping vegetation or for digging. The long, 

 stout tail was used, as by the kangaroo, for balancing 

 and bracing the body and, again, like that animal, they 

 held the tail off the ground when ruiming. Footprints are 

 more abundant than skeletons, because the Triassic red 

 beds afforded a poor environment for preservation, but 

 many Triassic mud-flats reveal the criss-cross trails of 

 running dinosaurs, the mud frequently spattered by rain- 

 drops that fell just before or just after the visitation. Two 

 groups of Triassic reptiles took to the sea as a permanent 

 abode: the dolphinlike ichthyosaurs and the turtlelike 



