THE ORDOVICIAN PERIOD 437- 



pelecypods, cystoids, graptolites, and corals became diversified and 

 of higher types ; and bryozoans, crinoids, and fishes are known for 

 the first time. During this period graptolites and cystoids attained 

 their climax and were never again important. In this period, too, 

 the straight cephalopods rapidly developed from small to gigantic 

 forms and into many species, but occupied a subordinate place after 

 the Silurian. Before the close of the period all of the great types of 

 life and most of the important subdivisions were present. 



When the faunas of the Ordovician stages of North America are 

 compared with those of Europe, it is found that, although the genera 

 are usually identical, the species are different though similar. 



Adaptation to environment was almost as well established then as 

 now. Certain species lived almost exclusively on muddy bottoms, 

 certain ones on sandy, and still others on calcareous bottoms. There 

 was also adaptation to shallow and deep water. 



The effect of isolation is noticeable when, for example, a portion of 

 an epicontinental sea was cut off by some barrier, such as a gentle 

 upfolding of a portion of the sea bottom, or a bar, or when ocean cur- 

 rents, because of their lower or higher temperature, prevented the life 

 of different portions of the sea from mingling, the isolation of the fauna 

 permitting an independent development without interference from 

 outside. It consequently sometimes happened that the faunas of ad- 

 joining seas differed considerably. When the barriers were removed, 

 a rapid and marked change in the fauna was often quickly brought 

 about. 



The evolution of the life of the period gave rise to many new species, 

 with the result that when the fauna of the earliest and latest Ordovi- 

 cian are compared, they are found to differ widely. It is because of 

 the appearance of new species that the Ordovician series of strata have 

 been divided into several stages, which are usually easily distinguished 

 by their contained fossils. 



Climate and Duration of the Ordovician. — Fossils found in Ordo- 

 vician strata of Arctic lands show that the climate there was not unlike 

 that of the temperate and tropical regions of the same time. During 

 the Middle Ordovician, and again later in the period, reef corals were 

 common from Alaska to Texas. The conclusion is that climatic zones 

 did not exist, but that the climate of the world was uniformly equable 

 and less diversified than now. 



The duration of the period was about the same as that of the Cam- 

 brian, perhaps 4,000,000 years. 



