V38 



The Earth 



Fig. 489 hnprint of a strange birdlike miinial 

 (Archaeornis) which lived in the Mesozoic era. 



(SMITHSONIAN INSTITUTION) 



pounds, its brain must have weighed lit- 

 tle more than a pound. So poorly devel- 

 oped a nervous system might be reason 

 enough for its becoming extinct when it 

 was obliged to compete with more intel- 

 ligent animals, but no doubt there were 

 many other factors which contributed 

 to its dying out. 



The "King Tyrant" reptile (Tyran- 

 nosaunis rex) with a monstrous body 

 and enormous mouth armed with great 

 teeth was probably a flesh eater and a 

 powerful fighter. Many of the flesh 

 eaters were apparently heavily armored, 

 and many were provided with long, 

 strong tails. Marks in the fossil bones in- 

 dicate that many bone fractures had oc- 

 curred while the animal was alive; this 

 leads us to the belief that these dinosaurs 

 must have been mighty fighters. You 

 will find Exercise 3 interesting. 



and Its Inhabitants Change unit x 



The dinosaurs died out completely in 

 what seems to us like a sudden catastro- 

 phe. There is not a single dinosaur fossil 

 in the rocks of the next era. But, if you 

 recall the length of a geologic era, you 

 will realize that the dinosaurs may have 

 died out gradually over a period of thou- 

 sands and thousands of years. 



The Age of Mammals. This is the sixth 

 (Cenozoic) era. It started about 60 mil- 

 lion years ago and lasted to within a mil- 

 lion years of the present. The dinosaurs 

 had become extinct at the close of the 

 last era but there were many other kinds 

 of reptiles. There were many species of 

 amphibians, thousands of species of fish, 

 and countless thousands of species of in- 

 vertebrates. But the earth in the sixth era 

 came to look very different from the 

 earth in the earlier eras. There were now 

 many kinds of flowering plants growing 

 in large numbers; a few of these had ap- 

 peared in the fifth era. Birds became com- 

 mon; and the mammals, of which there 

 had been only a few kinds in the age of 

 reptiles, were represented by thousands 

 of species. There were many more spe- 

 cies of mammals then than now; many 

 died out before modem times. 



Geologists have found evidence that 

 many changes in the earth itself occurred 

 during the age of mammals, just as in 

 previous eras. The Alps and the Hima- 

 layas were thrust up as mountain ranges. 

 The outlines of the continents changed. 

 During the early and middle part of the 

 age of mammals there seems to have 

 been a broad land connection between 

 North America and northern Asia and 

 also between North America and north- 

 ern Europe; many kinds of mammals 

 could have roamed from one continent 



