HASEMAN, GEOGRAPHICAL DISTRIBUTION IN SOUTH AMERICA 95 



highly probable that this antecedent type, and not Mesosaurus, was the 

 form which was distributed. 



The exponents of the Gondwana theory must assume that Mesosaurus 

 originated and died off on the Gondwana continent where its ancestors 

 are not known, and that it necessitated a continuous Gondwana Land 

 when its distribution is used to support a connection between Africa and 

 South America. They must also assume that Mesosaurus was the form 

 which was distributed and that Stereosternum evolved from it after it 

 arrived in South America. Two objections can be raised against this 

 view which are based on positive evidence. First, the nearest ancestral 

 cotylosaurians known are from the northern hemisphere, and none are 

 known from South America. Hence the positive evidence in one case 

 becomes negative evidence in the other. In other words, the positive 

 evidence is in favor of a northern origin of the ancestral form which gave 

 rise to Mesosaurus and Stereosternum. Secondly, Mesosaurus was a 

 good aquatic reptile and did not need a continuous Gondwana Land in 

 order to get into Africa and South America. 



It is evident, then, that the point of origin of the Mesosauria is un- 

 known. We only know the point of extinction of a few individuals of 

 three species of the genus. The genus was already very distinct from 

 other reptiles in the lower Permian, and its ancestral stock could easily 

 have arisen in the northern hemisphere from some unknown cotylo- 

 saurian. In fact, Moodie's paper on the Carboniferous air-breathing 

 vertebrates of the United States National Museum indicates that this is 

 probable, because Isodectes punctulatus Cope from the Allegheny series 

 and Sauravus cosiei Thevenin from the Carboniferous of France very 

 remotely point back to Microsauria on one hand and to Mesosauria on 

 the other. At any rate, the Mesosauria must have originated before the 

 beginning of the Permian, and the point of origin could have been in the 

 northern hemisphere just as well as in the Gondwana continent. The 

 fact that the Mesosauria are known only from Africa and South America 

 is in favor of the latter view, while the nearest related antecedent types 

 of Carboniferous Cotylosauria are in favor of the former. 



Even if we assume that the point of origin of the Mesosauria was in 

 either Africa or South America, their distribution offers absolutely no 

 argument for a connection between these continents, because the Meso- 

 sauria were aquatic reptiles, as is shown by their long snout, long needle- 

 like teeth, lack of scales, dorsal position of nares, unique ribs and webbed, 

 paddle-like feet. McGregor's reconstruction of M. brasiliensis indicates 

 that it could not have traveled overland, and inasmuch as it appears to 

 me that no river could have flowed from Africa into South America, or 



