98 THE AGE OF MAMMALS 



Toward the close of the Age of Reptiles, in late Cretaceous times, sea 

 and land still possessed a large reptilian fauna; the great marine ich- 

 thyosaurs and plesiosaurs had previously become extinct, but the giant sea 

 lizards, or mosasaurs, still survived. 



In Belgium, the very summit of the Cretaceous, the Danian or Mae- 

 strichtian Stage,^ a name given to the exposures around Msestricht, records 

 the existence in the seas of several mosasaurs, namely, the huge Mosa- 

 saurus giganteus and the lesser Platecarpus, as well as of great marine turtles. 

 On land there wandered the tall herbivorous dinosaurs known as Iguano- 

 dontia (Orthomerus dolloi Seeley) and their enemies, the carnivorous 

 dinosaurs (Megalosaurus bredai Seeley). 



At the same time in the Rocky Mountain region, where the land ani- 

 mals only are known, there existed several kinds of dinosaurs. Chief 

 among the herbivorous forms were the giant Iguanodontia (Trachodon) 

 or "duckbill" dinosaurs, the great paired-horned Ceratopsia (Triceratops) , 

 and the armored ankylosaurs (Ankijlosaurus). All these herbivorous forms 

 were subject to attack by the giant carnivorous megalosaurs of the genus 

 Tyrannosaurus. There were also smaller dinosaurs (Ornithomimus) , cur- 

 sorial, or of swift-running habit. These reptiles were in the climax of 

 specialization and grandeur; they moved amidst a stately flora of palms 

 and sequoias interspersed with bananas and fig trees, and a very rich de- 

 ciduous tree flora of modern south temperate type. 



A great many species of small mammals are known in these Upper 

 Cretaceous dinosaur beds of the Rocky Mountain region. They are with- 

 out exception of small size, and as compared with the reptiles, they are 

 humble and inconspicuous forms. 



We have no conception as to what worldwide cause occurred, whether 

 there was a sudden or a gradual change of conditions at the close of the 

 Cretaceous; we can only observe that the worldwide effect was the same: 

 the giant reptiles both of sea and land disappeared. Reptiles are so sen- 

 sitive to temperature that it is natural to attribute this extinction to a 

 general lowering of temperature, or refrigeration, but the flora shows no 

 evidence of this either in Europe or America; nor is there evidence of any 

 great geographic cataclysm on the surface of the earth, for the plant life 

 transition from one Age to the other in the Rocky Mountain region is alto- 

 gether gradual and gentle. 



Among the successive stages and formations in which this momentous 

 change from Age to Age is recorded are the following: 



' According to De Lapparent, Danian and Msestrichtian are not synonymous; the Mae- 

 strichtian, forming a part of the Aturian (or Upper Senonian), is earlier than the Danian. 



