300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



stantly changing conditions and are today the dominant terrestrial 

 vertebrates, the history of the reptiles has been quite different. In the 

 Mesozoic the reptiles were in a high degree of development and special- 

 ization, and highly diversified. There were, among others, gigantic, 

 chiefly herbivorous dinosaurs, great marine reptiles of various kinds, 

 and several types of flying reptiles, together with the more familiar 

 crocodiles, turtles, and snakes. The development of the reptiles 

 reached a culmination in the Cretaceous, but toward the end of that 

 period most of them disappeared. Up to the end of the Cretaceous 

 there were 19 orders of reptiles, but since the earliest Tertiary, when 

 there were a very few reminders of the exceedingly rich Mesozoic 

 fauna, only four of these have persisted, the Rhynchocephalia, repre- 

 sented only by Sphenodon or Hatteria, the tuatara of New Zealand, the 

 crocodilians, confined to tropical and subtropical regions, the turtles, 

 and the Squamata, including the lizards and snakes. 



The world-wide and practically simultaneous disappearance of most 

 of the reptiles, including all the giant terrestrial herbivorous and 

 carnivorous dinosaurs, the flying pterodactyls, and the large ichthyo- 

 saurs, plesiosaurs, and mososaurs, was followed by the evolution and 

 development from mammalian stock of a great variety of types oc- 

 cupying the former habitats of all the terrestrial reptiles, with the 

 cetaceans replacing the marine types. Presumably the place of the 

 flying reptiles was taken by certain birds. 



Any satisfactory explanation of the causes of the disappearance of 

 the dominant reptiles and their rapid replacement by mammals must 

 be applicable equally to all the continents and to all the oceans as well. 

 In view of the dependence of practically all modern reptiles, especially 

 those with heavy calcareous skeletons or dermal scutes such as the 

 hard-shelled turtles and the crocodilians, on abundant sunlight, and 

 the independence of sunlight characteristic of the largely nocturnal 

 mammals, it would seem that a radical change in the amount or charac- 

 ter of the sun's radiations reaching the earth may well have been the 

 chief factor in the disappearance of the reptiles leading to the domi- 

 nance of mammals. 



During the Cretaceous there was extensive inundation of the land 

 areas by the sea, the continents were much isolated, and the climate 

 presumably was warm and uniform. At the end of the Cretaceous 

 there seems to have been a great upheaval of the land in both the 

 northern and southern hemispheres. This was accompanied by local 

 and intermittent volcanic activity throughout the Eocene in the Rocky 

 Mountain region, Central America, the East and West Indies, and 

 southern Europe. Any marked increase in the land area would mean 

 a corresponding increase in dust in the atmosphere, inorganic dust 

 from arid areas, and pollen and organic dust from heavily vegetated 



