i86 



THE ORIGIN AND EVOLUTION OF LIFE 



is very similar to that of the solid-headed Amphibia (Steg- 

 ocephalia). Bone by bone its parts indicate a common descent 



from the skull type of the fringe- 

 finned fishes (Crossopterygia, 

 Fig. 53)- 



As revealed by the researches 

 of Cope, Williston, and Case, 

 the adaptive radiation of the 

 reptile life of western America 

 in Permian time is as follows: 

 First there is a variety of swift- 

 moving, alert, predaceous forms 

 corresponding to the fusiform, 

 swift-moving stage in the evolu- 

 tion of the fishes. Some of 

 these reptiles (Varanops) re- 

 semble the modern monitor liz- 

 ards (Varanus); others {Oplii- 

 acodon and Theropleura) are 

 provided wath four well-devel- 

 oped limbs and feet, the long tail 

 being utilized as a balancing 

 organ. These were littoral or 

 lowland reptiles, insectivorous 

 or carnivorous in habit. The 

 primitive, lizard-like pelycosaur Varanops, with a long tail 

 and four limbs of equal proportions, represents more nearly 

 than any known ancient reptile, apart from certain special 

 characters, a generalized prototype from which all the eighteen 

 Orders of the Reptilia might have descended; its structure could 

 well be ancestral to that of the lizards, the alligators, and the 

 dinosaurs. At present, however, it is not determined whether 



ARAEOSCELIS 

 PEPTILIA 



Fig. 66. Ancestral Reptilian T\tes. 



Two of the defenseless, swift-moving, 

 terrestrial reptilian types, Varanops 

 and Arwoscelis, of the Permo-Carbonif- 

 erous period of Texas. The skull and 

 skeleton of ArcBoscelis foreshadow the 

 existing lizard (Lacertilian) type and 

 Williston regards it as the most nearly 

 related Permian representative known 

 of the true Squamata (ancestors of 

 the lizards, snakes, and mosasaurs). 

 Restorations of Varanops and ArcBos- 

 celis modified from Williston. Drawn 

 for the author by Richard Deckert. 



